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Ingneous Rocks
Igneous rocks form from molten mixtures that erupt from deep within
the Earth as magma or volcanic lava.As the minerals cool, they form
interlocking crystals, giving the resulting rocks their strength.Some
minerals are heavier than others , or melst at higher temperatures , so
they tend to get left behind when the molten rock wells up.This means
than an ingneous rock is rarely the same as its parent rock , and usually
lighter.The process has created a wide variety of rocks from the same
raw material.
PERIDOTITE :This is the rock that forms much of the deep mantle beneath
the crust , and therefore 80 per cent of theplanet.It is rare on the surface ,
squeezed it up from beneath the ocean floor.It is very heavy and mainly consists
of dark green olivine , rich in magnesium and iron.
Basalt :
Dark , dense basalt forms the bedrock of the ocean floors.It erupts
from the spreading rifts of mind-ocean ridges , and also from hotspot volcanoes
like those on Hawaii.Itis created by partial melting of peridotite in the mantle , to
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form a very fluid lava that contains far less of the heavy , greenish olivine that is
such an important ingredient of peridotite.This makes basalt lighter , too.
ANDESITE :
Named after the Andes of South America , where it is abundant,
andesite is solidified volcanic lava that has erupted from deep belowthe
mountains.Here , basalt ocean floor is being dragged beneath the continent and is
melting. The molten rock that rises to the surface contains fewerr heavy minerals
than basalt , so andesite is a lighter rock. It is one of the main rocks that form
continents.
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GRANITE :
All rocks contain silica - the substance that we use to make glass.
This can form relatively light minerals that melt at much lower tempetatures than
the heavy minerals in rockslike basalt. As the rocks beneath continents are heated
, the silicate minerals may form sticky magma that rises and then cools , turning
into relatively light but very hard granite.Ot is mostly ple feldspar and quartz , with
very little dark , heavy material.
RHYOLITE :
The magma that become granite usually cools deep in the
crust.This takes a very long time , allowing big crystals to grow and form the
granite.But if the magma reaches the surface it erupts as very viscous lava that
cools quickly into fine-grained rhyolite.The only difference between the two rocks
istheir crystal size.In the same way , basalt that cools deep in the crust forms a
coarse-grained rock called gabbro.
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OBSIDIAN :
Obsidian is volcaniclava that has cooled too fast for crystals to
form.It can be created from any type of lava , but usually has the same mineral
composition as rhyolite or granite.When it breaks it has a rippling fracture pattern
like that of flint or glass , and equally sharp edges , so like flint it was used to make
stone tools in the past.Always very dark , it has also been used as a gemstone.
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PUMICE :
The lava erupted from volcanoes often comtains a lot of gas. The
gas usually boils out of very liquid basalt lava quite easily , but has more difficult
escaping from much stickier silica-rich lava such as rhyolite.If the rock then
solidifies with the gas bubbles still inside , it forms pumice. This has much the
same structure as plastic foam , and is so light that it floats on water.