Rocks

The Earth’s solid outer layer, its crust, is fundamentally comprised of rocks, which are naturally occurring solid formations made up of one or more minerals. Minerals are the basic constituents of rocks, defined by their precise chemical makeup and ordered internal atomic arrangement. This contrasts with rocks, which are typically heterogeneous mixtures lacking a fixed chemical formula. Understanding this distinction is key to appreciating the varied geological features found across our planet. Rocks are broadly classified into three major categories, each defined by its unique formation process: igneous, sedimentary, and metamorphic.

Igneous rocks, often referred to as “primary rocks,” originate from the solidification of molten material. When this molten rock, called magma, cools slowly beneath the Earth’s surface, it forms intrusive igneous rocks like granite, characterized by large, visible crystals. Conversely, when molten rock, known as lava, erupts and cools rapidly on the surface, it creates extrusive igneous rocks such as basalt, which typically exhibit fine grains or a glassy texture. Sedimentary rocks, also known as “secondary rocks,” develop from the accumulation, compression, and cementing together of fragments derived from the weathering and erosion of pre-existing rocks. These rocks often display distinct layering and can preserve the remains of ancient organisms in the form of fossils. Examples include sandstone, formed from sand grains, and limestone, often originating from the accumulation of organic material. Their formation can be mechanical, organic, or chemical. Metamorphic rocks arise when existing igneous or sedimentary rocks (or even other metamorphic rocks) undergo significant transformation due to intense heat, pressure, or the influence of chemically active fluids, without actually melting. This process alters their mineral composition, texture, or internal structure, leading to rocks like marble, formed from limestone, or slate, derived from shale, under conditions ranging from contact (thermal) to regional (dynamic) metamorphism.

The continuous transformation of these rock types from one to another is a central concept captured by the rock cycle. This dynamic process illustrates the profound interrelationships among igneous, sedimentary, and metamorphic rocks, driven by both the Earth’s internal thermal energy and surface phenomena such as weathering, erosion, and deposition. For instance, an igneous rock can be broken down by weathering into sediments, which then consolidate to form a sedimentary rock. If this sedimentary rock is subsequently buried deep within the Earth’s crust, it can be subjected to intense heat and pressure, metamorphosing into a new rock type. Should this metamorphic rock then melt, it would return to its molten state, thus completing the cyclical journey. This interconnected system underscores the constant evolution and recycling of the Earth’s crustal materials over vast geological timescales.

Exercises

I. Short Answer Questions 

Question 1.
State two points of distinction between rocks and minerals.
Ans:

ere are two distinct differences between rocks and minerals, phrased uniquely:

1. Fundamental Building Blocks vs. Assemblages: In contrast, a rock is typically an aggregation—a composite—of various minerals, or occasionally includes other non-mineral components (such as fossilized organic matter or solidified volcanic glass). Think of minerals as the distinct, individual Lego bricks, while rocks are the diverse structures built from these bricks.
2. Inherent Consistency vs. Variable Mixtures: Each mineral possesses a set of stable and predictable physical characteristics (like its specific hardness, consistent color, or how it breaks along planes), owing to its uniform internal crystalline arrangement and fixed composition. Regardless of where it’s found, a particular mineral will exhibit these consistent traits. Rocks, on the other hand, lack this intrinsic uniformity; their chemical makeup is not fixed, and their internal arrangement is a blend of their constituent parts. Consequently, a rock’s observable properties are a function of the specific minerals present, their relative amounts, and how they are arranged, leading to considerable variation even within the same broad rock category.

Question 2.

Name any three elements of the earth’s crust.

Ans:

The Earth’s crust is a complex geological layer, predominantly structured by three key elements: oxygen, silicon, and aluminum. Oxygen, constituting nearly half of the crust’s mass at 46%, rarely exists in its pure form due to its high reactivity. Its significance lies in its strong tendency to form chemical bonds, particularly with silicon, leading to the formation of pervasive silicate minerals that make up most crustal rocks, as well as various oxide compounds.

Silicon, the second most abundant element at 27-28% of the crust’s composition, provides the fundamental structural framework for numerous minerals. Its unique ability to form tetrahedral units that link in diverse ways gives rise to the extensive silicate mineral family, which accounts for over 90% of crustal material. This essential silicon-oxygen framework is crucial in shaping the properties and structural features of a wide range of common rock types.

Aluminum, the most abundant metal and third most common element at roughly 8% of the crust, is predominantly incorporated into the crystal structures of common rock-forming minerals. Within these structures, it frequently substitutes for silicon or other positively charged ions, thereby considerably influencing the minerals’ structural integrity, stability, and chemical behavior. The dynamic interactions among these three prevalent elements are central to understanding the intricate composition and ongoing geological processes that continuously shape the Earth’s outermost layer.

Question 3.

Name three types of rocks.

Ans:

Here’s a breakdown of the three fundamental rock types, expressed in a unique way:

1. Igneous Rocks: The “Born of Fire” Rocks

These are the foundational rocks of Earth’s crust, originating directly from the planet’s internal heat. They form when molten rock – either deep underground as magma or erupted onto the surface as lava – cools and solidifies. Imagine liquid rock freezing solid. The speed of this cooling dictates their texture: slow cooling allows large crystals to grow (like in granite), while rapid cooling results in fine-grained or even glassy textures (like in basalt). They are essentially a direct solidification of Earth’s fiery interior.

2. Sedimentary Rocks: The “Layered History” Rocks

These fragments, called sediments, are transported by wind, water, or ice, and then deposited in layers. Over vast stretches of time, the weight of overlying layers compacts these sediments, and natural cements bind them together, transforming loose material into solid rock. Sandstone, with its visible grains, and limestone, often rich in ancient shell fragments, are prime examples of these storied layers.

3. Metamorphic Rocks: The “Transformed Under Stress” Rocks

They begin as existing igneous, sedimentary, or even other metamorphic rocks that are subjected to extreme conditions – immense pressure from overlying rock, searing heat from deep within the Earth or nearby magma intrusions, or interactions with hot, chemical-rich fluids.Instead, the minerals within the rock recrystallize and rearrange themselves, sometimes developing new textures or entirely new minerals. This process gives rise to rocks like the beautifully patterned marble (from limestone) or the fine-grained, cleavable slate (from shale), each a testament to a dramatic geological past.

Question 4.

Why are the igneous rocks also called the primary rocks?

Ans:

Igneous rocks hold the designation of “primary rocks” due to their foundational role in Earth’s geological processes. Their genesis directly from the cooling and solidification of molten magma or lava marks them as the initial rock formations within the rock cycle. This direct crystallization from the Earth’s molten interior means they represent the very first stage of solid rock creation.

Essentially, the Earth’s primitive crust, billions of years ago, was entirely igneous in nature, serving as the fundamental platform upon which all subsequent geological transformations have occurred. This establishes igneous rocks as the bedrock, both literally and figuratively, for other rock types. Consequently, igneous rocks serve as the ultimate source material for the formation of sedimentary and metamorphic rocks. Sedimentary rocks are created from the breakdown and accumulation of fragments, frequently derived from pre-existing igneous formations. Similarly, metamorphic rocks arise when existing rocks, often igneous, undergo profound changes in response to intense heat, pressure, or chemical alterations deep within the Earth’s crust.

Question 5.

Give one difference between Extrusive igneous and Intrusive igneous rocks.

Ans:

The core difference between extrusive and intrusive igneous rocks lies in the environment where their molten material solidifies, which profoundly influences their crystalline structure. Extrusive igneous rocks form when lava erupts onto the Earth’s surface and cools rapidly in contact with the atmosphere or water. This swift cooling process inhibits the formation of large crystals, resulting in a fine-grained or even glassy texture where individual mineral grains are microscopic or absent.

In contrast, intrusive igneous rocks develop from magma that solidifies slowly deep within the Earth’s crust. This extended cooling duration allows ample time for mineral crystals to grow to a larger size, leading to a coarse-grained texture where distinct mineral components are easily visible to the naked eye. Thus, the rate of cooling, dictated by the location of solidification, is the primary factor differentiating these two fundamental types of igneous rocks.

Question 6.

Name any two chief characteristics of Igneous Rocks.

Ans:

Here are two primary characteristics of igneous rocks:

1. Molten Genesis: The most fundamental trait of igneous rocks is their direct formation from the cooling and solidification of intensely hot, molten rock. This molten material originates either deep within the Earth’s crust or mantle as magma, which solidifies underground to form intrusive (plutonic) igneous rocks, or it erupts onto the Earth’s surface as lava, crystallizing to create extrusive (volcanic) igneous rocks. This direct transition from a liquid to a solid state is the defining process unique to igneous rock formation.
2. Texture Reflecting Cooling Rate: Igneous rocks display distinct textures that are directly indicative of their cooling history. They commonly possess a crystalline texture, where visible mineral crystals have grown during solidification. The size of these crystals varies significantly: slow cooling allows for the development of large, well-formed crystals (phaneritic texture), typical of intrusive rocks, while rapid cooling results in small, fine-grained crystals (aphanitic texture), characteristic of extrusive rocks. Alternatively, extremely rapid cooling can prevent crystal formation entirely, leading to a glassy (vitreous) texture, where the rock resembles natural glass.

Question 7.

What are the main characteristics of Basic Igneous Rocks?

Ans:

Basic igneous rocks, also referred to as mafic rocks, are characterized by their specific chemical makeup and mineral content, a distinction reflected in their name, derived from “magnesium” and “ferric” due to their high concentrations of these elements. A key defining feature is their relatively low silica (SiO2) content, typically falling within the 45% to 55% range by weight. This lower silica concentration results in mafic magmas being less viscous and more fluid compared to silica-rich (felsic) magmas. This enhanced fluidity allows mafic lavas to spread more extensively and facilitates the easier escape of volcanic gases, generally leading to less explosive eruptions.

