Rocks
Rocks Key Words:
- Igneous: Rocks made from magma or lava.
- Sedimentary: Rocks made from sediments of other rocks and/or fossils/dead plant and animal remains.
- Metamorphic: Rocks formed when heat or pressure changes the composition of other rocks.
- Intrusive: Inside the earth’s crust.
- Extrusive: On top of/on the earth’s surface.
- Plutonic: Intrusive
- Volcanic: Extrusive
Past Exam Questions:
- Explain the formation of Igneous/Sedimentary/Metamorphic rocks.
(This question can contain just one type or sometimes you must explain two or you could be given a choice between two groups).
- Explain how different rock type produce distinctive landscapes in Ireland, with reference to examples you have studied.
- Humans interact with the rock cycle in a number of ways. Describe and explain how this interaction with the rock cycle takes place, with reference to one of the following: Mining, Extracting Building Materials, Oil/Gas Exploitation, Geothermal Energy Production.
You may also like: Complete Guide to Leaving Cert Geography
~Sample Answer
Igneous Rocks
Rocks are continually formed, changed, destroyed and reconstructed as part of the rock cycle. The rock cycle is the process by which each rock type can be changed into another. Rocks vary in their origin, formation and characteristics. There are 3 main rock categories, igneous, sedimentary and metamorphic. This essay will focus on the formation of igneous rocks, the examples I will use are granite and basalt. Igneous rocks are formed when magma or lava cools and hardens within or on the earth’s crust. They can be either intrusive (plutonic) or extrusive (volcanic) depending on where they form.
Intrusive igneous rocks are formed within the earth’s crust. This happens when magma cools slowly, deep inside the rock of the earth’s crust. Rocks like granite are formed this way, they contain minerals such as feldspar, mica and quartz. The amount of the minerals present will determine the colour of the rock. Granite can be black, white, pink or grey. Granite forms at destructive plate boundaries when the oceanic crust is subducted beneath the continental crust. The oceanic crust is pushed into the mantle causing the plate to melt, magma rises inside the buckled plate through a small fissure. As the magma gathers here inside the crust the rock that surrounds it acts as an insulating blanket. This blanket causes the rock to cool very slowly, forming large crystals and producing granite rock as it does so. Granite is a coarse grained rock with large crystals. It forms the core of many fold mountains e.g. the Caledonian Fold Mountains. Granite can be found in the Wicklow Mountains and the Mourne Mountains in Ireland.
Extrusive igneous rocks are formed on top of the surface of the earth. This happens when lava flows out from the vent of a volcano and cools and hardens on the cone or on the land that surrounds a fissure. The lava was once magma but forced its way towards the crack in the crust and between the layers of rock. As the cracks were narrow the magma reaches the surface as lava but cools quickly as it comes into contact with the atmosphere. This causes the rock to have only microscopic crystals because of the speed it cools at. The lava flows out without violence and spreads evenly, 5-6km thick. The thick layers of lava form basalt when they cool quickly on the surface. This process occurred at the Giants Causeway in Co. Antrim where lava spilled out on to the surface and cooled in hexagonal columns over a large area of land on the coastline of Antrim. Basalt is also a medium grained rock, usually a dark colour, charcoal or black. Basalt is used for building roads.
~Tip to remember: Intrusive means inside and so is formed by magma. Extrusive means outside (exit) and is formed by lava.
Sedimentary Rocks
Rocks are continually formed, changed, destroyed and reconstructed as part of the rock cycle. The rock cycle is the process by which each rock type can be changed into another. Rocks vary in their origin, formation and characteristics. There are 3 main rock categories, igneous, sedimentary and metamorphic. This essay will focus on the formation of sedimentary rocks, the examples I will use are limestone and sandstone. Sedimentary rocks are formed when sediments of other rocks, and or dead plants and animals are lithified, compressed and cemented together forming a rock with layers. Sedimentary rocks can be organic and inorganic.
Organic sedimentary rocks are those made from something that was once living such as plants or animals. An example of an organic sedimentary rock is limestone. Limestone was formed over millions of years by the remains or animals and plant compressing and cementing under great pressure. Irelands sedimentary rocks were formed at various stages as the Eurasian plate drifted north from its original location near the Antarctica about 500 million years ago. The climates Ireland experienced on its journey and the position above and below sea level helped the formation of limestone rock. Limestone formed under the sea near the equator. When shell sea creatures die, they remain on the sea bed sometimes a victim of the waves which break them up into tiny pieces and sometimes forming mud. This is what makes up the limestone, the sediments and mud. The sediment builds up and are compressed by their own weight cementing together and forming the rock. Limestone also forms when water containing calcium carbonates evaporates and deposits sediments on the sea floor. Over millions of years layers of sediment builds up, compresses and cements forming the rock. Limestone rock contains layers called strata, vertical joints and horizontal bedding planes. Irelands limestone is called carboniferous limestone and it is the most common rock in Ireland. As the rock contains calcium carbonate it is a victim of carbonation which is evident at the Burren Co. Clare, this is a karst region, an area of exposed limestone that has been formed and altered by chemical weathering.
