Volcanoes

Volcanoes are essentially natural vents in the Earth’s crust, acting as conduits for the escape of molten rock (magma), ash, gases, and steam from deep within the planet. The immense heat and pressure originating in the Earth’s mantle, where magma resides, coupled with the dynamic movement of tectonic plates, are the primary forces driving volcanic activity. As these colossal plates converge, diverge, or slide past one another, they generate fractures and weakened zones within the Earth’s crust. Through these vulnerabilities, magma, often superheated by groundwater that transforms into potent steam, is propelled upwards, eventually erupting onto the surface as lava. This cyclical process of eruptions and the subsequent cooling and solidification of expelled materials gradually constructs the characteristic conical or shield-like structures we identify as volcanoes. A notable concentration of such geological dynamism is observed within the “Pacific Ring of Fire,” a horseshoe-shaped belt encircling the Pacific Ocean, signifying intense interactions at plate boundaries.

Volcanoes are classified based on their eruptive patterns and physical configurations. In terms of activity, they can be categorized as active, exhibiting regular eruptions (e.g., Mount Etna); dormant, presently inactive but retaining the potential for future eruptions (e.g., Mount Vesuvius); or extinct, showing no signs of activity for millennia (e.g., Arthur’s Seat). These powerful eruptions also sculpt a variety of landforms, including highly fertile volcanic plateaus and plains, majestic volcanic mountains, and distinctive features such as caldera lakes, hot springs, and geysers.

While often associated with devastation, volcanoes also confer significant constructive benefits. Volcanic ash and weathered lava enrich soils, rendering them exceptionally fertile for agriculture. They serve as sources of valuable minerals and precious stones, and the geothermal heat they generate can be harnessed for energy. Moreover, the unique landscapes forged by volcanic activity frequently become tourist attractions, and crater lakes can provide consistent water sources. Nevertheless, their destructive capacity is undeniable. The release of ash and noxious gases pollutes the atmosphere, and subsequent heavy rainfall can trigger catastrophic floods and landslides, rendering affected regions uninhabitable for extended periods.

Exercises

I. Short Answer Questions.

Question 1.
What are known as volcanoes ?
Ans:

Volcanoes are essentially natural vents or openings in the Earth’s crust (or other planetary bodies) through which molten rock (called magma when underground and lava once it erupts onto the surface), ash, gases, and steam are expelled from deep within the planet.

Over time, the accumulation and solidification of these erupted materials often build up distinctive landforms, most commonly conical or shield-shaped mountains, around the vent. The activity of volcanoes is primarily driven by the immense heat and pressure within the Earth’s mantle and the dynamic movement of tectonic plates.

Question 2.

What is the difference between magma and lava ?

Ans:

The core distinction between magma and lava lies fundamentally in their geographical position relative to the Earth’s surface.

Magma refers to the molten rock found beneath the Earth’s crust. It resides in subterranean reservoirs, known as magma chambers, located within the crust or the upper mantle. This underground molten material is a complex amalgam, consisting of dissolved gases, suspended mineral crystals, and various trapped gas bubbles.

Conversely, lava is the term applied to this identical molten substance once it has been extruded onto the Earth’s surface, emerging from a volcanic vent or a fissure. 

Question 3.

Give one example each of an active volcano and a dormant volcano.

Ans:

Here’s an example for each:

• Active Volcano: Mount Etna in Sicily, Italy. It is one of the most active volcanoes in the world, with frequent eruptions that can range from mild lava flows to more explosive events. Its activity has been recorded for thousands of years.
• Dormant Volcano: Mount Fuji in Japan. While it hasn’t erupted since 1707, it’s considered dormant rather than extinct because scientists believe it still has the potential to erupt again in the future due to its geological setting and past activity.

Question 4.

What is the difference between a dormant volcano and an extinct volcano ?

Ans:

The classification of volcanoes as dormant or extinct primarily hinges on their history of eruption and their perceived potential for future activity. While there can sometimes be nuances and scientific debate, the core distinction is as follows:

A dormant volcano is one that has not erupted for a significant period of time, often spanning centuries or even millennia, but which still retains the potential to erupt again in the future. These volcanoes are essentially “sleeping” and are not currently active, yet geological monitoring might reveal subtle signs of internal activity, such as gas emissions, seismic tremors, or ground deformation, suggesting that their magma chamber is still active and could reactivate. Famous examples include Mount Vesuvius in Italy, which famously erupted in 79 AD after a long period of dormancy, and Mount Fuji in Japan. The key characteristic here is the potential for future eruption.

Conversely, an extinct volcano is one that scientists believe is highly unlikely to erupt ever again. This classification is usually made when there’s no record of eruption in human history, and more importantly, geological evidence suggests that the volcano has been cut off from its magma supply. Over vast stretches of geological time, the internal plumbing system of an extinct volcano typically cools and solidifies, making future eruptions physically improbable. Often, extensive erosion has significantly altered their original conical shape. 

Question 5.

What is the magma chamber of a volcano ?

Ans:

A magma chamber is best described as a substantial, underground reservoir of molten rock, or magma, positioned beneath a volcano. It serves as the fundamental storage facility for the hot, fluid material that ultimately powers volcanic eruptions.

This subterranean chamber originates when magma, formed deep within the Earth’s mantle due to extreme heat and pressure, gradually ascends through fissures and weak points in the crust. As it rises, the magma frequently encounters regions where the surrounding rock is either less dense or warmer, allowing it to accumulate and form a significant pool rather than proceeding directly to the surface.

Far from being a static feature, the magma chamber is a constantly evolving, temporary storage area. Pressure within the chamber escalates as additional magma flows in and as dissolved gases begin to separate from the molten rock. When this internal pressure surpasses the resistance of the overlying rock, it can propel the magma upward through the volcano’s vent, resulting in an eruption. The dimensions, depth, and chemical makeup of the magma chamber play a crucial role in determining the nature, frequency, and intensity of a volcanic outburst.

Question 6.

Name two types of landforms made by volcanoes.

Ans:

Volcanic activity sculpts a variety of distinctive landforms on the Earth’s surface. Two prominent examples include:

1. Volcanic Mountains (e.g., Composite Volcanoes or Stratovolcanoes): These are perhaps the most iconic volcanic landforms, characterized by their steep, conical shape. They are built up over many eruptions from alternating layers of viscous lava flows, ash, and other ejected rock fragments. 
2. Volcanic Plateaus (or Lava Plateaus): Formed by the outpouring of highly fluid lava from fissures or cracks in the Earth’s crust, rather than a central vent. This lava spreads out over vast areas, cooling and solidifying into extensive, relatively flat, elevated regions. 

Question 7.

What is called the Pacific Ring of Fire ? Why is it called so ?

Ans:

The Pacific Ring of Fire is an expansive, horseshoe-shaped region encircling the Pacific Ocean, characterized by an exceptional concentration of active volcanoes and earthquake epicenters. It is also the site of about 90% of the world’s earthquakes, including many of the most powerful seismic events. This geological belt extends along the western coasts of North and South America, through the Aleutian Islands, Kamchatka Peninsula, Japan, the Philippines, Indonesia, Papua New Guinea, and New Zealand.

The name “Ring of Fire” accurately reflects the intense volcanic activity and frequent earthquakes that punctuate its perimeter, creating a literal “ring” of geological “fire.” This remarkable dynamism is a direct outcome of plate tectonics. The Ring of Fire is situated at the boundaries where several major tectonic plates—most notably the vast Pacific Plate—interact with numerous surrounding plates, including the North American, South American, Eurasian, Indo-Australian, and Nazca plates. At these convergent, divergent, or transform boundaries, plates are in perpetual motion, either colliding, separating, or sliding past one another.

The predominant process driving the Ring of Fire’s activity is subduction, occurring at convergent plate boundaries. Here, a denser oceanic plate is compelled to slide beneath a lighter continental or another oceanic plate, descending into the Earth’s mantle. As the subducting plate plunges, it melts due to escalating heat and pressure, generating magma. The immense friction and stress generated by these grinding plate movements also trigger frequent and potent earthquakes. Consequently, the “fire” in its name vividly symbolizes both the visible eruptions and the subterranean heat and friction that energize this geologically volatile region.

Question 8.

Name the three types of volcanoes on the basis of the frequency of their eruption.

Ans:

Based on the frequency of their eruptions, volcanoes are categorized into three distinct types:

1. Active Volcanoes: These are volcanoes that erupt regularly or have shown signs of recent activity, indicating a high likelihood of future eruptions in the near future. Examples include Mount Etna in Italy and Stromboli, often called the “Lighthouse of the Mediterranean.”
2. Dormant Volcanoes: Also referred to as “sleeping volcanoes,” these have not erupted for a considerable period, perhaps hundreds or even thousands of years, but still possess the potential to erupt again. Mount Vesuvius in Italy, which famously buried Pompeii, is a well-known example of a dormant volcano that has erupted after long periods of inactivity.
3. Extinct Volcanoes: These are volcanoes that show no recorded history of eruptions for a very long time—typically tens of thousands of years or more—and are considered highly unlikely to erupt again. Their magma supply has either dried up or the conduit to the surface has sealed. Often, the craters of extinct volcanoes become filled with water, forming lakes, such as Arthur’s Seat in Edinburgh, Scotland.

Question 9.

What are known as Shield volcanoes ?

Ans:

Shield volcanoes are a distinct type of volcano characterized by their broad, gently sloping profiles, which strongly resemble the shape of a warrior’s shield lying flat on the ground. 

When this runny lava emerges from the central vent or fissures on the volcano’s flanks, it flows easily and spreads out over vast distances before solidifying. Unlike the thicker, stickier lava of other volcano types, basaltic lava doesn’t pile up steeply. Over countless eruptions occurring over long periods, layer upon thin layer of solidified lava accumulates, gradually building up the volcano’s wide base and shallow slopes. Eruptions from shield volcanoes are typically effusive and non-explosive, producing impressive lava flows rather than violent explosions. Prominent examples of shield volcanoes include the massive Mauna Loa and Kilauea in Hawaii, which are among the largest volcanoes on Earth by volume.

Question 10.

Mention any two extrusive landforms caused by volcanic eruptions.

Ans:

Extrusive landforms encompass geological features that originate when molten rock (lava) and other volcanic debris are expelled onto the Earth’s surface, subsequently cooling and solidifying.

Two prominent extrusive landforms resulting from volcanic activity are:

Volcanic Cones/Mountains (e.g., Composite, Shield, Cinder Cones): These represent the most iconic extrusive landforms, constructed through a series of eruptions. Their shape and dimensions vary considerably, influenced by the characteristics of the lava and the explosive nature of the eruption. For example, composite volcanoes (such as Mount Fuji) are distinguished by their towering, steep-sided conical shape, formed by alternating strata of viscous lava flows and fragmented pyroclastic material. Conversely, shield volcanoes (like Mauna Loa) exhibit broad, gently sloping profiles, a result of highly fluid basaltic lava spreading extensively across the landscape.

Lava Plateaus: These expansive, relatively flat topographic features arise when highly fluid lava erupts from elongated fissures (cracks) in the Earth’s crust rather than a singular central vent. The effusive lava flows then spread over vast regions, solidifying to form thick, stratified plains. A prime illustration of this phenomenon is the Deccan Traps located in India.

Question 11.

Name any two intrusive landforms made by volcanic eruptions.

Ans:

Intrusive landforms, often referred to as plutons, are geological structures formed when magma cools and solidifies beneath the Earth’s surface. Unlike extrusive landforms that result from lava reaching the surface, intrusive forms are revealed only after the overlying rock layers are eroded away over long periods.

Here are two prominent examples of intrusive landforms:

1. Batholiths: These are vast, irregularly shaped masses of intrusive igneous rock, typically granitic in composition. They represent the cooled and solidified magma chambers that fed ancient volcanoes. Due to their immense size, batholiths often form the core of mountain ranges, becoming exposed after millions of years of erosion.
2. Dykes (or Dikes): These are sheet-like intrusions of igneous rock that cut across existing rock layers (discordant). They form when magma injects into cracks or fissures and then solidifies. Dykes can range from a few centimeters to several meters in thickness and can extend for kilometers, often appearing as wall-like structures when exposed by erosion.

Question 12.

How are hot springs formed ?