Mineralogically, basic igneous rocks are rich in dark-colored ferromagnesian minerals such as pyroxene, olivine, and calcium-rich plagioclase feldspar. The prevalence of these iron and magnesium-bearing minerals is responsible for their characteristic dark hues, usually appearing black or dark gray, and contributes to their higher density compared to lighter, silica-rich rocks. Common examples illustrating these characteristics include basalt, an extrusive rock with a fine-grained texture, and gabbro, an intrusive rock exhibiting a coarser grain. Basalt is a primary constituent of the oceanic crust, while gabbro forms in deeper geological intrusions.

Question 8.

Name two important landforms made by Igneous Rocks.

Ans:

Here are two important landforms crafted by igneous rocks:

1. Volcanic Mountains/Shields: These prominent geological features arise directly from the ascent and eruption of molten rock. When magma breaches the Earth’s surface as lava, ash, and volcanic gases, it accumulates to construct distinct landforms. These can range from the classic, steep-sided cones of stratovolcanoes, such as Japan’s Mount Fuji, to the broad, gently sloping profiles characteristic of shield volcanoes, exemplified by those found in the Hawaiian Islands.
2. Exposed Plutons (e.g., Batholiths): While initially forming deep within the Earth’s crust, large intrusive igneous bodies, known as plutons (with batholiths being particularly massive examples), become significant landforms through prolonged erosional processes. These enormous masses of slowly cooled magma, once solidified, can be gradually revealed as overlying rock layers are stripped away. Such features frequently constitute the durable cores of extensive mountain ranges, as seen in the formidable Sierra Nevada range in California, where the underlying batholith forms its fundamental structure.

Question 9.

What are Sills ?

Ans:

In geology, a sill represents a distinct type of intrusive igneous rock formation. It is essentially a planar, sheet-like body of igneous material that develops when molten magma injects itself between pre-existing layers of rock, subsequently cooling and solidifying within that space. This formation is characterized by its concordant nature, meaning it lies parallel to the bedding or foliation of the surrounding host rocks, in contrast to dikes which cut across rock layers.

Sills typically form in a horizontal or gently dipping orientation, mirroring the pre-existing strata they intrude. As they are intrusive features, the magma cools slowly deep within the Earth’s crust, leading to the formation of larger mineral crystals and thus a coarser-grained texture compared to volcanic rocks that solidify rapidly at the surface. These underground formations can vary significantly in size, from a few centimeters to hundreds of meters in thickness, and can extend for many kilometers laterally, offering valuable insights into historical magma movements and crustal dynamics.

Question 10.

Which rocks are associated with ores of metals ?

Ans:

All three major rock types – igneous, sedimentary, and metamorphic – can be associated with ores of metals, though their roles and the specific types of deposits they host can vary significantly.

Igneous rocks are arguably the most directly associated with a wide range of metal ores. Many economically important metallic deposits are formed, directly or indirectly, from magmatic processes. As molten magma cools and solidifies, various metals (like copper, gold, silver, platinum, nickel, iron, and tin) can crystallize and become concentrated within the igneous rock itself or be carried by hot, mineral-rich fluids (hydrothermal solutions) that emanate from the cooling magma. These fluids can then deposit the metals in veins within the igneous rock or in surrounding “country rocks.” Examples include porphyry copper deposits, which are often found in intrusive igneous rocks, and magmatic sulfide deposits, rich in nickel and platinum group elements, typically associated with mafic and ultramafic igneous rocks like gabbro.

Sedimentary rocks can also host significant metal ore deposits. While they don’t form metals directly from molten rock, they play a crucial role in concentrating metals that have been weathered and eroded from pre-existing rocks. Metals can be transported in solution or as tiny particles and then precipitated or accumulated in sedimentary environments. Examples include banded iron formations (BIFs), which are major sources of iron, and some lead-zinc deposits that form in sedimentary basins. Laterites, formed from the intense weathering of mafic rocks in tropical climates, are also sedimentary deposits rich in iron, manganese, and aluminum.

Metamorphic rocks are transformed versions of existing igneous or sedimentary rocks, and during this transformation, pre-existing ore deposits can be modified or new ones can form. Intense heat and pressure can cause minerals to recrystallize, concentrating metals into new forms. Additionally, chemically active fluids associated with metamorphic processes can leach metals from one area and deposit them in another, often along faults or shear zones. For example, some gold deposits are formed through “lateral secretion” during metamorphism, where gold is liberated from deforming rocks and concentrated in zones of lower pressure. Certain metamorphic rocks like serpentinites, which form from the alteration of mafic and ultramafic rocks, can also host asbestos or even some metal deposits.

Question 11.

Which rocks are associated with fossil fuels ?

Ans:

Fossil fuels, such as coal, oil, and natural gas, are overwhelmingly associated with sedimentary rocks. This strong connection stems directly from the unique formation process of sedimentary rocks and the origin of fossil fuels themselves.

Sedimentary rocks are formed from the accumulation, compaction, and cementation of sediments, which often include vast amounts of organic matter from ancient plants and animals. Over millions of years, as these organic remains are buried under successive layers of sediment, they are subjected to intense heat and pressure. This geological process transforms the organic material into the carbon-rich compounds that constitute fossil fuels. The layered nature of sedimentary rocks creates ideal environments for the preservation of these organic materials and the subsequent migration and trapping of liquid and gaseous hydrocarbons.

Question 12.

Mention any two chief characteristics of Sedimentary Rocks.

Ans:

Here are two chief characteristics of sedimentary rocks:

1. Stratification (Layering): Sedimentary rocks are distinctively characterized by their formation in layers, or strata. This layering is a direct result of the deposition of sediments in successive beds over time, often reflecting changes in the type of sediment, the depositional environment, or the energy of the depositing medium (like water or wind). These visible layers can range from very thin laminations to thick beds and are a hallmark feature.
2. Presence of Fossils: Due to their formation process, which involves the accumulation and burial of organic matter along with mineral sediments, sedimentary rocks are almost exclusively the only type of rock in which fossils are found. The remains or traces of ancient plants and animals can be preserved within the rock layers, providing invaluable insights into past life forms, environments, and geological timelines.

Question 13.

Give two examples of Sedimentary Rocks.

Ans:

Here are two distinct examples of sedimentary rocks:

1. Sandstone: This clastic sedimentary rock is formed from the compaction and cementation of sand grains, which are typically composed of quartz. It often displays visible layering, known as bedding, and can vary in color depending on the minerals present in its cement (e.g., iron oxides can give it a reddish hue). Sandstone is commonly found in ancient riverbeds, deltas, and desert environments.
2. Limestone:Much of it forms from the accumulation of marine organism shells and skeletons (like corals and foraminifera), making it an organic sedimentary rock. However, it can also form chemically through the precipitation of calcium carbonate from water. Limestone is often permeable and can dissolve to form caves and sinkholes, and it’s widely used in construction and as a raw material for cement.

Question 14.

Name the rocks which are most widespread on the earth.

Ans:

When evaluating the most prevalent rock types on Earth, it’s essential to differentiate between their surface distribution and their overall volumetric contribution to the Earth’s crust. Sedimentary rocks dominate the Earth’s surface, blanketing roughly 75% of continental land. They form a comparatively thin layer over deeper igneous and metamorphic formations, a characteristic stemming from their genesis through the accumulation, compaction, and cementation of weathered and eroded material in basins.

Conversely, considering the total volume of the Earth’s crust, igneous and metamorphic rocks constitute the overwhelming majority, estimated at 90-95%. Igneous rocks, notably basalt, are fundamental to the oceanic crust and also contribute significantly to the continental crust through intrusive bodies such as granite batholiths. Metamorphic rocks, resulting from the alteration of pre-existing rocks under conditions of intense heat and pressure, are also profoundly abundant beneath the surface, particularly within the continental crust.

Question 15.

Name the three stages of lithification of Sedimentary rocks.

Ans:

The conversion of loose sediments into solid sedimentary rock is a multi-step geological process called lithification, which unfolds over extensive periods due to the effects of burial and subsequent alterations. This transformation begins with compaction. This pressure forces the individual sediment particles closer, diminishing the void spaces (pores) between them and expelling interstitial fluids. The result is a notable reduction in sediment volume and a corresponding increase in its density.

After compaction, the process advances to cementation. Here, minerals dissolved in groundwater circulating through the now-compacted sediments begin to precipitate within the remaining pore spaces. These precipitated minerals, commonly calcite, silica, or iron oxides, serve as a natural binding agent, effectively gluing the individual sediment grains together. This cementing action is what fundamentally converts the initially unconsolidated sediment into a cohesive, solid sedimentary rock.

While compaction and cementation are central to lithification, a third process, recrystallization, frequently occurs concurrently. This involves the reorganization or growth of existing mineral grains within the sediment, sometimes leading to the formation of new, more stable mineral phases or the development of an interlocking crystalline structure. Driven by elevated temperatures and pressures associated with deep burial, recrystallization further contributes to the solidification and enhanced stability of the newly formed rock.

Question 16.

Name the types Sedimentary rocks based on agents of formation.

Ans:

Sedimentary rocks are classified according to the primary agents and processes responsible for their formation, illustrating the varied origins of these geological structures—from fragmented rock accumulation to the precipitation of dissolved minerals or organic remains.

One significant category is Clastic Sedimentary Rocks, also known as mechanically formed or detrital rocks. These are created from the lithification and accumulation of solid fragments, or “clasts,” that result from the physical weathering and erosion of existing rocks. Various natural forces such as water, wind, glaciers, and gravity transport these fragments, which range in size from minute clay particles to substantial boulders. Over time, these sediments undergo compaction due to the weight of overlying layers and are cemented together by minerals that precipitate from groundwater, ultimately forming rocks like sandstone, shale, and conglomerate.

Another key type is Chemical Sedimentary Rocks, which develop when minerals previously dissolved in water precipitate out of solution. This precipitation can be triggered by shifts in water chemistry, temperature, or pressure, or simply through the evaporation of water, leading to a supersaturation of dissolved ions. Examples include rock salt (halite) and gypsum, which originate from the evaporation of saline water in oceans or lakes, and certain limestones that form through inorganic precipitation.

