Geology of the British Isles What rock is that? Have you ever wondered how to go about identifying rocks? Well, here's your chance! You too can be a geologist, but beware! You might have to do a bit of thinking along the way. Minerals A 'mineral' is a solid material, formed by natural processes and with a chemical composition that falls within certain narrow limits. Minerals are made up of atoms which are arranged in a regular pattern, so they form ‘crystals’ with characteristic shapes, like cubes, sheets or pyramids. Rocks A 'rock' is a solid collection of mineral grains. These may be fragments of crystal or whole crystals and they can be mm to cm in size. A 'rock' may have only one type of mineral, but usually it consists of several different minerals. So how do you know whether you're looking at a rock or a mineral? A rock is an aggregate of one or more types of minerals which may be present as crystals or grains. If your specimen is a single (or collection of) crystals, all made up of the same thing, your probably looking at some kind of mineral. If it’s metal, you could be in luck, it might be a native element like gold or silver, or more likely, it's one of the sulphide minerals like pyrite - often known as fool’s gold. Alternatively, there's a very small outside chance it's part of a meteorite! In which case you should send it with details of when and where you found it to either the Natural History Museum or to the Open University, and get us to identify it for you. (Don’t worry, we'll send it back!) If you're fairly sure you’re looking at a rock. Before you try to work out its mineral composition, you should first look at its texture. Texture Texture is the term used to describe the physical relationships between the particles from which the rock is made. Virtually all rocks have textures that are either crystalline, (made of crystals) or fragmental (made of grains). In rocks with a crystalline texture, the minerals have grown together as interlocking crystals. In rocks with a fragmental or granular texture, individual mineral and rock grains have been transported and deposited, and may be quite rounded. In either case the size of the individual grains can vary considerably. Rocks with crystalline texture are usually harder and more compact than those with granular texture. When crystalline rocks are broken they tend to fracture along smooth, angular surfaces within individual crystals, rather than between crystals. The result is that the broken surfaces of crystalline rocks have lots of flat faces that reflect and glint as they catch the light. Rocks with granular texture usually fracture between individual grains because the ‘cement’ holding them together is often weaker than the grains themselves. Layering After texture, grain size and colour, you should consider if there are any 'structures' in your rock. By 'structure' we mean are there any bands, sheets or layers? Layers can be bands of different coloured crystals, different sizes of grains, or the rock may simply split into thin sheets, like slate. Layers in rock appear in different ways - Ribbon-like layers of crystals Grain size layers Thin platy layers or sheets Ribbon-like layers of crystals: In some rocks different coloured minerals are lined up in ribbons. Usually in two colours, black and white, or green and white, or black and pink. Ribbon-like layers are often typical of metamorphic rocks where the minerals grow in layers in response to pressure. Bands and layers of crystals are usually an indication of a metamorphic rock. Grain size layers: In sedimentary rocks, grains of different sizes are deposited as the strength of the current waxes and wanes (gets stronger or weaker). Larger grains are deposited by stronger currents. Smaller grains are deposited by weaker currents. Often when grains are sorted by wind or water, they will be the same composition and shape, but different sizes; and therefore different shades of the same colour. Bands and layers of grains usually indicate a sedimentary rock. Thin platy layers or sheets: Some minerals grow naturally to form platy sheets - but it takes added temperature and pressure to bring them all into line so that they form sheets or layers. Clay minerals are typically flat and platy, but if they undergo metamorphism they will line up into sheets and form a rock which easily splits - like slate. For example: mudstone contains lots of flat platy clay minerals - but it doesn’t easily split into layers because the minerals are arranged in a random pattern. However, if you add heat and pressure all the clay minerals will align themselves so they are roughly parallel to each other and the rock will split very easily giving you “slate”. Fossils What are fossils? The word fossil comes from the latin word ‘fossilis’ which means 'dug up'. A fossil is the remains or impression of any plant or animal which has been buried in sediment and preserved in a sedimentary rock. Less commonly, fossils can also be preserved in ice, e.g. woolly mammoths, or even in lava flows and volcanic ash, e.g. charred tree