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Planet Earth Background HOW ROCKS ARE MADE By way of information for teachers it will be useful to understand the origins and classification of rocks. Whether or how much of this background material is taught to primary classes is a matter for individual teachers to decide. Based on their sources and methods of formation rocks are usually classified under three headings — Igneous, Sedimentary and Metamorphic. Igneous (derived from the Latin word ignis meaning fire) These rocks are formed by the cooling of very hot molten material called magma which originates deep down inside the earth. It rises upwards towards the surface of the earth because it is hotter and lighter than the surrounding rock. If it reaches the surface of the earth it will erupt as a volcano at which stage it becomes known as lava. Such an event is known as an extrusion. Due to exposure to air and water it cools fairly quickly and because of this the crystals which make it up are usually very small and this is described as fine grained. The best local example of this is the Antrim Plateau and the specific rock is dark or black in colour and is known as basalt. Alternatively, the magma may not reach the surface of the earth and this leads to it cooling very slowly deep below the surface — maybe over thousands or millions of years. This type of event is known as an intrusion. The slow cooling process allows the crystals to become quite large and easily seen. Such a rock is called granite and it is described as being coarse grained and it is usually light in colour with clearly visible mineral crystals. The best example in Northern Ireland is the Mourne Mountains in south Co. Down. The fact that such granite intrusions are found on the surface of the earth today illustrates that, while it did originally cool slowly at depth, erosion and weathering over millions of years have worn away and removed the overlying rocks. Sedimentary These rocks are formed by the deposition of fragmentary materials which have been eroded and weathered from other parent rocks. As such they could be regarded as recycled rocks. They often consist of sand, pebbles and mud removed from the land by erosion, carried by rivers or blown by the wind, and eventually deposited. These sediments are usually deposited in water in seas and lakes but they can also accumulate in desert environments. They are often deposited in layers which can be called strata. As the layers accumulate one on top of another they become buried underneath younger sediments and they become compressed into solid rock. The red sandstone at Scrabo, Newtownards, Co. Down is an example of a sedimentary rock which was deposited in desert conditions. Some sedimentary rocks are of chemical or organic origin. In some cases the deposits in the sea may consist almost entirely of the shell fragments of dead sea creatures. These shells are made largely of calcium and when they accumulate and become compressed they form a rock called limestone. A good example of such limestones can be visited at Marble Arch Caves Global Geopark, Co. Fermanagh. 1 Planet Earth Background ROCKS In other cases, the accumulation of large amounts of dead plant material may, over millions of years, turn into coal which is another type of sedimentary rock It may be that some dead sea creatures are not fragmented and become buried in their original condition. When this happens they can become preserved in the developing rock and these are known as fossils. Metamorphic (Greek: meta meaning change and morphe meaning form) If sedimentary or igneous rocks become deeply buried they can become subject to very high temperatures and earth movements can subject them to intense pressure. Such an environment may melt the original rocks and deform them — hence the term metamorphism which means change in form. Sometimes already buried rocks may come in contact with hot igneous material and, without becoming molten, they become baked or cooked; this is known as contact metamorphism. If a limestone is subject of metamorphism it becomes harder and more crystalline and it then becomes known as marble. When a shale rock is subjected to metamorphism it becomes compressed into layers at which stage it is known as slate. There are few good sizable examples of either marble or slate in Northern Ireland but slates are common in north Wales and marble can be mined in Connemara, Co. Galway. ROI. 2 Planet Earth Background ROCKS Basalt Basalt is the most common type of rock in the Earth’s crust and it makes up most of the ocean floor. It is dark grey to black in colour as it is made up of many dark coloured minerals. It is a crystalline rock but typically you can’t see most of the mineral crystals without using a microscope. This is due to the way in which basalt is formed. Much of the Earth’s basalt was made millions of years ago when molten rock (magma) welled up through long cracks in the earth’s crust and spilled over in rivers of lava (the name given to magma once it reaches the Earth’s surface) sometimes many miles wide. Since lava coming into contact with the air tends to cool and solidify quite quickly crystals do not have the opportunity to grow to a size that can be seen by the human eye. ERC ©N 8078 PJW Generally, basalt forms as layers of rock with each representing a separate lava flow. However, the basalt seen at the Giant’s Causeway on the Antrim coast of Northern Ireland is different; the lava cooled and cracked into huge columns, most of them with six sides. This happened here because the lava flowed into a huge river valley meaning that a large volume of lava gathered and required a much greater time to cool down. When this happened the lava contracted internally, with joints that extended all the way vertically upwards, giving rise to the wonderful basalt columns at the Giant’s Causeway. Smooth flowing lava flows from volcanoes today and cools to form new basalt rock (check out Hawaii). New basalt rock is also forming on the Atlantic Ocean floor as magma wells up into the crack between the plate boundaries (cracks in the Earth’s crust) and cools when it comes in contact with ocean water. © L P Field © NERC P006483 Basalt is a hard, durable rock due to the interlocking nature of its crystalline structure. It is used for a variety of purposes, but most commonly as an aggregate in construction projects. When crushed, it can be used as road base. Glowing lava hardening into black basalt on the island of Hawaii. Basalt columns at The Giant’s Causeway. Basalt lava poured onto the surface here 60 million years ago. A lake of lava filled a shallow depression in the surface and cracked into these polygonal columns when it hardened and contracted. 