Geology Reference
In-Depth Information
from temperatures in excess of 1,000 degrees Centigrade at depths
of several kilometres in the Earth
s crust and the slowness of the
cooling allowed large mineral grains to grow very gradually. Since
formation they have been uplifted and the overlying rocks eroded
away, with huge consequential changes in stress and temperature
conditions.
Granitic rocks are light-coloured and relatively light in weight
(unitweight27kN/m
3
, which is 2.7 times that of water). In terms
of mineralogy, granite has a high proportion of quartz and feldspar.
Quartz (SiO
2
) is hard, has no cleavage weaknesses and is much more
resistant to chemical weathering than feldspar. Feldspar (orthoclase
and plagioclase) is a much more complex silicate mineral with
cations of potassium, aluminium, sodium and calcium and relatively
weak cleavage directions that make it prone to chemical attack.
The feldspars therefore break down, primarily to form clays,
which are a series of minerals of essentially the same chemical
makeup as the feldspars but which are more stable at the Earth
'
'
s
surface temperature and chemistry. Minor minerals in many igneous
rocks include biotite, hornblende and magnetite, which contain iron,
which is released and then oxidised on decomposition, hence giving
the rust-red of weathered rock pro
les in many sub-tropical and
tropical parts of the world. Iron oxide and carbonate products
play an important role in cementing recently deposited sediments,
as discussed later. Granitic rocks are found in continental regions
and probably largely represent melted and reconstituted crust as
plates are subducted beneath mountain chains or in extension
zones (Davis & Reynolds, 1996).
Oceanic regions are made up of basic igneous rocks
-
mainly
basalt
fine-grained chemical equivalent of gabbro.
Basalt erupts along extensional plate boundaries such as that running
down the centre of the Atlantic Ocean as the European and African
plates move away from the American plates at a rate of about 20mm
each year. Basic igneous rocks are darker-coloured and heavier than
granite, largely because they are rich in iron and magnesium. Gabbro
has a unit weight of about 30 kN/m
3
. Basic rocks are sometimes
extruded in continental regions where faults allow basaltic magma to
rise as molten rock from great depth to the Earth
-
which is the
'
s surface. Basalt
'
rock that originated as lava
s Causeway in
Ireland and much of the other Tertiary volcanics of northern Britain.
The basaltic Deccan Traps in India cover an area of more than
50,000 km
2
.
Lavassuchasbasaltare
fields makes up the Giant
finer-grained than granite or gabbro
because they cool relatively quickly; where cooled extremely
quickly, say by extruding into water, they may form natural
volcanic glass. Basaltic lava has relatively low viscosity and
ows
alongway,unlikethemoreviscous pale-coloured acidic lavas
