Igneous rock

• Igneous rocks are formed when magma cools and hardens forming solid rock.
• There are 2 types of Igneous rocks:

  • Intrusive Igneous Rocks
  • Extrusive Igneous Rocks
• Intrusive rocks form inside the earth's crust
• Intrusive rocks are also known as Plutonic rocks.
• Granite is an example of an intrusive rock formed in the earth's crust
• In Ireland granite was formed 400 million years ago as part of the Caledonian fold mountain building
• Granite forms at a destructive boundary where 2 plates collide
• At a destructive boundary one plate sinks and melts in a process known as Subduction
• When subduction happens at a destructive boundary Magma rises into the plate above it but does not reach the surface
• As the magma is insulated by the surrounding rocks it will take a long time to cool
• As the magma takes a long time to cool large crystals will develop. ( intrusive )
• The large crystals on intrusive rocks are feldspar, mica and quartz.
• Large masses of granite found in the earths crust are called Batholiths.
• Granite can vary in colour. It can be grey to white to black to pink.
• Granite often is described as being similar to a fire lighter.
• Granite is a very decorative rock and is used for a variety of purposes.
• Granite can be cut and polished and used for worktops and fireplaces
• Granite formed inside the earths crust but due to weathering and erosion it may be exposed on the surface today.
• In Ireland, granite can be found in the Wicklow mountains.
• Extrusive rocks form on the earth's surface.
• Extrusive rocks are also known as volcanic rocks.
• Magma reaches the surface and is now known as lava, as it is exposed to the air it begins to cool quickly.
• Some extrusive igneous rocks form under water when magma rises onto a sea bed.
• Basalt is an example of an extrusive igneous rock.
• Basalt forms at constructive boundaries and hotspots where magma reaches the surface.
• The type of lava present at basalt forms is Basic lava which has a silica content of 55% or less.
• Basic lava is runny and can travel long distances.
• Granite is a dull brown or black colour.
• As the lava cooled quickly only microscopic crystals are present ( Basalt )
• Differences in the rate of cooling often causes basalt to split into columns of 6 to 8 sides.
• Basalt often has a stepped appearance as later lava flows do not travel as far as the first ones.
• Basalt is used in the construction of roads and as road chippings.
• In Ireland basalt is found in the Giants Causeway, Co. Antrim.