Slow cooling underground creates large-crystal rocks like granite, while rapid cooling above ground forms small-crystal or glassy rocks like basalt.
Sedimentary rocks are formed from the compression and cementation of accumulated sediments – fragments of other rocks, remains of organisms, or precipitated minerals (e.g., sandstone, limestone, coal). They often show layers and may contain fossils.
Metamorphic rocks are existing rocks transformed by intense heat, pressure, or chemical activity deep within the Earth. This alters their original minerals and textures. They can appear layered (foliated, like slate) or non-layered (non-foliated, like marble).
THINK AND ANSWER
Why is diamond clear and hard while graphite is soft ? Why is coal hard and chunky ?
Ans:
Diamond: Its transparency and extreme hardness stem from a strong, three-dimensional bonding arrangement where each carbon atom is linked to four others. This rigid framework and the absence of mobile electrons prevent light absorption, resulting in its clarity.
Graphite: The softness of graphite arises from its layered structure. Within each layer, carbon atoms are strongly bonded, but the connections between these layers are weak, allowing them to easily slip past one another. The presence of mobile electrons within these layers causes it to appear black.
Coal: The hardness and irregular, blocky shape of coal are a consequence of its formation under significant pressure and heat acting on plant material over geological timescales. This process increases its carbon concentration and compresses it. The presence of various minerals and the way it fractures under pressure also contribute to its solid, often chunky form, with the degree of hardness differing based on the specific type of coal.
Discuss
Discuss the importance of rocks for the contruction business.
Ans:
Construction heavily relies on rocks for their strength and abundance. They form the base of buildings and roads, are crushed into essential components for concrete and mortar, and are used as building stones for walls and facades. Large rocks stabilize infrastructure like bridges and coastlines. Essentially, rocks provide the fundamental, strong, and lasting materials needed for diverse construction projects.
VALUES & LIFE SKILLS
Rocks and minerals play a very important role in our lives.
Can you imagine modern life without minerals ?
What problems would you face in their absence ?
Ans:
Infrastructure Meltdown: No concrete (limestone, clay, gypsum), no steel (iron ore), meaning no buildings, roads, or bridges as we know them. Glass (quartz) and ceramics (clays) for windows, screens, and plumbing would vanish or be drastically inferior.
Tech Blackout: Smartphones, computers, TVs, and the internet rely on silicon, copper, gold, lithium, cobalt, and rare earths – all mined minerals. Modern communication and information technology would cease.
Energy Collapse: Traditional fossil fuels (coal, oil, gas extraction uses mining) and the shift to renewables (solar panels, wind turbines, electric car batteries need minerals) would be impossible.
Farming Disaster: No mined fertilizers (phosphorus, potassium) means mass food shortages. No metal for farm equipment.
Healthcare Catastrophe: Medical tools (steel), diagnostic machines, and even many medicines rely on minerals.
Everyday Life Vanishes: Plastics (mineral catalysts, fillers), cosmetics (talc, clays), and jewelry (gold, silver, gems) all depend on minerals.
Essentially, we’d revert to a pre-industrial existence, but far worse due to our population size and reliance on complex systems built on Earth’s mineral wealth. Our daily lives, from waking to sleeping, are fundamentally tied to minerals. Their absence would trigger unimaginable societal breakdown.
EXERCISES
1. Granite is an example of intrusive ________ rocks.
Ans: Igneous
2. A category of rock which is formed by the alteration of the parent rock due to heat and pressure is called: _____________.
Ans: Metamorphic rocks
3. Conglomerate is an example of _______ rocks.
Ans: Sedimentary
4. Sedimentary rocks are formed from ________ accumulated over long periods.
Ans: Sediments
5. Sedimentary rock such as limestone change into _______ .
Ans: Marble
B. Write True or False. Rewrite the false statements correctly :
1. Minerals are organic substances.
Ans: False.
Correct : Minerals are inorganic substances.
2. Basalt is an example of an intrusive igneous rock.
Ans: False.
Correct : Basalt is an example of an extrusive igneous rock.