stumps. There are two main kinds of fossil: a body fossil which is the actual remains of a plant or animal and a trace fossil, which records the activities of the animal, like footprints, burrows and bit marks. How do fossils form? In principal forming a fossil is quite easy. A plant or animal just has to get buried in sediment. Sadly the process is not that simple. Not all organisms get buried, and not all of those that are buried are preserved as fossils. Normally the first thing that happens is the death of the organism. The soft body parts then decay or get eaten by carnivores. Only in very rare and exceptional cases are the conditions right to preserve the soft parts of an organism. If the remaining hard parts, like the skeleton or shell, are then quickly buried by sediment they have a chance of being preserved. If not, they may be scattered by predators or broken up by wind and water. Even when buried, potential fossils may be destroyed by chemical solutions percolating through the sediment and dissolving the hard parts. As the sediment continues to compact and turn into sedimentary rock, the remaining hard parts may themselves undergo change. For example, the original skeleton may be dissolved and replaced by a new mineral. Why are fossils important? Fossils tell us something about the biology of now extinct plants and animals and sometimes also about the environment in which they lived. By comparing fossils with present animals and plants we can also begin to map out parts of the evolutionary tree of life. Fossils are also useful for the relative dating of sedimentary rocks and in some cases they also serve a practical economic purpose. The oil, gas and coal industries make extensive use of fossils in rock formations to help them date and correlate rocks so that they can then locate and exploit reserves. Grain Size Once you have decided if a rock is crystalline or granular it is helpful to work out the grain size - the size of crystals or grains in the rock. There are three broad categories: Coarse Grained: most of the rock is made of crystals or grains as large as rice, or larger. An example of a coarse grained rock is granite. Medium Grained: you can see the individual crystal grains without a magnifier, but most of the rock is made of grains smaller than rice, for example, sandstone. Fine Grained: the individual grains cannot be seen without a magnifier or microscope. Colour Rocks come in a multitude of colours, for simplicity we have divided them into dark, intermediate and light. - Dark coloured rocks: my be black, dark brown, dark grey or dark green. - Intermediate coloured rocks: are pale grey, reddish brown or green. - Light coloured rocks: are pale pink, pale green, light grey or white. Tip: the colour of a rock can also be influenced by its grain size. Very fine grained rocks often appear much darker than their coarse grained or coarsely crystalline equivalents. Granite Granite is a coarse-grained intrusive igneous rock. It contains large interlocking crystals which are randomly oriented. It is light in colour and contains a number of minerals, mostly feldspars, which are pale grey or pinkish, and quartz which is white or grey. It also contains small black specs of mafic (dark coloured) minerals. How was it formed? Granite formed deep in the Earth’s crust from large masses of cooling magma that never made it to the surface. Slow cooling produces the large crystals. Granite forms from magma that contains a lot of quartz silica. If you're interested in finding out more about geology, why don't you take a look at the Open University course SXR260 The Geological History of the British Isles? Microgranite Microgranite is a medium-grained intrusive igneous rock. It contains crystals, smaller than grains of rice, which are interlocking and randomly oriented. It is pale grey and can sometimes be pinkish in colour. It contains a number of minerals, mostly feldspars, which are pale grey or pinkish, and quartz, which is grey or white. It also contains small specs of mafic (dark coloured) minerals. How was it formed? Microgranite is the medium-grained equivalent of granite. The crystals are slightly smaller than granite indicating that the magma cooled more quickly. It usually occurs in smaller intrusions than granite. Microgranite forms from magma that contains a lot of quartz (silica). Rhyolite Rhyolite is a fine-grained extrusive igneous rock or volcanic rock. It is pale coloured, often light grey, tan or pinkish. Rhyolite is made up of quartz and feldspar crystals, and occasionally contains some matic (dark coloured) minerals. Usually the crystals are too small to see without magnification, but occasionally contains larger crystals, or small round pockets that were gas bubbles. Sometimes it can be banded. How was it formed? Rhyolite is a volcanic rock. It is fine-grained because it forms by the rapid cooling of magma, usually when it erupts onto the Earth's surface. When rhyolite erupts quietly it forms lava flows. If it erupts explosively it often forms pumice. Rhyolite forms from magma that