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Types of Rocks

A rock is a very popular term and one of the crucial parts of the earth. They are present worldwide in different climates and conditions, either dry, wet, cold or hot. The properties of Rocks vary depending on the biological activities and natural variations in various areas.

In this article, we are discussing the most widely present types of rocks. We also explain all the essential types and examples of each primary type. Before discussing the different rock types, let's understand the definition of rocks and how they are formed.

What are Rocks?

In geology, rock is a formation of the naturally occurring and successive collection of one or more minerals grains. The mineral grains in a rock can be so small that we can only see them with a microscope, or they can be as large as our fingernail or even finger. Rock consists of the basic units from which a solid Earth is formed. Rocks typically form recognizable and mappable volumes. Also, they are solid non-living mineral aggregates.

A unique arrangement of naturally occurred chemical composition, texture, grain size, mineralogy or other specific properties can form different rock types. There are wide varieties of rocks in nature, and different classification systems are used for each major type of rock. Different colors and textures seen in rocks are usually found based on the presence of different minerals. They are mainly identified by existing minerals and their texture (describing the shape, size, and arrangement of mineral grains).

Types of Rocks

Rocks are broadly classified into the following three categories:

  • Igneous Rocks
  • Sedimentary Rocks
  • Metamorphic Rocks
Types of Rocks

Each type of rock is a part of a rock cycle. The various physical and chemical changes in one rock type conditions can lead to another rock type development. Additionally, it can also form an entirely new and different rock of the same type. Generally, the different types of rocks are defined depending on how they are created or formed. Let's understand more about each type:

What are Igneous Rocks?

Igneous rocks are one of the three primary rock types. Volcanoes are the main source of formation for igneous rocks. Volcanoes generally spew out hot molten rock deep within the earth, or even as lava on the surface. When the magma cools down, it gets harder and forms igneous rock. Besides, a shiny and glassy rock is formed by cooling and freezing of lava. The rocks may also contain a few holes or spaces caused by bubbles trapped within the rocks during the cooling process. In this process, no crystals are formed. However, these types of rocks can form with or without crystallization.

The magma can be obtained from the partial melts of existing rocks in a crust or planet's mantle. Generally, there are three main reasons for melting: an increase in temperature, the change in a composition, or reduced pressure. The chemical composition of magma and the rate at which it cools determine which type of rock is formed when minerals cool and crystallize.

Examples of Igneous Rock

The most common examples of Igneous Rock include basalt, obsidian, granite, diorite, mica and quartz.

Types of Igneous Rock

Igneous Rocks are mainly classified as Intrusive and Extrusive:

  • Intrusive Igneous Rock: Intrusive Igneous Rocks crystallize below the surface of the earth. Because cooling occurs slowly for these crystals, the crystals grow larger. Some common examples of these rocks include granite, pegmatite, and diorite.
  • Extrusive Igneous Rock: Excessive rocks burst on the surface of the earth that produce small crystals. Besides, cooling is faster for these crystals. The cooling rate for some rocks is so high that they generate amorphous glasses. Some common examples of these rocks are pumice, basalt, and tuff.

What are Sedimentary Rocks?

Sedimentary rocks are the second type of rocks on the list. The accumulation of sediments generally forms these rocks. The sediments refer to a composition of various particles, such as pebbles, shells, sand, and other fragments of material. The process that helps different organic substances and minerals to settle in a place is known as sedimentation. In particular, sedimentary rocks are softer rock types and may easily break apart or crumble easily. We can often notice pebbles, sand, or stones in these rocks, and they are hardly the only type that includes fossils.

The sediments were formed due to the weathering and erosion of existed rock types. These exited rock particles were transported to larger water bodies by wind, rivers, ice, glaciers, or mass movements. The pieces of rocks settled at the surface over a long period (millions of years). The solid materials, solid rocks, were left behind after liquid evaporation. Additionally, Sedimentary rocks also contain several chemical precipitates. Sedimentation helps the rocks to form minerals that are precipitated by water solutions or shells of aquatic organisms.

Examples of Sedimentary Rock

The most common examples of Sedimentary Rock include conglomerate, limestone, halite, siltstone, and sandstone.