Ans:

Hot springs are a fascinating geological phenomenon that occurs when groundwater is heated by geothermal energy and subsequently rises to the Earth’s surface. The formation of hot springs typically involves a specific set of conditions:

Firstly, a source of heat is crucial. This often comes from magma or hot rock deep within the Earth’s crust, particularly in volcanically active regions or areas where the Earth’s crust is thin. Even in non-volcanic areas, the natural geothermal gradient (the increase in temperature with depth) can provide sufficient heat if water circulates deeply enough.

Secondly, percolating groundwater plays a vital role. Rainwater and surface water seep into the ground through cracks, fissures, and porous rock formations. As this water descends, it comes into contact with the heated rocks. 

Finally, geological pathways are necessary for the heated water to return to the surface. As the water heats up, it becomes less dense and begins to rise through a system of fractures, faults, or conduits in the rock. This natural buoyancy drives the heated water upwards, eventually emerging as a hot spring. The specific chemical composition of the water can also change as it interacts with various minerals in the rock formations during its underground journey.

Question 13.

What is called the Pacific Ring of Fire ? Why is it called so ?

Ans:

The Pacific Ring of Fire, also known as the Circum-Pacific Belt, is a vast, horseshoe-shaped zone encircling the Pacific Ocean. This region is remarkably distinguished by its exceptionally high concentration of active volcanoes and frequent, powerful earthquakes. Stretching approximately 40,000 kilometers (25,000 miles), it traces the boundaries of several major tectonic plates, including the massive Pacific Plate, as well as the North American, South American, Eurasian, Philippine Sea, Juan de Fuca, Cocos, and Nazca plates. About 75% of the world’s active and dormant volcanoes and approximately 90% of the world’s earthquakes occur along this dynamic belt.

It is called the “Ring of Fire” due to the intense and pervasive geological activity that manifests as a continuous, fiery arc of volcanoes around the Pacific basin. The name vividly describes the visual representation of numerous volcanic eruptions that historically have given the impression of a “ring of fire” along the coastlines. This fiery descriptor directly reflects the high density of volcanoes within this zone, which are a direct consequence of the constant movement and interaction of the Earth’s tectonic plates. Specifically, at these plate boundaries, processes like subduction (where one tectonic plate slides beneath another) lead to the melting of rock and the generation of magma, which then rises to the surface, fueling the frequent volcanic eruptions that characterize this fiery “ring.”

Question 14.

Give an example of each conical volcano and fissure volcano.

Ans:

Conical volcanoes are distinguished by their classic cone-shaped structure, which develops from numerous eruptions depositing successive layers of solidified lava, ash, cinders, and other fragmented volcanic materials around a central opening. The steepness of their slopes can vary, influenced by the characteristics of the erupted substances and the fluidity of the lava. A prime illustration of a conical volcano is Mount Fuji in Japan, which stands as an iconic stratovolcano (also known as a composite volcano), renowned for its almost perfectly symmetrical cone, sculpted by alternating flows of viscous lava and explosive releases of ash and rock.

Conversely, fissure volcanoes, or fissure vents, release molten material not from a single central point but from elongated cracks or fractures in the Earth’s crust. Rather than constructing a towering cone, these eruptions typically generate expansive, flowing lava that spreads across wide areas, ultimately forming extensive plains or plateaus of hardened lava. Although minor spatter cones might develop along the length of the fissure, these volcanoes fundamentally lack the pronounced conical edifice characteristic of their counterparts. A notable instance of fissure volcanism is the Laki fissures in Iceland. In 1783-1784, massive eruptions from these fissures unleashed immense quantities of basaltic lava, inundating vast regions and exerting substantial environmental and climatic effects globally. Iceland, situated on the Mid-Atlantic Ridge (a divergent plate boundary), is a quintessential setting for such fissure eruptions.

Question 15.
What is the difference between a dormant volcano and an extinct volcano ?
Ans:

The key distinction between a dormant volcano and an extinct volcano lies in their potential for future eruptions.

• Dormant Volcano: A dormant volcano is one that has not erupted for a long period of time (often centuries or even millennia), but is still considered to have the potential to erupt again in the future. While outwardly quiet, there might be underlying seismic activity, gas emissions, or ground deformation that suggests a magma chamber is still active beneath the surface. It’s essentially “sleeping” but could “awaken.” A famous example is Mount Vesuvius in Italy, which famously erupted in AD 79 after a long period of dormancy.
• Extinct Volcano: It has not erupted for tens of thousands or even millions of years, and geological evidence suggests it has been cut off from its magma supply or that the conditions necessary for an eruption no longer exist. The volcanic plumbing system beneath it is believed to have solidified, making future activity highly improbable. Arthur’s Seat in Edinburgh, Scotland, is a well-known example of an extinct volcano.

II. Match the following

Ans:

III. Fill in the blanks below

1. The forces arising from the interior of the earth are called ___________ forces.

Ans: endogenic

2. The molten rock that reaches the surface of the earth is called _________ .

Ans: magma.

3. A lava ____________ is made up of basic lava flows solidified away from the vent.

Ans: shield

4. __________ are intrusions of igneous rock that are vertical in shape.

Ans:  Vents

5. The Circum-Paciflc Belt is also called the ____________________ .

Ans: Pacific Ring of Fire.

IV. Long Answer Questions

Question 1.
Explain the various parts of a volcano.
Ans:

A volcano, in its typical conical form, comprises several distinct parts that facilitate the eruption of molten material from the Earth’s interior. Understanding these components helps in grasping the mechanics of a volcanic eruption.

At the very heart of the system lies the magma chamber, a vast, subterranean reservoir of molten rock, gases, and other superheated materials. This chamber is the source of the volcano’s power, accumulating magma under immense pressure from deep within the Earth’s mantle. Extending upwards from this chamber is the main vent or conduit, which acts as the primary pipeline through which magma, ash, and gases ascend towards the surface. This central pathway is the core of the volcanic structure.

As the conduit reaches the surface, it typically opens into a crater, a bowl-shaped depression at the summit of the volcano. This is the mouth from which eruptions primarily occur, expelling lava, ash, and pyroclastic material. Over time, the repeated accumulation and solidification of these erupted materials, such as lava flows, ash, and rock fragments, gradually build up the characteristic volcanic cone – the visible mountain structure we recognize. In addition to the main vent, some volcanoes may have secondary vents or fissures on their flanks, through which eruptions can also occur, sometimes forming smaller parasitic cones. During an eruption, a towering ash cloud or plume, consisting of pulverized rock, ash, and gases, is propelled high into the atmosphere, often spreading over vast distances. Finally, the lava flow itself refers to the stream of molten rock that flows down the slopes of the volcano during an effusive eruption, cooling and solidifying to add layers to the volcanic structure.

Question 2.

Describe the causes of volcanic eruptions.

Ans:

Volcanic eruptions, striking manifestations of Earth’s dynamic interior, are fundamentally driven by an intricate interplay of subterranean heat, immense pressure, and the incessant movement of tectonic plates. These colossal geological occurrences stem from several core mechanisms.

A primary catalyst is the extreme internal heat and pressure within the Earth’s mantle. As it ascends towards the surface, the pressure from the overlying rock column diminishes. This pressure reduction facilitates the exsolution and rapid expansion of dissolved gases within the magma, similar to the effervescence observed when a carbonated drink is unsealed. This substantial gas expansion generates immense upward force, compelling the magma to breach any available pathways within the crust.

Furthermore, plate tectonics provides the essential structural conduits for magma to reach the surface. The Earth’s lithosphere is fragmented into vast, continuously shifting tectonic plates. Volcanic activity is heavily concentrated along the boundaries where these plates interact. At divergent boundaries, where plates pull apart (such as at mid-ocean ridges or continental rift valleys), the thinning crust creates fissures through which magma can readily ascend and erupt. Conversely, at convergent boundaries, where one oceanic plate is forced to subduct beneath another (either oceanic or continental), the descending plate introduces water and volatile compounds into the mantle. This process lowers the melting point of the overlying mantle rock, generating magma that subsequently rises to form volcanic arcs. Even hotspots, instances of volcanic activity occurring far from typical plate boundaries, are linked to deep mantle plumes that transport superheated material closer to the surface, initiating localized melting and eruptions.

Finally, the behavior of volatile gases contained within the magma profoundly shapes the nature of an eruption. Magma holds dissolved gases, primarily water vapor, carbon dioxide, and sulfur dioxide. If the magma possesses low viscosity (is highly fluid), these gases can escape relatively easily, leading to effusive (flowing) eruptions. However, if the magma is highly viscous (thick and sticky), the gases become trapped, accumulating immense pressure. When this pressure overwhelms the strength of the overlying rock, it culminates in a sudden, violent, and often explosive eruption, fragmenting the magma into ash and pyroclastic debris.

Question 3.

Explain briefly the landforms created by volcanoes on the surface of the earth.

Ans:

Volcanic activity, a powerful force of geological transformation, sculpts a variety of distinctive landforms on the Earth’s surface. 

One of the most iconic landforms created are volcanic mountains or cones. These are built up over time by the repeated accumulation of lava flows, ash, and rock fragments ejected during eruptions. The shape of these mountains varies significantly based on the type of eruptive material and its viscosity, ranging from broad, gently sloping shield volcanoes to steep-sided composite cones.

Beyond mountains, extensive outflows of highly fluid lava can solidify to form vast, elevated flat-topped regions known as volcanic plateaus. Similarly, if these lava flows are particularly widespread and settle in lower-lying areas, they can create fertile volcanic plains.

Another dramatic feature is a caldera lake, which forms when the summit and upper flanks of a volcano collapse inward after a massive eruption, creating a large, bowl-shaped depression that subsequently fills with water. Furthermore, the heat associated with underground magma can manifest on the surface as hot springs, where geothermally heated groundwater emerges, or as spectacular geysers, which are intermittent eruptions of hot water and steam, both unique indicators of underlying volcanic heat.

Question 4.

Write any three destructive effects of volcanoes.

Ans:

Here are three destructive effects of volcanoes, presented uniquely and without plagiarism:

1. Widespread Devastation of Life and Property: Volcanic eruptions unleash incredibly destructive forces that can obliterate everything in their path. Hot, fast-moving lava flows incinerate vegetation, buildings, and infrastructure, making areas uninhabitable. Furthermore, eruptions often eject pyroclastic flows – superheated mixtures of gas and volcanic debris – that race down slopes at incredible speeds, vaporizing anything they encounter. The sheer force of explosions and falling ash and rocks can collapse structures and bury entire landscapes, leading to massive loss of human and animal life, as well as the destruction of homes, farms, and essential services.
2. Atmospheric Contamination and Environmental Impact: The gases, ash, and fine particulate matter expelled during an eruption can significantly pollute the atmosphere. Volcanic gases like sulfur dioxide, carbon dioxide, and hydrogen sulfide can be toxic to living organisms, leading to respiratory illnesses in humans and animals, and acid rain that damages crops, forests, and aquatic ecosystems. Extensive ashfall can blanket vast areas, disrupting air travel, reducing visibility, and accumulating on roofs, causing structural collapse. This atmospheric contamination can persist for extended periods, impacting climate patterns and reducing air quality far beyond the immediate vicinity of the volcano.
3. Triggering Secondary Hazards and Long-Term Displacement: Volcanic activity frequently initiates a cascade of other destructive natural hazards. Intense ashfall combined with heavy rainfall can generate lahars – destructive mudflows composed of volcanic debris and water – that surge down river valleys, inundating communities and infrastructure. Eruptions can also trigger landslides and tsunamis (if occurring near water bodies), exacerbating the destruction. In the long term, these combined effects often lead to the displacement of entire populations, forcing people to abandon their homes and livelihoods due to rendered land infertile, destroyed infrastructure, and lingering health risks, making recovery a prolonged and arduous process.

Question 5.

Describe the distribution of volcanoes in the world.

Ans:

The distribution of volcanoes across the globe is not random; instead, it exhibits distinct patterns closely tied to the dynamic processes of plate tectonics. The vast majority of the world’s volcanoes are found in specific zones where the Earth’s rigid lithospheric plates interact, either by colliding, pulling apart, or, less commonly, moving over fixed hotspots in the mantle.

The most prominent and active volcanic region globally is the Pacific Ring of Fire. This horseshoe-shaped belt, approximately 40,000 kilometers long, encircles the Pacific Ocean and is home to roughly 75% of the world’s active volcanoes. It’s characterized by numerous convergent plate boundaries, where oceanic plates are subducting (diving) beneath other oceanic or continental plates. As the subducting plate descends, it melts, and the resulting magma rises to the surface, forming arcs of volcanoes such as the Andes in South America, the Cascade Range in North America, and the island arcs of Japan, the Philippines, Indonesia, and the Aleutian Islands.

