An Introduction to 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.
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.
________________________.
Water is made of sand, gravel, clay and humus.
______________________________________.
Soil are of four types - sandy, manure, loam and clayey.
____________________________________________.
FAQs on Rocks and Soil
1. What is the main difference between rocks and soil?
The main difference lies in their composition and structure. Rocks are solid, naturally-formed aggregates of one or more minerals. In contrast, soil is the loose upper layer of the Earth's surface, consisting of a complex mixture of weathered rock particles, organic matter (humus), water, and air. Essentially, soil is what forms when rocks break down over a very long time and mix with living and dead material.
2. What are the three primary types of rocks found on Earth?
The three primary types of rocks are classified based on how they were formed:
- Igneous Rocks: Formed from the cooling and solidification of molten rock (magma or lava). Examples include Granite and Basalt.
- Sedimentary Rocks: Formed from the accumulation and cementation of small particles of sand, shells, and other materials over time. Examples include Sandstone and Limestone.
- Metamorphic Rocks: Formed when existing igneous or sedimentary rocks are changed by intense heat, pressure, or chemical reactions. Examples include Marble (from Limestone) and Slate (from Shale).
3. How does a hard, solid rock eventually become soft soil?
Solid rock turns into soft soil through a slow process called weathering. Over thousands of years, natural forces like wind, rain, ice, and temperature changes cause the rock to crack and break into smaller and smaller pieces. These tiny mineral particles then mix with humus (decomposed plants and animals), water, and air. This combination of mineral and organic components creates the fertile, soft layer we call soil. Therefore, rock is the parent material from which soil is born.
4. What are the common types of soil and which is best for growing plants?
The most common types of soil are sandy, clay, and loam. Sandy soil has large particles and drains water quickly. Clay soil has very fine particles and holds water well, but can become waterlogged. Loamy soil is considered the best for growing most plants because it contains a balanced mixture of sand, silt, and clay. This mix allows it to retain moisture and nutrients while also providing good drainage and aeration for plant roots.
5. Why is soil considered much more than just dirt?
Soil is a vital living ecosystem, not just inert dirt. It is crucial for life on Earth for several reasons. It anchors plant roots and provides them with essential nutrients and water, forming the foundation of our food chain. Soil also acts as a natural filter, purifying water as it seeps through. Furthermore, it is a habitat for billions of organisms, including bacteria, fungi, worms, and insects, which are essential for decomposing organic matter and recycling nutrients.
6. What makes some soils, like clay, feel sticky while others, like sand, feel gritty?
The texture of soil is determined by the size of its mineral particles. Clay soil is made of extremely fine, flat particles. When wet, these tiny particles slide over each other easily and stick together, giving the soil a smooth, sticky feel. In contrast, sandy soil consists of much larger, irregularly shaped, and coarse particles. These large grains do not stick together, which is why sand feels gritty and loose to the touch.
7. Can new rock be formed from old soil?
Yes, this is part of the continuous Rock Cycle. While soil forms from weathered rock, the materials in soil can eventually become rock again. Over millions of years, layers of soil and sediment can be buried deep within the Earth. The immense pressure and heat from burial can compact and cement these layers together, forming new sedimentary rock. This demonstrates the cyclical relationship between rocks and soil.
