WHAT ARE ROCKS?
All the hard or soft materials of the earth’s crust are termed as rocks. A rock is an aggregate of one or more minerals. Though there are about 110 elements found on the earth crust, yet about 98% of it is composed of 8 major rock-forming elements namely; Oxygen (47%), Silicon (28%), Aluminium (8%), Iron (5%), Calcium, Sodium, Potassium and Magnesium.
Rock may be hard or soft and in varied colours. For example, granite is hard, soapstone is soft. Gabbro is lack and quartzite can be milky white. Rocks do not have a definite composition of mineral constituents. Feldspar and quartz are the most common minerals found in rocks.
On the basis of formation, rocks are classified into:
1. Igneous rocks
2. Sedimentary Rocks
3. Metamorphic Rocks
These rocks are formed by cooling, solidification and crystallization of hot and molten magma found below the earth crust. These are granular and crystalline rocks. As igneous rocks form out of magma and lava from the interior of the earth, they are known as primary rocks. Igneous rocks are much less affected by chemical weathering but physical weathering affected them resulting in their disintegration and decomposition. About 90% of the crust is made up of igneous rocks.
On the basis of the mode of occurrence and structure, igneous rocks are classified into:
A. Intrusive Igneous Rocks B. Extrusive Igneous Rocks
Intrusive Igneous Rocks: It is formed when hot and molten magma gets solidified below the surface of the earth. These are of two types namely; plutonic and hypabyssal.
(i) Plutonic Intrusive Igneous Rocks: These are formed due to the cooling of magma very deep inside the earth. Due to excessive slow rate of cooling, these rocks have very large granules. Granite rocks are an example of such rocks.
(ii) Hypobyssal Intrusive Igneous Rocks: These are formed due to cooling and solidification of rising magma during volcanic activity in the cracks, pores, crevices and hollow spaces just below the earth’s surface. Dolerite and magnetite are examples of such rocks. Major forms of these rocks are Laccoliths, Phacoliths, Lopoliths, Batholiths, Sills, Dykes, etc.
Extrusive Igneous Rocks: These rocks are formed due to cooling and solidification of hot and molten lavas at the surface of the earth. These are also called volcanic rocks. These rocks have very small granules. Basalt is a good example of this type of rocks.
These rocks are formed on the surface of the earth due to the erosion and deposition of igneous and metamorphic rocks. Sedimentary rocks are layered or stratified rocks. Organic matters also play an important role in the formation of these rocks. These rocks are found over about 75% area of the crust. However, they contribute only 5% in the formation of the crust. These rocks contain fossils.
On the basis of the nature of the sediments, sedimentary rocks are classified into:
(i) Mechanically formed sedimentary rocks, such as; sandstones, conglomerates, clay rocks, shale and loess.
(ii) Organically formed sedimentary rocks, such as; limestone, coal and peat.
(iii) Chemically formed sedimentary rocks, such as; chalk rocks, gypsum and salt rock.
These rocks are formed due to complete alternation in the appearance and constitution of pre-existing rocks due to change in mineral composition and texture through temperature and pressure. These rocks are the hardest rocks and do not contain fossils.
Formation of Metamorphic Rocks:
(i) Metamorphic Rocks are formed through the sedimentary rocks. For example:
- Slate- from Shale
- Marble- from Limestone, Chalks and Dolomite
- Quartzite- from Sandstone and Conglomerate
(ii) Metamorphic Rocks are formed through the igneous rocks. For example:
- Gneisses- from Granites
- Amphibolites- from Basalt
- Schist- from Basalt
(iii) Metamorphic Rocks are formed by the metamorphosis of metamorphic rocks. For example:
- Phyllite- from Slate
- Schist- from Phyllite
- Serpentine- from Gabbro