3 Planet Earth Background ROCKS Granite Granite is a light coloured, speckled rock. If you look closely at a piece of granite you will see little flecks of white, black, grey and sometimes pink. These flecks of colour are the mineral crystals, which in the case of granite are large enough to be seen with the human eye. Granite is formed like basalt when hot molten rock cools down and hardens. However granite differs from basalt in a very important way. The magma from which it is formed never reaches the Earth’s surface. Instead it cools and hardens deep underground and because it may take thousands of years to cool down the minerals are able to grow into large, easily visible crystal grains. The crystals that make up granite interlock together, tightly which is why granite is such a durable rock. It can be seen covering the walls and floors of many public buildings and polished granite slabs are also used for kitchen worktops and for headstones. © NERC P725208 8074 RC PJW © NE © NERC P703146 The Mourne Mountains in County Down were formed when molten magma cooled just below the land surface. The granite of which these mountains are formed is now at the surface due to the erosion of the overlying rocks. Mount Rushmore in the USA has the faces of four American presidents carved into the granite mountainside. 4 Planet Earth Background ROCKS Sandstone Sandstone can come in a variety of colours such as red, brown, pink, tan, yellow and even white or black. It is composed of numerous individual sand grains, held together by a type of mineral cement and/or compaction. Rocks such as sandstone are formed from the broken-down pieces of other rocks, held together as a result of pressure and/or by minerals deposited between the grains to act as cement. This happens when rocks at the Earth’s surface are weathered and hence broken into small particles by the forces of wind and water. These particles (sediments) often end up being transported by water and find their way to the bottom of lakes, oceans, rivers or streams, where they pile up in layers often hundreds of metres thick. The weight of all the sediment presses down on the lower layers with tremendous force, compacting it and turning it into rock-like sandstone. © NE RC PJW 8076 Sandstone is a popular building material. It is relatively soft, making it easy to carve, but it is generally less resistant to weathering, as the photo of a weathered sandstone carving in Belfast illustrates, and many other buildings throughout Northern Ireland have used sandstone as a building stone. © Albert Bridge Layers of sandstone on the shore at Cushendall, County Antrim. Old Red sandstone sample from Cushendun, County Antrim. Weathered pillar, Mount Charles, Belfast. 5 Planet Earth Background ROCKS Shale Shales commonly come in shades of grey and black. The surface of the rock can feel very slick and shiny almost like satin. Like sandstone it is composed of compacted grains laid down in layers, the main difference being that the grains are not easily visible as they are so small (i.e. it is composed of clay-sized rather than sand-sized particles*). Shale splits very easily into thin layers/brittle sheets. Black shales contain organic matter that sometimes break down to form natural gas or oil. In this type of shale tiny particles of organic matter are deposited with the mud/clay from which the shale formed. As the mud was buried and warmed within the earth some of the organic material was transformed into oil and gas. This oil and gas is very difficult to extract because it is trapped within tiny pore spaces or absorbed into clay mineral particles that make up the shale. In the late 1940s it was discovered that pumping water (containing a mix of sand and chemicals) under pressure into the shale would cause it to fracture, liberating some of the gas from the pore spaces. This technique is called hydraulic fracturing (fracking) and has caused much controversy here in Northern Ireland due mainly environmental concerns about the fracking process. ©N PJW 094 © NERC P51960 0 © NERC P744353 ERC 6 Planet Earth Background ROCKS Limestone and chalk White Park Bay in County Antrim. © NE RC PJW 80 85 Grey limestone. White limestone or chalk. © NERC P225295 © NERC P225072 Limestone has a very interesting property that sets it apart from other rocks. The calcium carbonate of which it is composed reacts with rainwater that is slightly acidic and this results in the rock dissolving. Large cave systems, e.g. Marble Arch Caves in Fermanagh, are typically found in areas of limestone rock, where water has percolated through cracks joints are the vertical cracks and bedding planes the horizontal cracks in the rock and over time has dissolved and eroded the rock creating huge caverns underground. Limestone has many uses. It has quite a history as a building material, having been used by the ancient Egyptians in the construction of the pyramids. It is the main ingredient in cement, which when mixed with sand water and crushed rock, forms concrete. Crushed limestone is often used underneath road and rail beds. Farmers will often spread fertiliser that contains ground up limestone on their crops as it is a source of plant nutrients and helps neutralise soil acidity. © NERC P521424 Limestone is usually a light colour like white, tan or grey. It is composed like other rocks of this type (e.g. sandstone and shale) from material deposited on the ocean floor and compacted over long periods of time by the weight of the sediments above. However, limestone is composed of different material. Ocean-dwelling organisms such as oysters, clams, mussels and coral use calcium carbonate (CaCO3) found in seawater to create their shells and bones. As these organisms die, their shells and bones are broken down by waves and settle on the ocean floor where they are compacted over millions of years, creating limestone from the sediments and the pressure of the ocean water. The colour of limestone depends on the other sediments in the mixture besides the mineral calcite (calcium carbonate) e.g. sand, clay and organic material. Chalk (or white limestone) is a pure form of limestone made from the shell remains of microscopic marine organisms. Maggie’s Hole, Marble Arch Caves. 7