Lastly, Biochemical (or Organic) Sedimentary Rocks are formed from the accumulation of organic material or the remnants of living organisms. Organisms absorb dissolved ions from water to construct shells, skeletons, or plant structures. Upon their death, these organic remains accumulate, compact, and cement over geological time to form rock. Notable examples include the majority of limestones, frequently composed of shell fragments or microscopic marine organisms, and coal, which results from the extensive compaction and alteration of plant matter in swampy settings.

Question 17.

Which agents are responsible for deposition of sediments?

Ans:

Here’s a concise, unique summary of the agents responsible for sediment deposition:

• Water: The primary force, depositing sediments as its speed decreases. Rivers form deltas and floodplains; oceans create beaches and sandbars.
• Wind: Prevalent in arid zones, it transports fine sediments (sand, silt, dust) and deposits them when obstructed or slowed, leading to sand dunes and loess.
• Ice (Glaciers): Massive ice bodies carry vast amounts of debris. Upon melting, they deposit this material, forming moraines, outwash plains, and till.
• Gravity: A direct cause of deposition, particularly on slopes, where mass wasting events like landslides and rockfalls accumulate loose material at the base, creating talus and alluvial fans.

Question 18.

What are known as metamorphic rocks ? Give two examples.

Ans:

Metamorphic rocks arise from the profound alteration of pre-existing rocks, be they igneous, sedimentary, or even other metamorphic types. This transformative process, termed metamorphism, involves substantial heat, pressure, and/or the influence of chemically active fluids, reshaping the rock’s mineralogy, texture, or internal structure without causing it to melt. Such conditions are typically encountered deep within the Earth’s crust, frequently linked to tectonic plate interactions or the intrusion of molten rock. The original rock, known as the protolith, is essentially subjected to intense heating or compression, leading to a denser, more compact rock with newly configured mineral grains.

Two notable instances of metamorphic rocks include:

• Marble: This rock originates from the metamorphism of limestone or dolostone. Under the intense heat and pressure, the original calcite crystals within the limestone undergo recrystallization, growing into larger grains. The resulting marble is generally non-foliated, lacking distinct layers, and although often white, it can display a range of colors due to the presence of various impurities.
• Slate: A fine-grained, foliated metamorphic rock, slate is formed through the low-grade metamorphism of shale or claystone. The applied pressure causes the microscopic clay minerals to align perpendicularly to the stress, imparting slate with its distinctive cleavage, which allows it to be readily split into thin, flat sheets.

Question 19.

What is Mechanical Metamorphism ?

Ans:

Mechanical metamorphism, often referred to as dynamic metamorphism or cataclastic metamorphism, primarily involves the physical deformation and breakdown of rocks due to intense directed pressure and mechanical stress, with relatively little change in temperature over the long term. This process is particularly common in areas of high strain, such as along fault zones or in shear zones where large bodies of rock slide past one another.

Instead of new minerals growing due to significant temperature changes, the existing minerals in the rock are physically crushed, ground, pulverized, and deformed. This can lead to a reduction in grain size, the development of new textures like foliation (a planar alignment of mineral grains), and even the formation of very fine-grained, highly fractured rocks known as fault breccias or mylonites. While some heat can be generated by friction during this intense mechanical action, the dominant force driving the transformation is the mechanical stress.

Question 20.

What is meant by Rock Cycle ?

Ans:

The Rock Cycle describes Earth’s continuous process of transforming crustal materials. It connects the three main rock types:

• Sedimentary Rocks: Form from the accumulation and hardening of weathered rock fragments, minerals, or organic matter.
• Metamorphic Rocks: Form when existing rocks are altered by intense heat, pressure, or chemical reactions without melting.

Key processes in this cycle include:

• Weathering & Erosion: Breaking down and transporting rock fragments.
• Deposition: Settling of these fragments.
• Lithification: Compacting and cementing sediments into rock.
• Melting: Rocks transforming into magma deep underground.
• Crystallization: Magma or lava solidifying into igneous rock.
• Metamorphism: Existing rocks changing due to heat and pressure.
• Uplift: Buried rocks returning to the surface to restart the cycle.

Question 21.

What processes are involved in the formation of Igneous Rocks ?

Ans:

There are two primary processes involved, depending on where this solidification occurs:

1. Melting and Magma Generation:
   • Rocks deep within the Earth’s mantle or crust melt due to intense heat, pressure changes (decompression melting), or the addition of volatiles like water (flux melting). This molten material is called magma.
   • Magma is less dense than the surrounding solid rock, so it rises towards the Earth’s surface.
2. Cooling and Crystallization:
   • Intrusive (Plutonic) Rocks: If the magma cools and solidifies beneath the Earth’s surface, it does so slowly due to the insulating effect of overlying rock. This slow cooling allows mineral crystals ample time to grow large, resulting in a coarse-grained texture (e.g., granite).
   • Extrusive (Volcanic) Rocks: If the magma (now called lava) erupts onto the Earth’s surface (e.g., from volcanoes or fissures), it cools and solidifies very rapidly when exposed to air or water. This rapid cooling doesn’t allow much time for large crystals to form, leading to fine-grained or even glassy textures (e.g., basalt, obsidian).

II. Explain these terms associated with rocks.

Question 1.
Extrusive Igneous Rocks.
Ans:

The defining characteristic of these rocks is the rapid rate at which the lava cools. This quick cooling prevents the mineral crystals from growing large, resulting in a fine-grained texture where individual crystals are often too small to be seen without magnification. In some cases, the cooling is so instantaneous that no crystals have time to form at all, leading to a glassy texture. Familiar examples of extrusive igneous rocks include basalt, a dark, dense rock common in oceanic crust; rhyolite, a lighter-colored rock chemically similar to granite; and obsidian, a natural volcanic glass.

Question 2.

Laccoliths and Batholiths.

Ans:

Laccoliths and batholiths are both products of magma solidifying beneath the Earth’s surface, classifying them as intrusive igneous rock formations. Despite this shared origin, their distinct characteristics in terms of scale, morphology, and interaction with pre-existing rock strata set them apart.

A laccolith is a type of igneous intrusion characterized by its unique mushroom-like or dome-shaped structure. It arises when viscous magma intrudes between layers of sedimentary rock. Instead of breaking through to the surface, the magma exerts pressure on the overlying rock, causing it to arch upwards, forming a blister-like bulge. Laccoliths typically feature a relatively level base and a convex, or domed, upper surface. These formations are generally modest in size, and their subsequent exposure due to erosion can lead to the formation of hills or smaller mountainous features.

In contrast, a batholith represents a colossal, irregularly shaped body of intrusive igneous rock, most commonly composed of granitic or similar felsic compositions. Its formation involves the gradual cooling and solidification of substantial magma volumes deep within the Earth’s crust. Batholiths are notable for their immense scale, often spanning exposed surface areas exceeding 100 square kilometers, and their vertical extent remains largely undetermined. Unlike laccoliths, batholiths are discordant, meaning they cut across the existing rock layers rather than conforming to their stratification. These massive intrusions frequently constitute the foundational cores of mountain ranges, becoming visible at the surface only after significant uplift and the erosion of the overlying crust.

Question 3.

Fossil fuels.

Ans:

Fossil fuels, including coal, oil, and natural gas, are energy-rich geological deposits formed over millions of years from decomposed ancient organic matter (plants and animals) buried under intense heat and pressure. These carbon- and hydrogen-rich resources power electricity, transportation, and various industries.

Their formation involved dead organisms accumulating in oxygen-depleted environments, followed by deep burial and geological “cooking” that transformed them into hydrocarbons. Coal primarily came from ancient forests, while oil and natural gas originated from marine microorganisms.

While historically crucial due to their high energy density, burning fossil fuels releases greenhouse gases like CO2, contributing to global warming and climate change. Extraction also causes environmental damage. This necessitates a global shift towards renewable energy sources and sustainable practices to mitigate climate change and ensure future energy security.

Question 4.

Lithification of Rocks.

Ans:

Lithification is the natural process where loose sediments convert into solid rock, characteristic of sedimentary rock formation. It involves two main steps: compaction, where overlying weight compresses sediments, expelling water and tightening grain packing; and cementation, where dissolved minerals precipitate in pore spaces, binding grains into a cohesive rock.

Question 5.

Metamorphism.

Ans:

Metamorphism is the geological process where existing rocks (igneous, sedimentary, or other metamorphic rocks) transform into new rock types without completely melting. This transformation occurs due to changes in intense heat, pressure, and/or chemically active fluids, altering the rock’s mineral composition, texture, and structure. These conditions are typically found deep within the Earth’s crust, often associated with tectonic plate movements.

III. Distinguish between each of the following

P Q. Lava and Magma.
Ans:

While often used interchangeably in everyday conversation, “magma” and “lava” refer to the same molten rock, but their distinction lies purely in their location:

• Magma is molten or semi-molten rock found beneath the Earth’s surface. It exists in magma chambers within the Earth’s crust or upper mantle, under immense heat and pressure. 
• Lava is the term for molten rock that has erupted and flowed onto the Earth’s surface. When magma reaches the surface, the release of pressure causes dissolved gases to escape, and the molten rock then flows as lava. As lava cools and solidifies, it forms extrusive igneous rocks.

Question 1.

Plutonic and Volcanic rocks.

Ans:

Plutonic and volcanic rocks are both types of igneous rocks, meaning they form from the cooling and solidification of molten rock. The key difference lies in where this cooling occurs, which profoundly impacts their characteristics.

Plutonic Rocks (Intrusive Igneous Rocks):

• Formation Location: These rocks solidify beneath the Earth’s surface, deep within the crust.
• Cooling Rate: Due to being insulated by surrounding rock, magma cools very slowly over thousands to millions of years.
• Crystal Size: The slow cooling allows ample time for mineral crystals to grow large and well-formed, making them visible to the naked eye. This is known as a phaneritic texture.
• Texture: Typically coarse-grained and interlocking, without gas bubbles.
• Examples: Granite, gabbro, diorite.
• Exposure: They are usually exposed at the surface much later through processes of uplift and erosion, often forming the cores of mountain ranges.

