Weathering and Soil formation | ICSE Class 7 Geography Notes

This note covers rocks and their formation, the meaning and types of weathering, factors affecting weathering, soil formation, soil profiles, Indian soil regions, soil erosion and methods of soil conservation.

What are rocks, and how do the main types form?

A rock is a natural mass of mineral matter forming part of the earth’s crust, its outermost layer. Minerals are naturally occurring substances with definite chemical compositions and particular physical properties. Rocks differ in colour, size and texture, meaning the character of their grains.

How do igneous rocks form?

Igneous rocks form when molten rock cools and becomes solid. Magma is molten rock inside the earth. When it reaches the surface, it is called lava. The place and speed of cooling help explain differences between igneous rocks.

Extrusive igneous rocks form when lava cools rapidly at the surface. They have a very fine-grained structure. Basalt is an example, and the Deccan plateau is made up of basalt rocks.

Intrusive igneous rocks form when magma cools slowly deep inside the crust. Slow cooling allows large grains to form. Granite is an example. Thus, basalt and granite belong to the same main rock group but form under different cooling conditions.

How do sedimentary and metamorphic rocks form?

Sediments are small rock fragments. Wind and water transport and deposit them. When loose sediments are compressed and hardened into layers, they form sedimentary rocks. Sandstone forms from grains of sand.

Sedimentary rocks may contain fossils, the remains of dead plants and animals trapped in rock layers. The word “may” matters: the presence of fossils is a possible feature, rather than a requirement for every sedimentary rock.

Metamorphic rocks form when existing rocks change under great heat and pressure, the force pressing on material. Limestone changes into marble, while clay changes into slate. These examples describe changes in existing material, whereas igneous rock formation begins with cooling molten material.

How does weathering differ from erosion and the rock cycle?

Definition: Weathering is the mechanical breaking apart and chemical decomposition of rocks through the action of weather and climate. Mechanical breaking separates material into fragments; chemical decomposition changes its mineral material through reactions.

Weathering is an in-situ process, meaning that it happens on site. Very little or no movement of material takes place. A rock can therefore be weathered without its fragments being carried away to another place.

Erosion involves the removal and transport of earth material by agents such as running water, wind or moving ice. Deposition is the settling of transported material. These processes are linked, but each describes a different part of what happens to rock material.

Where does weathering fit into rock changes?

The rock cycle is the transformation of one rock type into another under suitable conditions. Weathering contributes by breaking rock into material that can later be transported and deposited. Compression and hardening of sediments then produce sedimentary rock.

Heat and pressure can change igneous and sedimentary rocks into metamorphic rocks. Melting produces magma, which can cool into igneous rock. Breaking apart, transporting, compressing, heating and melting should not all be described simply as weathering.

The drawing shows magma, igneous rock, sediments, sedimentary rock and metamorphic rock. Arrows connect these materials to show pathways of change. Follow each arrow while naming the process that links its starting and ending materials.

See Fig. 2.4 in your NCERT textbook

Note: Weathering helps erosion by providing loose material. However, weathering is not a precondition for erosion. Keep the distinction between rock breakdown and the removal of material clear.

How does mechanical weathering break rocks apart?

Mechanical weathering, also called physical weathering, breaks rocks through applied forces. Its central feature is physical disintegration, meaning separation into smaller pieces. Forces can act at the surface of a rock and within it.

How do expansion and contraction act?

Thermal expansion means expansion caused by heating. Contraction means shrinking. Repeated expansion and contraction place strain on rock material. Small and slow changes can cause great damage when they recur, producing continued fatigue in the rock.

1. Rock material experiences temperature changes that cause expansion and contraction.
2. The changes exert forces on the material at its surface and within it.
3. Repeated expansion and contraction subject the rock to continued strain.
4. The forces lead to fractures, breaking rock into smaller fragments.

Pressure release is another important cause of physical weathering. Most physical weathering processes are caused by thermal expansion and pressure release. This does not mean that these are the only forces involved.

Other physical forces include those associated with growing crystals, animal activity and water pressure during wetting and drying. A crystal is a solid with an orderly arrangement of its particles. The important connection is between a force and the breaking of rock.

