In this Article, We would like to study about Rocks and it’s type.All those substances found on the earth’s crust that are not metals are called rocks, whether it is soft like clay or hard like granite. The number of elements found in the upper part of the earth’s (i.e Crust) are 110 But only 8 elements make the crust.The contribution of these 8 elements in making the crust is around 98.6%.
The table given below represents these elements:
| S.N | Elements | Percentage |
| 1. | Oxygen | 46.8 |
| 2. | Silica | 27.7 |
| 3. | Aluminium | 8.13 |
| 4. | Iron | 5.0 |
| 5. | Calcium | 3.63 |
| 6. | Sodium | 2.83 |
| 7. | Potassium | 2.59 |
| 8. | Magnesium | 2.09 |
Types of rocks
1. Igneous rock
2. Sedimentary rock
3. Metamorphic rocks
1. Igneous rock
The Liquid that come out of the Earth during a volcano eruption in the form of magma are called igneous rocks. At the time of the eruption of the volcano, the magma that comes out of the earth after cooling down on the surface ,gets converted into igneous rock. These rocks were the first to be formed after the origin of the earth. These are also called primary rocks. Fossil fuels are not found in them.
Examples of Igneous rocks with their silica content are given in the table below.
| S.N | Rocks | Silica content |
| 1. | Granite rocks | 73% |
| 2. | Basalt rocks | 53% |
1.1 Types of Igneous rocks
1.1.1 Intrusive rocks
The magma cools down in the earth’s interior and becomes solid is called intrusive rocks.
When magma cools down deep in the earth and becomes solid, then it is called underground igneous rock. When the magma cools down on its way without reaching the surface, it is called an intermediate igneous rock.
Example – Granite (Most of the continental crust is made up of it)
1.1.2 Extrusive rocks
When the magma comes to the surface and cools down during the eruption of a volcano, it is called extrusive rock.
Example – Basalt rocks (Found on the ocean floor)
2. Sedimentary rock
Rocks are broken into small pieces by rolling, cracking and colliding with each other. These sediments get deposited by reaching from one place to another through various medium like air, water, etc. These depressions harden under pressure to form a layer of rock and these types of rocks are called sedimentary rocks.
Sedimentary rocks are formed when another type of rock, such as Igneous rock or Metamorphic rock, is broken down and deposited layer by layer.
Properties of Sedimentary Rocks:
i) These are layered.
ii) These are soft and porous.
iii) Fossils are found in it.
iv) Coal is found in it.
v) Petroleum substances are found in it.
These rocks are also called layered rocks. The remains of animals and plants get buried in these rocks which convert into fossil fuel with time, hence fossil fuels are found in this types of rocks. Sedimentary rocks are found on 75% of the entire earth’s crust and it is the most important rock from the commercial point of view Because fossil fuels, coal, marble, salt, petroleum, and stone chosen for making cement, etc. can be obtained from this rock.
Examples
Clay and shell, Limestone, Chalk and dolomite, Sandstone, Conglomerate
3. Metamorphic rocks
Due to high temperature and pressure, igneous and sedimentary rocks changes with time are called metamorphic rocks. The hardness of these Rocks is the highest and fossils are absent in this Rocks.
These rocks are formed by the effect of heat, pressure or chemical reaction with other types of rocks.They can be formed from igneous rocks or can be formed from sedimentary rocks or can also be formed from metamorphic rocks. Metamorphic rocks are formed when one type of rocks changes into another type of rocks as a result of intense heat, high pressure or chemical reaction.
One point to be noted here is that “Metamorphic rocks can be metamorphosed again”.
Example of Metamorphic rocks
Due to the effect of pressure, heat or chemical reaction, granite gets converted into gneiss rock.
By the effect of pressure, heat or chemical reaction, sandstone gets converted into quartzite.
Example
- Gneiss rocks are obtained from Igneous rocks. When igneous rock is subjected to high temperature and pressure, it is called gneiss rock.
- Amphibolite rocks and Schist rocks are obtained from Basalt rocks.
The list of rocks that have changed from sedimentary rocks to metamorphic rocks is given in the table below.
| S.N | Sedimentary Rocks | Metamorphic Rocks |
| 1. | Clay and shell | Slate |
| 2. | Limestone | Marble |
| 3. | Chalk and dolomite | Marble |
| 4. | Sandstone | Quartzite |
| 5. | Conglomerate | Quartzite |
Q1. Out of Igneous Rocks , Sedimentary Rocks and Metamorphic Rocks which is found in maximum quantity in the earth’s crust?
Answer : Igneous Rock