contains lots of silica (quartz) and is the fine-grained equivalent of granite. Diorite Diorite is a coarse-grained intrusive igneous rock. It contains large interlocking, randomly oriented crystals. It is a dark coloured rock, usually medium to dark grey, containing many mafic crystals. Mostly it looks like dark coloured granite. How was it formed? Diorite formed deep within the Earth’s crust from cooling magma that never made it to the surface. It usually occurs as quite small intrusions often associated with larger intrusions like granite. Slow cooling produces the large crystals. Diorite forms from magma that does not contain a lot of quartz (silica) or the light coloured minerals that make up granite. Microdiorite Microdiorite is a medium-grained intrusive igneous rock. It contains crystals that are smaller than grains of rice, which are interlocking and randomly oriented. It is dark grey or greenish brown. How was it formed? Like diorite, microdiorite forms from magmas that does not contain much quartz (silica) or the light coloured minerals that make up granite. The crystals are smaller than those in Diorite, indicating that the magma cooled more quickly. It usually occurs as small intrusions called ‘dykes’, which are sheet-like and cut through the surrounding rocks. Andesite Andesite is a fine-grained, extrusive igneous or volcanic rock. It is dark grey and made up of equal amounts of light and dark minerals, although the crystals are too small to be seen without a magnifier. Occasionally andesite may contain some larger crystals. Or small round pockets that were gas bubbles. How was it formed? Andesite is a volcanic rock. It is fine-grained because it forms by the rapid cooling of magmas usually when it erupts onto the Earth’s surface and forms lava flows. Andesite forms from magma that contains less quartz (silica) than rhyolite but more than basalt. It is therefore often referred to as 'intermediate' in composition. Andesite is the fine-grained extrusive equivalent of diorite. Gabbro Gabbro is a coarse-grained, intrusive igneous rock. It contains interlocking crystals, larger than rice grains, which are randomly oriented. It is dark green to black. It contains mainly mafic (dark coloured) minerals. How was it formed? Gabbro forms deep within the Earth’s crust, often from very large masses of magma that do not reach the surface. Slow cooling produces the large crystals. Gabbro forms under similar conditions to granite and diorite. But unlike granite, gabbro forms from magma that is rich in iron and magnesium, and poor in quartz (silica). Microgabbro or Dolerite Microgabbro is a medium-grained intrusive igneous rock. It contains crystals, smaller than rice grains, which are interlocking and randomly oriented. It is dark green to dark grey, with occasional rare paler crystals. How is it formed? Dolerite is the medium-grained equivalent of gabbro. The crystals are slightly smaller than gabbro, indicating that the magma cooled more quickly. It usually occurs as small intrusions called ‘dykes’ or ‘sills’ which are sheef-like and cut through the surrounding rocks. Like gabbro, dolerite forms from magma that is rich in iron and magnesium, and poor in silica (quartz). If you're interested in learning more about igneous intrusions, the Open University course S260 Geology covers the formation of igneous and metamorphic rocks and their associated structures. http://www3.open.ac.uk/courses/bin/p12.dll?C01S260 Basalt Basalt is a fine-grained, extrusive igneous or volcanic rock. It is usually black and contains mainly mafic (dark coloured) minerals, although the crystals are usually too small to be seen without magnification. Sometimes it may contain sparse crystals, or small round pockets that were gas bubbles. How is it formed? When ever you see TV footage of a volcano, it is usually basalt that you see flowing out over the surface and forming the lava flow. Most of the Hawaiian Islands, for example, are made up of basalt lava flows. Basalt is the fine-grained equivalent of gabbro. It is fine grained because the magma cools very rapidly when it flows out onto the surface. Basalt forms from magma that is rich in iron and magnesium and poor in silica (quartz). If you’re interested in learning more about basalt and lava, the Open University course S103 Discovering Science http://www3.open.ac.uk/courses/bin/p12.dll?C01S103 covers a wide range of topics, including earthquakes and volcanoes. Pumice Pumice is a fine-grained volcanic rock. It is very light grey to medium grey in colour. It contains a lot of empty gas bubbles, so it is very light and looks rather like a sponge. Sometimes pumice is so light that it will float on water. How is it formed? Pumice is formed when volcanoes erupt explosively. IT comes from the same kind of magma which would form granite or rhyolite, that is, a magma that contains lots of silica (quartz). Magma with lots of silica is usually thick and sticky. Some of the gases which cause the explosive eruption get trapped in the magma and form gas bubbles. These are preserved as holes when the