Volcanic Rocks (Extrusive Igneous Rocks):

• Formation Location: These rocks solidify on or very near the Earth’s surface, from erupted lava.
• Cooling Rate: Lava cools very rapidly when exposed to air or water.
• Crystal Size: Rapid cooling means crystals have little time to grow, resulting in very small (microscopic) crystals or even no crystals at all (forming volcanic glass). This is known as an aphanitic or glassy texture.
• Texture: Often fine-grained, glassy, or may contain vesicles (holes from trapped gas bubbles).
• Examples: Basalt, rhyolite, obsidian, pumice.
• Exposure: They are immediately exposed at the surface as lava flows or fragmented volcanic debris.

Question 2.

Thermal and Dynamic Metamorphism.

Ans:

Thermal (Contact) and Dynamic Metamorphism are two distinct processes that transform existing rocks, primarily differing in the dominant agent of change and their resulting characteristics.

Thermal Metamorphism is driven predominantly by heat. It occurs when rocks are heated by proximity to a hot igneous intrusion (magma or lava). This process involves high temperatures but generally low pressure, leading to the recrystallization of minerals without significant alignment. The resulting rocks are often non-foliated (lacking layers) and the effects are localized, forming a “baked” zone around the heat source. Examples include hornfels and marble.

In contrast, Dynamic Metamorphism is primarily caused by directed pressure and mechanical deformation (stress). This typically happens along fault zones where intense shearing forces crush and deform rocks. While some heat may be generated by friction, it’s the directed pressure that’s the main driver. This process leads to the alignment of mineral grains, producing foliated rocks with textures like mylonite or fault breccia. The effects are confined to narrow zones directly adjacent to the fault.

Question 3.

Sills and Dykes.

Ans:

In geology, sills and dykes (dikes) are both tabular intrusions of igneous rock that form when magma solidifies beneath the Earth’s surface. The key difference lies in their orientation relative to the existing rock layers (country rock):

• Sills: These are concordant intrusions, meaning they intrude and solidify parallel to the existing bedding planes or layers of the surrounding rock. Imagine a horizontal sheet of magma pushing its way between sedimentary layers.
• Dykes: These are discordant intrusions, meaning they cut across or intersect the existing layers or structures of the surrounding rock, often vertically or at steep angles. Think of a wall of magma cutting through horizontal rock beds.

Question 4.

Calcarious and Carbonacious rocks.

Ans:

Calcareous vs. Carbonaceous Rocks: A Distinction

While both terms relate to carbon, they highlight different primary compositions in rocks:

• Calcareous rocks are defined by their dominant content of calcium carbonate (CaCO3​). This mineral typically originates from the shells and skeletal remains of marine life. Limestone and dolostone are prime examples.
• Carbonaceous rocks, on the other hand, are characterized by a high proportion of organic carbon. This carbon comes from the preserved remains of ancient plants and animals. Common examples include coal and organic-rich shales.

Question 5.

Acid Igneous Rocks and Basic Igneous Rocks.

Ans:

Igneous rocks, formed from cooling and solidifying magma or lava, are broadly categorized based on their silica content, which influences their mineral composition, color, and density. This classification gives rise to “acid” and “basic” igneous rocks.

Acid Igneous Rocks (Felsic Rocks): These rocks are characterized by a high silica content, typically above 63-65%. They are rich in lighter-colored minerals like quartz and feldspar, leading to their generally light color (e.g., pink, white, light gray). Due to their composition, they tend to be less dense than basic rocks. The magma that forms acid igneous rocks is usually viscous (thick) and flows slowly, often leading to explosive volcanic eruptions or the formation of large, intrusive bodies deep within the Earth’s crust.

• Examples: Granite (intrusive), Rhyolite (extrusive).

Basic Igneous Rocks (Mafic Rocks): In contrast, basic igneous rocks have a lower silica content, typically ranging from 45% to 55%. They are rich in darker, iron- and magnesium-rich (ferromagnesian or “mafic”) minerals such as olivine, pyroxene, and calcium-rich plagioclase feldspar. This gives them a characteristic dark color (e.g., black, dark gray). Consequently, they are generally denser than acid rocks. The magma that forms basic igneous rocks is typically fluid (less viscous) and flows easily, often resulting in effusive volcanic eruptions (like lava flows).

• Examples: Basalt (extrusive), Gabbro (intrusive).

IV. State the types of rocks for the formation of which the following processes are involved.

Question 1.
Solidification of magma on the surface of the earth.
Ans:

When molten rock, known as magma, emerges onto the Earth’s surface, it undergoes a transformation and is then referred to as lava. This lava subsequently undergoes swift cooling and solidification, giving rise to what are called extrusive igneous rocks.

This geological process encompasses several key stages:

• Surface Extrusion: Fueled by its inherent buoyancy and the immense pressure from beneath, magma ascends, ultimately erupting through volcanic vents. These eruptions can manifest as quiescent, flowing streams or as forceful, explosive discharges.
• Accelerated Solidification: In stark contrast to magma that cools slowly beneath the Earth’s crust, surface lava is directly exposed to significantly lower ambient temperatures in the air or water. This exposure triggers its remarkably rapid hardening.
• Characteristic Textures:Microcrystalline (Aphanitic): Prompt cooling allows insufficient time for the development of large mineral crystals, leading to rocks with minute, often microscopic, crystalline structures (a prime example being basalt).
  • Vitreous (Glassy): Under conditions of exceedingly rapid cooling, crystal formation is entirely inhibited, resulting in the creation of natural volcanic glass (such as obsidian).
  • Porous (Vesicular): Gases dissolved within the lava expand as pressure decreases during eruption, forming bubbles. If these bubbles become entrapped within the solidifying lava, the resulting rock exhibits a porous texture, characterized by numerous voids (examples include pumice and scoria).
• Mineral Formation Sequence: Despite the rapid solidification, minerals still crystallize from the cooling lava in a generally predictable order, as described by Bowen’s Reaction Series. However, the accelerated cooling often restricts the complete growth of crystals and the full extent of mineral reactions.

Question 2.

Formation of large crystals, coarse texture and slow cooling and compaction.

Ans:

Intrusive Igneous Rock Formation: The Slow Cool Down

When magma (molten rock) forms deep underground, it often rises but gets trapped beneath the Earth’s surface. This trapped magma is insulated by the surrounding rock, leading to an incredibly slow cooling process—sometimes over millions of years. This prolonged cooling period is crucial because it provides ample time for individual mineral crystals to grow large. The result is a rock with a coarse, crystalline texture where you can easily see the individual mineral grains.

Compaction: A Sedimentary Story, Not Igneous

It’s important not to confuse this with compaction. Compaction is a process unique to sedimentary rock formation, where loose sediments are squeezed together by the weight of overlying layers, reducing pore spaces and eventually solidifying into rock. While crystals can settle in a magma chamber (magmatic compaction), the primary driver for crystal size in igneous rocks is the cooling rate, not external pressure.

Question 3.

Accumulation takes place over long periods of time in seas, lakes and streams.

Ans:

Option 1 (Focus on Sediments):

“Over extended geological timescales, vast quantities of eroded material, ranging from fine silts to larger rock fragments, gradually collect and settle within the tranquil environments of oceans, lakes, and flowing rivers. This slow, continuous deposition leads to the buildup of thick layers of sediment.”

Option 2 (Focus on Deposition):

“The process of deposition, occurring continuously over eons, sees the progressive build-up of particulate matter in aquatic settings such as marine basins, lacustrine environments, and fluvial channels. This sustained accumulation results in substantial stratified layers.”

Option 3 (More Concise):

“Extended periods witness the steady accumulation of diverse materials in marine, lacustrine, and fluvial systems. This long-term process forms extensive deposits within these water bodies.”

Option 4 (Descriptive):

“Across immense stretches of time, a patient process unfolds in the world’s watery realms: the gradual gathering and settling of transported earth materials. From the deepest ocean trenches to the quietest lakebeds and the winding paths of streams, these particulate burdens slowly amass, layer upon layer, over countless millennia.”

Key changes to ensure uniqueness:

• Varying vocabulary: Instead of just “accumulation,” I’ve used “collect and settle,” “buildup,” “deposition,” “progressive build-up,” “gathering and settling,” and “amass.”
• Different sentence structures: The phrasing and arrangement of clauses are different in each option.
• Adding detail/context: Words like “eroded material,” “geological timescales,” “particulate matter,” “aquatic settings,” and “transported earth materials” add specificity.
• Figurative language (Option 4): “Patient process unfolds” and “winding paths” add a unique touch.

Question 4.

Decomposition of organic matter at different stages and over different periods of time.

Ans:

This process is mainly carried out by decomposer organisms, primarily bacteria and fungi.

Stages of Decomposition

The breakdown of organic matter occurs in distinct stages:

• Fragmentation: Detritivores, like earthworms and insects, physically break down larger dead material into smaller fragments, increasing the surface area for microbial action.
• Leaching: Water dissolves and carries soluble nutrients, such as sugars and mineral salts, from the fragmented material into the surrounding soil.
• Catabolism: Microorganisms release enzymes that chemically break down complex organic molecules (e.g., cellulose, proteins) into simpler organic compounds.
• Humification: Partially decomposed organic matter gradually transforms into humus, a stable, dark substance that enhances soil fertility and water retention.
• Mineralization: Specialized microbes further break down humus and other remaining organic compounds, releasing inorganic nutrients (e.g., nitrates, phosphates) back into the soil, making them available for plant uptake.

Decomposition Over Time

The rate and extent of decomposition change significantly over time:

• Short-Term (Days to Weeks): Easily degradable compounds like sugars and amino acids break down rapidly. 
• Medium-Term (Weeks to Months): More resistant compounds, such as cellulose, decompose more slowly. Humification begins, forming a stable pool of organic matter, and nutrient release continues steadily.
• Long-Term (Months to Years/Decades): Very slow breakdown of highly resistant materials like lignin and established humus occurs. This stage is vital for long-term carbon storage in the soil, with minimal nutrient cycling.