What does exfoliation mean?

Exfoliation is the flaking away of more or less curved sheets from rock, leaving smooth, rounded surfaces. It is a result of weathering rather than a separate basic group of weathering. It can occur through expansion and contraction caused by temperature changes.

When explaining mechanical weathering, follow the cause through to the result: a force acts, strain develops, and the rock fractures or flakes. A description that stops at “the rock gets hot” leaves out the process connecting temperature changes with breakdown.

How do chemical and biological weathering act on rocks?

Chemical weathering changes rock material through chemical reactions. A chemical reaction changes the substances involved. Water, oxygen and acids act on minerals, causing decomposition or dissolving. Dissolving means becoming mixed into a liquid as a solution.

What helps chemical weathering?

Water and air, including oxygen and carbon dioxide, together with heat speed up chemical reactions. Oxygen and carbon dioxide are gases present in air. Decaying plants and animals also increase the quantity of carbon dioxide underground.

Acids are substances that can react with some rock minerals and help them decay or dissolve. Acids produced as plant and animal matter decays therefore link living things with chemical changes in rocks. Some elements become more soluble, meaning more able to dissolve.

The useful distinction is between changing the physical size of rock pieces and changing mineral material through reactions. Both kinds of change help prepare material for soil formation. They should not be treated as unrelated events.

How do organisms cause weathering?

Biological weathering is weathering associated with the growth, movement and activity of living organisms. Plant roots press against earth material and mechanically break it apart. Biological weathering can therefore include physical action.

Earthworms, termites and rodents burrow or force material apart. Their activity exposes fresh surfaces to chemical attack and helps air and moisture enter. Decaying organisms produce acids, so biological activity can also assist chemical weathering.

People contribute by disturbing vegetation and by ploughing and cultivating soil. These activities mix material and create new contacts between air, water and minerals. Biological activity therefore includes more than the action of roots alone.

Note: Very rarely does one weathering process operate completely by itself. Quite often one process dominates, while others also contribute. Biological activity can help both mechanical and chemical weathering.

Which factors affect weathering?

The rate and nature of weathering depend on climate, rock type and structure, the shape of the land, and vegetation. Climate means the characteristic weather conditions of an area over a long period. These controls explain why rocks do not weather identically everywhere.

How do climate and rock structure matter?

Temperature and precipitation, water reaching the ground as rain or snow, are important climatic controls. Temperature changes affect expansion and contraction. Moisture provides water for reactions and influences the activity of organisms.

Differences in the frequency of freezing and thawing, meaning becoming frozen and melting again, also influence weathering conditions. The amount and kind of precipitation matter. Weathering can vary within a climatic region as well as between different climatic regions.

Rock structure includes features such as cracks, layers and the arrangement of material. Cracks in rock are often called joints. The hardness of minerals and their susceptibility to chemical change also affect resistance to weathering.

A particular rock may resist one process but be less resistant to another. Under different climatic conditions, the same kind of rock may show different degrees of resistance. It is therefore misleading to rank all rocks as simply “easy” or “difficult” to weather.

What do slope and vegetation change?

Topography means the shape and arrangement of land features, including slopes. It influences exposure to sunlight and drainage, the movement of water over or through the ground. These conditions affect the environment in which weathering takes place.

Vegetation means plant cover. Roots apply pressure, and decaying plant material contributes acids. The density and type of vegetation therefore influence weathering. Plant cover itself is closely related to temperature and precipitation.

These factors work together. An explanation of weathering should connect the rock material with the conditions acting on it. Naming climate alone leaves out differences between rocks, while naming rock type alone leaves out changes in moisture, temperature and biological activity.

How does weathered rock become soil?

Soil is the thin layer of grainy material covering the earth’s surface. It contains minerals, weathered rock and organic matter, material derived from living things. It supports plants and many other organisms.

Weathering supplies the basic mineral material, but soil formation also involves living organisms and the addition of organic matter. A pile of newly broken rock fragments is therefore not a complete explanation of a developed soil.

What is the sequence of soil development?

