Introduction to rocks


A major part of the earth’s crust is composed of rocks. Rocks may be defined as an aggregate of minerals. Some rocks, such as quartzite (quartz) and marble (calcite), contain grains of one mineral only, but most are composed of a variety of different minerals, The rocks are broadly classified into three groups: (i) igneous rocks, (ii) sedimentary rocks, and (iii) metamorphic rocks.

(1). Igneous Rocks: 

Igneous rocks are formed by the cooling and solidification of magma. Typical igneous rocks are granite and basalt.

(2). Sedimentary Rocks: 

Sedimentary rocks are formed by the consolidation and cementation of the sediments deposited under the water. Typical sedimentary rocks are sandstone, limestone, and shale.

(3). Metamorphic Rocks

Metamorphic rocks are formed when the pre-existing rocks have been changed in texture and composition by increased temperature and pressure. Typical rocks of this kind are schist and gneiss.

Rock Cycle:

The rock cycle shows the relationship between the three types of rocks, that is igneous, sedimentary, and metamorphic rocks. One type of rock changes slowly to another type of different types of processes. Erosion and abrasion produce sediments from all three types of rocks. These sediments are then transferred and deposited into deep basin under the sea. Then it hardens to form sedimentary rocks. If these rocks are deeply buried, the internal temperature and pressure of earth turns them into metamorphic rocks. Intense heat at great depths melts metamorphic rocks and produced magma. The magma may rise up and reach the earth’s surface where it cools to from ”igneous rocks”. At the surface, igneous rocks are exposed to weathering and erosion, and a second cycle begins again. In the light of above discussion, following processes get involved during a single rock cycle;

(i). Weathering: The breaking down of the rock particles from the bed/parent rock is known as weathering.

(2). Erosion: The weathered particles are transported to the low-lying areas. This process is known as erosion.

(3). Deposition/ Sedimentation: The setting down of the transported particles/sediments of rocks is known as deposition/ sedimentation.

(4). Compaction: The deposited sediments are compacted by different newer sedimentary layers and water load.

(5). Cementation: The deposited sediments are cemented by addition of different chemicals, like salts, lime, etc.

(6). Metamorphism: Some of the sedimentary rock layers are buried deep into the surface. Extreme pressure and heat may convert these sedimentary rocks into metamorphic rocks. This process is known as metamorphism.

(7). Magmatism: Some of the sedimentary and metamorphic rocks go further deep into the earth surface, where there is intense heat. Due to this intense heat, these rocks melt. When magma extrudes out as a result of volcanism, or plate boundaries, forms the igneous rocks.

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