Inorganic sedimentary rocks are those made from lithified material e.g. other rocks that have been weathered down into sediments. The more common type of inorganic sedimentary rock is sandstone and the most common type of sandstone in Ireland is old red sandstone (ORS). This ORS was formed around 350 million years ago when Ireland was located at the same latitude as Southern African Deserts and has the same climate that they do today. The Caledonian mountains of Galway, Mayo, Donegal and Wicklow were victims of heavy rain from time to time. Flash floods flooded the lowland areas as the rain rushed down off the side of the mountains. Debris from the floods spread out over the lowland channels and floodplains in the form of beaches (along the seashore), gravel and sand. Over long periods of time and after a huge build-up of such sediments the deposits were cemented into larger particles. The cemented gravel made a rock called conglomerates and the cemented sand created sandstone. Large dunes of sand were also compressed into sandstone but later it was covered with limestone deposits. The sediments are laid down in layers and each successive layer compressed and compacted the next layer. Sandstone is exposed in the mountains of southern Ireland e.g. Macgillycuddy Reeks, the Galtees, and the Knockmealdowns. Sandstone is a coarse grained rock and usually brown or red in colour. The iron particles trapped in the rock give them the reddish colour as they become rusted.
Metamorphic Rocks
Rocks are continually formed, changed, destroyed and reconstructed as part of the rock cycle. The rock cycle is the process by which each rock type can be changed into another. Rocks vary in their origin, formation and characteristics. There are 3 main rock categories, igneous, sedimentary and metamorphic. This essay will focus on the formation of metamorphic rocks. The examples I will use are marble and quartzite. Metamorphic rocks are formed when igneous and sedimentary rocks come into contact with great heat (magma) and/or pressure (folding) and their chemical composition is changed forming a new rock type e.g. limestone changes to marble.
Metamorphism is the process in which a rocks composition is changed by great heat or pressure, there are two types; regional metamorphism and thermal metamorphism. Thermal metamorphism is the process of using heat alone and regional is where great heat and pressure will be applied over a large area e.g. fold mountain building.
Marble is created through regional metamorphism. Limestone changes to marble through great heat and pressure where the calcite in limestone changes and fossils disappear as interlocking grains grow. Marble is created at destructive plate boundaries where fold mountains are made. Most of Irelands metamorphic rocks were formed during the Caledonian fold mountain period. Here the North American and European plates collided causing the crust to buckle upwards and magma rose through the cracks inside the buckled rock. The pressure from the folding and heat from the magma caused limestone to change into marble. The metamorphism process changed the chemical composition of limestone. We know this as limestone is grey however Kilkenny marble is black and cork red. If the limestone is pure it forms white marble. However as limestone contains layers of clay and other materials it may form attractive flow banding (marble effect) and the other colours such as black, green and pink.
Quartzite is created through thermal metamorphism. Sandstone changes to marble through great heat. This type of metamorphism occurs when the heat from volcanic activity changes the rock that it comes into contact with. Quartzite in the Sugarloaf mountain in Co. Wicklow was formed when hot molten magma came into contact with sandstone changing the composition of the rock. Quartzite is a light coloured rock and can also be found in Croagh Patrick, Co. Mayo.
~Sample Answer (Human Interaction)
Humans interact with the rock cycle in many different ways, I will explain this process by examining Geothermal Energy in Iceland.
Iceland sits on the middle of the Mid-Atlantic Ridge, a constructive plate boundary where a lot of volcanic activity takes places. The Mid-Atlantic Ridge is a huge opening in the ocean floor created as the North American and Eurasian plates separate. Iceland was created when magma rose through this opening and cooled and hardened on the earth’s surface, over time it built up creating an island called Iceland.
Geothermal energy uses heat from rocks (igneous rocks) to heat water, turn into steam and resulting in the creation of energy. This type of energy is produced in Iceland due to the endless amounts of volcanic activity there. Over 90% of houses in Iceland are heated naturally by geothermal energy, this type of energy is one of the cheapest and cleanest forms of energy that exists today. The magma rises from the mantle and heats the surface of the earth to around 200 degrees Celsius, this heats the water below the rocks. In order to retrieve the boiling water a well is drilled into the rock. The hot water is collected in pipes that are placed in the mantle. The water is superheated but stays in liquid form due to the pressure from the rocks above. However, once this water reaches the surface it changes state and turns to steam, the steam turns turbines and electricity is generated from these turbines. If there is not enough groundwater present then large pipes of cold water must be pumped into the rock through another series of wells. This water will be around 10 degrees Celsius.
Hot springs can be found all over Iceland and the melt water created provides the country with the potential to create huge amounts of hydroelectric power. Geothermal energy has many benefits for Iceland as a country. It provides 89% of the electricity uses in Reykjavik (capital city) to homes and industry. The geothermal energy is used to heat greenhouses and therefore increasing the production of potatoes and other vegetables. This has creates a longer growing season and encourages horticulture to thrive in Iceland.
Geothermal energy has also created benefits in the tourism industry. Nowadays many tourists travel to visit the ‘blue lagoon’ spa which is a mineral rich, outdoor, heated pool. Tourists also go to Iceland to visit the springs and geysers which are created by the volcanic activity. This increase in tourism adds to the economy of Iceland generating capital in help the development of tertiary economic activities. Iceland has less need to use oil, gas and other non-renewable sources of energy reducing their carbon footprint and the negative impact they have on the environment.