(Source: VII NCERT Geography Chapter 2)
INTERIOR OF THE EARTH:
CRUST:
- The uppermost layer over the earth’s surface is called the crust.
- It is the thinnest of all the layers.
- It is about 35 km. on the continental masses and only 5 km. on the ocean floors.
- The main mineral constituents of the continental mass are silica and alumina. It is thus called sial (si-silica andal-alumina).
- The oceanic crust mainly consists of silica and magnesium; it is therefore called sima (si-silica and ma-magnesium).
MANTLE:
- Just beneath the crust is the mantle which extends up to a depth of 2900 km. below the crust.
CORE:
- The innermost layer is the core with a radius of about 3500 km.
- It is mainly made up of nickel and iron and is called nife(ni– nickel and fe– ferrous i.e. iron).
- The central core has very high temperature and pressure.
The crust
forms only 0.5 per cent of the volume of the earth, 16 per cent consists of the mantle and 83 per
cent makes the core.
The radius of the earth is 6371 km.
ROCKS
The earth’s crust is made up
of various types of rocks.
Any natural mass of mineral matter that makes up the earth’s crust
is called a rock.
Rocks can be of different colour, size and texture.
There are
three major types of rocks: igneous rocks, sedimentary rocks and metamorphic
rocks.
IGNEOUS ROCKS:
- Igneous is derived from Latin word Ignis meaning fire.
- When the molten magma cools, it becomes solid. Rocks thus formed are called igneous rocks.
- They are also called primary rocks.
- There are two types of igneous rocks: intrusive rocks and extrusive rocks.
- Lava is actually fiery red molten magma coming out from the interior of the earth on its surface.
- When this molten lava comes on the earth’s surface, it rapidly cools down and becomes solid.
- Rocks formed in such a way on the crustare called extrusive igneous rocks.
- They have a very fine grained structure. For example, basalt. The Deccan plateau is made up of basalt rocks.
- Sometimes the molten magma cools down deep inside the earth’s crust.
- Solid rocks so formed are called intrusive igneous rocks.
- Since they cool down slowly they form large grains.
- Granite is an example of such a rock. Grinding stones used to prepare paste/powder of spices and grains are made of granite.
SEDIMENTARY ROCKS:
- Sedimentary is derived from Latin word sedimentum meaning settle down.
- Rocks roll down, crack, and hit each other and are broken down into small fragments. These smaller particles are called sediments.
- These sediments are transported and deposited by wind, water, etc. These loose sediments are compressed and hardened to form layers of rocks.
- These types of rocks are called sedimentary rocks.
- For example, sandstone is made from grains of sand.
- These rocks may also contain fossils of plants, animals and other microorganisms that once lived on them.
METAMORPHIC ROCKS:
- Metamorphic is derived from Greek word metamorphose meaning change of form.
- Igneous and sedimentary rocks can change into metamorphic rocks under great heat and pressure.
- For example, clay changes into slate and limestone into marble.
The process of transformation of
the rock from one to another is known as the rock cycle.
Rocks are made up of
different minerals. Minerals are naturally occurring substances which have
certain physical properties and definite chemical composition. Minerals are
very important to humankind. Some are used as fuels. For example, coal, natural
gas and petroleum. They are also used in industries – iron, aluminium, gold,
uranium, etc, in medicine, in fertilisers, etc.