Junior Certificate Geography
Introduction to Rocks
• The crust of the earth is made of solid rock.
• There are three types of rocks.
• There are
•Igneous Rocks
•Sedimentary rocks
•Metamorphic rocks.
Igneous Rocks
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Igneous means ‘fire formed’.
Igneous rocks form when hot molten magma cools.
When magma cools ON the earth’s surface, basalt rock is made.
Volcanic eruptions cause magma to come to the earth’s surface where
it cools and hardens.
• This happened in the Giant’s Causeway where the basalt formed into
columns.
Igneous Rocks
• When magma cools IN the earth’s crust, granite is formed.
• Magma can rise into the crust but does not come onto the surface of
the earth.
• Sometimes this magma lies in a magma chamber which feeds volcanic
eruptions.
• If the magma in the chamber cools and hardens, it turns into granite.
• Granite can be found in the Wicklow Mountains.
Sedimentary Rocks
• Sedimentary Rocks are made from sediments.
• Sediments are tiny pieces of rock ( e.g. sand ) which were worn off
other rocks by the weather.
• When these sediments are weathered, they may be carried by rivers to
the sea where they settle on the ocean bed.
• Over time, these sediments build up in layers.
• As this happens, the lowest layers are compressed or squashed to form
rock.
• Sandstone is an example of a sedimentary rock made when layers of
sand were compressed on the ocean floor.
• Sandstone can be found in County Cork
Sedimentary Rocks
• Limestone is another example of a sedimentary rock but is different to
sandstone.
• Limestone is made up of the shells of dead sea creatures.
• When these creatures died, they fell to the ocean floor where they were
covered by more shells and other sediments.
• Over time the shells were compressed and hardened into limestone
rock..
• Limstone covers much of the Central Plain of Ireland
Metamorphic Rock
• Metamorphic means ‘changed’.
• Metamorphic rocks are other rocks which have been changed.
• Rocks can be changed by the influence of heat, pressure or heat and
pressure working together.
• If, when magma rises into the earth’s crust it comes into contact with
another rock it, will ‘cook’ the rock and change it into another rock.
• For example, Quartzite is a metamorphic rock which formed when
sandstone was exposed to the heat of magma..
• Quarzite can be found on the Great Sugarloaf Mountain in County
Wicklow.
• Similarly, great heat changes limestone to marble.
• Marble can be found in Connemara and Kilkenny
Metamorphic Rock
• Folding occurs when rock is squeezed due to the movement of the
plates of the crust.
• When folding happens, rock is put under great pressure.
• When Shale (made from mud and silt) is put under pressure, it
changes into Slate – the same stuff we put on our roofs.
Finish