This article is about the Rocks and their types (Igneous, Sedimentary & Metamorphic Rocks), and distribution of rocks in India. In this article, you will get to read about the definition of rocks, their types, their formation etc. This article is very important from the examination point of view.
Also Check: General Science MCQs
Rocks are an aggregation of one or more minerals held together by chemical bonds. It is a non living, naturally generated earth substance.
A mineral is a naturally occurring organic and inorganic substance, having an orderly atomic structure with a definite chemical composition and physical properties.
Important Facts:
- Earth’s surface is composed of rocks and about 98% of the earth’s crust is composed of oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium.
- The most dominant mineral in the earth’s crust is oxygen, while the most dominant mineral in the entire earth is iron.
Classification of rocks
On the basis of their hardness, the rocks are classified into two categories, viz hard rocks and soft rocks.
- The rocks which are resistant to erosion are known as hard rocks, while those that can be eroded easily are known as soft rocks.
- Though the strength is relative, still in general, igneous and metamorphic rocks are more resistant or hard rocks, and sedimentary rocks are less resistant.
On the basis of their formation, the rocks can be classified into three categories. They are;
- Igneous rocks
- Sedimentary rocks
- Metamorphic rocks.
These rocks are discussed in detail below;
1. IGNEOUS ROCKS
- These are the parent rocks or primary rocks because they were the first rocks formed during the cooling and solidification of magma and lava.
- They are crystalline in nature, and the size of the crystals in them depends upon the rate of cooling.
- If the rate of cooling is faster, the time for the formation of crystals is less, and therefore, they have small, fewer, or no crystals (e.g., Basalt).
- On the other hand, if the rate of cooling is very slow, then there is sufficient time for the development of crystals, and hence, the size of the crystals is larger (e.g., Intrusive rocks like Granite, Gabbro).
On the basis of place of formation, they are classified into:
- Extrusive igneous rocks; If they are formed outside the surface of earth.
- Intrusive igneous rocks; If they are formed under the surface of earth.
The intrusive igneous rocks are further classified into hypabyssal igneous rocks, which found nearer to the earth’s surface, and plutonic igneous rocks, which are found deep inside the earth.
On the basis of chemical properties, igneous rocks are of two types:
- Acidic igneous rocks: these have very high silica content (about 65% to 85%). These are hard, compact, massive and resistant to weathering. Examples are : Granite , Quartz and feldspar.
- Basic igneous rocks: The silica content in such rocks is less than 40%, and magnesia content is about 40%. These rocks are not so hard and are weathered relatively easy. Examples are Basalt, Gabbro etc.
Important Characteristics of Igneous rocks;
- They do not contain fossils, as high temperature destroys the fossil.
- They are found in massive forms and structures such as mountains, hills. These are compact rocks, hard rocks, and therefore not easily eroded or weathered.
SEDIMENTARY ROCKS
- Original rocks, due to external agents, are broken down to form sediments.
- These sediments may be transported and/or deposited by external agents.
- Over the period of time, these sediments are compacted and cemented to form sedimentary rocks.
- This process of formation of sedimentary rocks is known as lithification.
Classification of Sedimentary rocks;
On the basis of their origin, the sedimentary rocks can be classified into the following three categories;
- Physically formed or Clastic Sedimentary rocks.
- Chemically formed Sedimentary rocks.
- Organically formed Sedimentary rocks.
Physically formed Sedimentary rocks.
- They are formed due to mechanical force induced disintegration of existing rocks. The mechanical forces are running water, eind, ocean currents, ice etc.
- The sediments are broken and then compacted to form clastic sedimentary rocks.
- Examples; sandstone, clay, shale, loess
LOESS
- Very fine sedimentary rock which is rich in lime and yellowish in colour is known as loess.
- Loess is a very weak rock and hence it is easily eroded.
- Winds carry large amounts of loess and deposit them in the surrounding regions.
- Loess, being a weak rock, with high porosity, is easily eroded by the rivers, and hence the regions having loess deposits are known for rivers shifting their courses and badland topography.
- Example of regions with Loess;
- Yellow River Region
- Large amount of loess from Gobi desert is deposited in Huangho river region. Therefore, Huangho is known for flooding, shifting of the course and is known as sorrow of the China.
Chemically formed Sedimentary Rocks
- Formed by decomposition of existing rocks.
- The minerals which are soluble in water are carried by the water and deposited in some other place to form the chemically formed Sedimentary rocks.