rock cools as it comes flying out of the volcano. You can learn all about volcanoes in the new Open University Geohazards http://www3.open.ac.uk/courses/bin/p12.dll?C01SXG390 course. Gneiss Gneiss is a tough, hard, coarse-grained metamorphic rock. It looks like it has ribbons or stripes of different coloured minerals running through it. It is usually light in colour, but it can be quite dark. It can look similar to granite. But in granite the crystals are randomly aligned, whereas the crystals in gneiss are lined up in layers. The light coloured crystals in gneiss are usually quartz or feldspar, the dark coloured crystals are mafic minerals. How was it formed? Gneiss is formed from another metamorphic rock called schist, which itself started out life as a sedimentary rock called shale. To form a gneiss you need to subject the original rock to very great pressures and allow time for new large crystals to grow slowly. Because of the high pressure, the new crystals will tend to grow in layers or bands. Marble Marble is a fine-grained to very coarse-grained metamorphic rock. Marble is often pure white, and if you can see crystals it may look sugary. Marble can be streaked (or marbled) with grey, green, brown or red. It is quite soft and will not scratch glass. Don’t confuse marble with quartzite which is white and fine grained, but very hard and easily scratches glass. How was it formed? Marble is formed from the metamorphism of limestone. Like limestone, marble is mostly made up of the mineral calcite and when powdered it will fizz with white vinegar or dilute acid. Quartzite Quartzite is a tough, hard, fine-grained metamorphic rock. If the quartzite is made of pure quartz it is white, but it may have yellowish or reddish colour if it contains iron minerals. If you use a magnifier you may be able to see the grains of sand from which it is made. Quartzite breaks through the grains and not around them like sandstone. Don’t confuse quartzite with marble, which is much softer. Quartzite is very hard and will easily scratch glass. How is it formed? Quartzite is formed from the metamorphism of sandstone. Serpentine Serpentine is a metamorphic rock. It can be fine-grained and glossy or sometimes dull and coarse-grained. Serpentine is green, or greyish-green and it feels very slippery. It usually looks broken or layered and is made up of flat plates of green rock, which appear scratched. How was it formed? Rocks which can be metamorphosed to serpentine were originally formed from magmas that were very rich in iron and magnesium. These rocks are often found in the oceanic crust. They can be changed to serpentine during a collision of the Earth’s plates when giant slices of the oceanic crust are pushed up into the rocks of the continent. Slate Slate is a fine-grained metamorphic rock It can be black, grey, bluish, purplish or greenish grey. Slate contains many smooth thin flat layers (foliation) and it splits easily into sheets. How is it formed? Slate is usually formed from mudstone that has been put under pressure and heated up during plate collisions and mountain building. Pressure causes the platy clay minerals to line up parallel to each other and so the rock splits easily into sheets. Schist Schist is a medium to coarse-grained metamorphic rock. It is usually silvery green to dark green and grey. It contains thin layers which may be wavey, it usually splits easily along these layers, but does not form platy sheets like slate. How was it formed? Schists are formed by the metamorphism of mudrock and shale They often form during plate collisions, when the ocean floor plate is pushed up into or onto a continent. It is the mudrocks of the sea floor that get crunched up to form schist. Conglomerate Conglomerate is a coarse-grained sedimentary rock. It is a mixture of pebbles of many different sizes and sand all cemented together, mostly made up of quartz pebbles and quartz sand grains. How was it formed? On beaches, river banks and lake shores, sand and pebbles are deposited. They are usually rounded because they have been water-washed. Over time they become buried and compacted by the weight of sediments above them and are cemented together by chemicals dissolved in the water percolating through them. Sandstone Sandstone is a medium-grained sedimentary rock. It is pale yellow, grey or often red to brown. Composed of rounded grains of silica (quartz) that are all the same size, it is cemented together by silica, calcite or an iron mineral. Sandstones are often layered and can show colour variations between the layers. How is it formed? Sand sized grains of quartz are produced by the weathering of other rocks. These are transported and deposited by wind, waves and rivers. The original sediment may have been a sand bank, beach or desert sand dunes. When the sand is buried beneath other sediments it is compacted and cemented by chemicals dissolved in the water seeping through it. Sandstones formed in deserts are usually red in colour. Those formed on beaches or rivers are often yellow or grey. Mudstone Mudstone is a