Earth Rocks

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Rocks of Earth

Any aggregate of material particles that forms part of the earth’s crust is called a rock.

There are 3 major types of rock types :

Igneous Rocks

Formed by the solidification of molten magma from the interior of the earth.

Most abundant of the three types of rocks (95%).

They do not occur in layers. Most of them are crystalline and do not contain fossils.

All other types of rocks originate from these rocks, thus called Primary rocks.

They are classified on several grounds as mentioned below:

1. On the basis of mode of occurrence

• Intrusive Igneous Rocks: They are formed by the solidification of magma beneath the earth’s surface. They are further divided into plutonic and hypabyssal igneous rocks. Plutonic rocks cool deep beneath the earth. E.g., Granite. Hypabyssal rocks cool just beneath the earth’s surface. E.g., Batholith, laccolith, phacolith, sills, dykes, etc.
• Extrusive Igneous Rocks: They are formed due to cooling and solidification of hot and molten lava at the earth’s surface. E.g., Basalt, gabbro, etc.

2. On the basis of Silica Content

• Acidic igneous rocks having more silica. E.g. Granite.
• Basic igneous rocks having less silica. E.g. Gabbro.

Sedimentary Rocks

• Made up of weathered remains of igneous rocks. Also contains fossils of plants and animals.
• Comprise only about 5% of the earth’s crust but cover about 75% of the total land surface.
• The layers of sedimentary rocks hold all reserve of coal, oil and natural gas.
• Also known as Stratified Rocks because of the layers.
• Sedimentary rocks fall into three main groups:

1. Mechanically Formed: These are called clastic sedimentary rocks; the sediments are largely derived from pre-existing rocks that have been broken down and then transported by water, wind or ice to form rocks.
2. Organically Formed Rocks: These rocks are derived from remains of plants (e.g. peat, lignite, bituminous coal), or animals (e.g., chalk and coral).
3. Chemically Formed: E.g., Gypsum, salt rock, etc.

Metamorphic Rocks

• Sometimes igneous or sedimentary rocks metamorphize or change due to great ‘pressure, intense temperature or the action of water and chemical activity.
• Examples of metamorphic rocks formed from different rocks are:

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Paper IV [ A ]	Business Aptitude	50	50