Key Influencing Factors

The speed of decomposition is primarily influenced by:

• Litter Quality: Materials rich in simple sugars and nitrogen decompose quickly, while those high in lignin or waxes decompose slowly.
• Temperature: Warmer temperatures generally accelerate microbial activity and thus, decomposition.
• Moisture: Optimal moisture levels are essential; excessively dry conditions inhibit microbes, while waterlogging (lack of oxygen) slows down aerobic decomposers.
• Aeration (Oxygen): Aerobic (oxygen-requiring) decomposition is the most efficient form.
• pH: The acidity or alkalinity of the environment affects the types and activity of the dominant microbial communities.

V. Long Answer Questions

Question 1.
Distinguish between rocks and minerals.
Ans:

Minerals are the foundational components of Earth’s crust, akin to the individual ingredients in a complex dish. This consistent internal arrangement gives minerals their predictable physical properties, such as hardness, luster, and cleavage, which are crucial for their identification. Whether it’s the silicon dioxide in quartz or the calcium carbonate in calcite, every sample of a given mineral will share these fundamental characteristics, making them distinct and identifiable building blocks.

Rocks, on the other hand, are natural aggregates, or mixtures, of one or more minerals. Imagine them as the final geological dish, assembled from various mineral “ingredients.” Unlike minerals, rocks generally lack a fixed chemical formula and do not possess a single, overarching crystalline structure; their composition and texture are determined by the types and proportions of the minerals they contain and how those minerals are arranged. Rocks are broadly categorized by their formation processes into igneous (from cooling magma/lava), sedimentary (from compacted sediments), and metamorphic (transformed by heat and pressure). While a rock can be a blend of many different minerals, some, like limestone (primarily calcite), can be composed predominantly of a single mineral, yet they are still classified as rocks due to their aggregate nature and geological origin.

Question 2.

Describe how igneous rocks formed ? State their chief characteristics.

Ans:

The formation’s locus dictates the rock’s characteristics: intrusive (plutonic) rocks, like granite, crystallize slowly deep within the Earth, fostering large, visible mineral crystals. Conversely, extrusive (volcanic) rocks, such as basalt, form from rapidly cooling lava on the surface, yielding fine-grained or even glassy textures due to insufficient time for significant crystal growth.

While high temperatures, varying pressures, and volatile content (like water vapor and carbon dioxide) all influence their genesis, the cooling rate is paramount in defining an igneous rock’s texture. Slow cooling results in coarse-grained (phaneritic) textures, typical of intrusive rocks. Rapid cooling, characteristic of extrusive rocks, leads to fine-grained (aphanitic) or non-crystalline (glassy) textures.

Igneous rocks possess distinctive traits: they are typically crystalline with interlocking mineral grains, or can be glassy. Unlike sedimentary rocks, they generally lack fossils and layering, and their interlocking structure contributes to their notable hardness and durability. Classification is often based on silica content, which dictates mineral composition and color: felsic rocks (high silica) are light, mafic rocks (low silica, rich in iron and magnesium) are dark, and intermediate rocks fall in between.

Question 3.

How are igneous rocks classified on the basis of their chemical composition?

Ans:

Igneous rocks are fundamentally classified based on their chemical composition, primarily the percentage of silica (SiO2​) they contain, and by extension, the types of minerals that form within them. This chemical makeup dictates many of their observable properties, including color and density. This classification system divides igneous rocks into four main categories: Felsic, Intermediate, Mafic, and Ultramafic, representing a spectrum from high to low silica content.

Felsic rocks are characterized by a high silica content (typically >65% SiO2​), along with significant amounts of aluminum, sodium, and potassium. These rocks are generally light in color (e.g., pink, white) and have a lower density. Common minerals found in felsic rocks include quartz, muscovite mica, and potassium feldspar. Granite (intrusive) and rhyolite (extrusive) are classic examples of felsic igneous rocks. 

Moving along the compositional spectrum, Intermediate rocks have a silica content between 52% and 66% SiO2​. Diorite (intrusive) and andesite (extrusive) are typical intermediate igneous rocks, often exhibiting a salt-and-pepper appearance or a medium-gray color. Mafic rocks are relatively low in silica (45%−52% SiO2​) but rich in iron and magnesium. This makes them darker in color (often black or dark green) and denser. Key minerals include pyroxene, olivine, and calcium-rich plagioclase feldspar. Basalt (extrusive) and gabbro (intrusive) are prime examples of mafic rocks. Finally, Ultramafic rocks have the lowest silica content (<45% SiO2​) and are extremely rich in iron and magnesium, dominated by minerals like olivine and pyroxene. They are typically very dark green to black and are exceptionally dense. Peridotite, the primary rock of the Earth’s mantle, is an ultramafic rock, though extrusive ultramafic rocks (komatiite) are very rare on the surface.

Question 4.

Classify the igneous rocks on the basis of their place of origin.

Ans:

Igneous rocks originate from molten rock, and their ultimate texture is a direct consequence of their cooling environment. When molten rock, termed magma, crystallizes deep within the Earth’s crust, it gives rise to intrusive (or plutonic) igneous rocks. The considerable insulation from surrounding rock allows for a prolonged cooling period, providing ample opportunity for individual mineral crystals to grow to a size visible to the naked eye, thus exhibiting a coarse-grained (phaneritic) texture. Archetypal examples like granite and gabbro are products of these subterranean processes, often surfacing over geological eras through erosion.

In stark contrast, extrusive (or volcanic) igneous rocks form when molten material, now referred to as lava, solidifies rapidly on or very near the Earth’s surface following a volcanic eruption. In instances of exceptionally rapid cooling, such as when lava meets water, crystallization is entirely suppressed, leading to a glassy texture, exemplified by obsidian. Furthermore, the rapid escape of gases from cooling lava can create numerous trapped bubbles, imparting a vesicular texture, as seen in pumice. 

Question 5.

How are sedimentary rocks formed ?

Ans:

Sedimentary rocks chronicle Earth’s history, forming from the accumulation and cementation of rock fragments, organic matter, or precipitated minerals. This process begins with weathering, which breaks down older rocks into sediment of varying sizes. These sediments are then transported by natural forces like wind, water, and ice, eventually settling in layers as energy decreases. Over time, successive layers build up, compacting the material below. Finally, lithification occurs as pressure squeezes out water and dissolved minerals act as a natural cement, binding the loose sediments into solid rock, often preserving clues about past environments and life.

Question 6.

Explain the formation of sedimentary rocks on the basis of agents of formation.

Ans:

Sedimentary rocks originate from a sequence of steps, propelled by various natural forces. The initial stage involves weathering, a process of physical and chemical disintegration that transforms existing rocks into sediment.

Subsequently, these sediments are relocated by potent transporting agents. Water acts as a chief agent, conveying immense volumes of sediment within rivers, lakes, and oceans. As water flow decelerates, the sediment settles, forming stratified layers. Wind is responsible for transporting finer particles, such as sand and dust, frequently resulting in the formation of dunes or loess deposits. Ice, especially in the form of glaciers, possesses remarkable power, capable of transporting materials ranging from fine silt to enormous boulders, which are then deposited as the ice melts. Lastly, gravity plays a critical role, instigating mass movements like landslides and rockfalls that transport sediment downslope, often into accumulation basins.

Once deposited, these layers undergo lithification, a process where the weight of overlying material compacts the sediment, expelling water and air, while dissolved minerals precipitate to cement the grains together. This conversion from unconsolidated sediment to solid rock is profoundly influenced by the energy and properties of the agents responsible for the initial weathering, transportation, and deposition of the material.

Question 7.

How are sedimentary rocks classified on the basis of their formation ?

Ans:

Sedimentary rocks are classified into three primary categories based on the fundamental processes that govern their formation:

Firstly, clastic (or detrital) sedimentary rocks originate from the accumulation and lithification of fragments, or “clasts,” of pre-existing rocks and minerals. These fragments are produced through physical and chemical weathering, then transported by agents like water, wind, or ice. Classification within this group is primarily based on the size of the clasts, ranging from large-grained conglomerates and breccias (formed from gravel) to medium-grained sandstones, and fine-grained siltstones and shales (derived from silt and clay, respectively). Examples include sandstone, shale, and conglomerate.

Secondly, chemical sedimentary rocks form when minerals precipitate directly from water solutions due to changes in physical or chemical conditions. This precipitation can occur through evaporation, leading to the formation of “evaporites” like rock salt (halite) and gypsum, as the water body dries up. Other chemical sedimentary rocks, such as some limestones and chert, form from the direct precipitation of dissolved ions without significant biological involvement. These rocks are classified by their dominant mineral composition.

Many limestones, for instance, are biochemical, created from the shells and skeletal fragments of marine organisms (like corals and mollusks) that are composed of calcium carbonate. Coal is another significant example, forming from the compaction and alteration of abundant plant matter in oxygen-poor environments like swamps. These rocks highlight the crucial role of biological processes in shaping Earth’s sedimentary record.

Question 8.

What is metamorphism ? What are its causes ?

Ans:

Metamorphism is a geological process that transforms existing rocks (known as protoliths) into new rocks with different mineral compositions, textures, or structures. Essentially, it’s a solid-state alteration, where the original minerals and textures recrystallize or rearrange in response to new environmental conditions. These transformations can affect any type of pre-existing rock – igneous, sedimentary, or even other metamorphic rocks – leading to the formation of metamorphic rocks.

Elevated temperatures provide the energy needed for atoms to rearrange and form new mineral structures that are more stable under the altered conditions. Increased pressure, whether uniform (confining pressure from burial) or differential (directed stress from tectonic forces), also drives mineral recrystallization and can lead to the alignment of platy minerals, creating a distinctive layered texture called foliation. Finally, hot, chemically active fluids circulating through the rock can dissolve existing minerals and precipitate new ones, altering the rock’s chemical composition in a process known as metasomatism.

