Rocks are continuously being formed, changed/modified, destroyed, and reconstructed by forces within the Earth. This is known as the rock cycle.
Rocks are formed when magma rises, then cools and solidifies to form igneous rocks. This may occur on the Earth's surface or within the Earth's crust (intrusive or extrusive crystallisation). Igneous rocks may be destroyed/broken down by weathering and erosion (denudation) into sediments. The sediments are transported by moving water, ice and wind. Rocks are reconstituted where sediments are compressed, compacted, and cemented under the weight of water to form sedimentary rocks (lithification). They form layers known as strata. Sedimentary rocks may be changed/modified by great heat and pressure to form metamorphic rocks. Igneous rocks may also be changed by heat and pressure to form metamorphic rocks. Metamorphic rocks may be broken down into sediments by weathering and erosion. All rock types are destroyed at subduction zones where they melt back into the mantle. The cycle continues.
ROCKS ARE GROUPED ACCORDING TO THEIR FORMATIONIgneous Magma/lava cools e.g. basalt, granite(Latin for fire) and becomes solid pumice Sedimentary When sediments e.g. limestone, coal (plants, remains of sandstone, shale animals, rocks), conglomerate accumulate, compress, compact, and cement into a solid. Metamorphic When a rock is e.g. marble, quartzite(Latin for subjected to great heat slate, gneiss, schistchange) and/or pressure, it is changed.
Igneous rocks: Plutonic (intrusive) Volcanic (extrusive)
PLUTONIC (INTRUSIVE) ROCKSForm when magma cools slowly within the Earth's crust. It may cool at a depth of several kilometres e.g. granite. Granite has large crystals because the magma cooled slowly (takes longer to cool within the crust).Granite is coarse in texture. It usually has a grey, white, black, or pink colour. The colour can vary because of the differing percentages of the minerals mica, feldspar, and quartz.In Ireland, granite formed over 400 million years ago during the Caledonian foldings. Granite may be found in the Mourne mountains Co.Down and the Dublin-Wicklow mountains - The Leinster Batholith that was exposed due to denudation.Granite is a strong, resistant rock and is commonly used as a building material. It is often polished and used for kitchen worktops and fireplaces. When granite is broken down by weathering it produces clay.
VOLCANIC (EXTRUSIVE) IGNEOUS ROCKSForm when magma cools outside on the Earth's crust. These have smaller crystals because lava cools quickly when exposed to the elements e.g. basaltBasalt is a fine-to-medium-grained igneous rocks. It ranges in colour from dark grey to black. It is heavy and has small crystals (too small to be seen by the naked eye). It is the most common extrusive rock in Ireland.Basalt is composed of feldspar and quartz. It is a hard, resistant rock and is therefore used for road construction. It is too dark to use for buildings. In the Antrim-Derry Plateau, lava poured out of fissures in the ground (60 million years ago). It cooled and contracted to form six-sided columns of basalt e.g. The Giant's Causeway
Sedimentary rocks form from the build up of sediments over millions of years. The weight of these sediments and the weight of the overlying water causes the underlying layers to compress, compact, and cement into solid rock (lithification). As a result, they form in layers called strata and the weaknesses (horizontal) are known as bedding planes. The vertical weaknesses are known as joints. As a result, sedimentary rock is permeable (lets water through)
There are two types: Organic sedimentary rocks e.g. limestone and coal Inorganic sedimentary rocks e.g. sandstone and conglomerate
1. Organic sedimentary rocks come from the remains of once living organisms e.g. plants and animals e.g. limestone.Limestone is the most common rock in Ireland. It formed on the bed of warm tropical shallow seas about 350 million years ago when Ireland lay close to the Equator (during the Carboniferous era). Over millions of years the bodies of dead marine creatures e.g. shells, skeletons, teeth, coral, etc. accumulated on the sea floor. Over time the weight of the sediments and the great weight of water caused compression, compaction and cementing into solid limestone, a process known as lithification. The grains are cemented with calcium carbonate which comes from shells. Limestone forms in layers called strata which are separated by weaknesses called bedding planes, the vertical weaknesses are called joints and these make limestone permeable and therefore easily weathered by carbonation. Limestone may contain fossils as it forms at temperatures and pressures that do not destroy fossil remains. It varies in colour from white to grey to black depending on impurities in the water. It is commonly used to make cement and for construction. It is found across the central plain of Ireland and most famously in the Burren, Co.Clare. When weathered by erosion it forms a very distinctive Karst landscape.Chalk is a pure, white form of limestone.