fine-grained sedimentary rock . It is usually black or dark grey-brown and is often soft and crumbly. How is it formed? Mudstones form when very fine-grained clay particles are deposited in water. They tiny particles settle to the bottom of oceans, lake floors or lagoons or even in quiet stretches of rivers. As the mud is buried and compacted by overlying sediment, the water is squeezed out and it turns into mudstone. Shale Shale is a fine-grained sedimentary rock. It can be black, grey, brown or greenish/bluish. It is usually quite flaky. How is it formed? Shale is formed in the same way as mudstone. When clay particles settle to the floor in quiet lakes and lagoons. Shale is more flaky than mudstone. Limestone Limestone is a fine to medium-grained sedimentary rock. It is usually grey, white, yellowish or tan. IT can often be streaked red with iron or black with magnesium impurities. Limestone is made up of the mineral calcite, which fizzes in white vinegar. It does not contain any visible crystals but it can sometimes look sugary. Limestone typically contains fossil shells and corals which are themselves made up of calcite. How is it formed? Limestone can be formed by the burial of coral reefs, but mostly it forms from lime rich mud in warm tropical waters. Chalk Chalk is a fine-grained sedimentary rock. It is usually pure white and quite soft and crumbly. It often contains rounded lumps of dark coloured flint. How is it formed? Chalk is a type of limestone. It is made up of thousands of minute calcite and silica rich skeletons of tiny marine plankton, which settled through the water in warm tropical seas and accumulated on the sea floor. Siltstone Siltstone is a fine to medium-grained sedimentary rock. It is pale grey or brown and quite soft. Sometimes it contains thin layers which are darker or lighter in colour. Siltstones are made up of particles which are intermediate in size between sandstone and mudstone. How is it formed? Siltstones are formed by particles settling through water and accumulating on the ocean floor, river beds or lagoon and lake bottoms. As the silt is buried and compacted by overlying sediment, the water is squeezed out and it turns into siltstone. Flint (or Chert) Flint is a very fine-grained hard rock usually found as lumps (nodules) in chalk. It is black, dark grey or tan and will splinter and spark when struck. How is it formed? Flint forms during or after the chalk in which it is found. Flint nodules grow within the sediment by being precipitated from solutions which are rich in silica. The silica probably came from the silica-rich skeletons of the animals which make up the chalk. MINERALS Common Minerals found in Rocks Minerals are the building blocks of rocks. They can be present in rocks as grains or crystals. Rocks may be made up of just one kind of mineral, like limestone which is made out of the mineral calcite, and quartzite which is made out of the mineral quartz, or they can be obvious collections of different coloured minerals, like granite which is made up of grey/white quartz, pink feldspar and black mica. Here are some of the common minerals found in rocks: Quartz Feldspar Mica Amphibole Pyroxene Calcite Quartz is a hard grey/white mineral made of silica. In igneous rocks quartz is usually grey and because it is the last mineral to crystallize it often does not show good crystal shapes. However, quartz can also occur as veins where it has crystallized within cracks in the rock, here it can often show nice crystal shapes. Quartz can be tinted with impurities. In sedimentary rocks it can be yellowish or reddish, in metamorphic rocks it is usually grey or white. It can look glassy or waxy. Sometimes quartz can be of gemstone quality e.g. Rose Quartz. and Amethyst [which is violet coloured quartz containing iron impurities]. Feldspar is mineral containing silica, but also potassium, sodium and calcium. Feldspars are pink, tan or white. In igneous and metamorphic rocks the crystals are usually blocky and nearly rectangular and they show flat shiny faces on broken surfaces of rocks. Feldspar is harder to recognise in sedimentary rocks. Mica is a mineral containing potassium, aluminium and silica (pale coloured mica) and also magnesium and iron (dark coloured mica). Mica has thin layers that you can peel off with your finger nail. In rocks mica is usually present as flakes or layers of flakes. The colour can range from silvery to brown, to shiny black. Mica is found in igneous, metamorphic and sedimentary rocks. Amphibole is a mafic (dark) mineral containing lots of magnesium and iron, but also sodium, potassium and aluminium. It is usually dark green to black. It shows flat shiny crystal faces and the crystals are often rectangular or long thin needles. It is found in igneous rocks and dark coloured metamorphic rocks. Pyroxene is a mafic (dark) mineral containing calcium, magnesium, iron and some silica. It is dark green, dark brown or black and is easy to confuse with amphibole. It is found in igneous rocks and dark coloured metamorphic rocks. Calcite is usually white, but