Rocks
The crust of the earth is composed of rocks. These rocks vary in their origin,
formation and characteristics. Three major rock groups are recognised;
Igneous: for example, granite and basalt.
Sedimentary: for example limestone and sandstone.
Metamorphic: for example marble and quartzite.
Settings
Local – A common rock-type quarry.
National – Irish distribution of all three rock-groups.
Rocks
The crust of the earth is made up of rocks. Rocks are divided into groups depending on how they were formed.
(a)
Igneous: They were formed when hot, molten volcanic material cooled down and became solid. Examples
include granite and basalt.
(b)
Sedimentary: They were formed from the compressed remains of plants, animals and other rocks. Examples
include limestone, coal and sandstone.
(c)
Metamorphic: They were once sedimentary or igneous rocks, which were changed by great heat or pressure.
Examples include marble and quartzite.
Igneous
Magma (molten rock) comes to the earth’s surface when the plates pull apart or push together. Eventually the
magma cools and hardens and forms igneous rock.
Examples:
Granite – a hard coarse multi-grained rock, found in the Wicklow and Mourne mountains. Magma cooled slowly in
the crust leaving large crystals, it can be black to grey to pink in colour.
Basalt – a heavy black rock found on the Antrim plateau and the Giant’s Causeway. Lava cooled quickly on the
earth's surface, it has only tiny crystals.
Sedimentary
Sedimentary rocks take their name from sediments which are pieces of materials which sink to the bottom of liquids.
Dead plants , animals, shells and debris sank to the sea bed or the beds of lakes. Over millions of years more layers
were laid down eventually they were all compressed and cemented together.
Examples:
Limestone: Light to dark grey in colour. They are heavy and smooth with fine (small) grains. They may contain
fossils.
Limestone is laid down in layers called strata, layers between strata are called bedding plains. Limestone is
permeable (water can pass though it). Water dissolves limestone. Limestone is found in the Burren in Co. Clare.
Sandstone: This is a resistant rock (difficult to break down), it has a gritty texture because it is composed of
rounded sand grains cemented together. Sandstone is often red because of the mineral iron oxide in the cement.
Sandstone is found throughout Munster.
Metamorphic
Layers of sedimentary or igneous rock were folded by pressure when crustal plates moved together. Some of the
rock can be changed by this great heat or pressure.
Example: Limestone is changed to marble as a result of great heat or pressure.
Rocks and people
1. Rocks provide us with valuable natural resources.
2. Minerals: We extract iron from iron ore. Quartz is used in making watches and radios.
3. Rare minerals: Gold and silver are mined from rocks.
4. Fuels: Turf, coal, oil and gas are all fossil fuels, which we use to provide our energy needs. Kinsale gas field
has been operating for over 20 years, it pumps gas from 1000m below the sea bed. It provides employment
and energy.
5. Common rocks such as limestone are used in many trades; headstones and cement are made from rock, it is
also used in the production of iron and steel. They are used for buildings and in the resurfacing of roads.
Farmers even use lime as a soil conditioner.
Extraction
Drilling: Used to extract oil or gas from underground, can be offshore or onshore.
Open cast mining: If resource is near the surface of the earth the surface layer is removed and the resource
extracted. Ugly on the landscape.
Shaft mining: The resource lies in seams deep beneath the earth. Used to extract lead and zinc eg Navan.
Distribution of rocks in Ireland
Granite:
Basalt:
Sandstone:
Limestone:
Marble:
Quartzite:
Wicklow, Down, Galway and Donegal.
Antrim.
Munster.
The midlands and the Burren Co. Clare.
Connemara and Kilkenny.
Donegal, Mayo, Galway and Wicklow.
Read the information above carefully. For each heading you should look at the maps in your book and the diagrams
for each feature. If you are unsure of the diagrams you need to draw them onto this revision sheet.
When you have studied the information see if you can answer the questions below. Remember the format:
STATEMENT – DEVELOPMENT – EXAMPLE
Questions:
1. List the three rock groups and in the case of each give an example of a rock in that group.
2. Explain using a clearly labelled diagram how sedimentary rock is formed.
3. Explain using a clearly labelled diagram how igneous rock is formed.
4. Explain using a clearly labelled diagram how metamorphic rock is formed.
5. Where would you find the following rocks in Ireland; Granite, basalt, sandstone, limestone and marble.
6. List three uses for rocks.
Now go to studyclix.ie. Select Junior cert geography, then higher level, finally the section on rocks and weathering.
Attempt each question then check your answers. If you come across a question you can't do, ask in class.