Rocks // CLASS 11/ ISC GEOGRAPHY // WBSLST
About 98 per cent of the total crust
of the earth is composed of eight elements like oxygen, silicon, aluminium,
iron, calcium, sodium, potassium and magnesium, and the rest is constituted by
titanium, hydrogen, phosphorous, manganese, sulphur, carbon, nickel and other
elements.
Petrology is science of rocks. A
petrologist studies rocks in all their aspects viz., mineral composition,
texture, structure, origin, occurrence, alteration and relationship with other
rocks.
Rock may simply be defined as all
those materials which form the crust of the earth , whether hard like granite
or soft like clay, gravel or sandstone
Definition : A rock is an aggregate
of minerals , sometime chiefly or entirely of single mineral species as in case
rock salt and limestone , but more commonly of two or more different minerals
Classification of rocks
1.
Igneous rocks
2.
Sedimentary rocks
3.
Metamorphic rock
Igneous rocks
Meaning: the word igneous has been
derived from a latin word “ignis” means fire .
Definition: the type of rock that
formed due to upwelling of magma from mentle,and magma cools and solidifies as
lava is called igneous rock.
Naming: primary rock, parent rocks,
Characteristics: 1. formed due to
upwelling of magma from mentle,and magma cools and solidifies as lava forms
igneous
2. it is parent rock.
3. the process of cooling and
solidification can happened in the earth crust on the earth surface of the
earth.
4.there is no layer of this rocks
5. they are rich in minerals
6. they are free from fossils
7.there are no pores in the igneous
rocks and
Example: granite , grabbro,
pegmatite, basalt, volcanic breccia and tuff are example of igneous rocks.
CLASSIFICATION OF IGNEOUS ROCK
Generally igneous rocks are
classified on the basis of their chemical composition , texture and their
location.
CLASSIFICATION BASED ON CHEMICAL
COMPOSITION
1.
Mafic igneous rock: it contains ferromagnetism mineral
like pyroxsin, amphibole, olivine .example of mafic is basalt and gabbro
2.
Felsic igneous rock: it contains feldspar, sodium,
calcium and potassium
CLASSIFICATION BASED ON TEXTURE
Texture refers to sizes and patterns
of the mineral crystals present in them. The size of mineral crystal depends
largely on the rate of cooling magma,.
CRYSTALS IGNEOUS ROCK
1.
PHANERATIC TEXTURE CRUYSTAL:are large enough to seen
with naked eyes or with the help of hand lens
2.
APHAN TEXTURE CRYSTAL:are too small to be
distinguished without the aid of a microscope
3.
EQUIGRANULAR:all crystal are within same size range
4.
PORPHYRITIC: smaller crystal size
CLASSIFICATION BASED ON LOCATION
1.
EXTRUSIVE ROCK OR VOLCANIC ROCKS
2.
INSTRUSIVE ROCK OR PLUTONIC ROCK
EXTRUSIVE IGNEOUS ROCKS
Definition: When magma in its upward movement
cools and turns into solid form at the surface of the earth is called extrusive
igneous rocks
These are called volcanic rocks as
lava reaches the surface dur to volcanic eruption.
As lava cools down rapidly coming out
of the earth’s hot interior , the mineral crystal of extrusive rock change
their structure .
Example: basalt , grabbro
Location : north western part of
peninsular india
Instrusive igneous rock
Definition: When magma in its upward movement
cools and turns into solid form beneath the surface of the earth is called instrusive igneous rocks
Instrusive igneous rocks are two type
1.
PLUTONIC ROCKS
2.
HYPABYSSAL ROCKS
PLUTONIC ROCKS : the word plutonic
has been derived from a greek word “pluto” which means the “god of the
underworld”
the process of cooling and
solidification can happened in the earth crust on the earth surface of the
earth is called plutonic rocks. The rate of cooling is much slower and the size
of crystal is very large.
Granite is typical example of the
plutonic rocks
The texture of plutonic rock is
called HOLOCRYSTALLINE. Like granite and
pegmite
It is found in deccan plateau ,
chattisgarh , chotanagpur plateau , rajasthan
HYPABYSSAL ROCKS
Magma coming up from below , don’t reach
the surface and gets solidifies in cracks, and joints forming hyperbyssalrock
Dolerite is typical example of that
Crystalline of the hypabyssal rocks
are known as HYPOCRYSTALLINE
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