Humus is finely divided organic matter in soil formed from decaying remains. A weathering mantle is the layer of weathered material. Organisms colonise, meaning begin living in, this material or deposits brought from elsewhere.

1. Weathering breaks down rock and produces material from which soil can develop.
2. Bacteria and small organisms begin living in weathered material or transported deposits.
3. Dead remains of organisms and plants contribute to the accumulation of humus.
4. Roots enter the material, while burrowing animals move particles and help mix it.
5. The material develops spaces that retain water and allow air to pass, forming a mixture of mineral and organic products.

Porous means having spaces through which air or water can pass. As soil develops, it becomes a medium in which physical, chemical and biological activities continue. Soil is a changing system, rather than an unchanging covering.

Why do soils differ?

Parent material is the rock debris or deposits from which a soil develops. It influences texture, mineral content and chemical properties. Parent material can be weathered rock at the site or material transported and deposited there.

Climate supplies heat and moisture. Topography influences drainage and accumulation. Organisms add organic matter and mix particles. Time allows these processes to develop the soil. These factors act together and affect one another.

Table: Time needed to form a thin layer of soil.

Soils are thin on steep slopes and thick over flat upland areas. Gentle slopes with slow erosion and good movement of water into the ground favour soil formation. No specific length of time can be fixed for soils everywhere to develop and mature.

What does a soil profile show?

A soil profile is a vertical section showing soil layers and the material beneath them. A distinct soil layer is called a horizon. Reading a profile means following changes downwards, rather than treating the ground as one uniform mixture.

Which layers should be identified?

Topsoil is the upper soil layer. The profile shows it with humus and vegetation. Below it lies subsoil, shown with sand, silt and clay. These names describe mineral particles of different sizes: sand is coarser, silt is finer, and clay is finer still.

Beneath the subsoil is weathered rock material. The lowest part shown is parent rock, the underlying rock associated with the soil’s development. Distinguish weathered material from the unweathered rock beneath it.

Cartoon: Soil profile (NCERT Class 8 Figure 2.3). The cartoon presents a soil section as a smiling character. Its labels, from top to bottom, identify topsoil with humus and vegetation, subsoil with sand, silt and clay, weathered rock material, and parent rock.

Why is the profile important?

A profile connects the surface supporting vegetation with the rock material below. It helps explain why soil conservation must protect the upper soil, rather than merely leave some rock at the site. The layers have different characteristics and positions.

Time influences the development of a profile. Young soils may have no distinct horizons or only poorly developed ones. Do not assume that every soil has equally clear layers. A labelled profile is a guide to recognising vertical differences.

To draw one, keep the layers in their correct order and place labels beside them. Show vegetation above the upper soil and rock material below. Avoid adding fixed layer thicknesses, because soil development differs with parent material, climate, topography, organisms and time.

Where are some important soils found in India?

India has different soil types because its rocks, landforms, climate and vegetation vary. A soil map shows their broad distribution. It should be read alongside the factors of formation, rather than as a list of disconnected place names.

Where are alluvial and black soils found?

Alluvial soil develops from material deposited by rivers. It covers the northern plains, where the Indus, Ganga and Brahmaputra river systems have deposited it. It also occurs in eastern coastal plains, particularly in river deltas, areas of deposition near river mouths.

Alluvial soils contain varying proportions of sand, silt and clay. As a whole, they are very fertile, meaning able to support productive plant growth. Their origin illustrates that soil can develop in transported deposits as well as directly over weathered rock.

Black soil, also called regur, is typical of the basalt region of the northwestern Deccan plateau. It covers areas including Maharashtra and Madhya Pradesh. Its extremely fine clayey material is well known for holding moisture, and it is ideal for cotton.

The map uses pale green for alluvial soil and grey for black soil. Trace the broad northern alluvial belt and the black-soil area across the western and central peninsula, using the legend to identify each.

Reference: NCERT Class 10, India: Major Soil Types

What other patterns appear?

Red and yellow soils occur across parts of the eastern and southern Deccan plateau. Arid soils, soils of dry regions, are generally sandy and saline, meaning containing salts. Western Rajasthan provides an example of their distribution.

