Types of Rocks: Igneous, Sedimentary & Metamorphic Rocks

Rocks are naturally occurring solid masses of minerals or mineraloid matter. They are a collection of mineral grains that grow or are forced to be cemented together. Rocks can be found anywhere on earth. Rocks occur in different shapes, colours, strengths, sizes, weights, and textures.

Rocks can be distinguished based on minerals included, the way of their formation and their physical and chemical composition. Rocks are classified into three types based on their origin: igneous rocks, sedimentary rocks and metamorphic rocks.

Key Terms: Rocks, Minerals, Igneous Rocks, Sedimentary Rocks, Metamorphic Rocks, Types of Rocks, Magma, Sedimentation 

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What are Rocks?

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A Rock is a naturally occurring, consolidated material composed of grains of one or more minerals. Rocks have been used since the earliest humans existed, this time was called the 'stone age'. Stones were primarily and extensively used for constructing their homes and early infrastructure.

• A rock is made up of one or more minerals.
• Different types of rocks occur naturally in the earth's crust over a long period of time. Rocks are solid and small (Pebbles), large or medium.
• Rocks gradually change their shape and form over a period of time.
• Rocks are a crucial component and are economically vital. 
• The minerals extracted from rocks are essential and are used in every aspect of our lives.

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Types of Rocks

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Each rock consists of different properties and compositions which makes them different. Each rock is formed due to some physical changes such as melting, cooling, deforming, eroding, and compacting which are the crucial parts of the rock cycle.

According to the Geological classification of rocks, there are three main types of rocks:

1. Igneous Rocks
2. Sedimentary Rocks
3. Metamorphic Rocks

Read More: Difference between Magma and Lava

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Igneous Rock

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Igneous Rocks are rocks which form due to the solidification of melted lava and cooling of magma. These rocks can be formed without involving the crystallisation process. Their primary ingredient is magma. Magma melts down either due to high temperature or low pressure. After that, throughout a long time of the cooling process, large structures of crystals are formed. 

Some examples of igneous rocks are: 

• Granite Rock
• Obsidian Rock 
• Dacite Rock
• Basalt Rock
• Peridotite Rock

Types of Igneous Rocks

There are two subcategories of igneous rocks: 

• Intrusive Igneous Rocks: These rocks are formed or crystallised below the earth's crust and thus being below the crust, they take more time to cool down. For example, Diorite, Granite etc. 
• Extrusive Igneous Rocks: These rocks are formed when magma gets onto the surface in tiny crystals because the cooling process is much faster. Due to faster cooling, some rocks form an amorphous glass. Pumice and basalt are examples of extrusive igneous rock.

Uses of Igneous Rocks

• After polishing granite it is used as countertops and in constructing buildings.
• Basalt is used as an ingredient in cement.
• Pumice is used in lightweight construction.

Read More: Difference between Rocks and Minerals

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Sedimentary Rocks

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Sedimentary rocks are formed by deposition and consecutive cementation of a particular material within water bodies and upon the surface of the earth. This process of settling down minerals in a place is called sedimentation. Sedimentary rocks are formed from sediments of pre-existing rocks. The sedimentary rock formation is quite a lot different from igneous rocks and contains a large variety of minerals in them.

Some common sedimentary rocks are: 

• Halite
• Limestone
• Sandstone
• Siltstone

Types of Sedimentary Rock

The three types of sedimentary rocks are as follows: 

1. Clastic Sedimentary Rocks: They are formed from mechanical weathering debris. Some examples are sandstone and siltstone.
2. Chemical Sedimentary Rocks: They are formed from the dissolved materials which precipitate from the solution. For instance, iron ore and limestones.
3. Organic Sedimentary Rocks: They are formed from the accumulation of plant and animal debris. For example, coal and some dolomites are some organic sedimentary rocks.

Uses of Sedimentary Rocks

• Limestone is used in cement.
• Salt is the most common mineral used in every kitchen and household.
• Crude oil, natural gas and coal all are extracted from sediments.

Read More: Iron Extraction Blast Furnace Metallurgy

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Metamorphic Rocks

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Metamorphic Rocks emerge from either igneous or sedimentary rocks. These rocks are formed due to the transformation of existing rock types, in a process called metamorphism (change in form). In this process the original rock is subjected to high temperatures and the great pressure of the rock layers above it, causing profound physical and/or chemical change.

Uses of Metamorphic Rocks

• Marble is a fascinating rock used for making statues and antique items like vases.
• Schist and slate are used in buildings and sometimes as landscape materials.
• Ground-up marble is an ingredient used in toothpaste, paper etc.

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Things to Remember 

• Rocks are naturally occurring consolidated materials composed of one or more minerals. 
• Different rocks have different chemical compositions, mineralogy, grain size, texture, and other distinguishing characteristics. 
• There are three types of rocks- Igneous Rocks, Sedimentary Rocks and Metamorphic Rocks.
• Igneous rocks are formed by the cooling and solidification of magma or lava. 
• Sedimentary rocks are formed due to the deposition of material within bodies of water and at the surface of the earth.
• Metamorphic rocks are formed from existing rock types when subjected to extremely high temperature and pressure.