3. Sedimentary rocks are formed by the cooling and solidification of molten rock called magma.
Ans: False.
Correct : Igneous rocks are formed by the cooling and solidification of molten rock called magma.
4. Igneous and sedimentary rocks can change into metamorphic rocks.
Ans: True.5. Rocks are used in construction industries.
Ans: True.
What is crust ?
Ans:
It exists in two primary forms:
- Oceanic crust, situated beneath the seas, is relatively thin (5-10 km), denser (made of basalt and similar rocks rich in silicon and magnesium, often called “sima”), and geologically young (no more than about 180 million years old).
- Continental crust, forming the landmasses and their submerged edges, is significantly thicker (typically 30-50 km, reaching up to 70 km under mountains), less dense (composed mainly of granite-like rocks rich in silicon and aluminum, often called “sial”), and considerably older (containing Earth’s oldest rocks, around 4 billion years old).
Compositionally, the crust is dominated by elements such as oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium. These elements combine to create various minerals, with feldspars and quartz being the most prevalent.
Temperature within the crust rises with increasing depth, ranging from the surface temperature down to approximately 200°C to 400°C at its boundary with the mantle, and even higher in some localized areas.
The crust, along with the uppermost part of the mantle, constitutes the lithosphere. The movement and interaction of these plates are the driving forces behind major geological phenomena like earthquakes, volcanic eruptions, and the formation of mountain ranges, ultimately shaping the Earth’s surface.
Question 2.
Differentiate between SIAL and SIMA.
Ans:
| Feature | SIAL (Silica + Alumina) | SIMA (Silica + Magnesia) |
| Composition | Rich in silicon (Si) and aluminum (Al). Contains lighter silicate minerals like feldspar and quartz. | Rich in silicon (Si) and magnesium (Mg). Contains denser ferromagnesian minerals like pyroxene and olivine, as well as basaltic rocks. |
| Location | Forms the continental crust. It is thicker and underlies the landmasses. Can also be found as a thinner layer beneath some ocean basins. | Primarily forms the oceanic crust. It is thinner and underlies the ocean floors. Lies beneath the SIAL in continental areas. |
| Density | Lower density (around 2.7 g/cm³). | Higher density (around 2.9 to 3.3 g/cm³). |
| Thickness | Thicker (average 30-50 km, up to 70 km under mountains). | Thinner (around 5-10 km). |
| Rock Type | Predominantly composed of rocks similar to granite (felsic rocks). | Predominantly composed of basalt and gabbro (mafic rocks). |
| Age | Generally older. | Relatively younger (oceanic crust is continuously being formed and recycled). |
| Discontinuity | The boundary between SIAL and SIMA is sometimes referred to as the Conrad Discontinuity, though its global presence and nature are debated. | Lies beneath the SIAL, separated by the Conrad Discontinuity (if considered a distinct boundary). Above the Earth’s mantle, separated by the Mohorovičić Discontinuity (Moho). |
Question 3.
State two characteristics of the mantle.
Ans:
Composition and State: The mantle is primarily composed of silicate rocks rich in magnesium and iron. While predominantly solid, over geological timescales, the mantle behaves like a very viscous fluid, capable of slow movement and flow. This allows for processes like convection currents.
High Temperature: The temperature within the mantle increases significantly with depth, ranging from approximately 500°C (932°F) at the upper boundary with the crust to over 4,000°C (7,200°F) at the boundary with the core. This heat is generated by the Earth’s internal energy, including residual heat from its formation, radioactive decay of elements, and frictional heating.
Question 4.
What is the difference between the inner and the outer core ?