Laterite soil develops in tropical and subtropical climates with alternating wet and dry seasons. Heavy rain causes intense leaching.

Table: Characteristics of lateritic soils.

Forest soils occur in hilly and mountainous areas with sufficient rain forests. Their texture varies with the mountain environment. Soil distribution therefore reflects both the material available and the conditions under which soil develops.

The same map marks red and yellow soils with red diagonal shading, arid soil in pale yellow, and forest and mountainous soils with green diagonal shading. Compare the peninsular, northwestern and Himalayan patterns through the legend.

Reference: NCERT Class 10, India: Major Soil Types

Why does soil need conservation?

Soil conservation means protecting soil against loss and deterioration. Soil supports plant growth and living organisms, while its formation requires time. Protecting existing soil is therefore essential for maintaining the land’s ability to support life and cultivation.

Soil erosion is the removal of soil cover by agents such as water and wind. Soil formation and erosion happen simultaneously, and generally a balance exists between them. Human activities can disturb this balance.

What damages soil?

Deforestation, the removal of forests, and overgrazing, excessive grazing by animals, contribute to soil degradation. Degradation means deterioration in the condition of soil. Construction, mining and unsuitable farming practices can also increase soil loss.

Rain wash, floods and landslides are natural contributors to soil degradation. Overuse of chemical fertilisers or pesticides can also harm soil. Fertilisers supply plant nutrients; pesticides are substances used to control pests.

Ploughing directly up and down a slope creates channels through which water can flow quickly. Run-off is water flowing over the ground surface. Its movement can carry soil away, linking farming practice with the risk of erosion.

How does soil loss affect land?

Gullies are deep channels cut by running water. Land affected by severe gully erosion can become unfit for cultivation and is called bad land. In the Chambal basin, such lands are called ravines.

Conservation addresses the causes of loss: rapid water movement, wind movement, exposed ground and damaging land use. The appropriate method depends on the problem. A measure for a steep cultivated slope may differ from one used where wind removes loose soil.

How can soil be conserved on slopes and exposed land?

Conservation methods protect soil by reducing the force of moving water or wind, maintaining plant cover, and retaining moisture. Understanding the action of each method makes it easier to choose a suitable response to a particular form of soil loss.

Case study: How does Himalayan terrace farming reduce erosion?

Terrace farming uses broad, flat steps cut into steep slopes. These steps provide surfaces for growing crops and reduce surface run-off and soil erosion. Terrace farming is well developed in the western and central Himalayas.

The link between setting and method is clear: a steep slope is changed into a series of flatter cultivated surfaces. The conservation benefit is the reduction of run-off and erosion. When describing the method, connect the flatter steps with their role in reducing surface water flow.

Photograph: Terrace farming (NCERT Class 8 Figure 2.5). The photograph shows green cultivated steps following a hillside, with vegetation on surrounding slopes. The repeated steps illustrate how cultivation can be arranged across a slope.

Contour ploughing means ploughing parallel to contours, lines joining places at equal height. It slows the flow of water down a slope. Contour barriers are barriers of stones, grass or soil along contours, with trenches in front to collect water.

Case study: How do shelter belts protect western India?

Shelter belts are rows of trees planted to check wind movement and protect soil cover. They are used in dry and coastal regions. Shelter belts have contributed significantly to stabilising sand dunes and the desert in western India.

This example connects a wind-related problem with a protective row of vegetation. Strip cropping also reduces the force of wind: large fields are divided into strips, with strips of grass left between crops.

Which other methods help?

Afforestation means establishing tree cover. Along with proper grazing management, it can help to some extent. Conservation is most clearly explained by naming the method, describing what is done, and connecting that action with the soil problem it addresses.