2. Inorganic sedimentary rocks (mechanical) form from the sediments of pre-existing rocks e.g. sand, clay, pebblesThe sediments are transported by wind, water, and ice to the lakes, oceans, and seas. Here, they are compressed, compacted, and cemented under the weight of the sediments and the weight of the water, a process known as lithification e.g. sandstone.Sandstone is the most common inorganic sedimentary rock in Ireland. It is common to the mountains of Munster e.g. Caha and is known as old red sandstone. It is a coarse rock and usually brown or red in colour (due to the presence of iron oxide). When it formed Ireland had a desert type climate, 400 million years ago. It formed when grains of sand or quartz deposited on land or in water are compressed and cemented together. It forms in layers called strata. Strata are seperated from eachother by hairline cracks called bedding planes. Sandstone is porous which means it has spaces between the grains that can hold oil or gas. It is commonly used as a building rock e.g. front of houses and for road chippings. e.g. Comeragh mountains (Waaterford) and Magillycuddy's Reeks Co.Kerry
Other sedimentary rocks:Shale - clay compactedMudstone - mudConglomerate - gravel/pebblesCoal - vegetation
Metamorphic rocks formed when igneous or sedimentary rocks underwent change (texture, composition, appearance etc.) as a result of great heat and/or pressure. When heat causes these rocks to change/metamorphose, it is called thermal/contact metamorphism. When tectonic plates collide, the great pressure from this collision causes rocks to metamorphose - dynamic or regional (over great areas).
Limestone --> marble e.g. Connemara Co.GalwaySandstone --> quartzite e.g. Sugarloaf Mountain Co. Wicklow, Croagh Patrick Co. Mayo
QuartziteQuartzite is sandstone which changed due to great heat and pressure. The spaces between the grains of quartz are filled with silica (from magma). The quartz and silica are packed tightly together. As a result, quartzite is very resistant to erosion.Quartzite is a light-coloured rock, which is often white in colour, and is found on the peaks of mountains such as Croagh Patrick, Co. Mayo and the Sugarloaf Co. Wicklow. Because it is hard, it is commonly used for road surfacing.
Effects of metamorphismHARDNESS: Harder than their original rock type.COLOUR: colour of the rocks may change due to the effect of heat e.g. limestone is grey but when metamorphosed the resulting marble may be green, yellow, or white.FOLIATION: Mineral crystals may be rearranged to form foliated metamorphic rock such as gneiss. In this case the minerals are flattened in parallel layers. Some rocks have their sediments flattened and hardened into very thin layers e.g. slate.CHEMICAL COMPOSITION: Heat and pressure may cause the rock minerals to recombine to form different minerals in the metamorphic rock.
MarbleMarble is a hard crystalline rock of various colours. It is found in Connemara (green), Cork (red marble), and Rathlin Island, Antrim (white).Marble is a metamorphic rock formed by great heat and pressure on limestone or chalk. White marble is the result of pure limestone changing in composition. When impurities exist, the marble will not be pure white e.g. iron oxides, silt, sand, and clay. These impurities may have been present in layers in the original limestone.Marble can be cut and polished and is used for kitchen worktops, fireplaces, ornaments, and gravestones. People extract marble for building materials by quarrying.
The Rock Cycle