Another significant area of volcanic activity occurs along divergent plate boundaries, particularly at mid-ocean ridges. Here, tectonic plates are pulling away from each other, allowing molten rock from the mantle to rise and create new oceanic crust. This process, known as seafloor spreading, results in extensive underwater volcanic mountain ranges, like the Mid-Atlantic Ridge. While most of these volcanoes are submerged, some, like those in Iceland, emerge above sea level due to intense volcanic build-up. These eruptions are typically less explosive than those at convergent boundaries, characterized by more fluid basaltic lava flows.

Finally, a smaller but notable number of volcanoes are found in intraplate regions, far from plate boundaries. These are known as hotspot volcanoes, where plumes of abnormally hot mantle material rise from deep within the Earth, melting the overlying crust. As the tectonic plate moves over this stationary hotspot, a chain of volcanoes forms, with the youngest and most active volcanoes located directly over the plume. The Hawaiian Islands are the classic example of such a volcanic chain, where the Pacific Plate has moved over the Hawaiian hotspot for millions of years, creating a progression of islands from older, extinct volcanoes to the active ones currently forming.

Question 6.

Describe important volcanic landforms on earth.

Ans:

Volcanic activity significantly sculpts the Earth’s topography, yielding a diverse array of landforms that arise from either extrusive eruptions on the surface or intrusive solidification of magma beneath.

Extrusive Volcanic Landforms: These are the most immediately recognizable features, created when molten rock, ash, and gases erupt onto the Earth’s surface and subsequently solidify. Classic examples include Volcanic Mountains or Cones, which are built up through successive eruptions and vary in shape. Shield Volcanoes, like Hawaii’s Mauna Loa, are characterized by their broad, gently sloping profiles, formed by highly fluid basaltic lava. In contrast, Cinder Cone Volcanoes, such as Arizona’s Sunset Crater, are smaller, steep-sided cones composed of loose fragments ejected during explosive, often singular, eruptions. The most common and often symmetrical are Composite Volcanoes (Stratovolcanoes), exemplified by Japan’s Mount Fuji, which are constructed from alternating layers of viscous lava flows, ash, and rock fragments resulting from varied eruptive styles. Beyond cones, Volcanic Plateaus (Flood Basalts) represent vast, elevated, flat-topped expanses formed by immense volumes of fluid basaltic lava issuing from fissures and spreading over extensive areas, as seen in India’s Deccan Traps. Calderas, like Oregon’s Crater Lake, are large, basin-shaped depressions that form when a volcano’s magma chamber is largely emptied during a colossal eruption, causing the overlying structure to collapse. Lastly, Lava Domes are steep-sided, dome-shaped masses created by the slow extrusion of very thick, viscous lava piling over a vent, while Hot Springs and Geysers, such as Yellowstone’s Old Faithful, are geothermal features reflecting underlying volcanic heat causing superheated groundwater to rise or periodically erupt.

Intrusive Volcanic Landforms: These features originate when magma solidifies beneath the Earth’s surface and become visible only after overlying rock has been removed by erosion. Batholiths are the largest of these, vast bodies of coarse-grained rock that solidify deep within the crust, frequently forming the core of mountain ranges after extensive uplift and erosion (e.g., the Sierra Nevada Batholith). Sills are flat, sheet-like intrusions that solidify horizontally between existing rock layers, while Dikes are wall-like intrusions that cut vertically or diagonally across rock strata. Laccoliths are mushroom-shaped intrusions where viscous magma has pushed up overlying sedimentary layers into a dome. Finally, Volcanic Necks (or Plugs), like New Mexico’s Shiprock, are the solidified magma that once filled a volcano’s central conduit, left standing as prominent, isolated pinnacles after the softer surrounding cone has eroded away over millions of years.

Practice Questions (Solved)

Question 1.
Name three causes of volcanic eruptions.
Ans:

Volcanic eruptions, intricate geological phenomena, are fundamentally propelled by three interconnected mechanisms.

The Ascent of Buoyant Magma

Deep within the Earth, intense heat causes rocks to melt, forming magma. This molten rock is inherently less dense than the surrounding solid rock, much like a hot air balloon is lighter than the cooler air it displaces. This inherent buoyancy naturally drives magma upwards, where it collects in subterranean reservoirs known as magma chambers. As magma continues to accumulate, pressure within these chambers steadily increases, persistently seeking an exit.

Pressure from Dissolved Gases

Magma contains various dissolved gases, including water vapor, carbon dioxide, and sulfur dioxide, all held under immense pressure. As magma ascends closer to the Earth’s surface, the confining pressure diminishes. This reduction in pressure causes these dissolved gases to “exsolve,” or separate from the magma, forming bubbles. These gas bubbles expand rapidly as they approach the surface, generating a powerful upward thrust and significantly elevating the internal pressure within the magma chamber and conduit. When this internal gas pressure surpasses the strength of the overlying rock, it can trigger a forceful, explosive eruption.

Tectonic Plate Movement and Crustal Weaknesses

The Earth’s lithosphere is fragmented into colossal tectonic plates, which are in perpetual motion. The vast majority of volcanic activity occurs at the boundaries where these plates interact.

• Divergent Boundaries: At locations where plates pull apart, such as mid-ocean ridges, the Earth’s crust thins. This thinning creates fissures and weak points, providing easy pathways for magma to rise and erupt.
• Convergent Boundaries: Where plates collide and one slides beneath the other (a process known as subduction), the descending plate melts due to the extreme heat and pressure. The resulting magma, frequently enriched with water squeezed from the subducting plate, becomes buoyant and ascends, leading to the formation of volcanic arcs.

Question 2.

Name the largest active volcano in the world.

Ans:

Mauna Loa, located on the Big Island of Hawaii, stands as the world’s largest and most voluminous active volcano. While its peak rises over 4,000 meters (13,681 feet) above sea level, its true immense size becomes apparent when considering its foundation, which extends far below the ocean’s surface. From its base on the seafloor, Mauna Loa ascends to an astonishing total elevation exceeding 10,000 meters (33,500 feet), thereby surpassing Mount Everest in its overall vertical measurement. This formidable geological feature is classified as a shield volcano, characterized by its wide, gently sloping silhouette, a form sculpted over millennia by its highly fluid lava flows. Mauna Loa boasts an extensive eruptive past, with its most recent activity noted in November 2022.

Question 3.

Which volcano is known as the ‘light house of the Mediterranean ?

Ans:

The volcano affectionately dubbed the “Lighthouse of the Mediterranean” is indeed Stromboli.

Situated on its namesake island just off Sicily’s northern coast in Italy, this stratovolcano has earned its moniker through its remarkably persistent and frequently observable explosive activity. Its regular eruptions, which propel incandescent volcanic material skyward, create a vivid, natural beacon easily discernible from afar at night. This consistent glow has historically served as a vital landmark for mariners navigating the Mediterranean Sea. The distinct style of these eruptions is so characteristic that it has even lent its name to a specific type of volcanic activity, known scientifically as “Strombolian eruptions.”

Question 4.

Name the three belts where volcanoes are found.

Ans:

Volcanoes are not randomly distributed across the Earth’s surface but are largely concentrated in distinct belts, primarily correlating with the boundaries of tectonic plates. The three prominent belts where the majority of the world’s volcanoes are found are:

1. The Circum-Pacific Belt (or Pacific Ring of Fire): This is by far the most extensive and volcanically active belt, shaped like a horseshoe around the rim of the Pacific Ocean. It stretches along the western coasts of North and South America, across the Aleutian Islands, Kamchatka Peninsula, Japan, the Philippines, Indonesia, and New Zealand. This region is characterized by intense plate convergence and subduction, where oceanic plates are forced beneath continental or other oceanic plates, leading to the formation of numerous volcanoes and frequent earthquakes. Approximately 75% of the world’s active and dormant volcanoes are located in this belt.
2. The Mid-Atlantic Belt: This belt runs along the Mid-Atlantic Ridge, which is a major divergent plate boundary located in the middle of the Atlantic Ocean. Here, tectonic plates are pulling apart, allowing magma to rise from the mantle and create new oceanic crust, forming a chain of submarine volcanoes. While most of these volcanoes are underwater, some rise above sea level to form volcanic islands, such as Iceland, the Azores, and St. Helena.
3. The Mid-Continental Belt (or Alpine-Himalayan Belt): This belt stretches across central Europe and Asia, encompassing the Mediterranean Sea, the Alps, and the Himalayan mountain ranges, extending into parts of East Africa. While not as volcanically active as the Pacific Ring of Fire, it still hosts significant volcanoes, particularly in the Mediterranean region (e.g., Mount Vesuvius, Mount Etna, Stromboli). Volcanism in this belt is often associated with the collision of the African, Eurasian, and Indian plates. The East African Rift Valley, a part of this broader region, is also a significant area of volcanic activity due as the African plate slowly splits apart.

Question 5.

In which belt most of the volcanoes of the world are found ?

Ans:

This expansive zone wraps around the Pacific Ocean, extending from the western edges of the Americas, across the Bering Strait, and continuing through Japan, the Philippines, Indonesia, and down to New Zealand.

This area is marked by vigorous tectonic plate movement, where numerous major and minor plates either collide, pull apart, or grind past one another. The frequent process of oceanic plates diving beneath continental or other oceanic plates along this “ring” generates immense heat and pressure. This intense geological activity leads to the melting of rock and the subsequent creation of magma. This molten rock then ascends to the surface, resulting in a dense cluster of active volcanoes and frequent seismic events. Roughly 75% of the Earth’s active and dormant volcanoes, alongside approximately 90% of the globe’s earthquakes, manifest within this exceptionally dynamic and geologically crucial belt.

Question 6.

Name three causes of Earthquakes.

Ans:

Earthquakes are primarily caused by the sudden release of energy in the Earth’s crust, resulting in seismic waves. Here are three distinct causes:

1. Tectonic Plate Movement: The most prevalent cause of earthquakes is the dynamic interaction of the Earth’s colossal tectonic plates. These massive slabs of the lithosphere are constantly, albeit slowly, moving, driven by convection currents within the mantle. As they converge, diverge, or slide past one another, immense stress builds up along their boundaries. When this accumulated stress exceeds the strength of the rocks, they suddenly fracture and slip, releasing a burst of energy that propagates as seismic waves. This sudden movement is what we perceive as an earthquake.
2. Volcanic Activity: While less frequent than tectonic earthquakes, volcanic eruptions and the movement of magma beneath the Earth’s surface can also trigger seismic events. As magma ascends through the crust, it can fracture surrounding rock, causing small to moderate earthquakes. These “volcano-tectonic earthquakes” are often precursors to eruptions, indicating the upward migration of magma. The collapse of a volcano’s summit or caldera after a major eruption can also generate seismic tremors.
3. Human-Induced Activities (Anthropogenic Causes): Increasingly, human activities are recognized as catalysts for seismic events. Large-scale construction projects like the impoundment of water in massive reservoirs behind dams can alter stress on underlying faults, sometimes leading to “reservoir-induced seismicity.” Similarly, deep-well injection of wastewater from oil and gas operations, particularly fracking, can lubricate existing faults and increase fluid pressure, thereby triggering earthquakes. Mining activities, especially the collapse of mine shafts, and even underground nuclear testing have also been linked to localized seismic disturbances.

Question 7.

What is epicentre ?

Ans:

The epicenter refers to the specific point on the Earth’s surface situated directly above the hypocenter, or focus, which marks the true subsurface origin of an earthquake.

To conceptualize this, envision the precise location deep within the Earth where an earthquake’s seismic rupture initially propagates – this is the hypocenter. Now, visualize an imaginary vertical line extending from this deep-seated point straight up to the Earth’s surface. 

This concept holds paramount importance in seismology because the epicenter typically corresponds to the region on the surface where the earthquake’s impact is experienced with the greatest intensity, often leading to the most significant damage. Consequently, when earthquake locations are reported in the news, they almost invariably refer to the epicenter.

Question 8.

Indicate the world distribution of active volcanoes.

Ans:

The global distribution of active volcanoes is not arbitrary; rather, it is strongly linked to the active boundaries of Earth’s tectonic plates, with some exceptions occurring at isolated “hotspots” found within these plates.

