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How is granite
formed?
Scientists have concluded that most granite is formed by
slow cooling and crystallization of magma, a molten material.
The chemical composition of this magma is the same as granite.
It forms from rocks that melt at twenty five to forty
kilometers below the surface of the continents. At
temperatures ranging between 6500C and 9000C,
these rocks melt. Because the magma is lighter than the
surrounding solid rocks, it begins to rise. As it rises, it
cools. Coarse granite crystals are formed beneath the earth’s
surface. This is due to the slow cooling of the granite magma.
Sometimes granitic
magma erupts from volcanoes. When it cools too quickly, large
crystals are formed. The resultant rock is called rhyolite,
which has the same mineral |
A granite quarry |
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composition as
granite. One difference is that rhyolite is fine grained, as
against the coarse grains that make up the regular granite. |
The constituent
minerals
Feldspar is the main constituent of granite. It contains
abundant quantities of both plagioclase feldspar and alkali
feldspar. In fact, granite is classified based on the ratio of the
dominant feldspar to the subdominant feldspar. Another chief
constituent mineral is quartz. These minerals, namely alkali
feldspar, plagioclase feldspar and quartz impart the colors white,
pink or grey to the granite.
Granite also contains
small amounts of dark brown, dark green or black minerals such as
hornblende or biotite mica.
Classification
Scientists have classified granite as an igneous rock. As for
further classification, granites in which the quantity of
plagioclase feldspar is higher may be commonly found in large
regions of western United States. They are thought to be
characteristic of the great series of batholiths that stretch from
Alaska and British Columbia to Mexico, via Idaho and California.
Granites, which have a
greater composition of alkali feldspar (as compared to plagioclase),
are found in New England. Smaller masses of these types of granites
are also found in many sites in British Tertiary rocks and in the
Oslo region of Norway. They have been extensively quarried in
northern Nigeria.
The minerals in granite
are interlocked like the pieces in a jigsaw puzzle. The grains of
the minerals are very large, sometimes measuring as much as half a
centimeter in width.
Granite is a very strong
and durable rock. It is used in the construction of buildings and
tombstones. In the building industry, granite is used for pillars,
flooring (tiles) and making kitchen platforms
and sinks. |
