Types of Rocks

1. ROCKS
Mr. Toradmal Agastirishi B.
Asst. Professor
Department of Geography
Dada Patil Mahavidyalaya, Karjat, Dist.- Ahmednagar

2.  The earth's crust is composed of different rocks.
These rocks vary in their mineral content, hardness,
porosity, texture, chemical composition and
coherence.
 What is a rock
 Commonly, the term 'rock is applied only to those
materials of the earth's crust that ire hard and
resistant. But in the strict geological terms, all
materials, whether hard or soft, that have formed
the crust are known as rock. They contain hard
massive and resistant rocks like granite and marble
as well as soft materials tike silt and clay.

3.  Minerals
Rocks are made up of different minerals. A mineral
is a natural product and it is formed by inorganic
processes. It has a definite chemical composition
and an ordered internal atomic structure. The
colour, shape and appearance of these minerals give
rocks their characteristic appearance. The most
important rock forming minerals are silica,
chlorides, Sulphide, oxides of iron and carbonates

4.  CLASSIFICATION OF ROCKS
 The rocks are generally classified into three
groups on the basis of their origin. They are
Igneous Rocks
Sedimentary Rocks
Metamorphic Rocks.

5.  Igneous Rocks
The word 'igneous' is derived from the Latin word 'ignis' meaning fire. These rocks
are formed from the solidification of hot molten material known as magma. These
rocks have made nearly 95% of the outer 16 km of the earth's mass. Igneous rocks
were the first rocks that became solid and so formed the outer layer of the earth.
Hence, they are also known as 'primary ' rocks.
Geologists believe that rock material exists at very high temperatures below the
solid crust of the earth. This material is under enormous pressure. When this
pressure is relieved locally, this rock material becomes liquid and tries to come to
the surface of the earth. It finds an outlet along the lines of weakness, like joints
and faults. This molten al together with it&£ns content is known as magma. When
it reaches the surface of earth, the gases escape and now this material is known as
lava. The solid rocks formed cooling of magma or lava are known as igneous rocks.

6.  Classification of Igneous Rock
 On the basis of the places where the cooling of
molten matter and solidification take; place, the
igneous rocks are divided into the following
groups (Diag.

7.  1. PLUTONIC ROCKS
 These rocks are formed due to the cooling of magma at considerable
depth in the earth's crust. The cooling process of magma is very slow
and hence there is time for the crystallization of different minerals.
Thus, the crystals in these rocks are large. These rocks are also known
as intrusive rocks as they have intruded into the surface rocks from
the magma pockets below. These rocks are compact and coarse in
texture. Coarse grained granite is the example of plutonic rocks.
 In India, they are found in the States of Rajasthan, Madhya Pradesh,
Karnataka and Tamil Nadu. Diorite and gabbro are the other examples
of plutonic rocks. These plutonic rocks are found in the form of
enormous masses. They arc known as Batholiths.

8.  2. HYPABYSSAL ROCKS
 Many a times, the magma is unable to reach the surface of the earth. It enters the
cracks, faults and joints of the other rocks of the- earth's crust and cools down. The
cooling of this magma is comparatively faster than that of the intrusive rocks and
hence, the crystals of these rocks are smaller in size than thus intrusive ones.
These rocks are formed at shallow depths beneath the surface of the earth.
 These rocks take various shapes as they are formed in the spaces found in the
earth's crust. On the basis of their shape, they are commonly divided into three
types:
 Dyke :
 When the magma is trapped in vertical joints, these rocks are vertical in shape like
a pillar or a wall.
 Sills:
 When the magma enters horizontal joints, the igneous rocks are formed along a
horizontal direction. If they are considerably thick, they are known as sills.