Glossary

• Rock — A natural mass of mineral matter forming part of the earth’s outer crust.
• Igneous rock — Rock formed when molten magma or lava cools and becomes solid.
• Sedimentary rock — Rock formed when deposited sediments are compressed and hardened into layers.
• Metamorphic rock — Rock formed when existing rock changes under great heat and pressure.
• Weathering — The mechanical disintegration and chemical decomposition of rocks at or near their original site.
• Mechanical weathering — Physical breakdown of rock through forces such as repeated expansion and contraction.
• Chemical weathering — Breakdown of rock minerals through chemical reactions involving water, gases and acids.
• Biological weathering — Weathering associated with organisms, including root pressure, burrowing and the effects of decay.
• Humus — Finely divided organic matter in soil derived from the decay of living material.
• Parent material — Weathered rock debris or transported deposits from which a soil develops.
• Soil profile — A vertical section showing soil layers and the rock material beneath them.
• Soil erosion — Removal of soil cover by moving agents such as water and wind.
• Contour ploughing — Ploughing along lines of equal height to slow water movement down slopes.
• Terrace farming — Cultivation on broad, flat steps cut into steep slopes to reduce erosion.
• Shelter belts — Rows of trees planted to check wind movement and protect the soil cover.

Common errors and misconceptions

• Misconception: Weathering and erosion mean the same thing. Correct: Weathering breaks down rock with very little or no movement; erosion removes and transports material.
• Misconception: All igneous rocks cool at the surface. Correct: Extrusive rocks cool at the surface, while intrusive rocks form when magma cools slowly inside the crust.
• Misconception: All sedimentary rocks contain fossils. Correct: Sedimentary rocks may contain fossils. Fossils are a possible feature, not a requirement for every sedimentary rock.
• Misconception: Each type of weathering acts entirely alone. Correct: Very rarely does one operate completely by itself; biological activity can assist physical and chemical changes.
• Misconception: Soil is simply crushed rock. Correct: Developed soil contains mineral material and organic matter, and supports continuing physical, chemical and biological activity.
• Misconception: Every soil has clearly developed layers. Correct: Young soils may have no distinct horizons or only poorly developed ones; profiles develop over time.
• Misconception: Ploughing up and down a slope protects soil. Correct: It can create channels for rapid water flow. Contour ploughing follows lines of equal height and slows water movement.

Exam-style questions with model answers

Q1. Define weathering and explain how it differs from erosion. [2 marks]

1. Weathering is the mechanical breaking apart and chemical decomposition of rocks, with very little or no movement of material.
2. Erosion involves removing and transporting earth material, for example by running water or wind, rather than simply breaking it down on site.

Q2. Basalt forms when lava cools rapidly at the surface; granite forms when magma cools slowly inside the crust. Identify the igneous category of each and connect its grain size with its cooling conditions. [4 marks]

1. Basalt is an extrusive igneous rock because the given lava cools at the earth’s surface.
2. Its rapid cooling produces a very fine-grained structure, linking its small grains with the stated cooling conditions.
3. Granite is an intrusive igneous rock because the given magma cools inside the earth’s crust.
4. Its slow cooling allows large grains to form, explaining the contrast with rapidly cooled basalt.

Q3. Explain mechanical, chemical and biological weathering, giving an action associated with each. [3 marks]

1. Mechanical weathering physically breaks rock through applied forces. Repeated expansion and contraction due to temperature changes can strain and fracture the material.
2. Chemical weathering changes rock minerals through reactions. Water, oxygen and acids can help decompose or dissolve the mineral material.
3. Biological weathering involves living organisms. Growing roots mechanically break earth material apart, while burrowing organisms expose fresh surfaces to moisture and air.

Q4. Explain how weathered rock material develops into soil, from the initial breakdown of rock to a mixture containing mineral and organic material. Give five stages. [5 marks]

1. Weathering breaks down the original rock and supplies weathered mineral material. This provides the basic material in which soil-forming processes can begin.
2. Bacteria and other small organisms begin living in the weathered material or in deposits brought from elsewhere, introducing biological activity.
3. Dead plants and organisms add organic remains. Their decay contributes to the accumulation of humus within the developing material.
4. Roots grow into the material and burrowing animals move particles, helping to mix it and create spaces for air and water.
5. A developed soil becomes a mixture of mineral and organic products, with spaces that help retain water and permit air to pass.

