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Igneous Rocks
- Introduction
- Igneous landforms
- Economic uses
Igneous rocks are formed when molten rock from deep below the crust
(magma) cools and solidifies. If the magma reaches the surface, it may
spread out to form a lava plateau or it may erupt from a single point to
form a volcano. Magma that reaches the surface and cools forms
extrusive or volcanic igneous rocks and landforms.
Sometimes, the magma does not reach the surface but instead remains
trapped within the rocks of the Earth’s crust. Here it cools slowly to form
crystalline intrusive igneous rocks such as granite and landforms such as
batholiths, sills and dykes.
Note: Add diagram
Igneous Landforms
Intrusive igneous landforms
If the magma is intruded along vertical joints, it forms dykes whereas
if it moves along horizontal bedding planes, it cools to form sills. If
the overlying rocks are eroded away, these features may be exposed at
the surface. Sometimes, large areas of intrusive granite (called
batholiths) are exposed by erosion to form moorlands such as
Dartmoor in south-west England. The surface of the granite moorland
may be weathered to form distinctive tor landforms.
Extrusive igneous landforms
When large quantities of magma pour on to the surface through fault
lines, the lava flows that result can spread over huge areas and over
time this may be repeated to give a great thickness of igneous rock.
Examples can be found in Northern Ireland and in the Deccan Plateau
of India. There are thousands of examples of volcanoes formed when
lava is extruded through a vent on to the surface. Famous examples
include Mt St Helens in the USA and Mount Etna in Italy. The shape
of the volcano is determined by the type of lava that is extruded.
Economic uses
Many types of igneous rocks are used as building stone, facing stone, and
decorative material, such as that used for tabletops, cutting boards, and
carved figures.
Pumice is used as an abrasive material in hand soaps, emery boards, etc.
Gneiss, Schist and Gabbro are very hard and widely used as crushed stone for
concrete aggregate, road surfaces and railroad ballast.
Igneous rocks may also contain many important ores such tin or valuable
minerals such as diamonds.