CLASS 7TH | GEOGRAPHY | TYPES OF ROCKS | REVISION NOTES

What are Rocks?

Rocks are mineral aggregates with a combination of properties of all the mineral traces. Any unique combination of chemical composition, mineralogy, grain size, texture, or other distinguishing characteristics can describe rock types. Additionally, different classification systems exist for each major type of rock. There are different types of rocks existing in nature.

Rocks which are found in nature rarely show such simple characteristics and usually exhibit some variation in the set of properties as the measurement scale changes.

Types of RocksTypes of Rocks in India

Types of Rocks

There are three types of rocks:

• Igneous Rocks
• Sedimentary Rocks
• Metamorphic Rocks

Igneous Rock

Igneous rock is one of the three main rock types. Igneous rock is formed through the cooling and solidification of magma or lava. Igneous rock may form with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks.

This magma can be derived from partial melts of existing rocks in either a planet’s mantle or crust. Typically, the melting is caused by one or more of three processes: an increase in temperature, a decrease in pressure, or a change in composition.

Types of Igneous Rock

Following are the two types of igneous rock:

1. Intrusive igneous rock: These rocks crystallize below the earth’s surface resulting in large crystals as the cooling takes place slowly. Diorite, granite, pegmatite are examples of intrusive igneous rocks.
2. Extrusive igneous rock: These rocks erupt onto the surface resulting in small crystals as the cooling takes place quickly. The cooling rate is for a few rocks is so quick that they form an amorphous glass. Basalt, tuff, pumice are examples of extrusive igneous rock.

Igneous Rock Examples

Basalt	Diorite
Granite	Mica and quartz

Sedimentary Rock

The sedimentary rocks are formed by the deposition and subsequent cementation of that material within bodies of water and at the surface of the earth. The process that causes various organic materials and minerals to settle in a place is termed as sedimentation.

The particles that form a sedimentary rock by accumulating are called sediment. Before being deposited, the sediment was formed by weathering and erosion from the source area and then transported to the place of deposition by water, wind, ice, mass movement or glaciers, which are called agents of denudation. Sedimentation may also occur as minerals precipitate from water solution or shells of aquatic creatures settle out of suspension.

Types of Sedimentary Rock

Following are the three types of sedimentary rock:

1. Clastic sedimentary rocks: These rocks are formed from the mechanical weathering debris. Sandstone, siltstone are examples of clastic sedimentary rocks.
2. Chemical sedimentary rocks: These rocks are formed from the dissolved materials that precipitate from the solution. Iron ore, limestones are examples of chemical sedimentary rocks.
3. Organic sedimentary rocks: These rocks are formed from the accumulation of plant and animal debris. Coal, some dolomites are examples of organic sedimentary rocks.

Sedimentary Rock Examples

Halite	Limestone
Sandstone	Siltstone

Metamorphic Rocks

The metamorphic rocks make up a large part of the Earth’s crust and are classified by texture and by chemical and mineral assemblage. They may be formed simply by being deep beneath the Earth’s surface, subjected to high temperatures and the great pressure of the rock layers above it.

Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism, which means “change in form”. The original rock is subjected to heat with temperatures greater than 150 to 200°C and pressure around 1500 bars, causing profound physical and/or chemical change.

Types of Metamorphic Rock

Following are the two types of metamorphic rock:

1. Foliated metamorphic rocks: These rocks are produced by the exposure to heat and pressure which makes them appear layered. Phyllite, gneiss are examples of foliated metamorphic rocks.
2. Non-foliated metamorphic rocks: These rocks don’t have layers. Marble, quartzite are examples of non-foliated metamorphic rocks.

Metamorphic Rock Examples

Marble	Quartzite
Slate	Phyllite

Types of Rocks in India

Following are the classification of rocks in India:

1. Rocks of the Archaean system: These rocks get this name as they are formed from the hot molten earth and are the oldest and primary rocks. Gneiss is an example and is found in Karnataka, Andhra Pradesh, Tamil Nadu, Madhya Pradesh, Orissa and some parts of Jharkhand and Rajasthan.
2. Rock of Dhawar system: These are formed from the erosion and sedimentation of the Archaean system and are the oldest sedimentary rocks. These are mainly found in Karnataka.
3. Rocks of Cuddapah system: These are formed from the erosion and sedimentation of Dhawar system. Sandstone, limestone and marble asbestos are the examples and are mainly found in Rajasthan.
4. Rocks of the Vindhyan system: These are formed from the silt of river valleys and shallow oceans. Red sandstone is an example and is mainly found in Madhya Pradesh.
5. Rocks of Gondwana system: These are formed from the depressions in the basins. Coal is an example and is mainly found in Madhya Pradesh.
6. Rocks of Deccan trap: These are formed from the volcanic eruption. Dolorite and basalt are examples and are mainly found in Maharashtra and parts of Gujarat, Tamil Nadu and Madhya Pradesh.
7. Rocks of Tertiary system: These rocks are found mainly in the Himalayan regions.
8. Rocks of the Quarternary system: These rocks are found in the plains of the Indus and Ganga.