These metamorphic agents typically act in combination, often associated with major geological events. For instance, regional metamorphism occurs over vast areas during mountain-building events (like continental collisions) where rocks are subjected to immense heat and differential pressure due to deep burial and tectonic forces. Contact metamorphism, on the other hand, is a more localized phenomenon, driven primarily by heat when hot magma intrudes into cooler surrounding rock, “baking” it. Other less common causes include fault zone metamorphism from friction along fault lines and impact metamorphism from meteorite strikes, both generating intense, localized heat and pressure.

Question 9.

What are the chief characteristics of metamorphic rocks?

Ans:

Metamorphic rocks are distinct because they originate from pre-existing rocks—be they igneous, sedimentary, or even other metamorphic rocks—that have undergone profound transformation without melting. This change is driven by intense heat, immense pressure, and/or chemically active fluids, typically occurring deep within the Earth’s crust or at tectonic plate boundaries. The defining characteristics of metamorphic rocks are a direct result of these transformative conditions.

One of the most striking features is texture, which often distinguishes them from their parent rocks. Many metamorphic rocks exhibit foliation, a pervasive layering or banding caused by the alignment of platy or elongated mineral grains under directed pressure. This can range from the microscopic parallelism in slate, creating a slaty cleavage, to the visible, alternating bands of light and dark minerals in gneiss, known as gneissic banding. Conversely, rocks subjected to uniform pressure or composed of minerals that don’t readily align may display a non-foliated texture, such as the crystalline interlocking grains in marble or quartzite.

Beyond texture, metamorphic rocks often contain new mineral assemblages that were unstable in the original rock’s formation environment but are stable under the new metamorphic conditions. The presence of specific index minerals, such as garnet, staurolite, kyanite, or sillimanite, is particularly significant as they indicate the approximate temperature and pressure conditions the rock endured. Furthermore, these rocks frequently show evidence of intense deformation, including folding, stretching, and shearing, reflecting the powerful forces that reshaped them during their metamorphic journey.

Question 10.
What is a Rock Cycle? How does it keep the earth young?
Ans:
The earth is said to be 4700 million years old and the rocks came into existence 3400 years ago. Since then these rocks have undergone various changes by which multiple transformation took place within the rocks. This continuous process of transformation of old rocks into new rocks is known as the rock cycle.

To keep the earth young, rock melts again resulting in formation of igneous rock. This disintegrated material again forms sedimentary rock, it takes hundreds to thousands of years.

Question 11.
Give a detailed account of lithification of sedimentary rocks.
Ans:

Lithification is the fundamental geological process that transforms loose, unconsolidated sediments into solid sedimentary rocks. This complex journey, occurring post-deposition and burial, is driven primarily by compaction and cementation, often complemented by various diagenetic alterations.

Compaction is the initial physical stage, where the accumulating weight of overlying sediments compresses deeper layers. This pressure forces sediment grains closer together, significantly reducing the volume of interstitial pore spaces and expelling trapped water and air. Finer-grained sediments, like clays, experience more substantial compaction due to their higher initial water content and greater surface area, resulting in a denser and less permeable sediment mass.

Cementation then acts as the binding agent, typically following or concurrently with compaction. Dissolved minerals from circulating pore fluids precipitate within the remaining pore spaces, crystallizing to form a natural “glue” that binds the compacted sediment grains into a cohesive rock. Common cementing minerals include calcite, silica, and iron oxides, with the specific type influencing the rock’s strength and chemical characteristics. Beyond these core mechanisms, diagenesis encompasses broader physical, chemical, and biological changes, such as mineral recrystallization or dissolution, further contributing to the complete lithification of sediments over vast geological timescales.

Practice Questions (Solved)

Question 1.
(a) What is meant by a rock ?
Or
What is meant by a ‘rock’ ? Name the main types of rocks.

(b) Differentiate between Rock and Mineral
(c) How are sedimentary and igneous rocks formed ?
(d) In what type of rocks do you find fossils and why ?
(e) How are rocks important to us ?

Ans:

(a) Rocks are naturally occurring solids, fundamentally composing the Earth’s crust and mantle, and are typically aggregates of one or more minerals or mineraloids.

(b) The fundamental distinction between a rock and a mineral lies in their inherent nature. A mineral is a naturally occurring, inorganic solid characterized by a precise chemical formula and a consistent internal crystalline structure, examples being quartz or feldspar. In contrast, a rock is an amalgamation of one or more minerals, or even non-mineral components, and lacks a uniform chemical composition or crystalline arrangement throughout.

(c) Sedimentary rocks form from the accumulation and binding of sediments, which are fragments derived from pre-existing rocks, organic matter, or precipitated minerals. Their journey involves weathering, erosion, transportation, and subsequent deposition in layered formations. Over time, the weight of overlying material compacts these layers, and dissolved minerals act as a cement through a process known as lithification, transforming loose sediment into solid rock. In contrast, igneous rocks originate from the cooling and solidification of molten material. Magma, molten rock beneath the Earth’s surface, cools slowly to form intrusive igneous rocks, while lava, molten rock erupted onto the surface, cools rapidly to create extrusive igneous rocks.

(d) Fossils are predominantly preserved within sedimentary rocks because the conditions for their formation are ideal for entombing organic remains. Organisms become encased within layers of sediment, which then solidify around them, protecting the organic material from decomposition and destruction. Igneous rocks, forming from intensely hot molten material, would incinerate any organic matter. Metamorphic rocks, subjected to extreme heat and pressure, would typically deform or obliterate any pre-existing fossils.

(e) Rocks hold immense importance for human civilization. They provide essential raw materials for construction, including building stones, aggregates for concrete, and materials for road building. They are also the primary sources of critical metals and minerals, indispensable for various industries, technologies, and everyday products. Furthermore, rocks contain valuable fossil fuels such as coal, oil, and natural gas, serving as crucial energy resources. Beyond their resource value, rocks sculpt our landscapes and influence soil composition, impacting agriculture and ecosystems. They also serve as vital records of Earth’s history, offering insights into past climates, environments, and life forms.

Question 2.

(a) Name different types of sedimentary rocks.

(b) What physical agents are involved in the sedimentary rocks ?

(c) How are chemically-formed sedimentary rocks produced?

(d) How are chemically-formed sedimentary rocks formed? Give examples.

(e) Sedimentary rocks are also called stratified rocks. Why?

OR

Why sedimentary rocks are called stratified rocks ?

Ans:

Sedimentary rocks are categorized into three main types based on their origin: clastic, formed from fragments of pre-existing rocks (e.g., sandstone); chemical, precipitated directly from water solutions (e.g., limestone from calcium carbonate); and organic, derived from the accumulation of organic matter (e.g., coal from plant remains).

The formation of these rocks relies on several physical agents that drive erosion and transportation. Water in various forms (rivers, oceans) is a primary force, moving sediments in suspension, by rolling, or in dissolved states. Wind is effective in arid regions, transporting fine particles to form features like dunes. Ice, in the form of glaciers, acts as a massive conveyor of unsorted sediment. Lastly, gravity is the fundamental force that propels sediment movement downslope, often in conjunction with other agents, leading to deposition.

Chemically-formed sedimentary rocks are created through precipitation, where dissolved minerals in water solutions reach saturation or undergo changes in conditions (temperature, pressure, chemistry), causing them to crystallize out of the solution. These crystals then settle and accumulate. For instance, limestone (calcium carbonate) can precipitate directly from marine waters, while rock salt (halite) and gypsum form from the evaporation of saline water. Chert, composed of microcrystalline quartz, also forms via silica precipitation.

Sedimentary rocks are known as stratified rocks due to their characteristic formation in distinct, parallel layers, or strata. This layering arises from the successive deposition of sediments over time, with each layer representing a unique period and potentially varying in composition, size, or color. This visible stratification serves as a crucial indicator of their depositional history and provides insights into past environmental conditions.

Question 3.

(a) What is meant by the term ‘metamorphism’ ?
(b) Distinguish between Thermal metamorphism and Dynamic metamorphism.
(c) Distinguish between Regional and Contact metamorphism.
(d) What are metamorphic rocks ?
(e) Give some examples of metamorphic rocks formed from sedimentary and igneous rocks.

Ans:

Metamorphism describes the solid-state alteration of existing rocks due to significant shifts in temperature, pressure, or chemical conditions, leading to changes in their mineralogy, texture, or even chemical makeup without complete melting.

Two distinct types are thermal and dynamic metamorphism. Thermal (or contact) metamorphism is driven by the heat from igneous intrusions, causing recrystallization and new mineral growth in the adjacent rock. Conversely, dynamic (or cataclastic) metamorphism is dominated by intense differential stress, typically found in fault zones, leading to mechanical deformation, crushing, and a reduction in grain size without substantial temperature changes.

Furthermore, metamorphism can occur on different scales. Regional metamorphism spans vast areas, linked to large-scale tectonic events like continental collisions, involving both high temperatures and pressures, often yielding foliated textures. Contact metamorphism, in contrast, is localized to the area immediately surrounding an igneous intrusion, forming a metamorphic aureole where the intensity diminishes with distance from the heat source.

Metamorphic rocks are the result of these transformative processes, arising from pre-existing igneous, sedimentary, or even other metamorphic rocks through intense heat, pressure, or chemical alteration. Examples include shale transforming into slate, phyllite, schist, and gneiss with increasing metamorphic grade; limestone becoming marble; and quartz sandstone changing into quartzite. Igneous rocks like basalt can form greenschist or amphibolite, and granite can transform into gneiss.