Ans:
| Feature | Inner Core | Outer Core |
| State | Solid | Liquid |
| Thickness | Approximately 1,220 km (radius) | Approximately 2,300 km (thickness) |
| Composition | Primarily iron (Fe) and nickel (Ni), possibly with some lighter elements. May be purer iron than the outer core. | Primarily iron (Fe) and nickel (Ni), with a significant presence of lighter elements like oxygen, sulfur, silicon, carbon, and hydrogen. The presence of these lighter elements makes it less dense than pure iron. |
| Temperature | Estimated to be around 5,200°C (9,392°F) to 5,700 K (5,430 °C; 9,800 °F), similar to the surface of the Sun. | Ranges from approximately 4,500°C (8,132°F) in the outer regions to about 6,000°C (10,832°F) near the inner core. |
| Pressure | Immense pressure (around 3.6 million atm) | Very high pressure, but less than the inner core. |
| Density | Higher density (around 12.8 – 13.1 g/cm³) | Lower density (around 9.9 – 12.2 g/cm³) |
| Movement | Solid, but evidence suggests it can rotate slightly faster than the rest of the Earth (super-rotation) and potentially undergo other complex movements. | Liquid and in constant, turbulent motion due to convection currents driven by heat from the inner core. |
| Magnetic Field | Does not generate the Earth’s magnetic field directly due to its solid state. However, its properties influence the outer core. | The movement of the molten iron and nickel in the outer core is believed to be the primary source of Earth’s magnetic field through a process called the geodynamo. |
| Heat Transfer | Primarily conducts heat outwards to the outer core. | Transfers heat through convection, which drives the geodynamo. |
| Boundary | Surrounded by the liquid outer core at the Lehmann Discontinuity. | Surrounds the solid inner core and is surrounded by the solid mantle at the core-mantle boundary. |
Question 5.
What are minerals?
Ans:
Solid: Minerals exist in a solid state at standard temperature and pressure conditions on Earth.
Inorganic: Minerals are not composed of organic carbon compounds (though they may contain carbon in simple forms like carbonates). They are typically composed of elements or simple compounds.
Specific chemical composition: Each mineral has a relatively fixed chemical formula, which can be expressed using chemical symbols. For example, quartz is always silicon dioxide (SiO2), and halite (rock salt) is always sodium chloride (NaCl). While some minerals can have a range of compositions due to the substitution of one element for another within their structure (forming a mineral series), this variation is generally within defined limits.
Characteristic crystalline structure: The atoms within a mineral are arranged in a highly ordered, repeating three-dimensional pattern called a crystal lattice. This internal arrangement determines the external shape of well-formed crystals and influences the mineral’s physical properties, such as hardness, cleavage, and fracture.
Question 6.
Give an example of an intrusive igneous rock.
Intrusive igneous rocks form from magma that cools and solidifies beneath the Earth’s surface. The slow cooling process allows for the formation of large crystals, which are typically visible to the naked eye. Granite is a common intrusive rock known for its coarse-grained texture and its composition of minerals like quartz, feldspar, and mica.
Question 7.
Classify the following rocks as igneous, sedimentary, and metamorphic: gneiss, coal, shale, granite, gypsum, marble, sandstone, basalt, limestone, schist
Ans:
Minerals — Rocks
Gneiss — Metamorphic
Coal — Sedimentary
Shale — Sedimentary
Granite — Igneous
Gypsum — Sedimentary
Marble — Metamorphic
Sandstone — Sedimentary
Basalt — Igneous
Limestone — Sedimentary
Schist — Metamorphic
D. Answer the following questions in one or two paragraphs :
Question 1.
Explain the structure of the earth’s interior ?
Ans:
The Earth’s internal structure comprises three primary concentric zones:
- Crust: The outermost, relatively thin, and rigid layer, existing in two forms: the thicker, less dense continental crust composed predominantly of granitic rocks, and the thinner, denser oceanic crust mainly made of basaltic rocks.
- Mantle: The most substantial layer by volume, situated beneath the crust and extending inward. It is largely solid, although the upper portion includes the asthenosphere, a partially molten and viscous zone that enables the movement of tectonic plates.
- Core: The Earth’s innermost region, primarily composed of iron and nickel. It is differentiated into a liquid outer core, whose movement is believed to generate the planet’s magnetic field, and a solid inner core, resulting from immense pressure at the Earth’s center.
Question 2.
How are rocks different from minerals?
Ans:
Minerals: Natural, uniform solids possessing a specific chemical identity and ordered atomic arrangement.