The most prominent and volcanically active region globally is the Pacific Ring of Fire. This expansive, horseshoe-shaped arc encircles the Pacific Ocean, following the western coastlines of North and South America, and continuing through the Aleutian Islands, Kamchatka, Japan, the Philippines, Indonesia, Papua New Guinea, and New Zealand. Its intense seismic and volcanic activity is a direct result of the continuous collision and subduction of numerous major tectonic plates beneath adjacent ones.

In addition to the Pacific Ring of Fire, other significant areas of active volcanism include the Mid-Atlantic Ridge. This marks a divergent plate boundary where new oceanic crust is constantly being created. While largely submerged, volcanic activity surfaces in places like Iceland, which is entirely volcanic and highly dynamic, as well as the Azores. The Mediterranean Belt, also known as the Alpine-Himalayan Belt, is another key zone. Stretching across Southern Europe (including regions such as Italy with Mount Etna and Mount Vesuvius, and Greece with Santorini) and into parts of Asia Minor, this belt is a consequence of the ongoing collision between the African and Eurasian plates. Furthermore, the East African Rift Valley illustrates a divergent continental plate boundary, where the African continent is slowly splitting apart, giving rise to a chain of active volcanoes, such as Mount Kilimanjaro and Mount Nyiragongo. Finally, some volcanoes develop away from plate boundaries due to “intraplate hotspots.” These are locations where extremely hot magma plumes rise from deep within the Earth’s mantle. 

Question 9.

Give two reasons why tremors occur inside the earth?

Ans:

Earthquakes, fundamentally, are the vibrations or shaking of the Earth’s surface resulting from the abrupt release of energy within its crust. Their occurrence can be attributed to two main geological processes.

The predominant cause is the movement along tectonic plate boundaries. The Earth’s rigid outer layer is fractured into numerous large segments known as tectonic plates, which are perpetually in gradual motion. These plates engage in dynamic interactions at their interfaces: they can converge, diverge, or slide laterally past each other. Due to significant friction and irregularities along these boundaries, the plates frequently become locked. As the slow yet continuous motion of these plates persists, considerable stress and strain accumulate in the rocks along fault lines—fractures within the Earth’s crust. 

A secondary, though generally less intense, cause of tremors is volcanic activity. The ascent of magma and gases within the Earth’s crust beneath a volcano can induce pressure shifts and fracturing in the adjacent rock. As magma forces its way upward, or as subterranean volcanic chambers undergo cycles of filling and emptying, the overlying ground can experience vibrations. These tremors, often termed “volcano-tectonic” or “long-period” earthquakes, serve as a crucial indicator that a volcano might be on the verge of erupting or that internal activity is underway within its magmatic plumbing system.

Question 10.
Distinguish between :

1. Seismology and Volcanology.
2. Volcanic Dust and Volcanic Ash.

Ans:

Seismology vs. Volcanology: Distinct Yet Interconnected Earth Sciences

The dynamic nature of our planet is explored through two unique, yet deeply intertwined, scientific disciplines: seismology and volcanology.Seismologists meticulously analyze these waves, effectively using them as an internal diagnostic tool, to unravel the planet’s internal structure, pinpoint the origins and mechanisms of seismic events, shed light on the powerful forces of plate tectonics, and even locate hidden resources. Their core focus lies in the oscillations and movements occurring within the Earth’s solid crust and mantle, revealing the profound tremors that continually reshape our world.

In contrast, volcanology specifically investigates volcanoes, their associated phenomena, and the myriad geological processes that accompany them. Volcanologists delve into fundamental questions regarding why and how volcanic eruptions occur, meticulously examine the chemical composition of magma and lava, classify the diverse styles of volcanic explosions, trace the formation of various volcanic landforms, and assess the inherent dangers posed by volcanic activity. Their field of study predominantly revolves around the forceful ascent and expulsion of molten rock and gases from the Earth’s depths to its surface, observing the fiery transformation of internal geological forces into visible, often dramatic, surface events.

Volcanic Dust vs. Volcanic Ash: A Spectrum of Ejected Particles

When a volcano unleashes its power, it ejects a range of particulate matter, often broadly referred to as “volcanic ash,” but with a finer distinction for its most minute components. Volcanic dust specifically refers to the exceedingly fine particles of pulverized rock, minerals, and solidified volcanic glass propelled into the atmosphere during explosive eruptions. These microscopic fragments typically measure less than 2 millimeters in diameter, often being even smaller than 0.063 millimeters. Their minuscule size allows volcanic dust to remain suspended in the atmosphere for extended periods, drifting vast distances from the eruption site, contributing to vibrant sunsets, and, in significant concentrations, potentially influencing global climate by partially blocking sunlight.

Conversely, volcanic ash serves as a more all-encompassing term for all fragmented material ejected from a volcano during an eruption. While it certainly includes the ethereal volcanic dust, volcanic ash also incorporates slightly larger fragments, generally defined as having a diameter under 2 millimeters. Possessing a gritty, abrasive texture, its color can range from light grey to deep black. The descent of volcanic ash can present considerable hazards, including respiratory issues, structural damage to buildings, and severe disruptions to air travel, owing to its abrasive, corrosive, and electrically conductive properties. 

Question 11.

Describe the materials thrown out during volcanic eruptions.

Ans:

Volcanic gases, predominantly water vapor, carbon dioxide, and sulfur dioxide, are crucial to the eruptive force. These dissolved gases expand and escape as magma rises, propelling the eruption and potentially forming acid rain or affecting climate.

Molten rock, known as lava upon surfacing, varies in viscosity based on silica content and temperature. Low-silica lavas are highly fluid, forming smooth (Pahoehoe) or jagged (Aa) flows, while high-silica lavas are viscous, creating slow-moving flows or lava domes.

Pyroclastics, or “fire-broken” fragments, encompass various sizes of solid material ejected during explosive eruptions. These range from fine volcanic ash (under 2mm), which travels far and impacts aviation, to lapilli (2-64mm), and larger volcanic blocks (angular, over 64mm) or volcanic bombs (molten, over 64mm, acquiring aerodynamic shapes). Other forms include porous pumice and scoria.

Question 12.

Describe the effect of volcanic eruption of Karakatoa in 1883.

Ans:

The 1883 eruption of Krakatoa, an Indonesian volcano located in the Sunda Strait, was a cataclysmic event with both immediate, devastating local consequences and significant, far-reaching global impacts.

Locally and regionally, the eruption caused immense destruction and loss of life. The most significant impact was the generation of colossal tsunamis, with waves reaching heights of up to 42 meters (138 feet) in some areas. These monstrous waves ravaged the coastal towns and villages of Java and Sumatra, virtually wiping out 165 settlements and leading to an official death toll of over 36,000, though modern estimates suggest higher figures. The sheer force of the tsunamis was so great that the steamship “Berouw” was carried nearly a mile inland on Sumatra, killing its entire crew. Pyroclastic flows, hot avalanches of ash and gas, also incinerated everything in their path. The volcano itself largely collapsed into a submarine caldera, drastically altering the geography of the surrounding islands. The sound of the climactic eruption was arguably the loudest ever recorded, heard clearly over 4,800 kilometers (3,000 miles) away in Perth, Australia, and even causing pressure fluctuations detectable around the globe.

This atmospheric veil led to a noticeable cooling of global temperatures for several years following the eruption, with average Northern Hemisphere summer temperatures dropping by approximately 0.4°C (0.72°F). The fine particles also caused spectacular and prolonged vivid sunsets and unusual atmospheric optical phenomena, like “blue moons,” observed across the globe for months, even years, after the event. The eruption’s far-reaching effects on atmospheric circulation patterns also contributed to abnormal weather conditions in various regions, including unusually cold winters in the Northern Hemisphere. The Krakatoa eruption of 1883 thus became one of the first truly global catastrophes, its effects widely documented due to the then-new worldwide telegraphic network.

Question 13.

Why is volcanic activity often associated with mountain building ?

Ans:

Volcanic activity and the grand process of mountain formation are intrinsically intertwined through plate tectonics, the overarching mechanism governing the movement and interaction of Earth’s colossal lithospheric plates. Although not every mountain owes its existence solely to volcanic forces, nor does all volcanic activity directly sculpt vast mountain ranges, their interrelation is notably evident in several crucial geological contexts.

A prime illustration of this connection unfolds at convergent plate boundaries, particularly within subduction zones. Here, a more dense oceanic plate dives beneath another plate, which can be either oceanic or continental. As this oceanic plate descends into the mantle, the extreme heat and pressure, coupled with the liberation of water from the sinking slab, trigger the melting of the overlying mantle rock, generating magma. This less dense magma then ascends towards the surface, culminating in the formation of a chain of volcanoes on the overriding plate. Over numerous eruptions, the successive layering of lava, ash, and volcanic debris constructs imposing structures recognized as volcanic arc mountains. Excellent examples include the Andes Mountains in South America, forged by the subduction of the Nazca Plate beneath the South American Plate, and the Cascade Range in North America. 

Beyond convergent zones, even at divergent plate boundaries where plates move apart, volcanic activity plays a role in mountain building. These mid-ocean ridges, though predominantly submerged, are fundamentally extensive, underwater volcanic mountain ranges stretching for thousands of kilometers globally. In specific cases, like the East African Rift Valley, continental rifting can also instigate volcanic activity and the emergence of fault-block mountains as the crust stretches and thins, enabling magma to ascend. Consequently, whether through the direct construction of volcanic cones or the broader tectonic forces that uplift and deform the Earth’s crust, volcanic activity remains a foundational element in the ongoing narrative of our planet’s mountain formation.

Question 14.

Describe the materials thrown out during volcanic eruptions.

Ans:

During a volcanic eruption, a variety of materials are powerfully expelled from the Earth’s interior, distinguished by their physical state, size, and chemical composition. These ejected substances can generally be grouped into three main categories: molten rock, solid fragments, and gases.

The properties of this lava, including its chemical makeup and temperature, determine how it flows. Highly fluid, low-viscosity lava, typically basaltic, can spread rapidly across wide expanses, leading to the creation of broad, gently sloping shield volcanoes. Conversely, thick, high-viscosity lava, such as andesitic or rhyolitic types, moves slowly, building up around the vent and often contributing to more explosive eruptions due to trapped gases.

fragmented solid materials, collectively referred to as tephra or pyroclastic material, are launched into the atmosphere. These vary in size from microscopic particles to large boulders. Volcanic ash consists of incredibly fine rock, mineral, and glass fragments, capable of traveling thousands of kilometers from the eruption site, influencing global climate and air travel. Larger fragments include lapilli, which are pea to walnut-sized rock pieces, along with volcanic bombs and blocks. Bombs are molten or semi-molten fragments that solidify into aerodynamic shapes as they fly through the air, while blocks are solid, angular pieces torn from the volcano’s conduit or vent during the explosive event.

Finally, gases make up a significant portion of volcanic emissions. The most abundant volcanic gas is water vapor (H2​O), often comprising the majority of the gaseous output. Other common gases include carbon dioxide (CO2​), sulfur dioxide (SO2​), hydrogen sulfide (H2​S). These gases are crucial in generating the explosive force of eruptions. While some, like water vapor, are relatively benign, others, such as sulfur dioxide, can form sulfuric acid aerosols, contributing to acid rain and impacting atmospheric chemistry and climate. The rapid release of large volumes of these gases also poses considerable dangers, including asphyxiation from carbon dioxide or respiratory illnesses from sulfur dioxide.

Question 15.

Why are Earthquakes related to volcanoes ?

Ans:

Earthquakes and volcanoes are intrinsically linked through the dynamic processes occurring within the Earth’s lithosphere, primarily driven by plate tectonics. The vast majority of both phenomena occur along plate boundaries, where the Earth’s massive tectonic plates interact.

The movement and interaction of these plates—whether they are colliding (convergent boundaries), pulling apart (divergent boundaries), or sliding past each other (transform boundaries)—generate immense stress and strain in the Earth’s crust. When this accumulated stress exceeds the strength of the rocks, they suddenly fracture and slip, releasing energy in the form of seismic waves, which we experience as earthquakes. These seismic events are essentially the planet’s way of releasing built-up tension from plate motion.

Simultaneously, these very same plate boundaries often serve as pathways for magma to rise from the Earth’s mantle to the surface. At divergent boundaries, magma rises to fill the gap created by separating plates, forming new crust and often effusive volcanoes. At convergent boundaries, especially where an oceanic plate subducts beneath another plate, the subducting plate melts, generating magma that then rises to form volcanic arcs. Even within active volcanic systems, the movement of magma and gases beneath the surface can cause numerous small earthquakes, known as volcano-tectonic earthquakes, as the magma forces its way through existing rock or creates new fractures. Therefore, earthquakes are not only a consequence of plate interactions that lead to volcanism but can also be direct indicators of impending or ongoing volcanic activity.

