Chapter 2 — Inside Our Earth

The three layers are the crust (outermost and thinnest), the mantle (extends up to 2900 km below the crust), and the core (innermost, with a radius of about 3500 km).

Sial refers to the continental crust, which is mainly made of silica and alumina. Sima refers to the oceanic crust, which mainly consists of silica and magnesium.

The crust is about 35 km thick on continental masses and only about 5 km thick on ocean floors.

Nife is the name for the earth's innermost core. It is called nife because it is made of nickel (ni) and iron (fe, from the Latin word ferrous). The core has a radius of about 3500 km.

Igneous rocks (also called primary rocks) are formed when molten magma cools and solidifies. When lava cools rapidly on the surface, fine-grained extrusive rocks like basalt form. When magma cools slowly deep inside the crust, large-grained intrusive rocks like granite form.

Extrusive igneous rocks form when lava cools rapidly on the earth's surface; they have a very fine grained structure (e.g., basalt — the Deccan Plateau is made of basalt). Intrusive igneous rocks form when magma cools slowly deep inside the crust, forming large grains (e.g., granite, used for grinding stones).

Rocks break down into small fragments called sediments. These sediments are transported and deposited by wind and water, then compressed and hardened over time into layers of sedimentary rocks. Sandstone is an example, formed from grains of sand.

Fossils are the remains of dead plants and animals trapped in the layers of rocks. Sedimentary rocks often contain fossils of plants, animals, and other microorganisms.

Metamorphic rocks are formed when igneous or sedimentary rocks change under great heat and pressure. For example, clay changes into slate, and limestone changes into marble.

The rock cycle is the continuous process by which rocks transform from one type to another. Molten magma cools to form igneous rocks; these break down into sediments that form sedimentary rocks; heat and pressure convert them into metamorphic rocks; metamorphic rocks under extreme conditions melt back into magma, and the cycle continues.

To reach the centre of the earth, one would need to dig a hole 6000 km deep on the ocean floor, which is not possible. The deepest mine in the world (in South Africa) is only about 4 km deep, and the deepest oil drilling hole reaches only about 6 km.

The crust forms only 1% of earth's volume, the mantle makes up 84%, and the core makes up 15%.

Minerals are naturally occurring substances with certain physical properties and definite chemical composition. They are used as fuels (coal, natural gas, petroleum), in industries (iron, aluminium, gold, uranium), in medicine, and in fertilisers.

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