Rocks and Soil

We can find rocks and soil all around us, in different shapes, sizes, colours and forms. The Earth’s crust is primarily made of rocks and soil, which is present inside the Earth. The formation of soil occurs by the breakdown of rocks. In this chapter, we will study about rocks and soil, their types, process of formation and importance.

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Types of Rocks and Their Formation

Earth is approximately 4.5 billion years old and most of the rocks found on Earth are 100 million years old. Now, let's understand the type of rocks and their process of formation.

• Igneous Rocks - These rocks are formed when the molten magma cools down. These rocks are hard and strong.

• Sedimentary Rocks - These rocks are formed when small particles of rock settle at the bottom of a lake or sea and over time the particles become ‘cemented’ together. These are soft rocks and are found in layers. 

3. Metamorphic Rocks - The intense heat and pressure present inside earth changes the igneous and sedimentary rocks into metamorphic rocks overtime. 

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Uses of Rocks

• Coal - It is used in power plants to generate electricity.

• Limestone - It is used to manufacture cement, paper and glass. 

• Shale - It is a component of bricks and cement. 

• Granite - It is used in architectural construction and monuments. 

Formation of Soil 

Soil is formed when the force of water, wind and changes in weather break the rock into very small pieces. Soil consists of two main parts - small rock pieces and rotted plant and animal pieces. The plants and animals become part of the soil, when they die and their bodies rot, eventually giving rise to humus. Humus is dark brown in colour and fertile.

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Composition of Soil

If we take garden soil in a transparent bottle, pour water in it, shake the bottle after closing the lid, and let it sit for some time; we could see different layers of soil. 

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Types of Soil

• Clayey Soil - This soil contains clay. It consists of very small particles that are sticky and smooth to touch. It holds water.

• Sandy Soil - This soil contains sand. It consists of large particles having lots of space between them. This soil feels rough, loose and cannot hold water. The main difference between soil and sand is that soil is the loose and topmost layer of earth whereas sand is the loose material that consists of rocks or mineral grains.

• Silt Soil - This soil has a texture of talcum powder in absence of moisture and a bit slimy texture in its presence. The particles of this soil are very fine, and therefore susceptible to erosion.

• Loamy Soil - This soil contains a mixture of clay, sand and silt. This soil has the best ability to retain moisture and nutrients and is considered best for farming.

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Soil and Crops

Importance of Soil

• It is important for growing plants.

• The minerals present in soil have unique properties such as different colours. These colours can be used to make pigments for dyes, paints and make-up.

• Clayey soil is used in pottery.

• Some minerals are mined from Earth to make products like sunscreens and jewellery. 

• The exotic minerals in some soils are used to make mud spas and mud baths.

Solved Questions

1. What type of rocks are formed by the sedimentation of plants and animals over a long period of time?

Ans: Sedimentary rocks are formed when particles of rock, dead plants and animals settle at the bottom of a lake or sea. With time, the particles become ‘cemented’ together and form sedimentary rocks.

2. Which soil is called the ‘Gardener’s best friend’?

Ans: Loam or loamy soil is called the ‘gardener’s best friend’ because it has the best ability to retain moisture and nutrients, making it the best choice for growing crops.

Fun Facts

• Meteorites are the types of rocks that land on Earth from Space.

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• Pumice rock is filled with a lot of air that it floats. 

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• Rocks and minerals are also found in things we use as home like soap, toothpaste, makeup and batteries.

Summary

In this chapter, we learnt about rocks and soil, their types and process of formation. We also discussed the importance of rocks and soil. 

Learning By Doing

• Spot five items in your home that are made of rocks.

• Change the underlined words to correct the following sentences. Rewrite the correct sentences in the space provided.

• Humus makes the soil barren.

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• Water is made of sand, gravel, clay and humus.

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• Soil are of four types - sandy, manure, loam and clayey.

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