Question 16.

What is a geyser ? Give two of its main characteristics. Name a well known geyser.

Ans:

These natural phenomena are driven by unique geothermal conditions where groundwater is superheated by underlying magma or hot rocks, and then rapidly expands and erupts due to pressure build-up.

Two main characteristics of geysers are:

1. Intermittent Eruptions: Unlike a regular hot spring that flows continuously, a geyser erupts at irregular or, in some cases, remarkably predictable intervals. These eruptions involve a sudden burst of water and steam, followed by a period of quiet recharge before the next eruption.
2. Underground Plumbing System: Geysers require a complex, constricted underground network of fissures, cracks, and chambers. This “plumbing” allows water to collect, get heated to extreme temperatures under pressure (preventing it from boiling until a certain point), and then build up enough steam to force an eruption.

Question 17.

State two ways in which lava may come out of the earth’s crust giving an example of each type of these volcanic activities.

Ans:

I need to provide two ways lava can erupt from the Earth’s crust, along with an example for each type of volcanic activity. I will use a search query to confirm common eruption types and their examples to ensure accuracy and uniqueness in my answer. Lava, the molten rock that erupts onto the Earth’s surface, can emerge in primarily two distinct ways, each leading to different types of volcanic activity:

1. Through a Central Vent (Cone Eruptions): This is the most commonly recognized form of volcanic activity, where magma ascends through a single, pipe-like conduit and erupts from a distinct opening or crater at the summit of a volcano. The nature of the eruption here can vary greatly depending on the lava’s viscosity and gas content.
   • Example: A classic example is the eruption of Mount Fuji in Japan. It’s a composite volcano (stratovolcano) that builds its steep, conical shape through alternating eruptions of viscous lava flows and explosive pyroclastic material from its central vent. Similarly, Mount Etna in Italy frequently exhibits effusive lava flows from its summit craters.
2. Through Fissure Vents (Fissure Eruptions): In this type of eruption, lava emerges from long, linear cracks or fractures in the Earth’s crust, rather than from a single, centralized opening. These fissures can extend for many kilometers, allowing highly fluid lava to spread out over vast areas.
   • Example: The eruptions in Iceland are a prime illustration of fissure volcanism. Iceland lies on the Mid-Atlantic Ridge, a divergent plate boundary where the North American and Eurasian plates are pulling apart. This stretching of the crust creates extensive fissures from which basaltic lava pours out, forming vast lava plains and plateaus, such as those seen during the Holuhraun eruption (2014-2015).

Question 18.

(a) What do you understand about ‘Vulcanism’ ?
(b) What are ‘Volcanoes’ ?
(c) How are volcanoes formed ?
(d) Differentiate between active dormant and extinct volcanoes.
(e) What is‘magma’?
(f) What do you understand by ‘Crater of the Volcano’ ?

Ans:

Understanding Volcanic Phenomena

(a) What is ‘Vulcanism’? Vulcanism encapsulates the comprehensive process by which the Earth’s (or another celestial body’s) internal heat drives the generation of molten rock (magma), gases, and solid fragments, ultimately expelling them onto the surface or into the atmosphere. This fundamental geological force is responsible for shaping planetary landscapes, enriching atmospheres with released gases, and is intricately linked to the dynamics of plate tectonics and the continuous rock cycle.

(b) What are ‘Volcanoes’? Volcanoes are geological conduits or fissures in a planet’s crust, serving as outlets through which subsurface molten rock (known as lava once it reaches the surface), ash, and various gases are discharged. They frequently manifest as conical landforms, progressively built up by the accumulation of these erupted materials over successive events.

(c) How are volcanoes formed? The genesis of volcanoes primarily stems from the upward movement of molten rock from the Earth’s deep interior to its surface. This formative process unfolds through several stages:

• Magma Genesis: Within the Earth, intense geothermal heat and immense pressure cause pre-existing rocks to melt, giving rise to magma. This melting typically occurs in specific tectonic environments:
  • Convergent Boundaries (Subduction Zones): As one tectonic plate dives beneath another, water released from the descending plate lowers the melting point of the overlying mantle rock, facilitating magma formation.
  • Divergent Boundaries (Rift Zones): Where tectonic plates pull apart, a decrease in pressure on the underlying mantle allows it to melt and ascend.
• Magma Ascent: Being less dense than the surrounding solid rock, the buoyant magma initiates an upward journey, often pooling in subterranean reservoirs known as magma chambers.
• Eruption: As magma continues its ascent, dissolved gases within it exert increasing pressure. This internal pressure, coupled with the magma’s inherent buoyancy, compels it to force its way through existing fissures and conduits to the surface. Upon reaching the surface, it is violently expelled as lava, pyroclastic material (ash and rock fragments), and gases.
• Volcano Edification: With each successive eruptive event, layers of solidified lava flows, ash, and other ejected debris accumulate concentrically around the vent, gradually constructing the characteristic edifice of the volcano.

(d) Differentiate between active, dormant, and extinct volcanoes. The classification of volcanoes broadly indicates their perceived likelihood of future eruptive activity:

• Active Volcanoes: These are volcanoes that are currently undergoing an eruption, or have demonstrated eruptive activity in recent geological history (typically within the last 11,700 years, the Holocene epoch), and exhibit ongoing signs such as gas emissions, seismic tremors, or ground deformation, indicating a high probability of future eruptions.
• Dormant Volcanoes: These volcanoes are not presently erupting but possess a well-documented history of past eruptions and retain the underlying magmatic system necessary for future activity. They are essentially “sleeping giants,” capable of reawakening after periods of quiescence.
• Extinct Volcanoes: These volcanoes are considered to have permanently ceased eruptive activity. Their deep-seated magma supply is presumed to have solidified, or the specific geological conditions that led to their formation have fundamentally altered. While generally inert and often heavily eroded, the definitive declaration of “extinction” carries a slight degree of scientific humility, as rare instances of unexpected reawakening have been observed in geological history.

(e) What is ‘magma’? 

It is a heterogeneous mixture comprising liquid silicate rock, suspended solid mineral crystals, and dissolved volatile gases. The moment this subsurface molten material breaks through to the Earth’s exterior, it undergoes a name transformation and is thenceforth referred to as lava.

(f) This depression directly overlies the central vent or vents from which volcanic materials are forcefully ejected during eruptive phases. Craters are sculpted by the intense forces of explosive eruptions and can vary significantly in their dimensions and morphology. Over time, craters may undergo further modification through subsequent eruptions, partial collapse (potentially forming larger depressions known as calderas), or erosional processes, and can sometimes accumulate water to form picturesque crater lakes.

Question 19.

(a) Describe the distribution of volcanoes in the world.
(b) What are the influences of volcanic eruptions on man ?

Ans:

(a) Volcanoes aren’t randomly distributed across Earth’s surface; instead, their locations are strongly linked to the movement and interaction of tectonic plates. Most volcanic activity is concentrated in specific geological environments:

Pacific Ring of Fire

This vast, horseshoe-shaped region encircles the Pacific Ocean and is Earth’s most volcanically active area, home to about 75% of all active volcanoes. It’s characterized by extensive subduction zones, where oceanic plates dive beneath other plates. This process generates intense heat and pressure, leading to abundant volcanic eruptions. Notable examples include the volcanic arcs found in the Andes, Japan, Indonesia, and the Cascades mountain range.

Mid-Ocean Ridges

These are underwater mountain ranges where tectonic plates are pulling apart, a process known as divergence. As the plates separate, magma from the Earth’s mantle rises to fill the gap, creating new oceanic crust and forming numerous submarine volcanoes. The Mid-Atlantic Ridge is a prime example, with Iceland representing a significant landmass formed by this type of volcanic activity.

Rift Zones

Located on continents, rift zones are areas where continental plates are actively splitting apart. This separation allows magma to ascend, resulting in volcanic eruptions. The East African Rift Valley is a prominent illustration of this continental rifting and associated volcanism.

Hotspots

Unlike the other settings, hotspots are isolated areas of volcanism that occur within the interior of tectonic plates, far from plate boundaries. Scientists attribute these to fixed plumes of superheated magma originating deep within the Earth’s mantle.

(b) Volcanic eruptions, while awe-inspiring demonstrations of Earth’s power, bring both severe challenges and surprising benefits to human life.

Negative Impacts of Volcanic Eruptions

Immediately, eruptions pose direct threats like loss of life and injury from burns, suffocation, and trauma from ejected materials and fast-moving pyroclastic flows. Widespread destruction of property occurs as lava, ash, and mudslides (lahars) engulf homes, businesses, and infrastructure. Even at a distance, health issues arise from fine volcanic ash causing respiratory and skin problems, and toxic gases posing a silent danger. Essential services are disrupted, with power outages, contaminated water, and severe impacts on air travel due to ash clouds. On a global scale, large eruptions can lead to climate impacts, potentially causing a “volcanic winter” that lowers global temperatures and disrupts agriculture.

Positive Influences of Volcanic Activity

Despite the risks, volcanic activity offers long-term advantages. Volcanic ash creates exceptionally fertile soils, rich in minerals and nutrients, making volcanic regions prime agricultural areas. Volcanic processes are also responsible for forming valuable mineral deposits, concentrating precious metals like gold, silver, and copper. Lastly, the dramatic, unique landscapes sculpted by volcanoes attract tourism, boosting local economies and creating jobs.

Question 20.

(a) What is an ‘earthquake’ ?
(b) Give two major causes of earthquakes.
(c) Describe the world’s distribution of earthquakes.
(d) Mention some of the main effects of earthquakes.
(e) Name the major earthquakes of India from 1991 to 1997.

Ans:

(a) What is an ‘earthquake’ ? An earthquake is a sudden, often violent, tremor of the Earth’s surface. It arises from the rapid discharge of accumulated stress within the Earth’s rigid outer layer, the lithosphere.

(b) Give two major causes of earthquakes.

1. Plate Tectonic Movement: The primary driver of most earthquakes is the dynamic interaction of Earth’s colossal tectonic plates. These enormous, rigid segments of the Earth’s crust and uppermost mantle are in perpetual motion—colliding, pulling apart, or grinding past one another. Immense stress builds up along their boundaries, known as faults. When this stored stress overwhelms the frictional forces locking the plates together, a sudden, rapid slip occurs, releasing energy and generating an earthquake.
2. Volcanic Activity: Earthquakes can also be a consequence of the subterranean movement of magma. As molten rock migrates, collects in chambers, or is forced through fissures beneath or within a volcano, it exerts pressure on surrounding rock. This pressure can induce fracturing and displacement, leading to ground tremors known as volcanic earthquakes.

(c) Describe the world’s distribution of earthquakes. Earthquakes exhibit a distinct, non-random global distribution, primarily concentrated along the boundaries of the Earth’s tectonic plates. The vast majority of seismic events occur within these well-defined belts:

• The Pacific Ring of Fire: This highly active arc, encircling the Pacific Ocean, is responsible for approximately 90% of the world’s earthquakes. It’s characterized by intense subduction (one plate sliding under another), collision, and transform (plates sliding past each other) plate movements.
• Mid-Oceanic Ridges: These extensive underwater mountain ranges, where new oceanic crust is continuously formed as plates diverge, are sites of frequent, typically shallower earthquakes.
• Alpine-Himalayan Orogenic Belt: Stretching from the Mediterranean region across parts of Asia, this zone experiences significant seismic activity. It’s a direct result of the ongoing collision between the African, Arabian, and Indian plates with the Eurasian plate.

While less common, minor earthquakes can also occur within continental interiors, often along ancient or reactivated fault lines responding to broader regional stress fields.

(d)

• Ground Shaking: The most immediate and destructive impact, leading directly to the oscillation, fracturing, and ultimate collapse of buildings, bridges, and other critical infrastructure.
• Landslides and Rockfalls: Seismic vibrations can destabilize slopes, triggering large-scale movements of soil, rock, and debris, particularly in hilly or mountainous terrain.
• Tsunamis: Powerful underwater earthquakes (or those causing significant vertical displacement of the seafloor) can generate immense ocean waves. These waves traverse vast ocean distances and can cause catastrophic inundation and destruction upon reaching coastal areas.
• Liquefaction: In areas where saturated, loose, granular soils (like sands or silts) are present, intense ground shaking can cause the soil to lose its structural integrity and behave like a fluid. This phenomenon can cause buildings and other structures to sink, tilt, or even float.
• Fires: Earthquakes frequently lead to the rupture of gas lines, electrical cables, and fuel storage facilities, often sparking widespread and challenging-to-control fires in urban environments.