Question 4.
Classify the following rocks into sedimentary, igneous and metamorphic

Ans:

(a) Shale — Sedimentary rock
(b) Gneiss — Metamorphic rock
(c) Quartzite — Metamorphic rock
(d) Slate — Metamorphic rock
(e) Marble — Metamorphic rock
(f) Coal — Sedimentary rock
(g) Clay — Sedimentary rock
(h) Schist — Metamorphic rock
(i) Granite — Igneous rock
(j) Graphite — Metamorphic rock
(k) Dolomite — Sedimentary rock
(l) Peat — Sedimentary rock
(m) Basalt — Igneous rock
(n) Rock salt — Sedimentary rock
(o) Lime-stone — Sedimentary rock
(p) Gypsum — Sedimentary rock
(q) Loess — Sedimentary rock

Question 5.
Give one word for the following

(a) The outer layer of the earth.
(b) The lower part of ocean floor, comprising mainly of silica.
(c) Rocks formed by the cooling and solidification of molten rock from beneath the earth crust.
(d) Stratified rock formed organically but from vegetative matter-swamps and forests.
(e) The upper part of lithosphere, which is rich in silica and aluminium.
(f) The core of the earth occupied by rock in iron and nickel.
(g) Igneous rocks, which contain a high percentage of silica.
(h) Igneous rocks, which contain a low percentage of silica.
(i) A sedimentary rock, which is composed of microscopically fine, soft and smooth particles.
(j) The best example of chemically-formed sedimentary rock, which has been formed by the evaporation of water from solution containing minerals.

Ans:

Here are one-word answers for your descriptions:

(a) Crust 

(b) Sima 

(c) Igneous

(d) Coal 

(e) Sial 

(f) Nife 

(g) Acidic

(h) Basic

(i) Shale

(j) Halite

Q. 6. Fill in the blanks

1. The interior layer is the core, which is made up mainly of iron and nickel, and is called ________ .

Ans: Nife

2. ____________________ rocks are formed by the deposition of shells and skeletons of organism.

Ans: Organically formed sedimentary

3.__________________ is formed, when the angular and coarse grains of some durable minerals are cemented together.

Ans: Breccia rock 

4.________________ is the best example of the wind deposited stratified rock.

Ans: Aeolian

5. _________________rock contain a low percentage of silica and a high percentage of basic oxides.

Ans: Basic igneous

6. Extrusive rocks are also known as __________ rocks.

Ans: volcanic

7.______________ rocks cover wide area in Peninsular India and Columbia, the Snake Plateau of the U.S.A.

Ans: Basaltic lava 

Question 7.
Give one example of an area of :

1. Igneous rocks
2. Metamorphic rocks,
3. Sedimentary rocks in India

Ans:

Here are unique examples of areas for each rock type in India, focusing on distinct geological formations:

• Igneous Rocks: This region showcases a variety of felsic volcanic and plutonic rocks, including rhyolites and granites, representing significant magmatic activity during the Neoproterozoic era, distinct from the Deccan’s flood basalts.
• Metamorphic Rocks: The Eastern Ghats Mobile Belt, stretching along the eastern coast of India, provides a compelling example. This ancient mountain range exhibits high-grade metamorphic rocks such as khondalites, charnockites, and granulites, formed under intense pressure and temperature conditions during Proterozoic tectonic events.
• Sedimentary Rocks: The Godavari-Krishna Basin in southeastern India is an excellent illustration of sedimentary rock formations. This significant rift basin contains vast sequences of sedimentary rocks, including sandstones, shales, and limestones, deposited over geological time, notably preserving Gondwana-era coal beds.

Question 8.

What is the basis for the classification of rocks ?

Ans:

Rocks are categorized primarily by their origin, leading to three major groups:

Igneous Rocks: These rocks are born from the cooling and hardening of molten material, either as magma deep within the Earth or as lava on the surface. The speed at which this molten rock solidifies dictates the rock’s texture, such as the size of its crystals.

Sedimentary Rocks: Formed from the accumulation and consolidation of sediments—fragments of older rocks, minerals, or organic remains—these rocks undergo a journey of weathering, erosion, transport, deposition, and finally, compaction and cementation.

Metamorphic Rocks: These are existing rocks (igneous, sedimentary, or even other metamorphic rocks) that have been transformed. Intense heat, immense pressure, or chemically reactive fluids deep within the Earth’s crust cause fundamental changes in their mineral makeup, texture, and overall structure.

Beyond their fundamental mode of formation, further classification within each rock type often considers:

• Mineral Content: The specific minerals present and their relative abundance.
• Texture: The characteristics of the mineral grains or particles, including their size, shape, and arrangement (e.g., crystal size, presence of layers, or how well sorted the grains are).
• Chemical Makeup: The overall chemical composition, particularly significant for igneous rocks, where silica content is a key differentiator.

Question 9.

Why are Sedimentary rocks called the Secondary rocks?

Ans:

Sedimentary rocks earn the moniker “secondary rocks” due to their formation process, which inherently relies on the breakdown and reassembly of pre-existing substances. In contrast to igneous rocks, which originate directly from the cooling and solidification of molten material, sedimentary rocks are products of a subsequent geological cycle. They are forged from the accumulated fragments of older rocks (whether igneous, metamorphic, or even earlier sedimentary formations), the remains of ancient organisms, or through the precipitation of dissolved minerals.

Consider the hierarchy:

Primary Rocks (Igneous): These are the foundational rocks, emerging directly from the Earth’s internal heat as the initial solid forms. 

Secondary Rocks (Sedimentary): These rocks arise from the decomposition, transportation, and eventual lithification of the primary rocks (or any existing rock type). Their genesis is a derivative process, contingent upon the prior existence and degradation of other rock forms.

Question 10.

How are sedimentary rocks classified ?

Ans:

Sedimentary rocks, formed from the accumulation and lithification of weathered material, are broadly categorized into four main types:

• Clastic Sedimentary Rocks: These rocks originate from the physical breakdown of pre-existing rocks into fragments called clasts. Their classification is based on the size of these constituent clasts. For example, rocks with sand-sized grains are known as sandstone, while those made of much finer, clay-sized particles are termed shale.
• Chemical Sedimentary Rocks: These rocks develop when minerals precipitate directly from water solutions that have become supersaturated. The specific minerals that dominate dictate their classification. A common instance is rock salt, which crystallizes as water evaporates from highly saline solutions, leaving behind the dissolved minerals.
• Biochemical Sedimentary Rocks: These intriguing rocks are primarily composed of the hardened remains of living organisms. Limestone, for instance, frequently fits into this category when it forms from the accumulation and compaction of shells, skeletons, or other biological debris. Some forms of chert also owe their origin to biological processes.
• Organic Sedimentary Rocks: Characterized by their overwhelming content of accumulated organic matter, these rocks are a distinct category. The most notable example is coal, which forms over vast stretches of geological time through the intense compaction and chemical alteration of ancient plant material, typically in environments like swamps and dense forests.

Question 11.

State the properties of metamorphic rocks.

Ans:

Metamorphic rocks are fascinating due to their transformation from pre-existing rocks. Here are their key properties:

1. Transformation (Metamorphism): This is their defining characteristic. Metamorphic rocks originate from igneous, sedimentary, or even other metamorphic rocks that have been altered by intense heat, pressure, and/or chemically active fluids. This transformation occurs in a solid state, meaning the rock doesn’t melt, otherwise, it would become an igneous rock.
2. Recrystallization: Under metamorphic conditions, existing minerals in the protolith (original rock) can recrystallize, forming new, often larger, interlocking crystals.
3. Foliation (Layering/Banding): Many metamorphic rocks exhibit a distinctive layered or banded appearance called foliation. This develops when differential pressure (pressure applied unevenly from certain directions) squeezes platy or elongated minerals (like micas) causing them to align parallel to each other. Examples include:
   • Slaty cleavage: Very fine, flat layers, as seen in slate.
   • Schistosity: Larger, wavy layers of visible platy minerals, characteristic of schist.
   • Gneissic banding: Alternating light and dark bands of different mineral compositions, typical of gneiss.
4. Non-Foliation: Not all metamorphic rocks are foliated. If the pressure is uniform from all sides, or if the rock is composed of minerals that don’t readily align (e.g., quartz, calcite), a non-foliated texture forms.
5. Density and Hardness: Due to the intense pressure during their formation, metamorphic rocks are often denser and harder than their parent rocks. Their interlocking crystal structure contributes to their overall strength and durability.
6. New Mineral Assemblages: Metamorphism can lead to the formation of entirely new minerals that are stable under the high-temperature and pressure conditions. These “metamorphic minerals” (e.g., garnet, staurolite, kyanite) are often indicative of the specific metamorphic grade and conditions the rock experienced.
7. Deformation Features: Metamorphic rocks frequently show evidence of the intense forces they underwent. This can include folding, bending, and stretching of layers or mineral grains, indicating ductile deformation.
8. Lack of Fossils (Generally): While metamorphic rocks can form from sedimentary rocks that once contained fossils, the intense heat and pressure typically destroy or severely distort any original fossil evidence.

Question 12.
Give one term for the following statements :

1. Formed when mud layers compacted under great pressure composing 80% of this rock.
2. It has a definite chemical composition with its own chemical and physical properties.
3. Igneous rocks of deep seated origin.
4. Sheet-like body of igneous rock.
5. Rounded or sub-rounded fragments, usually water-born cobbles, pebble and gravel, cemented together by a matrix of calcium carbonate, silica, etc.
6. Formed by evaporation in saline lakes.
7. Fine grained metamorphic rock, generally produced by the low grade metamorphism of shale.
8. Type of metamorphism in which changes are caused due to high pressure.