Q5. Describe a soil profile from the surface downwards using these labels: topsoil, subsoil, weathered rock material and parent rock. Give one feature of each. [4 marks]

1. Topsoil lies at the surface. It is the upper soil layer shown with humus and vegetation.
2. Subsoil lies below the topsoil. It contains mineral material represented in the profile by sand, silt and clay.
3. Weathered rock material occurs beneath the subsoil. It consists of rock material that has been affected by weathering.
4. Parent rock is shown at the base. It is the underlying rock beneath the weathered material in the profile.

Q6. Explain five controls on soil formation: parent material, climate, topography, organisms and time. [5 marks]

1. Parent material supplies the rock debris or deposits from which soil develops. Its mineral composition and texture influence the resulting soil.
2. Climate supplies temperature and moisture conditions. These influence weathering and the chemical and biological activities involved in the development of soil.
3. Topography affects sunlight exposure, drainage and soil accumulation. Steep slopes have thin soils, while gentle slopes with slow erosion favour formation.
4. Organisms contribute organic matter and mix soil particles. Dead remains add humus, while roots and burrowing animals change the developing material.
5. Time allows the soil-forming processes to act and horizons to develop. Young soils may show no horizons or only poorly developed ones.

Q7. Explain how each method conserves soil: terrace farming, contour ploughing, shelter belts, mulching and rock dams. [5 marks]

1. Terrace farming makes broad, flat steps on steep slopes. These provide surfaces for cultivation and reduce the run-off that contributes to soil erosion.
2. Contour ploughing follows lines of equal height across a slope. It slows water movement down the slope and helps reduce erosion.
3. Shelter belts consist of rows of trees planted to check wind movement. They protect soil cover in dry and coastal regions.
4. Mulching covers bare ground between plants with organic matter such as straw. This cover helps the soil retain moisture.
5. Rock dams are piles of rocks that slow flowing water. They help prevent the formation of gullies and further loss of soil.

Q8. Water may wash topsoil away as a sheet, or cut deep channels called gullies; wind may blow loose soil away. Name each form of erosion and describe its distinguishing action. [3 marks]

1. Sheet erosion occurs when water flows as a sheet across a slope and washes away topsoil over an area.
2. Gully erosion occurs when running water cuts deep channels into soil. The channels, called gullies, distinguish it from sheet removal.
3. Wind erosion occurs when wind carries loose soil away from flat or sloping land. The moving agent here is wind rather than flowing water.

Key takeaways

• Igneous, sedimentary and metamorphic rocks differ in formation: cooling, compression of sediments, and alteration by great heat and pressure.
• Weathering happens on site with very little or no movement; erosion involves removal and transport of earth material.
• Mechanical, chemical and biological weathering interact, and very rarely does one process operate completely by itself.
• Climate, rock characteristics, topography and vegetation influence weathering, so the same rock may behave differently under different conditions.
• Weathering supplies mineral material, while organisms, humus, air, water and time contribute to the development of soil.
• A soil profile shows vertical differences, from topsoil and subsoil down to weathered material and parent rock.
• Soil conservation protects a resource that supports plants and other organisms and requires time to develop.
• Terraces and contours reduce water-related loss; shelter belts check wind movement, while mulching helps retain soil moisture.

Test yourself

What is the difference between magma and lava?

Magma is molten rock inside the earth; lava is molten rock that has reached the surface.

What does in-situ weathering mean?

It means weathering happens on site, with very little or no movement of the material.

Why is root action classed as biological weathering?

Roots belong to living plants, and their pressure can mechanically break earth material apart.

What is humus, and where does it come from?

Humus is finely divided organic matter in soil, formed through the decay of organic remains.

Which factors control soil formation?

The main controls are parent material, climate, topography, biological activity and the time available for development.

What lies between topsoil and weathered rock material in the profile?

Subsoil lies between them and is shown containing sand, silt and clay.

How does contour ploughing differ from ploughing up and down a slope?

Contour ploughing follows lines of equal height and slows water. Ploughing up and down a slope can create channels for rapid water flow.

Which conservation method covers bare ground with straw?

Mulching covers bare ground between plants with organic matter such as straw to retain moisture.