(e) The significant earthquakes that affected India between 1991 and 1997 include:

• 1991 Uttarkashi Earthquake: This event occurred in October 1991, impacting the Garhwal region of Uttarakhand (which was then part of Uttar Pradesh).
• 1993 Latur Earthquake: A particularly devastating seismic event that struck central Maharashtra in September 1993.
• 1997 Jabalpur Earthquake: An earthquake that took place in May 1997, centered near Jabalpur in Madhya Pradesh.

Question 21.
What are the following

(a) Fissure type of volcanoes
(b) Spine or plug
(c) Caldera
(d) Mud volcanoes
(e) Epicentre
(f) ‘Ring of Fire’
(g) Cinder Cone

Ans:

(a) Fissure-Type Volcanoes

Forget the classic cone shape; fissure-type volcanoes defy that image entirely. Instead of erupting from a single, centralized vent, these volcanoes manifest as long, linear cracks or ruptures in the Earth’s crust, often stretching for many kilometers. Molten rock, or lava, doesn’t explode skyward but rather oozes out relatively calmly along these extended fractures, flowing like a wide river across the landscape. This results in the formation of vast, flat plains or expansive plateaus composed of solidified lava, rather than the familiar mountainous structures. A prime historical example is the Laki eruption in Iceland (1783-1784), which dramatically reshaped the terrain with its immense lava flows.

(b) Volcanic Spine or Plug

It’s a stiff, column-like formation of extremely thick, hardened lava that was too viscous to flow horizontally when it erupted. Instead, this exceptionally sticky molten rock solidified within or directly above the volcano’s conduit. Over eons, as the softer, surrounding volcanic rock erodes away due to weathering, the more resilient, solidified spine is left standing tall and prominent, a testament to the volcano’s ancient plumbing. Shiprock in New Mexico is an iconic illustration of such a feature.

(c) Caldera

A caldera is a vast, often circular or oval-shaped basin that forms at the summit of a volcano. It’s not a typical crater, but rather a much larger depression created when a volcano’s underlying magma chamber is substantially emptied during a colossal eruption. With the withdrawal of so much molten rock, the overlying ground loses its structural support and collapses inward. These impressive geological depressions can stretch for many kilometers across and sometimes collect water, forming stunning caldera lakes, such as the renowned Crater Lake in Oregon.

(d) Mud Volcanoes

Mud volcanoes are geological curiosities that stand apart from their fiery counterparts. Their activity is powered by subterranean gas and fluid pressure forcing saturated sediments upward, and they operate at significantly cooler temperatures than magmatic volcanoes. You’ll often find them linked to active fault lines, areas with geothermal heat, or regions rich in underground petroleum deposits. The infamous Lusi mud volcano in Indonesia serves as a stark modern example.

(e) Epicenter

The epicenter pinpoints the exact location on the Earth’s surface directly above where an earthquake originates. This true point of rupture deep underground is known as the hypocenter (or focus). 

(f) “Ring of Fire”

The “Ring of Fire” is an immense, horseshoe-shaped zone encircling the Pacific Ocean, notorious for being a hotbed of seismic and volcanic activity. This vast arc, stretching approximately 40,000 kilometers, is home to a staggering 75% of the world’s active and dormant volcanoes and accounts for about 90% of all global earthquakes. This intense geological dynamism is a direct consequence of the continuous, energetic interactions—where several major tectonic plates collide, pull apart, or slide past one another—along its extensive boundaries.

(g) Cinder Cone

It forms when frothy, gas-rich lava is explosively ejected from a single vent, solidifying rapidly into porous, pebble-sized fragments called cinders or scoria. These lightweight fragments fall back down around the vent, accumulating to create a relatively steep, symmetrical conical hill, typically with a distinct bowl-shaped crater at its summit. Cinder cones are generally modest in size, making Mexico’s Parícutin a celebrated example due to its rapid and observable growth in the 20th century.

Question 22.

(a) Distinguish between the following pairs of terms associated with vulcanicity

1. Lava and Magma
2. Acidic Lava and Basic Lava
3. Cinder Cone and Composite Cone
4. Fissure-type Volcanoes and Central-types Volcanoes
5. Crater and Caldera
6. Laccolith and Lapolith
7. Geysers and Hot Springs

(b) Distinguish between the following pairs of terms associated with crustal movement of the earth

1. Graben and Horst
2. Tilted Block mountains and Lifted Block mountains

Ans:

(a)

Question 23.

Give a brief account of ‘Plate Tectonics’.

Ans:

These colossal plates are in continuous, albeit slow, motion, a process driven by convection currents circulating within the asthenosphere, a semi-fluid layer directly beneath the lithosphere. This mechanism involves warmer, less dense material rising, cooling, and then descending, establishing a persistent circulatory pattern that propels the massive tectonic plates across the globe.

The interactions at plate boundaries are crucial in shaping most of Earth’s significant geological characteristics. At divergent boundaries, plates separate, allowing molten rock (magma) to rise and form new crust, as observed in mid-ocean ridges and continental rift valleys. Conversely, convergent boundaries involve plate collisions. This can lead to an oceanic plate descending beneath either a continental plate or another oceanic plate (a process known as subduction), resulting in the formation of deep ocean trenches, volcanic arcs, and potent earthquakes, famously exemplified by the Pacific “Ring of Fire.” When two continental plates collide, their comparable densities prevent subduction, causing the crust to deform and thicken, which consequently creates immense mountain ranges like the Himalayas. Lastly, transform boundaries are characterized by plates sliding horizontally past each other. This movement neither produces nor destroys crust but generates considerable stress, frequently triggering powerful earthquakes, such as those along the San Andreas Fault.

Question 24.
Give reasons for the following

1. The Belts of volcanic activity and earthquakes are roughly the same.
2. Basic lava cones are broader than Acid lava cones.
3. The Circum-Pacific Belt of volcanoes is called ‘The Ring of Fire’.

Ans:

Here’s a rephrased explanation of the provided information, aiming for uniqueness while retaining the core concepts:

Correlation Between Volcanic Activity and Earthquakes

The geographical distribution of volcanic activity and earthquakes largely coincides because both are fundamentally products of plate tectonics. The Earth’s outermost solid shell, known as the lithosphere, is fragmented into colossal plates that are in perpetual motion. The overwhelming majority of both volcanic eruptions and seismic events occur at the junctures where these plates engage in various interactions: colliding (convergent boundaries), separating (divergent boundaries), or grinding past each other (transform boundaries). These dynamic interactions generate immense stress and frictional forces, which are the primary drivers of seismic activity (earthquakes) and simultaneously create pathways for molten rock (magma) to ascend to the surface, resulting in volcanic eruptions.

Distinction Between Basic and Acid Lava Cone Shapes

The varying profiles of volcanic cones, specifically why basic lava cones are more expansive than acid lava cones, can be attributed to the inherent viscosity (or resistance to flow) of the molten rock.

• Basic (Mafic) Lava: Characterized by a low silica content, basic lava exhibits low viscosity, meaning it is quite fluid and flows readily. This allows it to spread out extensively and rapidly across the landscape before solidifying. The repeated outpouring of such fluid lava constructs broad, gently sloped edifices commonly known as shield volcanoes.
• Acid (Felsic) Lava: In contrast, acid lava possesses a high silica content, rendering it highly viscous—thick and sticky. Its slow flow rate causes it to accumulate in layers around the volcanic vent and solidify quickly. This characteristic behavior leads to the formation of steep-sided, conical volcanoes, often referred to as stratovolcanoes or composite cones.

The “Ring of Fire”

The Circum-Pacific Belt of volcanoes is aptly named “The Ring of Fire” due to the extraordinary concentration of volcanic and seismic events encircling the Pacific Ocean basin. This region is defined by numerous subduction zones, where one oceanic tectonic plate is forced to dive beneath another continental or oceanic plate. This powerful subduction process is a prolific source of magma generation, which in turn fuels a continuous chain of active volcanoes. The frequent and often powerful earthquakes that also originate along these active plate boundaries further underscore the intensely dynamic and “fiery” nature suggested by the moniker.

Question 25.

Match the items given in Column A with the correct ones in Column B.

Ans:

Question 26.
Give one word for each of the following :

1. A narrow block elevated between two normal faults.
2. The funnel is shaped hollow at the top of a volcanic cone.
3. The lava which is poor in silica and rich in iron and magnesium.
4. A volcano which has the possibility of erupting in future.
5. A large sill of acid lava which has solidified gradually giving a dome – like shape.
6. A volcano where magma reaches the surface through a vent or a pipe.
7. A volcano whose eruption buried and destroyed two Roman towns.
8. An instrument used for recording all the earth tremors and earthquakes.
9. The surface position immediately above the origin of an earthquake.
10. The region where there are the highest number of geysers and hot springs.

Ans:

1. Horst
2. Crater
3. Mafic
4. Quiescent
5. Laccolith
6. Stratovolcano
7. Vesuvius
8. Seismograph
9. Epicenter
10. Hydrothermal

Question 27.

(a) Which type of lavas weather into more fertile soil. Name also one useful feature of volcanicity other than soil fertility.
(b) Which four of the following words are connected with volcanic activity :  Karst, crater, drumlin, stalactite, gully, potholes, ash, basalt, swallow, holes, dyke, domes, bluffs.

Ans:

(a) Lavas that are rich in basic minerals, often termed mafic lavas, tend to decompose into highly productive soils. The abundance of ferromagnesian minerals within these lavas contributes to their fertility, as these minerals release vital plant nutrients such as iron, magnesium, and calcium upon weathering.

Beyond enhancing soil fertility, volcanism offers another significant benefit: geothermal power. Regions with volcanic activity frequently exhibit elevated heat transfer from the Earth’s interior. This heat can be effectively utilized for power generation or for direct applications like heating residential buildings and agricultural greenhouses.

(b) The following four terms from your list are directly associated with volcanic processes:

• Crater: This refers to the characteristic depression, often bowl-shaped, found at the summit of a volcano, serving as the vent for erupted materials.
• Ash: These are the minute fragments of rock, minerals, and volcanic glass that are ejected during a volcanic eruption.
• Basalt: A prevalent type of dark, fine-grained igneous rock that typically forms from the rapid cooling of mafic lava flows, especially common in effusive eruptions.
• Dyke: This describes a planar body of igneous rock that cuts across pre-existing rock layers, formed when magma intrudes into fissures.

Question 28.

What are tectonic movements ? How are these classified?

Ans:

The Earth’s rigid outer layer, the lithosphere, is divided into tectonic plates that constantly drift over the semi-fluid asthenosphere, driven by mantle convection. This continuous, slow movement shapes our planet, causing earthquakes, volcanoes, and the formation of mountains and ocean basins.

Plate interactions at their boundaries define three main types:

• Divergent boundaries (constructive): Plates pull apart, allowing magma to rise and create new crust. Examples include mid-ocean ridges and continental rift valleys.
• Convergent boundaries (destructive): Plates collide.
  • Oceanic-continental collisions result in subduction of the denser oceanic plate, forming trenches, volcanic arcs, and strong earthquakes.
  • Oceanic-oceanic collisions lead to one plate subducting beneath the other, creating island arcs and ocean trenches.
  • Continental-continental collisions cause crustal crumpling and folding, forming large mountain ranges.
• Transform boundaries (conservative): Plates slide horizontally past each other. Crust is neither created nor destroyed, but significant friction often leads to powerful earthquakes.

Question 29.
Give reasons for the following :

1. Earth movements have modified the Earth’s surface.
2. Internal processes are different from external processes.
3. Folding and faulting frequently go together.
4. Earth as a whole does not expand.

Ans:

Here are concise and unique reasons for each statement:

Earth movements have modified the Earth’s surface. Earth movements, driven by tectonic forces, cause the crust to buckle, fracture, and shift. This leads to large-scale uplifting of mountains, subsidence of basins, and the formation of rift valleys, directly sculpting the planet’s topography over geological timescales.

Internal processes are different from external processes. Internal processes, like volcanism and plate tectonics, originate from the Earth’s internal heat and create new landforms or deform existing ones by building up or breaking down crust from within. External processes, such as weathering and erosion, are powered by solar energy and gravity, and primarily act on the Earth’s surface to wear down and transport material.

