Weathering and Soil Formation

Rocks are aggregates of one or more minerals, without having a definite chemical composition. The rocks may be composed of hard materials like granite and marble or soft like chalk, clay and sand.

The main types of rocks are:

1. Igneous rocks or Primary rocks:
   1. Intrusive igneous rocks — For example, granite and dolerite
   2. Extrusive igneous rocks — For example, basalt, pumice
2. Sedimentary rocks — For example, sandstone and limestone
3. Metamorphic rocks — For example, marble and quartzite

When igneous or sedimentary rocks are subjected to excessive heat and pressure inside the earth, changes occur in their colour, hardness, texture and mineral composition. Such changed rocks are called metamorphic rocks.

Two characteristics of metamorphic rocks are:

1. These rocks are very hard and have a high specific gravity.
2. These rocks do not have empty spaces in them.

Mechanical weathering involves disintegration of rocks without the change in their chemical composition. It is mainly brought about by:

1. Changes in temperature
2. Freezing action of water

Freezing action of water — In mountainous regions or in areas with cold climate, the breaking of rocks is caused by the freezing action of water. The cracks and joints in the rocks fill up with water. At night, when the temperature falls, the water in the cracks or joints freezes. The conversion of water into ice increases its volume, thereby requiring more space. This causes widening of cracks and joints and finally leads to their disintegration. This is also called frost action.

Role of plants — The penetration of roots in the rock cracks widens the cracks and disintegrates the rocks. The uprooting of trees by winds also breaks the rocks.

Role of animals — Animals such as worms, rabbits and moles dig burrows in the ground. In the process, they loosen the ground and disintegrate the rocks.

The process of soil formation gives rise to four well-marked horizontal layers in the soil profile. These horizontal layers are called soil horizons. The soil horizons are:

1. Horizon A (topsoil)
2. Horizon B (subsoil)
3. Horizon C (Rock Fragments)
4. Horizon D (Parent Rock)

The intrusive igneous rocks cool below the earth's surface.

Limestone metamorphoses into marble.

Exfoliation is also known as onion peeling.

When water chemically reacts with a mineral, it is called hydration.

Soil formation is a very slow process.

Soil profile is the vertical section of soil from the surface to the parent rock.

1. SANDSTONE — (c) The Wave Rock
2. LIMESTONE — (a) Pamukkale
3. BASALT — (b) Giant's Causeway

Igneous rocks are called primary rocks because these rocks were the first to be formed.

The rocks are heated during the day and cooled during the night. Different minerals present in the rocks expand and contract at different rates according to the changes in temperature. As a result of this unequal expansion and contraction of minerals in the rocks, a series of cracks develop on the outer surface of the rocks.

Lithosphere

Two types of igneous rocks are:

1. Intrusive igneous rocks
2. Extrusive igneous rocks

Sandstone is an example of sedimentary rocks.

True

Two uses of rocks are:

1. In ancient times, rocks were used to make tools and weapons.
2. Rocks are widely used in the construction of roads, houses and buildings.

The decay of rocks as a result of chemical action is called decomposition.

Two factors affecting weathering are:

1. Types of Rocks
2. Climatic conditions

True

The different types of chemical weathering are:

1. Solution
2. Oxidation
3. Hydration
4. Carbonation

Plants, animals and human activities are responsible for biological weathering.

igneous

a metamorphic

geological

carbonation

deforestation

overgrazing