9.  Sheet
 when the hyperbyssel rocks are thin and run along a horizontal direction in the
earths crust they are known as sheets.
 LAVA OR VOLCANIC OR EXTRUSIVE ROCKS
 Lava reaches the surface of the earth and comes in contact with the cool
atmosphere and solidification takes place. Lava is poured on the surface of the
earth by volcanic activity and these rocks are commonly known as volcanic rocks.
The cooling of the lava is rapid, hence crystallization is also rapid, and so the
crystals found in these rocks are very small in size- Basalt is the best example of
this type. Rhyolite, andesite and obsidian are also other examples of volcanic
rocks. The north-western part of the Deccan Plateau comprising large areas of
Gujarat, Madhya Pradesh, Maharashtra and Karnataka are made up of basalt. This
region is also known as the "Deccan Trap". Basalt is largely used for the
construction of road and houses.

10.  Chemical Composition of Igneous Rocks:
 The chemical composition of igneous rocks is extremely complicated. On the basis ol the silica
content in the rock, they are commonly divided into two types :
 1. Acid Rocks ; When the proportion of silica is high, i.e. usually more than 65%, the rocks are
known as acid rocks. Granite and obsidian are acid rocks.
 2. Basic Rocks : When the proportion of silica is less than 55%, the igneous rocks are known as
basic rocks. These rocks are darker in colour and heavier than the acid rocks. Gabbro and basalt
are the basic rocks.
 General Characteristics of Igneous Rocks
 Igneous rocks are made up of crystals of various shapes and sizes and their arrangement is
not symmetrical.
 Igneous rocks do not form layers like sedimentary rocks. They are compact, massive and hard.
 These rocks are not the subject of folding but joints and faults develop into them due to
compression and tension.
 These rocks are not porous and rain water percolates only through their cracks and joints'.
 Igneous rocks do not contain any fossils of plants or animals.

11.  SEDIMENTARY ROCKS
 Sedimentary rocks have an external origin. They originate from the pre-existing
rocks. Rocks that are exposed on the surface of the earth are subject to mechanical
and chemical weathering and erosion. This is carried out through different agents
like running water, moving ice, etc. These agencies break and erode the rocks and
the broken material like sand, silt and clay is carried to depressions like the lakes,
seas and oceans through rivers, wind and moving ice. This material is known as
sediments. This loose material also contains cementing material like silica and
calcium carbonate. The consolidated material or sediments become hard and
compact duo to pressure of water from above and also due to the presence of the
cementing material. The loose fragments of sediments settle down according to
their weight and size. Thus, the rocks that are formed from the sediments are
known as sedimentary rocks. The deposition of sediments usually.

12.  takes place in the form of layers or strain, hence, they are also known
as stratified rocks (Diag. 4.2).
Diag. 4.2: Sediments are assorted according to their size
A large part of today's land surface is made up of sedimentary rocks and it is
estimated that nearly 75% of the surface rocks of the earth are sedimentary in
origin.

13.  Classification of Sedimentary Rocks
 The sedimentary rocks are classified in different ways but commonly they
are
 grouped as follows :
 1. Mechanically derived rocks
 2. Organically derived rocks
 3. Chemically derived rocks.
 I. Mechanically Derived Rocks
 These rocks are mainly made up of inorganic material. The weathered
material of the rocks is deposited in low-lying areas like river basins, deltas,
on the beds of the sea and oceans. The deposits are made up of clay, silt, sand
and gravel. On the basis of the size of particles, these rocks are divided into
following groups.

14.  Argillaceous Rocks :
These rocks are mainly made up of minute clay particles and
hence, are soft. They are readily weathered but they are non-
porous and impermeable. Shale is an example of argillaceous
rocks. In India these rocks are found in the Northern Great Plain.
 Arenaceous Rocks :
The proportion of sand is more in these rocks than clay and silt.
The sand is mainly made up of hard quartz and hence, these rocks
are hard and are resistant to erosion. Sandstone is the best
example of these rocks. In India sandstone is found in the
Vindhyan Plateau, the Aravallis in Rajasthan and Kurundwad in
Maharashtra.