Folding and faulting frequently go together. Both folding and faulting are responses of rock to stress, typically compressive. When rock is subjected to immense pressure, it first deforms elastically, then plastically (folding). If the stress exceeds the rock’s strength, or if the rock is brittle, it will fracture (faulting). Often, a combination of ductile and brittle behavior occurs, or as rocks fold, internal stresses can accumulate leading to rupture.

Earth as a whole does not expand. While plate tectonics involves new crust being generated at mid-ocean ridges, an equal amount of old crust is consumed back into the mantle at subduction zones. This continuous recycling process maintains a relatively constant surface area and volume for the Earth, preventing overall expansion.

Question 30.

How the theory of plate tectonics has explained the formation of mountains like Himalaya or Alps and of the volcanic islands.

Ans:

Plate tectonics drives the creation of mountains and volcanic islands.

Mountains (Himalayas, Alps): These form at convergent boundaries where continental plates collide. For instance, the Himalayas resulted from the Indian plate crashing into the Eurasian plate. Since neither plate easily subducts, the immense pressure causes the crust to fold, buckle, and thrust upwards, building massive mountain ranges. 

Volcanic Islands:

• Oceanic-Oceanic Convergence: When one oceanic plate slides beneath another (subduction), the descending plate melts, generating magma. This magma rises, erupting to form volcanoes that eventually emerge as island arcs (e.g., Japan).
• Hotspots: Some islands, like Hawaii, form over mantle plumes (hotspots) independent of plate boundaries. As a plate moves over a stationary hotspot, magma punches through, creating a chain of volcanoes.

Question 31.

(a) Describe the distribution of volcanoes in the world.

OR

Name the important belts of volcanoes.

(b) What are the influences of volcanoes eruption on man?

OR

What is the importance of volcanoes ?

OR

Mention adverse and beneficial effects of volcanoes.

Ans:

(a) Distribution of Volcanoes: The most prominent belt is the “Pacific Ring of Fire,” encircling the Pacific Ocean, where oceanic plates are subducting beneath continental or other oceanic plates. Other significant belts include the Mid-Atlantic Ridge, where plates are pulling apart, and areas within continents like the East African Rift Valley, associated with rifting. A smaller number of volcanoes occur at “hot spots,” isolated from plate boundaries, like the Hawaiian Islands.

(b) Influences of Volcano Eruptions on Man (Adverse and Beneficial Effects):

Adverse Effects:

• Destruction: Lava flows, ash falls, and pyroclastic flows can devastate infrastructure, agriculture, and natural ecosystems.
• Health Hazards: Volcanic ash and gases (like sulfur dioxide) can cause respiratory problems, eye irritation, and acid rain.
• Climate Impact: Large eruptions can inject ash and aerosols into the stratosphere, temporarily lowering global temperatures.

Beneficial Effects:

• Fertile Soil: Volcanic ash and weathered lavas enrich soils with minerals, leading to highly productive agricultural lands.
• Geothermal Energy: Volcanic regions are sources of geothermal energy, a clean and renewable power source.
• Mineral Deposits: Volcanic processes bring valuable minerals (e.g., gold, silver, copper) closer to the Earth’s surface, forming ore deposits.
• Tourism: Volcanic landscapes, hot springs, and geysers attract tourists, boosting local economies.
• New Land Formation: Volcanic eruptions, particularly underwater, can create new landmasses.

Question 32.
What are the following :

(a) Fissure type of volcanoes
(b) Spine or plug
(c) Caldera
(d) Mud volcanoes
(e) Epicentre
(f) ‘Ring of Fire’
(g) Cinder Cone

Ans:

(a) Fissure-Type Volcanoes

Forget the classic cone shape; fissure-type volcanoes defy that image entirely. Instead of erupting from a single, centralized vent, these volcanoes manifest as long, linear cracks or ruptures in the Earth’s crust, often stretching for many kilometers. Molten rock, or lava, doesn’t explode skyward but rather oozes out relatively calmly along these extended fractures, flowing like a wide river across the landscape. This results in the formation of vast, flat plains or expansive plateaus composed of solidified lava, rather than the familiar mountainous structures. A prime historical example is the Laki eruption in Iceland (1783-1784), which dramatically reshaped the terrain with its immense lava flows.

(b) Volcanic Spine or Plug

It’s a stiff, column-like formation of extremely thick, hardened lava that was too viscous to flow horizontally when it erupted. Instead, this exceptionally sticky molten rock solidified within or directly above the volcano’s conduit. Over eons, as the softer, surrounding volcanic rock erodes away due to weathering, the more resilient, solidified spine is left standing tall and prominent, a testament to the volcano’s ancient plumbing. Shiprock in New Mexico is an iconic illustration of such a feature.

(c) Caldera

A caldera is a vast, often circular or oval-shaped basin that forms at the summit of a volcano. It’s not a typical crater, but rather a much larger depression created when a volcano’s underlying magma chamber is substantially emptied during a colossal eruption. With the withdrawal of so much molten rock, the overlying ground loses its structural support and collapses inward. These impressive geological depressions can stretch for many kilometers across and sometimes collect water, forming stunning caldera lakes, such as the renowned Crater Lake in Oregon.

(d) Mud Volcanoes

Mud volcanoes are geological curiosities that stand apart from their fiery counterparts. Their activity is powered by subterranean gas and fluid pressure forcing saturated sediments upward, and they operate at significantly cooler temperatures than magmatic volcanoes. You’ll often find them linked to active fault lines, areas with geothermal heat, or regions rich in underground petroleum deposits. The infamous Lusi mud volcano in Indonesia serves as a stark modern example.

(e) Epicenter

The epicenter pinpoints the exact location on the Earth’s surface directly above where an earthquake originates. This true point of rupture deep underground is known as the hypocenter (or focus). 

(f) “Ring of Fire”

The “Ring of Fire” is an immense, horseshoe-shaped zone encircling the Pacific Ocean, notorious for being a hotbed of seismic and volcanic activity. This vast arc, stretching approximately 40,000 kilometers, is home to a staggering 75% of the world’s active and dormant volcanoes and accounts for about 90% of all global earthquakes. This intense geological dynamism is a direct consequence of the continuous, energetic interactions—where several major tectonic plates collide, pull apart, or slide past one another—along its extensive boundaries.

(g) Cinder Cone

It forms when frothy, gas-rich lava is explosively ejected from a single vent, solidifying rapidly into porous, pebble-sized fragments called cinders or scoria. These lightweight fragments fall back down around the vent, accumulating to create a relatively steep, symmetrical conical hill, typically with a distinct bowl-shaped crater at its summit. Cinder cones are generally modest in size, making Mexico’s Parícutin a celebrated example due to its rapid and observable growth in the 20th century.

Question 33.
Distinguish between the following pairs of terms associated with vulcanity

1. Magma and Lava
2. Acidic Lava and Basic lava
3. Cinder Cone and Composite Cone
4. Fissure-type Volcanoes and Central Type Volcanoes
5. Crater and Caldera
6. Laccolith and Lapolith
7. Geysers and Hot springs
8. Active Volcano and Dormant Volcano
9. Folding and Faulting
10. Volcanic Cone and Volcanic Plateau
11. Seismic Focus and Epicentre
12. Dykes and Sills

(b) Distinguish between the following pair of terms associated with crustal movement of the Earth

1. Graben and Horst
2. Tilted Block Mountains and Listed Block Mountains

Ans:

(a)

1. The core difference between magma and lava is simply where the molten rock is found.

• Magma: This refers to the molten and semi-molten rock mixture that exists below the Earth’s crust. It’s the subterranean form of this superheated material, often collecting in vast chambers within the Earth’s interior. Magma typically carries dissolved gases and fragments of solid minerals.
• Lava: This is what that identical molten rock becomes after it has been expelled onto the Earth’s exterior. Once magma breaches the surface, usually through volcanic vents or cracks, it is then termed lava. As it flows, lava undergoes a process of gas release and cooling, eventually solidifying to create various above-ground volcanic features.

2. The primary differentiator between acidic and basic lava is their silica concentration, which fundamentally governs their fluidity and, consequently, their eruption characteristics and the types of volcanic structures they create.

Acidic Lava: Characterized by an elevated silica content, acidic lava exhibits significant stickiness and resistance to flow. This high viscosity impedes the easy escape of volcanic gases, often leading to pressure buildup and powerful, explosive eruptions. The resultant lava flows are typically slow-moving and pile up, constructing prominent, steeply sloped volcanic edifices.

Basic Lava: In contrast, basic lava possesses a comparatively low silica content, rendering it highly mobile and able to flow with considerable ease. This low viscosity permits volcanic gases to dissipate readily, usually leading to more effusive, less violent eruptions. The rapid and widespread spreading of basic lava forms expansive, gently inclined landforms, such as broad shield volcanoes.

3. Cinder Cone: A smaller, simpler volcano, typically built from loose, ejected volcanic fragments (cinders) during a single, relatively short, explosive eruption. It forms a steep-sided, often symmetrical, cone with a bowl-shaped crater.

Composite Cone: A larger, more complex volcano (also called a stratovolcano) constructed over many eruptions from alternating layers of viscous lava flows and explosive pyroclastic material. This layering gives it a classic, tall, and steep conical shape, capable of powerful, infrequent eruptions.

4. Fissure-type volcanoes erupt from elongated cracks in the Earth’s surface, producing widespread, flat lava flows without building a distinct cone.

Central-type volcanoes erupt from a single, localized vent, leading to the formation of conical mountains as lava and ash accumulate around that central point.

5. While both are volcanic depressions, craters are smaller, bowl-shaped openings formed by the outward expulsion of material directly from a volcano’s vent during eruptions. They represent the “mouth” of the volcano.

In contrast, calderas are significantly larger, basin-shaped depressions that form when a volcano’s underlying magma chamber empties (usually from a massive eruption or withdrawal of magma), causing the overlying volcanic structure to collapse inward. Calderas represent a more profound structural change and are often associated with rare, powerful eruptions.

6. While both laccoliths and lopoliths are types of concordant igneous intrusions (meaning they intrude parallel to existing rock layers), their defining difference lies in the shape of the intrusion and how they deform the overlying rock.

• Laccolith: Imagine a mushroom or a dome. A laccolith forms when viscous magma intrudes between sedimentary rock layers and, due to the pressure, pushes the overlying strata upwards, creating a distinct dome-shaped or lens-shaped bulge. The base of a laccolith tends to remain relatively flat, while its upper surface is convex. They typically have a feeder pipe from below.
• Lopolith: Instead of pushing the overlying rocks up into a dome, the magma in a lopolith causes the overlying and underlying rock layers to sag downwards in a basin-like depression. Lopoliths are generally much larger than laccoliths and are often associated with the intrusion of mafic (iron and magnesium-rich) magma, which is denser and can cause subsidence as it cools and crystallizes.

7. While both geysers and hot springs are geothermal features where groundwater is heated by the Earth’s internal heat (often from shallow magma or hot rocks), their key distinguishing factor lies in their mode of discharge.

• Hot Springs: These are characterized by a continuous, non-explosive flow of naturally heated groundwater to the Earth’s surface. The water typically rises to the surface through open channels or fractures, allowing heat and dissolved gases to escape gradually. The flow is generally tranquil, forming pools or gentle streams, and the temperature can vary widely, from merely warm to boiling.
• Geysers: These are a specialized type of hot spring that exhibit intermittent, explosive eruptions of superheated water and steam. This violent discharge occurs due to a unique “plumbing system” beneath the surface, which involves constricted underground conduits. Water becomes superheated under pressure within these confined spaces. When a portion of this water flashes into steam due to a slight drop in pressure (e.g., from an initial release of hot water), it rapidly expands, forcing the remaining water and steam explosively out of the vent in a towering jet. After an eruption, the system refills with water, and the cycle repeats.

8. The essential difference between an active and a dormant volcano hinges on their current eruptive status and future potential.

An active volcano is one that is either presently erupting or exhibiting strong, quantifiable indicators of an imminent eruption. These signs might include heightened seismic activity, noticeable ground swelling, or increased releases of volcanic gases. Many experts also classify a volcano as active if it has erupted within the last 10,000 years (the Holocene geological epoch), even if it’s presently calm. The defining characteristic is that its internal magmatic system remains vibrant and capable of eruption.

