WBBSE Notes For Class 7 Geography Chapter 6 Rock And Soil

Chapter 6 Rock And Soil

In geography, any natural solid material which forms the crust of the Earth is called a rock. A rock is simply an aggregate of various minerals but in the scientific sense, rocks are the materials that make up the earth’s crust.

A rock has no definite chemical composition as minerals. The hardness, color, shape, and rigidity of rocks depend upon their minerals.

Different Types Of Rocks: On The Basis Of Their Mode Of Formation, Rocks Are Classified Into Three Types:

1. Igneous Rocks,
2. Sedimentary Rocks and
3. Metamorphic Rocks.

1. Igneous Rocks: Igneous rocks are those rocks that have been formed by cooling. and solidification of lav The word ‘Igneous’ is derived from the Latin word ‘Ignis’ meaning ‘fire’. In the beginning, when the Earth was born, it was just a glowing ball of gas.

Gradually it cooled down and its molten volatile surface solidified.

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In this way, the igneous rocks formed at the first on the earth’s crust. Hence it is called ‘Primary Rock’. It has no layer and that is characteristics of igneous rocks why it is called ‘Unstratified rock’.

study of rocks and soil

Characteristics :

1. Igneous rocks are of thermal origin.
2. Igneous rocks are hard and resistant. They do not break down easily.
3. These rocks are compact, massive, and crystalline.
4. There are no layers in this rock.
5. They do not contain any fossils.
6. Metallic minerals are common in this rock.

Examples: Granite, Basalt, Dolerite, and Gabro et are examples of igneous rocks. Granite is used for house building and idol-making. Basalt, Dolerite is quarried for road-making.

 

 

2. Sedimentary Rocks: A rock composed of sediments and generally having a layered (Stratified) appearance is called ‘sedimentary rock’.

Sedimentary rocks are formed from the sediment accumulated over a long period, usually underwater at the bottom of the sea, ocean, and lakes in layers. Layers are known as strat So, these rocks are also called ‘Stratified Rocks’.

 

Characteristics :

1. Sedimentary rocks are formed by the sedimentation of organic or inorganic materials.
2. These rocks are stratified.
3. These rocks are porous.
4. These rocks are soft and break down easily.
5. Fossils are found in only this type of rock.
6. Organic matter, such as coal, petroleum, limestone, etc are found in this type of rock.

Examples: Sandstone, Mudstone, Limestone, Dolomite, Gypsum, and Rock salt et are. important sedimentary rocks. Sandstones and Mudstones are used as house-building materials. Limestone is used as raw materials in cement, chemical and iron, and steel industries.

 

 

3. Metamorphic Rocks : Metamorphic rocks are those rocks that have been changed in their form, and color et due to intense heat, tremendous pressure, and chemical action.

Characteristics:

1. Metamorphic rocks are changed due to heat, pressure, and chemical action.
2. These are hard resistant rocks.
3. These rocks are highly crystalline.
4. These rocks are heavier than other rocks.
5. No fossils are found.
6. Pre-existing igneous sedimentary and also metamorphic rocks are changed to metamorphic rock.

Example: Examples of metamorphic rocks include-Granites changes into gneiss, Basalt changes into Amphibolite, Limestone changes into Marble, Sandstone changes into Quartzite and Coal changes into Graphite.

Marble is extensively used by architects. The famous Taj Mahal is made of marble. Slates are used in making blackboards and can be used as a substance for writing.

 

 

Chapter 6 Rock And Soil From Rocks to Soils

Soils are the top covering of the earth’s surface and have been formed by the wear and tear of rocks and also vegetable and animal matter. The change from hard and rugged rocks to soft, fine-grained soil takes a long time.

The first step in this direction is the disintegration or weathering of rocks.

Weathering contributes much to the formation of soil. Mechanical or physical and chemical weathering plays a very important part in the development of soils, so essential to sustaining every form of life on the earth.

The soil formation process is an extremely slow one. For instance, the development of a soil layer of 2.5 cm may have taken well over a thousand years.

The disintegration of rock by weathering often produces a layer of loose rock materials.

It remains overlying on the bed-rocks, i.e. the primary rock bed of the region. This covering of loose materials over the primary rock bed is known as ‘regolith’. Soil is derived from regolith by different natural processes.

Biological weathering by plants and animals often plays an important role in information soil. Bacteria, fungi, and protozoa cause the decay of plants and animals.

Burrowing animals also help in it. When all these die and decay, minerals and humus are added to the upper layer of soil (The organic matter in the soil which differentiates from regolith and which comes from animal and vegetable decay, is called humus).

The formation of soil can be understood from the chart given below.

 

 

While the chemical action produces the elements so essential for plant growth, it is the mechanical action or the impact of physical forces that gives structure and texture to the soil.

