Rocks and Soils — Class 8 Social Science (Samacheer Kalvi)
TN State Board (Samacheer Kalvi) Class 8 Social Science, Geography — Chapter 1. The rocks beneath our feet and the soil that feeds us.
1. About this lesson
This lesson covers the three types of rocks, weathering and the formation of soil, the soil profile and composition, the soil types of India, and soil conservation.
2. Types of rocks
| Rock type | How it forms | Note |
|---|---|---|
| Igneous | by the solidification of molten magma/lava | the "primary" rock; e.g. granite, basalt |
| Sedimentary | by the deposition and hardening of sediments in layers | fossils are found in these; e.g. sandstone, limestone |
| Metamorphic | when igneous or sedimentary rocks are changed by heat and pressure | e.g. marble (from limestone), slate (from shale) |
3. Weathering and soil formation
- Weathering is the breaking down of rocks by sun, water, wind and living things.
- Soil is the product of the weathering of rocks mixed with humus (decayed organic matter).
4. Soil profile and composition
- The soil profile is the arrangement of soil in layers (horizons); the top layer is the topsoil, the most fertile layer containing humus.
- Soil is made of about 45% minerals, 5% organic matter, 25% water and 25% air.
5. Soil types and conservation
- Major Indian soils: alluvial (most fertile, river plains), black (regur — ideal for cotton), red and laterite.
- Soil conservation is protecting soil from erosion to keep it fertile. Methods: afforestation, controlled grazing, building dams, crop rotation, terrace and contour farming.
6. Worked examples
Example 1. How are igneous rocks formed? By the solidification of molten magma/lava.
Example 2. In which rocks are fossils found? In sedimentary rocks.
Example 3. Which soil is best for growing cotton? Black soil (regur).
7. Book-back questions (Samacheer Kalvi)
I. Choose the correct answer
- Igneous rocks are formed by the — (a) solidification of magma / (b) deposition of sediments. Ans: (a) solidification of magma.
- Fossils are found in — (a) igneous / (b) sedimentary rocks. Ans: (b) sedimentary.
- Metamorphic rocks form under — (a) heat and pressure / (b) only rain. Ans: (a) heat and pressure.
- The ideal soil for growing cotton is — (a) black soil / (b) laterite soil. Ans: (a) black soil.
- Soil is the product of the ____ of rocks — (a) weathering / (b) melting. Ans: (a) weathering.
II. Fill in the blanks 6. The top layer of soil is called the topsoil. 7. Soil contains about 45% minerals. 8. The decayed organic matter in soil is called humus.
III. Answer briefly 9. Name the three types of rocks with one example each. 10. What is soil conservation? Mention two methods.
8. Common mistakes
- Mistake: Saying fossils are found in igneous rocks. Fix: Fossils form in sedimentary rocks (heat and pressure destroy them in igneous rock).
- Mistake: Confusing weathering with erosion. Fix: Weathering breaks rocks in place; erosion then carries the pieces away.
- Mistake: Mixing up the soil types for cotton. Fix: Black soil (regur) is best for cotton.
9. Quick revision
- Geography Ch 1 · rocks and soils.
- Igneous (magma solidifies), sedimentary (layers, fossils), metamorphic (heat + pressure).
- Soil = weathered rock + humus; topsoil is most fertile.
- Composition: 45% minerals, 5% organic matter, 25% water, 25% air.
- Soils: alluvial, black (cotton), red, laterite. Conservation: afforestation, crop rotation, dams, controlled grazing.