- The Chemically formed Sedimentary rocks are formed either due to the process of:
- Solvation: interaction of solvent with the solute particles dissolved in it.
- Carbonation: The process of carbonation occurs in calcium-rich rocks. Atmospheric carbon dioxide reacts with water forming weak carbonic acid which then dissolves calcium-rich rocks or sediments.
- e.g., limestone, gypsum
- Oxidation: It occurs in the rock rich in iron. These rocks undergo rusting especially in tropical and subtropical regions. Therefore, the rocks develop the reddish-brown covering over crust.
- ex: Common type of process in India
Organically formed Sedimentary Rocks
- The rocks that have been derived due to physical disintegration or chemical decomposition of organic sediments are known as organically formed sedimentary rocks.
- The remains of plants and animals are buried under sediments and due to intense heat and pressure from the overlying layers, their composition changes.
- If the rocks are rich in carbon, it will result in the formation of coal.
- e.g.,
- Limestone: The bones of the dead and decomposed organic matter of the animals rich in calcium when they are buried over the period of time.
- Chalk: A soft rock which is rich in calcium carbonate
Characteristics of Sedimentary Rocks:
- They are arranged in layers, i.e., stratified, and the layers are known as bedding planes of sedimentary rocks.
- They hold the most valuable information about the geological records and are fossiliferous (containing fossils) in nature.
- They are less resistant to erosion, so they are found in smaller forms and deposited in faraway lands in the form of soil.
- They cover the largest surface area of the globe or the earth, but they account for only 5% of the mass of the earth’s crust.
- Sedimentary rocks are economically important because they provide food resources, energy resources, as well as various industrial raw materials, because the sedimentary rocks are permeable, surface water percolates, and these regions are rich in ground water.
3. METAMORPHIC ROCKS
- Metamorphosis = change in form.
- Metamorphic rocks are the rocks formed due to the transformation of original rock as a result of temperature and pressure changes.
- When rocks are forced down to lower layers by tectonic processes or when molten magma rising through the crust comes in contact with the crustal rocks, or the underlying rocks are subjected to huge pressure by the weight of the overlying rocks, metamorphosis occurs.
- During the course of metamorphosis, some rocks develop an arrangement of grains or minerals, which is known as foliation or lineation.
- The metamorphic rocks showing foliation are called foliated rocks and the ones not showing it are known as non-foliated rocks.
- Metamorphic rocks are hard rocks and hence similar to the igneous rocks. They occur in massive forms.
Types of Metamorphism shown by Metamorphic rocks
- Thermal / Contact Metamorphism
- Thermal metamorphism is caused due to high temperature.
- During volcanic eruption when magma rises upward, it changes the rocks which come in contact with it into a new metamorphic rock.
- Example: Limestone → Marble, Coal → Diamond (carbon)
- Dynamic / Regional Metamorphism
- It occurs due to intense pressure.
- In the area of fold mountains, regional metamorphism is common because of great pressure exerted by the plates.
- Example: limestone —-> marble
The table given below show which rock changes into which kind of metamorphic rocks under the effect of pressure and temperature.
| Igneous/sedimentary rock | Agent | Metamorphic rock |
|---|---|---|
| Granite | Pressure | Gneiss |
| Clay, Shale | Pressure | Schist |
| Sandstone | Heat | Quartzite |
| Clay, Shale | Heat | Slate or Phyllite |
| Coal | Heat | Anthracite or Graphite |
| Coal | Heat & Pressure | Diamond |
| Limestone | Heat | Marble |
Characteristics of Metamorphic rocks
- Metamorphic rocks. do not contain fossils due to high temperature & pressure.
- Metamorphic rocks are economically most important because they contain the storehouse of minerals, especially metallic minerals such as iron ore, gold, copper, manganese, etc.
Distribution of rocks in India
- The peninsular plateau has the oldest landmass in the world, and therefore the rocks are metamorphic, i.e., Archean gneiss and schists.
- The Deccan lava plateau has basaltic rock due to the Reunion hotspot.
- Himalayas are made up of gneiss and schists due to intense folding.
- The sediments brought down by the Himalayan rivers were deposited to give rise to the Great Plains of India stretching between the Indus to Brahmaputra.
So, this was all about the Rocks: Igneous, Sedimentary & Metamorphic Rocks, and distribution of rocks in India. We are sure that you have loved reading this post.