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 Home » Tutorials » Geography » Physical Geography » Rocks & Minerals

Rocks & Minerals


The earth’s solid layer, called lithosphere, is about 100 km in thickness. The word ‘lithosphere’ literally means ‘a sphere of rocks’. Down to a depth of 16 km from the earth’s surface, 95 per cent of the earth materials consist of rocks. The rocks are made of individual solid substances called minerals. Each mineral usually contains two or more elements, of which the whole earth is made of.

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Petrologists (scientists who study rocks) define a rock as any natural mass of mineral matter that makes up the earth’s crust. All rocks are not hard. Clay, for example, is a type of rock that is soft. Chalk is still softer.

Rock Types

There are three main groups of rocks: Igneous; Sedimentary; and Metamorphic Rocks.

Igneous Rocks

These are formed from lava hurled out of a volcano or from the cooling of hot magma below the crust. Granite is coarse-grained igneous rock that is formed by the slow cooling of magma. Basalt is fine-grained igneous rock, almost black, that is formed by quick cooling of lava. Igneous rocks are classified on the basis of chemical composition and texture. Chemical differentiation of magma gives rise to mafic and felsic types of igneous rocks. Texture relates to the sizes and patterns of the mineral crystals present in the rock. The size of mineral crystals in an igneous rock depends largely upon the rate of cooling of magma. As a general rule, rapid cooling results in small crystals and slow cooling in large crystals. Extremely sudden cooling will result in the formation of a natural glass which is noncrystalline. Large bodies of magma, trapped beneath the surface, cool very slowly because the surrounding rock conduct the heat slowly. Rapid cooling occurs in lava that loses heat rapidly to the atmosphere or to the overlying ocean water.

The crystals large enough to be seen with naked eyes or with the help of a hand lens are called phaneritic textured crystals whereas those too small to be distinguished without the aid of a microscope are called aphanitic textured crystals of the igneous rocks. Where crystals in the rock are all within the same size range, the texture is described as equigranular. Where a few large crystals, called as phenocrysts, are embedded in a martix or groundmass of smaller crystals, the texture is porphyritic.

Sedimentary Rocks

These are made from sediments formed by the erosion and weathering of other rock types. Wind, water and snow erode rocks and carry the sediments to low lying areas. When deposited in the sea they are compressed and hardened to form layers of rocks. Sediment is fragmented mineral and organic matter derived directly or indirectly from pre-existing rocks and from life processes, transported and deposited by air, water or snow/ice. Sandstone is made from grains of sand, which have been naturally cemented together. Chalk is made up of millions of tiny calcium carbonate (lime) skeletons of microorganisms.

Throughout the geologic past, layers of sediment have accumulated to great thickness in certain favourable areas on the continents and on the ocean floor. As new layers are added, the underlying layers experience progressively deeper burial. Under increasing pressures imposed by the overlying load, water is excluded from the sediment. They become denser and strongly coherent. Ultimately, hard rock layers are produced, a process termed lithification. In some instances, chemical changes also occur in the sediments following deposition. All processes of physical and chemical change affecting sediments during its conversion into solid rock are called diagenesis.

The first level of classification of sedimentary rocks is into clastic and non-clastic divisions. The adjective “clastic” comes from the Greek word “klastos”, meaning “broken” and describes clastic sediments consisting of particles removed individually from a parent rock source. The naming of clastic rocks depends in part on the size of component mineral grains. A system of grading of mineral grains has been established by geologists, which is known as Wentworth Scale.

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