The Formation of Rocks in Ireland

It is the most widespread type of intrusive igneous rock at the earth’s surface. Igneous rocks form when molten rock is erupted at a volcano and then cools and hardens to form solid rock (Holden, 2012). Granite rock is created when magma is forced between other rocks deep within the earths crust. The magma then cools due to the drop in temperature and crystallizes in caverns deep within the earth. The molten rock cools quite slowly over time, which allows the crystals to grow and develop inside the molten rock. Granite is transported to the surface of the earth by plate tectonics.

The collision of continental plates moves the granite from deep within the crust of the earth to the surface. Granite is found in Ireland in the Wicklow Mountains. (http://www. talktalk. co. uk/reference/encyclopaedia/hutchinson/m0015252. html) Granite is distinguished by three features. Firstly, granite is composed of large mineral grains that bind tightly together. Secondly, granite always contains the minerals of quartz and feldspar. (http://www. windows2universe. org/earth/geology/ig_granite. html) These minerals give granite a light complexion, generally a pink or white colour.

Thirdly, almost all granite is igneous and plutonic. This is because it formed from a fluid state, which was magma and it formed in a huge, deeply buried body also known as a pluton (http://geology. about. com/od/more_igrocks/a/granite. htm). Basalt Basalt is a common extrusive igneous rock. Basalt is commonly dark in colour. It has a fine-grained mineral texture which is due to the rapid cooling of the molten rock after it is erupted at a volcano. The rapid cooling occurs due to being exposed to the cooling air or water on the surface of the earth. Holden, 2012) Basalt is formed generally in three different environments: Oceanic divergent plate boundaries, oceanic hotspots and at hotspots and mantle plumes beneath continents. Basalt forms at oceanic divergent plate boundaries at mid-ocean ridges. At mid-ocean ridges convection currents pull the plates apart creating a rift valley. This allows hot rock from deep within the earths mantle to melt and erupt onto the seafloor forming basalt. (Honan, 2011) At oceanic hotspots basalt is also formed. Hotspots are areas in the cente of tectonic plates where hot plumes of magma force there way to the surface.

Basalt is formed at these hotspots when an eruption occurs on the ocean floor. If the eruptions are repeated enough times the basalt can build the volcanic cone larger and larger to form an island. The Hawaiian Islands are an example of this. (http://geology. com/rocks/basalt. shtml) Basalt is also formed in a continental environment where mantle plumes or hotspots can deliver large quantities of magma up to the earths surface. These eruptions generally form from fissures or vents and they produce some of the largest basalt flows on the continents. (http://geology. com/rocks/basalt. html) Basalt can be found in Ireland in the Giant’s Causeway in County Antrim. Limestone Limestone is a sedimentary rock. Sedimentary rocks are created by the processes of chemical or physical weathering . The sediment which is created from the processes of weathering can accumulate over time and eventually build up and harden to form solid rock. (Holden, 2012) Limestone in Ireland formed about 300 to 350 million years in warm shallow seas when Ireland lay close to the equator. Over millions of years the bodies of dead marine creatures such as shellfish and coral fell to the seafloor and built up on the seabed.

Mud and sand particles from rivers also fell to the seafloor and added to the sediment. Over time the weight of the accumalting sediment compressed the sediment. Water and air was pushed from the pore spaces and calcium carbonate from the sediment binded the rock together. The process by which loose grains of sediment are compressed to form solid rock is called lithification. (Honan, 2011) Chemical weathering and eroison of limestone rock produced unusual landscapes in Ireland called karst landscapes. An example of a karst landscape in Ireland is the Burren in County Clare. Honan and Mulholland , 2007) Sandstone Sandstone is an inorganically formed sedimentary rock. It forms when particles of igneous, metamorphic and other types of sedimentary rocks are transported by rivers and gather on the seafloor or in lakes. The sediment layers then become compacted and over time cemented together by silica to form sandstone rock. (Honan S. , 2011) Sandstone in Ireland was formed 400-350 million years ago when Ireland was situated 30 degrees south of the equator. During this period Ireland was experiencing some very dry desert conditions.

Many large rivers flowed across the dry landscape carrying the sediments which over time formed Old Red Sandstone which can now be found in the Cork and Kerry Mountains. (Honan and Mulholland , 2007) Schist Schist is a metamorphic rock. Metamorphic rocks are produced from existing sedimentary or igneous rocks usually as a result of partial melting and recrystallization. The changes normally occur where there is high pressure such as under hundreds of metres of bedrock or where rock is crushed at the meeting point of tectonic plates. Many metamorphic rocks have a layered structure due to this intense external pressure.

Metamorphic rocks are often harder and more resistant than sedimentary rocks. (Holden, 2012) Schist in Ireland was formed through regional metamorphism. This is when large areas of rock covering many thousands of square kilometres are metamorphsed. Schist in Ireland was formed about 400 million years ago during the Caledonian folding period. The rocks changed due to huge pressure on the rocks created by plate movement. This deformed the rocks and forced them deep into the earth’s crust where they were heated to temperatures up to 1,000 degrees.