IGNEOUS ROCKS
Igneous is a Latin
word that means “origin by fire”. These rocks are formed from the cooling of
molten magma or lava. This can happen both inside the earth and on the surface
of the earth. If cooling happen inside earth the resulting rock is called
“Intrusive Igneous Rock” and if it happens outside on earth surface the
resulting rocks are called “Extrusive Igneous Rocks”. 90% of the earth crust is
made up of Igneous Rocks.
CLASSIFICATION
OF IGNEOUS ROCKS
These rocks can
be classified in various ways
ON BASIS OF
COMPOSITION
On this base
Igneous Rocks are of two types
ACID
IGNEOUS ROCKS
These are rich
in silica (80%) whereas other 20% portion contain Potassium, Magnesium or
Sodium. These rocks are light in color. These are fast cooling and are produced
from slow moving lava. Their rate of erosion is very low. Granite is a good
example of this type of rock.
BASIC
IGNEOUS ROCKS
These are Basic
in nature and are composed of Magnesium (40%), Silica less than 40% and 20% of
Iron, Aluminum and Potassium. The flow of lava that makes these types of rocks
is fast and its cooling is slow. These are dark colored rocks and produce
Plateau e.g. Deccan Plateau of India. Examples of such type of rocks are Basalt
and Dole site etc.
ON BASIS OF FORM
On this base
Igneous Rocks are of two types
EXTRUSIVE
IGNEOUS ROCKS/ VOLCANIC ROCKS
When magma comes
out of volcano as lava and solidify externally it forms Extrusive Igneous Rocks
e.g. Basalt, Rhyolite etc.
INTRUSIVE
IGNEOUS ROCKS/ PLUTONIC ROCKS
When magma
solidify inside the earth curst it forms Intrusive Igneous Rocks e.g. Granite.
The Intrusive Igneous rocks are further of two types
Concordant
Intrusive Igneous Rocks
When the magma
do not disturb an existing structure of rocks and solidify in the already
existing cracks and faults, then such rocks formed are called Concordant
Intrusive Igneous Rocks e.g. Sill, Lacolith.
Discordant
Intrusive Igneous Rocks
If molten magma destroys
or dissolves the existing structure of rocks and solidifies to form a new rock,
it is called Discordant Igneous Rock e.g. Dyke, Batholith and Stock.
ON BASIS OF
FORMATION
On this base
these rocks are classified as
BATHOLITH
It is a massive
“pluton (A body of Plutonic rock)” that is formed after the destruction of existing
rocks by the molten magma.
STOCK
It is a mound
shaped accumulation of Igneous Rocks which is formed after solidification of
magma
LACOLITH
It is a Doom
shaped structure of the top that is supplied by the magma through a small pipe
like conduct from below
LAPOLITH
It is saucer
shaped structure which is also supplied by a small pipe through asthenosphere
PHACOLITH
It is a wave
like structure where the molten magma solidify
SILL
Sometimes the
magma rises up and fails to cross the surface of the earth and solidify in a
thin sheet in horizontal position just below earth surface, it is called Sill
DYKE
It is the
vertical intrusion of the molten magma and solidify in the earth and cause damage
to already existing rocks.
PROPERTIES OF
IGNEOUS ROCKS
·
These
rocks possess the property known as Jointing. It is the tendency of rock to
develop parallel fractures without any obvious movement. These may be
rectangular in shape like in Granite, or columnar as in Basalt, jointing in
form of concentric shells is also observed. Another
property of these rocks is Exfoliation. Sometimes the jointing occurs in form
of concentric shells and the outer shells peel away due to the erosion exposing
lower layers. This phenomenon is known as exfoliation.
·
These
are hard and compact.
·
Non-porous
to air and water.
·
No
fossil formations occur in them.
·
Rich
in minerals (Fe, Ni, Cu, Pb, Mn etc)
·
Slow
Erosion Rate
·
Large
crystalline structure in case of slow cooling and vice versa
ECONOMIC
IMPORTANCE
·
These
rocks form fertile soils like that of Deccan Plateau
·
These
are used as raw material and construction material in Industry and
construction.
LANDSCAPE FORMED
FROM IGNEOUS ROCKS
Igneous rocks
are quite resistant to erosion however the Intrusive Igneous Rocks form certain
landscapes when their overburdens are removed by erosion and weathering.
Similarly when a “Dike” is exposed after the erosion of surrounding overburden it produces the Landscape of a tower like shape.
For instance a
short Igneous Intrusive Rock “Sill” is covered by a softer cover of Sedimentary
Rocks. Over a long span of time this overburden erodes away while the “Sill”
resists erosion and thus is exposed giving a table like landform which is now
called “Mesa”.