Though soils are derived from rocks there are some differences between rocks and soils. These are as follows:

 

Rocks	Soils
1. Any natural solid material which forms the crust of the Earth is called a rock.	1. The uppermost layer of the Earth’s crust covering loose materials is called soil.
2. Rocks are aggregates of minerals.	2. Rock particles make soil.
3. Rocks are formed by cooling and solidification of lava sedimentation of silts and metamorphism.	3. It forms slowly over a long period by the mechanical and chemical weathering of bedrock.
4. Rocks do not have definite layers.	4. The soil has definite layers.
5. The rock consists of different types of minerals.	5. The soil consists of mineral particles, organic matter (humus), water, air, and living organisms (bacteria).
6. Rocks have a greater depth.	6. Soils are found up to a depth of 2-3 m.
7. Rocks are harder than soils.	7. Soils are softer than rocks.
8. Specific gravity of rock is higher than soil.	8. Specific gravity of soil is lower than a rock.

 

What Is Soil Made Of :

In the non-living part of the soil, rock waste is produced by weathering and erosion. Its particles are usually composed of gravel, silt, and clay. The living part of the soil contains the roots of the plants, fungi, bacteria, insects, small biting animals et

Living and non-living colloids play an important chemical role in the development of soils.

 

 

 

Chapter 6 Rock And Soil Factors Of Formation Of Soil

The main factors in the formation of soils are

1. parent rocks,
2. Relief,
3. Climate,
4. Organic matter (plants and animals) and
5. Time.

 

 

1. Parent rocks: Parent materials over which the rock waste and soil develop are the most important factor. The texture and structure of soils depend on the parent rock.

 

 

2. Relief: Ordinary slopes where water drainage is good to have the ideal condition for the development of soil. Steep slopes have thin soil layers, whereas the valleys and lowlands possess thick fertile soils.

3. Climate: Climate is considered to be the most important factor in the development of soil. The most important elements of climate are-

1. Water or Humidity,
2. Temperature and
3. Air or wind.

“the study of rocks and soil “

1. Water or Humidity: Water or humidity is the most important element for the chemical and biological processes of soil development.

2. Temperature: When the temperature is high, chemical and biological activities are accelerated.

3. Air/wind: The role of air/wind is not very important. Air/wind increases evaporation from the soil. The winds erode the upper layers of the soil and transport them to distinct places.

4. Organic Matters (Plants And Animals): The action of animals and plants over the soil is very important. Vegetation is of two types-

1. Molecular vegetation-Fungi, bacteria et and
2. Massive vegetation-trees, bushes grasses et The soil gets humus from plants. Animals’.also, help in the development of soil. Earthworms, burrow in the soil and upturn the soil of the lower layer. Ants, termites, dogs, squirrels et also do the same thing.

5. Time: The soil formation process is an extremely slow one. It does not form in a day. For instance, the development of a soil layer of 2.5 cm may be taken well over a thousand years. Soils in wet areas require less time to develop than in dry areas.

 

Chapter 6 Rock And Soil Soil Types

Soil texture refers to the size of its grains or particles. On the basis of texture, the soil can be divided into the following types.

1. Sand Or Sandy Soil: The rock particles ranging in diameter from 0.02 to 2.00 mm make sand. The sandy soil is dominated by sand. It contains about 80% of sand particles and 20% of clay or silt. Sandstone is a rock, composed of sand grains. Sandy soil has bigger grains and bigger spaces between grains. It absorbs water in no time. It is not suited for agriculture.

2. Clay Or Clayey Soil: The soil composed of the finest rock particles is called clay the size of rock particles is about 0-002 mm the soil contains about 28% of clay and the rest particles or another size. The grains and pore spaces are very small. Water logging is common in such soils. They are better suited to agriculture than sandy soil.

3. Loam/ Loamy Soil: Loamy soils are a mixture of both sand and clay. It contains equal portions of sand and clay. Depending upon a preponderance of the two, they are called either sandy loams or clayey loams. This soil is very well aerated and watered. So, this type of soil is the best suited for cultivation.

 

Soil Types	Silt contains
Clay	0.45
Clay loam	0.42
Loam	0.42
Silt	0.82
Sandy loam	0.14
Sand	0.12

 

Clay contains	Sand contains
0.45	0.1
0.25	0.33
0.18	0.4
0.13	0.05
0.11	0.75
0.08	0.8

 

“rocks and soils “

Different Characters Of Different Soils :

 

Types of Soil	Grain size	Capacity to retain wates	Crops
Clayey	Plate-shaped grains are found in compact and fine grains	Water cannot move rapidly through it	Rice, Watermelon.
Loamy	The grain size of the soil is smaller than sandy soil and bigger than clay because it is a mixture of clay and soil.	It is well watered.	The best soil for plant growth as well as agricultural crops. Sugar-cane, pulses, wheat, rice, oil seeds, etc.
Sandy	Sand grains are rounded or irregular in shape. They are bigger grains than loams and clay. 1-0-005	It does not hold water well and has large pore spaces between the grains water drains off rapidly.	Wheat, oil seeds, melon, etc.