15.  II. Organically Derived Rocks
 The proportion of organic matter is more than the inorganic matter in these
rocks. These rocks arc composed of the remains of various organisms and
plants. They are commonly divided into two groups :
 (a) Calcareous Rocks :
Different organisms like coral and oysters live and die in the sea water. When
these organisms die, their fleshy parts either decompose in water or are eaten by
other organisms. But their shells and skeletons which are made up of calcium
carbonate, settle on the floor of the sea and ocean. In due course, they become
compact and hard. They are known as calcareous rocks as they are mainly made
up of calcium carbonate. Limestone, chalk, coral and diatomite rocks are thus
formed from the remains of minute marine organisms. Limestone is used widely
for constructional purposes and also for making cement. In India, it is found in
Rajasthan.

16.  Carbonaceous Rocks :
 These rocks are formed from the remains of plants and the proportion of carbon
content is more in them. In many swampy areas of the world, the climate was
favourable for the growth of luxuriant vegetation. In course of time, these plants
were buried under various sediments and the woody substance turned into
carbon. Coal is a carbonaceous rock. Peat and lignite arc the inferior varieties of
coal an the carbon content in them is less and their calorific value is low. On the
other hand, bituminous and anthracite are the good varieties of coal as their
carbon content is high and so also their calorific value.
 In India coal is found in basins of rivers Mahanadi, Godavari and Damodar. It is
found in the States of West Bengal, Bihar, Orissa, Madhya Pradesh, Chhattisgarh
and Maharashtra.

17.  Chemically Derived Rocks
 All water found on the surface of the earth contains salts in solution. These salts
present in water are either precipitated due to evaporation of water or by
chemical interaction. Rock salt and gypsum are formed in this way. Stalactites
and stalagmites are formed in underground caves when evaporation of lime
mixed water occurs. Flint and travertine are other examples of rocks. All
chemically-derived rocks are found in a consolidated state and are not found
over large areas.
 General Characteristics of Sedimentary Rocks
 Sedimentary rocks are comparatively softer than igneous or metamorphic rocks.
 These rocks are found in layers or strata.
 Being softer than igneous rocks, these rocks are easily weathered by the
different agents of erosion.
 Sedimentary rocks contain fossils of animals and plants and hence, arc a record
 of past animal and plant life.

18.  Metamorphic Rocks
 The word "metamorphic" is derived from the Greek word "meta-morphe"
which means "change in form". Igneous and sedimentary rocks may
undergo change. This change is of two types :
 There is a change in the form of a rock.
 There is a change in the physical and chemical property of a rock.
Thus, due to the change in both appearance and character, an
entirely new rock comes into existence. This rock is the result of
metamorphosis and hence, is known as Metamorphic Rocks.
This metamorphism takes place due to the pressure and heat of the
interior of the earth. Thus, the origin of these rocks is closely associated
with earth movements like folding, faulting and vulcanicity.

19.  This metamorphism is of two types:
 Contact Metamorphism (Diag 4.3).
This is associated with volcanic activity. When the eruption
of magma takes place , the rocks that lie close to the volcanic
pipe come in contact with the hot magma and due to the
tremendous heat, changes occur. There is no change in their
appearance but there is a change in their properties. When
limestone comes in contact with the hot magma, it gets
baked and thus turns into marble. Coal turns into graphite.
Similarly, sandstone changes

21.  Regional Metamorphism
 This metamorphism is associated with mountain building activities like folding and
faulting. The tremendous tension, pressure and the resultant heat produced in the
interior of the earth, changes the rocks over large areas. General Characteristics of
Metamorphic Rocks
 1. These rocks in general are compact.
 2. They are resistant to erosion.
 Metamorphic rocks are useful in a number of ways, e.g. slate is used for roofing.
Graphite is used in making pencils and crucibles. Marble is used in making statues
and ornamental buildings. The famous Taj Mahal is made of white marble.
 In India, metamorphic rocks are found in different areas. Marble is mainly found in
 Rajasthan and Madhya Pradesh. Slate is found in many ranges of the Himalayas.
 Graphite is found in Andhra Pradesh and Orissa.