FORM FIVE SELECTIONS 2021** FORM 5 JOINING INSTRUCTIONS
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The igneous rocks are commonly classified based
either on mineralogical composition, textural characteristics or chemical
composition.
i) Chemical composition
Classification by chemical composition
normally is based on the percentage of silica in a rock.
a) Acid rocks: These are
igneous rocks with more than 65% silica. Examples are granite and granodiorite
b) Intermediate rocks: These
are rocks with 55-65% silica. Examples are syenite and diorite
c) Basic rocks: Cover all
igneous rocks with 44-55% silica (eg. Basalt)
d) Ultrabasic rocks: Include
all igneous rocks with less than 44% silica. A rock like anorthosite belongs to
this category. Ultrabasic rocks having higher magnesian content are known as
ultramafic (eg. Kimberlite)
ii) Textural characteristics
The textures of igneous rocks give an indication of
the cooling history of the magma. This classification is based therefore on the
cooling history of the magma and the modes of occurrence of the rocks.
a) Volcanic rocks
These are extrusive igneous rocks that crystallize
from lava. The magma that flows out of the earth’s surface while still wholly
or partly molten is called lava.
The rocks formed from lava are largely glassy
and fine–grained. Others exhibit fragmental and vesicular textures. Examples
are Phonolite, pumice, agglomerate, etc
b) Hypabyssal
rocks
These are rocks found in
minor intrusions such as sills and dykes. Dykes are sheet–like intrusions that are vertical or nearly so and
cut sharply across bedding or foliation. Sills are sheet–like intrusions that are essentially horizontal and
usually follow bedding or foliation. Igneous rocks associated with those minor
intrusions are generally medium
grained. Typical example is dolerite
c) Plutonic rocks
These are igneous rocks that form major intrusions
such as batholiths and stocks. The batholiths are large, cross–cutting
intrusions, having steeply dipping contacts and no apparent floor. Cover thousands of square kilometers. The
stocks are smaller than batholiths
occupy areas of few square kilometers. The plutonic igneous rocks are
characteristically coarse – grained.
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