GATEWAY TO CGL MAINS: GEOGRAPHY STUDY NOTES

ROCKS AND MINERALS

About 98 per cent of the total crust of the earth is composed of eight elements like oxygen, silicon, aluminium, iron, calcium, sodium, potassium and magnesium, and the rest is constituted by titanium, hydrogen, phosphorous, manganese, sulphur, carbon, nickel and other.

1) The three types of rocks are

a) igneous rocks (formed directly from liquid rock),

b) metamorphic rocks (formed by direct alteration of existing rocks), and

c) sedimentary rocks (formed by eroded materials from other rocks).

i. Igneous Rocks

1) Igneous rocks solidify from a liquid magma as it cools. When magma cools rapidly, mineral crystals do not have time to grow very large. On the other hand when magma cools slowly crystals grow to several millimeters or more in size.

Granite and basalt are the examples of IR. Igneous rocks are classified as

a) Extrusive Rocks

Extrusive igneous rocks solidify from molten material that flows over the earth’s surface (lava).

Common extrusive rocks are

i)    basalt,

ii)   andesite, and

iii)   rhyolite.

b) Intrusive Rocks

Intrusive rocks form from molten material (magma) that flows and solidifies underground.

Common rock types within the intrusive category are granite and diorite.

ii. Sedimentary Rocks

These are types of rocks created from deposition of layers upon layers of sediments over time. These types of rocks are formed on the Earth's surface, as well as underwater.

Examples – Sandstone, limestone, stromatolites , oil shale and coal shale, gypsum, shale, and conglomerate.

iii. Metamorphic Rocks

Metamorphic rocks are any rock type that has been altered by heat, pressure, and/or the chemical action of fluids and gases.  When igneous rocks, or sedimentary rocks, or even metamorphic rocks get buried very deep under the earth's surface, a process that takes millions of years, they get changed into something else by the enormous pressure and heat inside the earth.

“Some examples of metamorphic rocks are:

•     Limestone being changed into marble     

•     Shale turning into slate

•     Granite being changed into gneiss          

•     Sandstone turning into quartzite