Rocks: Naturally occurring solid masses composed of one or more mineral types, lacking a singular, consistent chemical identity.
Fundamental distinction: Minerals are the basic chemical constituents; rocks are assemblages of these constituents.
Question 3.
How are igneous rocks formed?
Ans:
Igneous rocks, often called “fire rocks” (from the Latin word “ignis” meaning fire), are formed through the cooling and solidification of molten rock.
Here’s a breakdown of the process:
- Melting: Deep within the Earth’s mantle or crust, existing rocks can melt due to:
- Increased Temperature: Higher geothermal gradients in certain areas can cause rocks to exceed their melting point.
- Decreased Pressure: A reduction in pressure (decompression melting), such as at mid-ocean ridges or mantle plumes, can lower the melting point of rocks.
- Change in Composition: The introduction of volatiles like water or carbon dioxide can lower the melting temperature of surrounding rocks.
- Movement of Magma/Lava: The molten rock, now less dense than the surrounding solid rock, rises towards the Earth’s surface.
- Cooling and Solidification (Crystallization): As the molten rock cools, the atoms within it begin to arrange themselves into orderly structures, forming crystals of different minerals. The rate of cooling significantly influences the size of these crystals:
- Slow Cooling (Intrusive Rocks): Magma that cools slowly beneath the Earth’s surface has ample time for large crystals to grow, resulting in coarse-grained igneous rocks (also called plutonic rocks). Examples include granite, diorite, and gabbro.
- Fast Cooling (Extrusive Rocks): Lava that erupts onto the Earth’s surface cools rapidly due to contact with the atmosphere or water. This rapid cooling limits the time for crystal growth, resulting in fine-grained igneous rocks (also called volcanic rocks). Examples include basalt, andesite, and rhyolite.
Question 4.
How do acidic igneous rocks differ from basic igneous rocks ?
Ans:
Acidic Igneous Rocks:
- Abundant silica (greater than 65%).
- Typically light in hue (e.g., pink, pale gray).
- Predominantly composed of quartz and orthoclase feldspar.
- Exhibit lower density values.
- Molten state is characterized by high viscosity.
Basic Igneous Rocks:
- Limited silica content (45-55%).
- Typically dark in hue (e.g., dark gray, black).
- Primarily composed of calcium-rich plagioclase, pyroxene, and olivine.
- Exhibit higher density values.
- Molten state is characterized by low viscosity.
Fundamental Distinction: The proportion of silica is the primary determinant, influencing mineral composition, coloration, density, and the flow properties of their molten forms.
Question 5.
How are sedimentary rocks formed ?
Ans:
Sedimentary rocks are created through the sequential processes of:
- Sedimentation: The layering and build-up of fragmented materials (derived from pre-existing rocks, organic debris, or chemical precipitation).
- Compression: The reduction of pore space between sediment particles due to the increasing weight of overlying deposits.
- Lithification: The hardening of sediments into solid rock through the precipitation of dissolved minerals that act as a binding agent.
Question 6.
What are metamorphic rocks ?
Ans:
Metamorphic rocks form from pre-existing rocks altered deep within Earth. Intense heat, pressure, and reactive fluids cause mineral recrystallization, yielding new textures and compositions.
E. Picture study
The photograph shows you a type of rock that is widely used in the construction industry.
Question 1.
Name the type of rock.
Ans:
The type of rock is chemically formed sedimentary rock as limestone becomes marble. It is marble.
Question 2.
How is it formed ?
Ans:
Marble originates from the metamorphic alteration of limestone (or dolostone) under conditions of significant heat and pressure within the Earth’s crust. This process induces the recrystallization of calcite grains, resulting in a more compact and crystalline rock structure. The presence of trace elements within the initial rock dictates the diverse colors and patterns observed in marble.
LET’S DO SOMETHING
With the help of your teacher, collect different types of rocks such as granite, sandstone, limestone, marble, slate, graphite, etc. and prepare a chart containing information about their colour, hardness, texture, and permeability.
Ans:
Do it Yourself.