Types of Sedimentary Rock

The Sedimentary Rocks are mainly classified into the following three types:

  • Classic Sedimentary Rock: Mechanical weathering debris led to the formation of classic sedimentary rocks. Examples of these rocks are sandstone and siltstone.
  • Chemical Sedimentary Rock: Dissolved materials precipitated from the solution formed chemically sedimentary rocks. Examples of these rocks are limestone and iron ore.
  • Organic Sedimentary Rock: Organic sedimentary rocks formed due to the accumulation of plant and animal debris. Examples of these rocks include coal and some types of dolomite.

What are Metamorphic Rocks?

Metamorphic rocks are types of rocks that are formulated deep below the earth's surface. They are formed when the existent rocks' minerals are replaced by intense heat and pressure (squeeze) within the earth. The mineralogy, texture, and chemical composition of existed rocks change with exposure to these extreme conditions. Metamorphic rocks mainly originate from other types of existing rocks. For example, a sedimentary rock named shale can be metamorphosed (changed) into a metamorphic rock like gneiss or slate.

In particular, the existent rock is subjected to heat at higher temperatures of about 150 to 200 ° C with a pressure of about 1500 bar, causing profound chemical and physical properties changes. Rocks resulting from these extent changes often contain ribbon layers and contain shiny crystals, formed by minerals that grow slowly over time on their surface.

Examples of Metamorphic Rock

The most common examples of Metamorphic Rock include marble, gneiss, slate, phyllite and quartzite.

Types of Metamorphic Rock

The Metamorphic Rocks are mainly classified into the following two types:

  • Foliated Metamorphic Rock: Foliated metamorphic rocks are formed due to exposure to heat and pressure. These conditions made these rocks layered. Examples of these rocks include gneiss and phyllite.
  • Non-foliated Metamorphic Rock: Unlike Foliated metamorphic rocks, where rocks are layered, non-foliated metamorphic rocks don't have layers.

Types of Rocks in India

India has a wide variety of rocks. They can be classified as below:

Rocks of the Archaean System

The rocks of the Archaean system are formed from hot molten earth. They are considered to be the primary and oldest types of rocks. Gneiss is an example of this type of rock. Such rocks are most commonly found in Orissa, Karnataka, Andhra Pradesh, Madhya Pradesh, and Tamil Nadu. Also, such rocks are present in some places of Rajasthan and Jharkhand.

Rocks of the Dharwar System

The rocks of the Dharwar system are usually formed by erosion and sedimentation of the Archaean system. These types of rocks are considered to be the oldest types of sedimentary rocks. Such rocks are mostly found in Karnataka.

Rocks of the Cuddapah System

The rocks of the Cuddapah system are most commonly found in Rajasthan. These rocks are formed due to the erosion and sedimentation of the Dharwar system. Some examples of these rocks include limestone, sandstone and marble asbestos.

Rocks of the Vindhyan System

The rocks of the Vindhya system are mainly found in Madhya Pradesh. These rocks are mainly formed by the silt of river valleys and shallow oceans. Red sandstone is a prime example of this type of rock.

Rocks of the Gondwana System

The rocks of the Gondwana system are primarily found in Madhya Pradesh. These rocks are formed due to the depressions in the basins. Coal is a prime example of this type of rock.

Rocks of the Deccan Trap

The rocks formed because of the volcanic eruption are known as rocks of the Deccan Traps. These rocks are mainly found in Maharashtra. Also, such rocks are present in some places of Madhya Pradesh, Tamil Nadu and Gujarat. Some examples of this type of rock are basalt and dolerite.

Rocks of the Tertiary System

The formation of this type of rock is considered to be about 66 million years back. Specifically, these rocks belong to the Cenozoic era. These rocks are most commonly found in some parts of the Himalayan regions.

Rocks of the Quarternary System

These rocks are mainly found in the plains of the Indus and Ganga. These rocks were formed during the Pleistocene and Eocene.

Importance of Rocks

Rocks are the essential parts of the earth and contribute to the beauty and wonderment of nature. Rocks also help in the formation of soils, which help grow trees, plants and even forests. Since rocks include various minerals, they play a vital role in different natural systems or cycles. To maintain a healthy lifestyle and strengthen the body, humans must consume sufficient amounts of minerals regularly. The vegetables and fruits provide some such minerals for humans.

Besides biological importance, rocks are also important for humans in various ways. They help manufacture cement, lime, and other construction or building materials. Rocks are also widely used to produce petrochemicals, paper, insecticides, fiberglass, linoleum, building roads, ornamental stones, and monuments. Rocks also make it easier to study and learn more about Earth materials, structure, natural systems, etc.


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