TYPES OF ROCKS

TYPES OF ROCKS

Rocks differ in their properties, size of particles and mode of formation. On the
basis of mode of formation rocks may be grouped into three types:
(a) Igneous
(b) Sedimentary and
(c) Metamorphic
Igneous Rocks
The word igneous is derived from the Latin word ‘ignis’ meaning fire. Igneous rocks are formed by the cooling of highly heated molten fluid material, known as
magma. The word magma is derived from a Greek word which means ‘dough’. It
requires a greater quantity of heat to melt the rocks under overlying pressure than
at the surface. We do not know the exact depths at which magma forms but
probably it is formed at different depths not exceeding 40 km. Molten rocks
produce an increase in volume which is responsible for causing fractures or cracks
in the crust. The overlying pressure gets weakened along these openings, thus
forcing out the magma through them. Otherwise it can’t escape due to great overlying
pressure.
When magma is ejected to the surface, it is called lava. Igneous rocks are formed
from solidified molten magma below or on the earth’s surface. As they comprise
the earth’s first crust and all other rocks are derived from them, these are called
the parent of all rocks or the ‘primary rocks’. In simple words, all rocks can be
described as of igneous origin because at one time or another, they were erupted

to the surface: A younger series of igneous rocks is still being formed. About 95%
of the volume of outermost 16 km of the earth is composed of them. These are
largely hard and massive because of their magmatic origin and are crystalline in
appearances.
On the basis of their mode of occurrence, igneous rocks can be classified as :
extrusive or volcanic rocks and intrusive rocks.
(i) Extrusive igneous rocks are formed by cooling of lava on the earth’s surface.
As lava cools very rapidly on coming out of the hot interior of the earth, the
mineral crystals forming these rocks are very fine. These rocks are also called
volcanic rocks. Gabbro and basalt are very common examples of such rocks.
These rocks are found in volcanic areas. Deccan plateau’s regur soil in India
is derived from lava.
(ii) Intrusive igneous rocks are formed when magma solidifies below the earth’s
surface. The rate of cooling below the earth’ s surface is very slow which
gives rise to formation of large crystals in the rocks. Deep seated intrusive
rocks are termed as plutonic rocks and shallow depth intrusive rocks are
termed as hypabyssal. Granite and dolerite are common examples of intrusive
rocks. From this point of view, therefore, igneous rocks can, in accordance
with their mode of formation, be classified as (a) Plutonic, (b) Hypabyssal
and (c) Volcanic rockmasses. The huge blocks of coarse granitic
rocks are found both in the Himalaya and the Decean Plateau It illustrates that magma, on cooling, produces rocks of different shapes and sizes, depending on the space available after it forces itself
into the crust. Common forms of instrusive igneous rocks are batholiths, sills and
dykes etc. Batholiths are huge masses of solidified magma. They vary in size;
some are as much as several hundred kilometers across and thousands of kilometers
thick. They generally form the core of the major mountains, as shown in this
diagram. Their irregular dome shaped roofs sometimes appear on the surface
after erosion of millions of years. Sill is the horizontal intrusion of solidified magma
between the layers of pre-existing rocks. Dyke is similarly a more or less vertical
formation from few metres to several kilometers in length and from few centimeter
to hundreds of metres in thickness.
 On the basis of chemical properties, igneous rocks are classified into acidic and
basic rocks. These are formed as a result of solidification of acidic or basic lava.
Acidic igneous rocks are composed of 65% or more of silica. These rocks are
light coloured, hard and very strong. Granite is an example of an acidic rock.
Basic igneous rocks contain less than 55% of silica and have more of iron and
magnesium. These rocks are dark coloured and weak enough for weathering.
Gabbro, basalt and dolerite are examples of basic rocks MORE