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• Guttenberg Discontinuity : It separates mantle and core. This is named after the world famous
geologist Guttenberg.
• Mohorovicic Discontinuity : It is a layer found in semi molten state between the crust and
mantle. It is named after the famous geologist Mohorovicic.

matErials of thE Earth’s Crust

All solid materials of the earth’s crust, whether hard

or soft, that make up the earth’s crust are called
Rock. They are of different size, texture and colours.
Rocks are generally solid mass but also include loose

materials such as sand, soil, silt.

Minerals are substances which occur in rocks. The
main characteristics are as follows:
• They have a definite chemical composition.
Each mineral contains two or more simple Rocks and Minerals
substances called elements.
• They are non-living solid substances.

• They provide us metals and chemicals.
• A rock in which a particular mineral is found in large percentage is called an ORE.

On the basis of formation, the rocks are classified into three types : igneous, sedimentary
and metamorphic.

1. igNEous rocks
The oldest type of all rocks is the igneous rock. The word
“igneous” has been derived from a Greek word which

means “fire”. These rocks are formed by the cooling and
solidification of molten rocks. When a volcano erupts,
molten rock called magma comes out on the earth’s
surface and when the magma cools and solidifies, it
leads to the formation of igneous rocks. Examples are :
Intrusive lgneous Rock granite, basalt, pumice, obsidian and flint.

Igneous rocks are also called primary rocks. There are two types of igneous rocks :

Intrusive rocks : All igneous rocks do not cool the same way. Some cool slowly, deep under
the earth’s surface. These are called intrusive igneous rocks. Granite is an example of a
rock that cools slowly and has large crystals.

Extrusive rocks : Other rocks formed when the magma erupted from a volcano or reached
the earth’s surface through long cracks. Magma is called lava when it reaches the earth’s
surface. Lava cools quickly and forms rocks with small crystals. They are called extrusive
igneous rocks. Basalt, obsidian are the examples of this type of rock.

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