can be coloured red/brown or black by impurities. It can form good crystals, with flat shiny faces shaped like parallelograms. It is soft and can easily be scratched with a steel point. The test for calcite is that it will fizz in white vinegar or dilute acid. It is principally found in sedimentary rocks and is the main mineral making up Limestone. UK MAP AND TIMELINE Depending on where you live in the UK, the rocks beneath your feet will be quite different. They formed at different times and in very different environments. It is not always easy to work out what the rocks in an area are made of because they are often covered by soil and vegetation, but below the surface in every landscape there are rocks. In mountainous regions such as Scotland, Wales and the North of England, the rock is usually exposed at the surface, and the mountains will show evidence of glaciation. In the south of England, however, much of the rock is covered by boulder clay and other soils that have been deposited by glaciers. Good places to look at the geology of your area are road cuttings and local quarries. If you live near the sea, look along the cliffs. Your local church or other old buildings could also provide clues, as they may be made of local stone. Highlands The Scottish Highlands have spectacular mountains made of old igneous and metamorphic rocks. The oldest rocks are gneiss in the Outer Hebrides and the extreme north west of Scotland. The youngest rocks are 50 million year old basalt lavas on the islands of Skye and Mull. Around the Moray Firth there is a lot of Old Red Sandstone which is Devonian in age and was laid down in a desert environment. This is often used as building stone. Midland Valley The rocks of the Midland Valley are mainly Old Red Sandstone which is Devonian in age and was laid down in a desert environment, together with Carboniferous age sediments such as sandstone, limestone and coal measures. There are also some Carboniferous age volcanic rocks such as the basalt lavas at Salisbury Crags and Arthur’s Seat in Edinburgh. Southern Uplands The Southern Upland region of Scotland has rocks which are mostly metamorphosed siltstones and mudstones which are folded and faulted. North England The Pennines in North England are made up of limestones, mudstones, siltstones and sandstones which are Carboniferous in age. In the Lake District there are older volcanic and metamorphic rocks, and in the Cleveland Hills there are younger sediments, which are sandstones and shales of Jurassic age. Central England In central England the rocks generally get younger from west to east. In the west the rocks are mostly sandstones mainly of Permian and Triassic age. In the east are clays and limestones of Jurassic and Cretaceous age, finishing up with upper Cretaceous age chalk on the coast. The Peak District to the north is largely made of Carboniferous limestone and sandstone. Millstone Grit is a well known rock type from the Peak District. There are also coal measures which used to be mined for coal in Nottinghamshire and South Yorkshire. South East England The south east of England is a region in which the rocks have been gently folded. The youngest rocks are in the Thames Valley and Southern Hampshire, these are Tertiary age shallow water sandstones and mudstones. With the exception of some spectacularly tilted and folded limestones and shales of Jurassic age in Dorset, the oldest rocks are in the Weald of Sussex and Southern Kent. These are Cretaceous sandstones and mudstones. In between these areas is Cretaceous age ‘chalk’ which forms the Chilterns and the North and South Downs. Where the North Downs meet the sea they form the impressive White Cliffs of Dover, and where the South Downs meet the sea is the equally spectacular Beachy Head. South West England The rocks of south west England are mainly strongly deformed Old Red Sandstone of Devonian age, together with Carboniferous sandstones and siltstones. The moors, such as Bodmin and Dartmoor are formed where granite intruded these younger rocks. The granite can be seen as granite tors out on the moors. Granite also forms the spectacular coast around Lands End. The Lizard Peninsula in Cornwall is something special. The rocks here are metamorphic serpentine, gabbro, schist and gneiss, which were pushed up from the ocean floor. Wales The oldest rocks in Wales are in the extreme north west on Anglesey and on the Lleyn peninsula. These are rocks which are Precambrian in age and include lavas, gneiss and quartzite. In north Wales are the volcanic rocks of Snowdonia, and slates of Ordovician and Silurian age. In south east Wales, Devonian age Old Red Sandstone forms the Brecon Beacons. The Carboniferous age rocks of the south Wales coal fields are geologically slightly younger than the sandstone. Ireland The north and north west coast of Ireland from Antrim round to Connemara is mostly metamorphic rocks and granite, similar to the rocks of the central highlands of Scotland, except where they are covered by 50 million year basalt lavas which stretch from Belfast to the