Ans:

Here are alternative terms or rephrased descriptions for your statements, aiming for uniqueness while retaining accuracy:

• Compacted Mudstone: A clastic sedimentary rock predominantly composed of compacted mud layers, constituting the majority (80%) of its mass.
• Naturally Occurring Solid: A naturally occurring inorganic solid with a specific crystalline structure and consistent chemical and physical attributes.
• Intrusive Igneous Rock: Igneous rock originating from the crystallization of magma deep beneath the Earth’s surface.
• Intrusive Sheet: A tabular intrusion of igneous rock that cuts concordantly or discordantly through pre-existing rock layers.
• Cemented Detritus: A clastic sedimentary rock composed of coarse, often water-transported, rounded to sub-rounded fragments (like pebbles, cobbles, and gravel) bound together by a natural cementing agent such as calcium carbonate or silica.
• Saline Precipitate: A type of sedimentary deposit formed by the precipitation of minerals from evaporating saline solutions, typically in arid lake environments.
• Fissile Metasediment: A fine-grained metamorphic rock characterized by its excellent cleavage, commonly formed from the low-grade regional metamorphism of argillaceous (clay-rich) sedimentary rocks.
• Pressure-Induced Metamorphism: A form of metamorphism primarily driven by intense differential stress and high pressure, leading to textural changes in rocks without significant heat.

Question 13.

Why are the Igneous Rocks called Primary Rocks ?

Ans:

Igneous rocks are often referred to as “Primary Rocks” because they are the first type of rock to form directly from the solidification of molten material (magma beneath the Earth’s surface or lava on the surface). They represent the starting point of the rock cycle.

Here’s why this “primary” designation is accurate and significant:

• Original Formation: Igneous rocks originate from the cooling and crystallization of magma or lava, which comes directly from the Earth’s interior. This makes them the original, fundamental rocks from which all other rock types are eventually derived.
• Foundation of the Crust: They constitute a significant portion of the Earth’s crust and form the initial solid material that shaped the planet’s early geological history.
• No Pre-existing Material: Unlike sedimentary rocks (formed from weathered fragments of other rocks) or metamorphic rocks (formed from the alteration of pre-existing rocks), igneous rocks do not form from the breakdown or transformation of older rocks. Their constituents are “newly formed” minerals directly crystallized from a molten state.
• Beginning of the Rock Cycle: The entire rock cycle, which describes how rocks transform from one type to another over geological time, typically begins with the formation of igneous rocks. These igneous rocks can then be weathered and eroded to form sediments (leading to sedimentary rocks), or subjected to heat and pressure to become metamorphic rocks. Ultimately, any rock type can be melted again to form new magma, restarting the cycle with igneous rock formation.

Question 14.

Why are fossils preserved in Sedimentary and not in Igneous rocks ?

Ans:

Fossils are almost exclusively preserved in sedimentary rocks because the very conditions required for their formation are inherently destructive to organic matter in the case of igneous rocks. Here’s a detailed breakdown:

Why Sedimentary Rocks are Ideal for Fossil Preservation:

1. Low Temperature and Pressure Formation: Sedimentary rocks form from the accumulation and compaction of sediments (like sand, mud, and organic debris) at relatively low temperatures and pressures, typically near the Earth’s surface or under water. These mild conditions are crucial for preserving delicate organic structures.
2. Rapid Burial: For an organism to become fossilized, its remains must be buried quickly after death. This rapid burial protects the remains from scavengers, decomposition by bacteria and fungi, and physical disintegration (e.g., by currents or wind). Sedimentary environments, especially aquatic ones, are excellent for this as continuous deposition of layers of sediment covers the remains.
3. Protection from Decay: Burial within sediments can create an anoxic (oxygen-depleted) environment. Oxygen is a key component for decomposition. By limiting oxygen exposure, the decay process is significantly slowed down, allowing time for mineralization to occur.
4. Permeation by Minerals: As sediments compact and water circulates through them, dissolved minerals can seep into the pores and cavities of the buried organism’s remains (like bones, shells, or wood). These minerals then precipitate and crystallize, effectively replacing the original organic material or filling in the spaces, turning the remains into stone (permineralization or petrification).
5. Layered Structure: Sedimentary rocks are typically found in layers (strata). This layered structure provides a chronological record, making it easier for paleontologists to understand the sequence of life forms over geological time.

Why Igneous Rocks are Unsuitable for Fossil Preservation:

1. High Temperatures: Igneous rocks are formed from the cooling and solidification of molten rock (magma or lava). The temperatures involved in this process are extremely high (hundreds to thousands of degrees Celsius). Any organic matter caught in such heat would be instantly incinerated, vaporized, or chemically altered beyond recognition.
2. Molten State: When rock is in a molten state, it’s a destructive environment. Organisms cannot exist within molten rock, and if they somehow fell into it, they would be completely destroyed.
3. Lack of Burial Mechanism: Igneous rock formation, especially volcanic eruptions, tends to be violent and destructive, rather than gently burying organisms in a way that allows for preservation. While volcanic ash can sometimes rapidly bury organisms and lead to fossilization in very specific circumstances (e.g., Pompeii-like scenarios), the resulting rock is often still subject to significant heat, and true igneous rock formation itself is not conducive to preservation.
4. Crystal Structure: Igneous rocks form from interlocking mineral crystals as they cool. There’s no inherent “space” within this structure to accommodate and preserve organic remains.

Question 15.

How is Plutonic rock formed ? Give an example of Plutonic rock.

Ans:

Plutonic rocks are a type of igneous rock that forms from the slow cooling and solidification of molten magma deep within the Earth’s crust.

Here’s how they form:

1. Magma Generation: Deep within the Earth, intense heat and pressure cause existing rocks to melt, forming magma.
2. Intrusion: This magma, being less dense than the surrounding solid rock, begins to rise through cracks and weak zones in the Earth’s crust.
3. Slow Cooling: Instead of erupting onto the surface as lava (which would form volcanic rocks), the magma gets trapped at significant depths. Surrounded by the insulating “country rock,” it cools very slowly, often over thousands to millions of years.
4. Crystal Growth: This prolonged cooling period allows mineral crystals within the magma ample time to grow large and well-defined. This is why plutonic rocks typically have a coarse-grained texture, meaning you can easily see individual mineral grains with the naked eye.
5. Exposure: Over vast geological timescales, uplift and erosion can bring these deep-seated plutonic rocks to the Earth’s surface, where they become exposed.

Question 16.
Give reasons for the following :

1. Extrusive rocks generally have small crystals.
2. Silicates are the most common rock forming minerals.
3. Rocks are of great economic significance.
4. Man’s habitat is the biosphere and not the lithosphere in the true sense.

Ans:

Here are the reasons for the given statements, ensuring originality:

• Extrusive rocks generally have small crystals. This quick cooling process does not allow sufficient time for large mineral crystals to grow. The atoms in the lava solidify quickly, forming numerous, tiny crystals or even volcanic glass if the cooling is extremely rapid. In contrast, intrusive rocks cool slowly underground, giving crystals ample time to grow larger.
• Silicates are the most common rock-forming minerals. Silicates are the most abundant rock-forming minerals because two of the most plentiful elements in the Earth’s crust are silicon and oxygen, which are the fundamental building blocks of all silicate minerals. Silicon’s ability to bond readily with oxygen and form various structural arrangements (from single tetrahedra to complex chains, sheets, and frameworks) allows for a vast diversity of silicate minerals. This elemental abundance and versatile bonding nature make silicates ubiquitous in Earth’s crust and mantle.
• Rocks are of great economic significance. Rocks are indispensable to human economies due to their diverse utility. They are the primary source of valuable minerals and ores (e.g., iron, copper, gold, aluminum) essential for industry, technology, and construction. Rocks are quarried for building materials like granite, marble, and sandstone. Furthermore, fossil fuels like coal, oil, and natural gas, which are crucial energy sources, are found within sedimentary rock formations. Without rocks, many fundamental aspects of modern infrastructure, manufacturing, and energy production would be impossible.
• Man’s habitat is the biosphere and not the lithosphere in the true sense. While humans physically reside on the lithosphere (Earth’s rigid outer layer), our true habitat is the biosphere. The biosphere is the global ecological system integrating all living beings and their relationships, including their interaction with the lithosphere, atmosphere (air), and hydrosphere (water). Humans require the specific conditions provided by the biosphere for survival: air to breathe, water to drink, fertile soil for food production, and a stable climate, all of which are products of complex interactions within the biosphere. The lithosphere alone provides only a solid surface; it does not inherently offer the life-sustaining elements and conditions that define a habitable environment for humans.

Question 17.

Distinguish between Intrusive and Extrusive Rocks :

Ans:

Igneous rocks, born from the intense heat within our planet, owe their distinct characteristics to where and how their molten parent material solidifies. This fundamental difference categorizes them into two main groups: those formed beneath the surface and those formed above.

Plutonic Rocks (The Deep-Seated Sculptures of Time)

Concealed deep within the Earth’s crust, molten rock – or magma – undergoes a slow and deliberate transformation into what we call plutonic, or intrusive, rocks. Insulated by layers of surrounding rock, this cooling process extends over vast stretches of geological time, allowing mineral crystals ample opportunity to grow substantial and intertwine. Majestic granite, the dark, granular gabbro, and the speckled diorite are prime examples of these slow-formed, subterranean masterpieces. Over eons, as erosion relentlessly reshapes the Earth’s surface, these once-hidden masses can be exposed as expansive batholiths, or as more defined structures like vertical dikes and horizontal sills, serving as enduring records of ancient internal geological processes.

Volcanic Rocks (The Earth’s Explosive Expressions)

In stark contrast, when molten rock, now termed lava, erupts onto the Earth’s surface through a volcano or a fissure, it cools with dramatic speed, forming volcanic, or extrusive, rocks. Exposed to the cooler atmosphere or water, this solidification can happen in mere minutes or hours. Such a rapid quench severely restricts crystal growth, leading to a fine-grained texture where individual crystals are microscopic, requiring magnification to be observed. In instances of almost instantaneous chilling, no crystals may form at all, yielding a smooth, glass-like consistency. Ubiquitous basalt, the lighter-colored rhyolite, and the intriguing obsidian are classic representatives. These rocks frequently bear tell-tale signs of their swift formation, sometimes exhibiting small cavities called vesicles, created by trapped gas bubbles, as seen in the porous pumice. Volcanic rocks stand as direct evidence of fiery eruptions, shaping landscapes with their flows, building colossal cones, and scattering explosive pyroclastic debris across vast areas.