Weathering and Soil Formation Chapter Notes | Geography Class 7 ICSE PDF Download

Introductuon

Rocks

• Rocks are natural masses of mineral matter forming the Earth's crust.
• Composed of one or more minerals, cemented, squeezed, heated, or cooled.
• Vary in color, size, and texture.
• Classified into three main types: igneous, sedimentary, and metamorphic.
• Rocks transform from one type to another through the rock cycle.

Igneous Rocks

• Formed by cooling and solidification of magma or lava.
• Magma is molten rock inside the Earth's crust; lava is magma that reaches the surface.
• Coarse-grained rocks (e.g., granite, gabbro) form when magma cools slowly, allowing large crystals.
• Fine-grained rocks (e.g., basalt, rhyolite, obsidian) form when lava cools quickly on the surface.
• Other examples include pumice, tuff, and andesite.

Types of Igneous Rocks

• Extrusive Rocks: Form on Earth's surface, cool rapidly, have fine-grained textures (e.g., basalt).
• Intrusive Rocks: Form below the surface, cool slowly, have larger crystals (e.g., granite).

Sedimentary Rocks

• Formed by cementing of weathered material deposited through erosion.
• Develop on Earth's surface, in water or on land, in layers called strata.
• Composed of rock fragments, minerals, or plant/animal material.
• Often formed near water, but can also be shaped by wind, rivers, or oceans.
• Examples include sandstone, limestone, shale, gypsum, conglomerate, coal, and flint.
• Found in large deposits, such as at the Grand Canyon, USA.

Metamorphic Rocks

• Formed when igneous or sedimentary rocks undergo intense heat or pressure.
• Transform into denser, more compact rocks without melting.
• Examples include slate (from shale), marble (from limestone), and gneiss (from granite).

Rock Cycle

• A continuous process where rocks change from one type to another.
• Igneous rocks form from cooled magma, then break down into sediment through weathering.
• Sediments accumulate in lakes or oceans and harden into sedimentary rocks.
• Sedimentary rocks under heat and pressure transform into metamorphic rocks.
• Metamorphic rocks may melt into magma under extreme conditions, restarting the cycle.

Weathering

• Process where rocks and minerals change texture or composition at the Earth's surface.
• Caused by environmental forces like water, wind, temperature changes, and biological activity.
• Occurs in situ (in place) without movement of rock material.
• Differs from erosion, which involves the movement of rock or soil particles by wind, water, or ice.

Factors Affecting Weathering

• Climate: Weathering is faster in hot, wet climates (e.g., tropical rainforests) and slower in cold, dry areas.
• Type of Rock: Quartz-rich rocks weather slowly; feldspar, calcite, or iron-rich rocks weather quickly.
• Topography: Elevated or sloped areas weather faster due to exposure to sun, wind, and rain.
• Vegetation: Vegetation protects rocks from weathering; bare rocks weather more.
• Surface Area: Smaller rock pieces with more surface area weather faster than larger blocks.
• Pollution: Acid rain from pollution (containing nitric and sulfuric acids) accelerates weathering.

Types of Weathering

• Mechanical
•  Chemical
• Biological

Mechanical or Physical Weathering

• Breaks rocks into smaller pieces without changing their composition.
• Caused by physical forces like ice wedging, pressure release, plant root growth, and abrasion.

Ice Wedging

• Water seeps into cracks in rocks and freezes, expanding as it turns into ice.
• The expansion force is strong enough to split rocks apart, breaking them into smaller pieces.
• Common in areas where temperatures frequently rise above and fall below 0°C (32°F), the freezing point of water.
• In cold regions, water enters rock cracks during the day, freezes and expands at night, and continues this freeze-thaw cycle, causing rocks to splinter.

Pressure Release

• Rocks deep inside the Earth are under high pressure from surrounding rocks.
• When these rocks reach the surface due to Earth’s forces or erosion of overlying material, the pressure on the surface is released.
• The release of pressure causes the rock to expand, forming cracks.
• This process is called exfoliation, also known as onion-skin weathering, because the rock surface peels off in thin layers like an onion.

Abrasion

• Water wears down rocks in riverbeds and along shorelines through friction.
• Abrasion happens when moving water or tumbling rocks rub against each other or the riverbed, grinding and wearing them down.
• Ocean waves also cause abrasion by beating against rocky shores, gradually wearing them down.

Chemical Weathering

• Changes the composition of rocks through chemical reactions with water or air.
• Common processes include oxidation, hydration, carbonation, and solution.
• Main processes include oxidation, hydration, carbonation, and solution.

Oxidation

• A chemical reaction between minerals in rocks and oxygen in the air.
• Changes iron minerals in rocks from light grey to brown-red, a process called rusting.
• The change in color shows the rock's composition has changed, causing it to weaken and break apart.

Hydration

• Water reacts with rock minerals, dissolving them and forming new compounds through a process called hydrolysis.
• For example, in granite, feldspar crystals react with water to form clay minerals.
• The clay weakens the rock, making it more likely to break apart.

