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