In contrast, a dormant volcano is presently quiescent, not erupting, and has not erupted for a considerable span (often exceeding the 10,000-year benchmark, though specific definitions can vary). Despite its current inactivity, it crucially possesses the inherent capacity to erupt again in the future. It’s akin to being in a “sleep” phase, yet its underlying conduits are intact, and magma could potentially ascend once more. Forecasting the reawakening of a dormant volcano presents a significant challenge for scientists, as some can remain inactive for hundreds or even thousands of years before an eruption.

9. Both folding and faulting are natural processes that alter the Earth’s crust due to immense geological forces, but they differ fundamentally in how rocks respond to stress.

Folding is the process where rock layers contort and bend into wavelike shapes without fracturing. Picture a malleable material being slowly squeezed from opposing sides, causing it to undulate rather than snap. This typically occurs when rocks, particularly those under high pressure and temperature deeper within the Earth’s crust, behave in a ductile or pliable manner under compressional stress. The resulting structures are known as folds, characterized by upward arches called anticlines and downward troughs termed synclines. This “plastic” deformation is a major contributor to the creation of extensive mountain ranges, such as the Rockies or the Andes.

Faulting, in contrast, involves the fracturing and subsequent displacement of rock masses along well-defined planes of weakness called faults. Envision a rigid object being subjected to excessive force, causing it to crack and separate. This type of deformation arises when rocks, often brittle ones closer to the Earth’s surface where temperatures and pressures are lower, reach their breaking point under various stresses. The sudden movement along these fracture lines can release significant energy, leading to earthquakes. Faults are categorized by the direction of relative movement (e.g., normal faults from pulling apart, reverse faults from pushing together, strike-slip faults from sliding past each other). Faulting is responsible for features like block mountains and rift valleys.

10. Volcanic Cone vs. Volcanic Plateau

While both are results of volcanic processes, volcanic cones are distinct, often symmetrical, elevated structures formed by eruptions from a single, localized vent, accumulating material around that point to create a hill or mountain. Their size varies, but they’re identifiable as individual peaks.

In contrast, a volcanic plateau is an expansive, largely flat, elevated landform resulting from immense volumes of fluid lava erupting from multiple fissures or cracks in the Earth’s crust, spreading out to cover vast geographical areas. These are regional features, far larger in scale than individual cones.

11. Seismic Focus vs. Epicentre

The seismic focus (hypocenter) marks the exact subterranean origin point where an earthquake rupture begins, acting as the true source of seismic energy release.

Conversely, the epicenter is merely the surface projection directly above this focus, serving as the primary location where an earthquake’s impact is experienced most severely.

12. Dykes vs. Sills

Both dykes and sills are solidified magma bodies within older rock, but they differ fundamentally in how they interact with the surrounding rock layers.

A dyke is a wall-like igneous intrusion that slices across the pre-existing rock strata, forming a sharp, often near-vertical cut through the host rock’s fabric.

In contrast, a sill is a sheet-like igneous intrusion that spreads parallel to the existing rock layers, inserting itself as a distinct, often horizontal, bed or layer within the surrounding rock.

(b)

Graben vs. Horst

Graben: A graben is a segment of the Earth’s crust that has dropped down relative to adjacent blocks, bounded by parallel normal faults dipping inward. It forms a geological depression or valley, indicative of crustal stretching.

Horst: Conversely, a horst is a block of the Earth’s crust that has been uplifted or remained elevated compared to the surrounding down-dropped grabens, also defined by parallel normal faults that dip away. It represents a geological ridge or a block mountain.

Tilted Block Mountains vs. Uplifted Block Mountains

Tilted Block Mountains: These mountains arise when a crustal block is not only uplifted along faults but also undergoes a significant rotational tilt. This results in an asymmetrical profile, featuring one very steep, faulted side and a more gently sloping, tilted upper surface.

Uplifted Block Mountains (or Lifted Block Mountains): In this formation, a crustal block is primarily vertically raised between faults, largely retaining its original horizontal orientation. Consequently, these mountains typically exhibit a relatively flat or gently inclined summit, flanked by abrupt, steep fault scarps on their sides.

Question 34.
Give reasons for the following :

1. The belts of volcanic activity and earthquakes are roughly the same.
2. Basic lava cones are broader than the Acid lava cones.
3. The Circum-Pacific Belt of volcanoes is called ‘The Ring of Fire’.

Ans:

1. Volcanic Activity and Earthquakes Share Common Belts

The global mapping of both volcanic eruptions and earthquake epicenters reveals a striking overlap, a congruence directly attributable to the Earth’s dynamic plate tectonics. The lithosphere, our planet’s rigid outer shell, is fragmented into colossal, perpetually moving pieces called tectonic plates. The vast majority of both seismic shaking and magma expulsion are concentrated along the active boundaries where these colossal plates engage.

At Collision Zones (Convergent Boundaries): When plates collide, one often dives beneath the other in a process known as subduction. This descent generates immense friction, a primary trigger for powerful earthquakes. The melting of the subducting plate fuels the ascent of magma, leading to the formation of volcanoes at the surface.

At Separation Zones (Divergent Boundaries): Where plates pull apart, magma from the mantle surges upward to fill the void, creating new crust through effusive volcanic activity. This spreading motion also induces fracturing and seismic events, typically less intense than those at convergent zones.

At Sliding Zones (Transform Boundaries): Along these boundaries, plates scrape horizontally past each other. This lateral shearing generates substantial friction and frequent, often shallow, earthquakes. Significant volcanism is usually absent unless other tectonic forces are also at play.

In essence, the worldwide distribution of volcanism and seismic activity precisely mirrors the intricate web of plate boundaries, which serve as the primary arenas for crustal deformation and magma generation.

2. The distinct profiles of volcanic cones formed by basic and acidic lavas are a direct consequence of their respective viscosities, a property primarily determined by silica content.

Basic Lava Cones (e.g., Shield Volcanoes): Basic lava, rich in ferromagnesian minerals and low in silica, possesses exceptionally low viscosity (it flows readily). Upon eruption, this runny material spreads out rapidly and widely across the landscape before solidifying. This expansive flow allows it to accumulate into broad, gently sloped edifices resembling a warrior’s shield. The low viscosity also facilitates the easy escape of dissolved gases, resulting in typically non-explosive, effusive eruptions.

Acid Lava Cones (e.g., Stratovolcanoes/Composite Cones): Acidic lava, characterized by its high silica content, is exceptionally viscous (thick and sticky). This pasty material resists flowing far from its source, instead piling up steeply around the eruptive vent. This characteristic behavior leads to the construction of tall, steep-sided, conical structures. The high viscosity also traps volcanic gases effectively, building up immense internal pressure that often culminates in violent, explosive eruptions, which contribute significant amounts of ash and rock fragments alongside the lava, further building the vertical cone.

3.  The Circum-Pacific Belt has earned its famous epithet, ‘The Ring of Fire,’ due to the extraordinary concentration of active volcanoes and the frequent occurrence of powerful earthquakes that delineate its vast, arc-shaped perimeter. This intense geological dynamism is inextricably linked to the pervasive convergent plate boundary interactions throughout this immense region.

The ‘Ring of Fire’ primarily encompasses zones where dense oceanic plates (like the Pacific Plate itself) are relentlessly descending beneath lighter continental or other oceanic plates through the process of subduction. 

Profound Volcanism: As an oceanic plate plunges into the Earth’s mantle, it undergoes partial melting due to escalating temperature and pressure, often facilitated by the release of water from its hydrated minerals. The resultant magma, being less dense, ascends toward the surface, fueling the emergence of extensive chains of highly active and frequently explosive volcanoes, collectively known as volcanic arcs.

Prevalent Earthquakes: The immense friction and stress generated by the continuous grinding and slipping between the subducting and overriding plates trigger frequent and often exceptionally powerful earthquakes. This includes some of the largest “megathrust” quakes recorded globally, making the region a seismic hotspot.

Question 35.

(a) Name one useful feature of vulcanicity other than soil fertility.
(b) Out of the following words write down the four that are connected with volcanic activity.
Karst, crater, drumlin, stalactites, gully, pot holes, ash, basalt, swallow holes, dyke, domes, bluffs.

Ans:

Here’s a rephrased version of your points, ensuring uniqueness and accuracy:

(a) Beyond contributing to fertile soils, another significant advantage of volcanic activity is its role in providing geothermal power. Areas with volcanic influence often offer abundant underground heat, which can be effectively utilized for generating electricity or directly supplying warmth to various applications.

(b) The four terms from the given selection that relate to volcanic processes are:

• Crater: This refers to the characteristic bowl-shaped hollow found at the peak of a volcano, typically created by the force of explosive eruptions.
• Ash: These are the minute fragments of rock, minerals, and volcanic glass that are forcefully expelled from a volcano during its eruptive phase.
• Basalt: A prevalent dark, fine-grained igneous rock formed from rapidly cooled lava, commonly associated with more fluid, effusive volcanic flows.
• Dyke: This describes a planar intrusion of igneous rock that cuts perpendicularly or sharply across the pre-existing rock layers, representing magma that solidified within a fissure.

Question 36.
Give reasons for the following :

1. Earth’s movements have modified the Earth’s surface.
2. Earth as a whole does not expand.

Ans:

Earth’s movements have modified the Earth’s surface.

The Earth’s surface is constantly reshaped because its lithosphere is fragmented into dynamic tectonic plates. These plates are in continuous, albeit slow, motion, driven by convection currents in the underlying mantle. Where plates converge, diverge, or slide past each other, massive forces are generated. These forces lead to folding (bending of rock layers), faulting (breaking and displacement of rock), volcanism (eruption of molten rock), and earthquakes. These processes collectively build mountains, create ocean basins, form rift valleys, and alter landscapes over geological timescales, thus perpetually modifying the Earth’s surface.

Earth as a whole does not expand.

The Earth does not expand as a whole despite ongoing geological activity because the processes of crustal creation are effectively balanced by processes of crustal destruction. While new oceanic crust is continuously generated at mid-ocean ridges through seafloor spreading (divergent plate boundaries), old oceanic crust is simultaneously subducted and consumed back into the mantle at deep-ocean trenches (convergent plate boundaries). This global recycling mechanism, known as plate tectonics, ensures that the Earth’s overall volume remains remarkably constant, preventing any significant expansion.

Question 37.
Answer the following :

1. Some volcanoes erupt explosively
2.  Some volcanoes develop parasitic cones.
3. Hot springs are common in volcanic regions.
4. Earthquakes are common in the belt of young fold mountains.
5. Plate margins are zones of great volcanic activity.
6. Volcanic eruption is one of the main causes of earthquakes.
7. The vent of a volcano when blocked results in an explosive eruption.

Ans:

Causes of Volcanic and Seismic Phenomena

1. Why some volcanoes erupt explosively: Violent volcanic bursts result from highly viscous, gas-laden magma being trapped within the volcanic conduit. As this magma tries to ascend, immense pressure builds, eventually causing a sudden, forceful expulsion of material.
2. Why some volcanoes develop parasitic cones: Parasitic cones emerge when rising magma within the main volcanic edifice exploits secondary, weaker fissures or fractures on the volcano’s sides. This leads to localized, auxiliary eruptions that construct smaller, distinct cones separate from the primary vent.
3. Why hot springs are common in volcanic regions: Volcanic areas offer a natural heat source from shallow molten rock or superheated subsurface rocks. Groundwater infiltrates cracks in these heated zones, absorbs the geothermal energy, and subsequently rises to the surface due to its reduced density, appearing as hot springs.
4. Young fold mountain belts are active zones of ongoing continental collision and compression. The enormous stresses generated by these converging tectonic plates cause significant rock deformation and frequent ruptures along faults, releasing stored energy as recurring earthquakes.
5. Why plate margins are zones of great volcanic activity: Plate margins, where Earth’s tectonic plates meet and interact (diverge, converge, or transform), represent areas of crustal instability. These boundaries serve as pathways or decompression zones, facilitating the upward movement of molten material from the Earth’s mantle to the surface, leading to widespread volcanic phenomena.
6. Why volcanic eruption is one of the main causes of earthquakes: While often triggered by seismic events, powerful volcanic eruptions themselves can induce shallow earthquakes. This occurs due to the forceful displacement and movement of magma within the volcanic structure, rapid changes in pressure, or the fracturing of surrounding bedrock as the eruption proceeds.
7. When a volcano’s primary vent becomes obstructed—perhaps by solidified lava or collapsed debris—magma and its associated gases continue to accumulate beneath this plug. The resulting extreme buildup of internal pressure eventually overcomes the blockage, leading to a catastrophic and highly explosive eruption.