Carbonation

• Carbon dioxide mixes with water to form carbonic acid, which causes chemical weathering.
• Important in cave formation, as carbonic acid dissolves minerals like calcite in limestone.
• The dissolved minerals are washed away, hollowing out the rock and creating caves.
• Limestone is especially vulnerable to carbonation due to its calcite content.

Biological Weathering

• Caused by living organisms, leading to both decomposition and disintegration of rocks and minerals.
• Influenced by the surrounding environment, especially in tropical and subtropical regions.

Humans and Animals

• Humans break rocks for construction, such as building roads, dams, or channels, increasing the surface area for chemical weathering.
• Animals, birds, insects, and worms create holes in rocks, aiding the weathering process.
• In tropical and subtropical areas, ants and termites build galleries and passages in rocks, carrying materials and releasing acids that speed up disintegration.
• Rabbits burrow into soft rocks, moles dig into the ground, and dead animal bodies release substances that react with minerals, helping decay rocks.

Higher Plants and Roots

• Roots of trees and plants grow into cracks and crevices of rocks, exerting force that can break hard rocks apart, like a peepal tree splitting walls or rocks.
• Grass roots form a sponge-like mass, holding soil in place, preventing erosion, and allowing moisture and air to enter rocks for further weathering.
• Some roots grow deep into soil, creating drainage channels, especially in limestone and marble, where they release acids that dissolve carbonates.
• Dead roots and plant residues decompose, producing carbon dioxide that contributes to weathering.

Soil

• A vital natural resource, forming the top layer of the Earth's crust (1-3 meters deep).
• Fertile soil supports plant growth and is essential for ecosystems.

Soil Formation

• Depends on weathering and three additional processes: organic activity, leaching and precipitation, and eluviation and illuviation.
• Some effects of weathering include landslides, mudflow, and soil formation.

Organic Activity

• Involves the accumulation of plant and animal material in the soil.
• Organisms like termites, ants, and worms mix this material into the top soil layer through burrowing and sorting.

Leaching and Precipitation

• Leaching is the removal of soluble mineral matter, like ions, from the upper soil layer to the lower layer.
• Precipitation occurs when this soluble matter forms solid material in the subsoil after being washed down.

Eluviation and Illuviation

• Eluviation is the downward movement of fine particles, like clay, into groundwater.
• Illuviation is the accumulation of these fine particles in the lower soil layers.

Factors Controlling Soil Formation

• Parent Rock: The mineral or organic material from which soil forms determines its properties; for example, if the parent rock is red sandstone, the soil will also be red and share similar traits.
• Climate: A key factor in soil formation, as temperature, rainfall, and available organic material affect the rate of weathering and soil development; climate also influences the type of vegetation, which impacts the soil.
• Relief: Refers to the landscape's position and slopes; steep, long slopes allow water to run down quickly, eroding the surface and leading to poor soil formation on slopes, while richer soil forms at the base.
• Time: Soil formation takes many years; young soils may still show traits of their parent material, but over time, they gain organic matter and change due to moisture and environmental factors, sometimes transforming into a different soil type.

Soil Composition

• Consists of weathered rock particles, organic matter, water, and air.
• Weathered rock particles are the main component, varying by parent rock (e.g., granite vs. limestone).
• Water and air: 20-30% of soil volume each.
• Organic matter: About 5%, from decayed plants/animals (humus).
• Composition affects what can grow, be built, or how rainwater interacts with soil.

Soil Profile

• Soil is divided into horizontal layers called horizons, forming the soil profile.
• A Horizon (Topsoil): Dark, rich in humus, contains the most organic matter.
• B Horizon: Below A, brownish/reddish, contains clay and minerals washed from A.
• C Horizon: Deepest soil layer, light yellowish-brown, least weathered rock particles.
• D Horizon: Parent rock below C, not soil but the source of C horizon particles.

Soil Erosion

• Removal of fertile topsoil by physical (wind, rain, slopes) or cultural factors (overgrazing, deforestation, unscientific farming).
• Reduces soil fertility, impacting plant growth.

Soil Conservation

• Essential for maintaining soil’s role in supporting plants, filtering air/water, and decomposing waste.
• Protects ecosystems, supports animals, and ensures clean water through natural filtration.

Importance of Soil Conservation

• Soil is the foundation for plant life; without healthy soil, plants would die, impacting the entire ecosystem.
• Provides air for gas exchange between plant roots and the atmosphere.
• Supports the animal kingdom; without plants, animals depending on them would suffer, and soil is also home to many living creatures.
• Essential for clean water supply; soil filters water as it flows through, removing toxins before it reaches its destination.

Methods of Soil Conservation

• Afforestation: Planting trees to hold soil with roots, preventing erosion by rain or wind.
• Checking Overgrazing: Controlling cattle grazing to protect soil and restore pastures.
• Terrace Farming: Cutting slopes into steps to retain topsoil against rainwater runoff.
• Crop Rotation: Growing different crops to prevent mineral depletion and maintain soil fertility.
• Windbreaks: Planting dense rows of trees around farmland to reduce wind erosion.

Soils of India: Types and Regional Distribution