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Igneous Rocks
Igneous rocks are formed when hot molten rock
material called magma solidifies. Magmas are
developed when melting occurs either within or
beneath the Earth’s crust, that is, in the upper
mantle. Igneous rocks are composed principally
of silicate minerals.
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Igneous rocks may be divided into intrusive and
extrusive types, according to their mode of
occurrence. Intrusive (plutonic) form below the
ground surface, cool slowly creating coarse
crystal structure. Extrusive (volcanic) form at
ground surface and cool rapidly.
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Form of Intrusive Igneous Rocks
Concordant bodies
When the magma is unable to cut across and
disturb the existing intruded rocks, it may get
cooled and solidified within the fissures and
cavities prevailing in existing rocks. Such
intrusions which are not influenced, by the
structural features of the intruded rocks, will
produced Concordant bodies.
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Form of Intrusive Igneous Rocks
Concordant bodies may occur in different forms
like Sills, Laccoliths
Discordant bodies
If the intrusive magma solidifies after disturbing
and cutting across the existing intruded rocks, it
will produce Discordant bodies.
Discordant bodies may occur as Dikes/Dykes,
Volcanic neck and Batholiths.
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Form of Intrusive Igneous Rocks
when the new rock forms within and parallel to
the bedding of a layers rock, it is called a sill.
A dike or dyke in geological usage is a sheet of
rock that formed in a fracture in a pre-existing
rock body.
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Form of Extrusive Igneous Rocks
The extrusive igneous rocks may occur in two
major forms i.e. Flows and Pyroclastics.
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Plutonic rocks:
Granite, Syenites, Diorites, Gabbros, Peridotities
Hypabyssal rocks:
Dolerites
Volcanic rocks:
Rhyolites, Andesite, Basalts
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Texture
Fast cooling on the surface results in many
small crystals. It results in aphanitic texture
(crystals cannot be distinguished with the naked
eye). Slow cooling results in few much larger
crystals of phaneritic texture.
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Texture
Aphanitic texture consists of small crystals that
cannot be seen by the eye with or hand lens
Phaneritic textured rocks are comprised of large
crystals that are clearly visible to the eye with or
without a hand lens or binocular microscope
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• Granite (PINK)
Intrusive, contains primarily feldspar and quartz,
widely used for construction materials and also for
monuments
• Basalt
Extrusive, primarily ferromagnesium minerals, very
hard good to use as road construction material,
rapidly cooling results in formation of joints
• Diorite (WHITE)
Moderate in hardness, similar to granite with
plagioclase feldspar instead of orthoclase and little
quartz
Andesite, Rhyolite, Gabbro, Pumice
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