Giant's Causeway. The region around County Down is composed of Ordovician and Silurian aged rocks. These are mostly metamorphosed siltstones and mudstones which are faulted and folded. The Wicklow Mountains are slates of Ordovician and Silurian age which are folded and faulted and similar to those in Wales. Most of the rest of Ireland, including the centre and south west is largely limestone and sandstone of Carboniferous and Devonian age. These become more strongly buckled and folded towards the south. THE ROCK CYCLE Why is it important to know about rocks? For human kind, rocks and minerals have great economic value, whether it is good hard stone for building, or minerals and metals for weapons and jewellery. All the Earth's processes depend on the properties of rocks and minerals in some way. Most of the Earth's major events, such as volcanic eruptions, mountain building, weathering, erosion and even earthquakes all involve rock and minerals. So a knowledge of the Earth's basic building blocks is essential to understanding the Earth. Every rock contains clues about the environment in which it formed. If a rock is made up of small fossil shell fragments, that tells us it was formed in a shallow marine environment. Other rocks may contain clues that show they were formed by a volcanic eruption, or deep within the Earth during mountain building. Rocks provide a record of events that occurred during the 4.5 billion years of the Earth's life. The rocks of the Earth are constantly being transformed into new types of rock and recycled by a number of processes which together make up the rock cycle. Magma Some rocks form by the hardening of molten rock or “magma”. This can happen either at the Earth's surface after a volcanic eruption, where magma might flow out onto the surface to form lava, or beneath the Earth's surface where the magma slowly crystallises to form granite. These kinds of rocks are known as igneous rocks. Sediment New rocks, however, can also form by the Earth's processes acting on existing rocks. Weathering and erosion by wind, water and ice produce vast quantities of rock and mineral particles called sediment, which is subsequently deposited on the Earth’s surface or in the oceans. Over time sediments become buried and compressed to form sedimentary rocks. Sedimentary Rocks These sedimentary rocks can themselves become eroded to form new sediments. Completely new sedimentary rocks can form from biological processes such as the accumulation of shells or the growth of coral reefs. Metamorphic rocks Igneous and sedimentary rocks can be changed or altered by high temperature and pressure into metamorphic rocks. This often happens at the boundaries of the Earth’s tectonic plates, where for example, two continents may collide forming new mountain ranges. Under very extreme conditions, metamorphic rocks can begin to melt and produce new magma. Landscape Features Every geological event of this ancient land has left its mark. Traces of a volcanic eruption millions of years ago still exist as well as beaches left high and dry by a retreating ice age. The Giant's Causeway, County Antrim, Northern Ireland Around 50 or 60 million years ago a volcano erupted in this part of what is now Northern Ireland. Basalt lava flowed out onto the surface of the ground, and as it cooled, it formed thousands of spectacular polygonal shaped columns, the tallest of which is now 40 feet high. In some places the solidified lava is 90 feet thick. The tops of the lava columns form stepping-stone type structures, which leads from the cliff foot and into the sea. These are about 30 cm across and are mostly hexagonal, although some five-sided examples can be seen. The Giant's Causeway is made up of about 40,000 of these pillars of basalt, creating a spectacular landscape feature that is the focus of local legend. Successive eruptions created layers of basalt, which are visible in the cliff's edge. Raised Beaches of Scotland 100,000 years ago Scotland was covered by a gigantic ice sheet. The massive weight of the ice depressed the land beneath it and for thousands of years the coastline etched its signature into the rock. When the last ice age ended, about 10,000 years ago, the retreating ice sheet released its water and the land started a process known as the “glacial rebound”. The land began to rise, taking with it the cliffs and beaches that for centuries formed the coastline. Over the millennia the coastline has changed, leaving a stepped landscape that illustrates ancient sea levels. The result are the magnificent raised beaches of Scotland. Much of the rock in this area is sedimentary, formed on the ocean floor, and later deformed as faults in the Earth’s crust tilted the strata to vertical. Some areas are made of granite, where molten rock pushed its way into the sediments and cooled slowly. The softer sedimentary rock has eroded away to leave the granite outcroppings. The Wastwater Screes, Lake District The valleys of the Lake District have been scoured and scarred by the movement of glaciers, producing the well known 'U'-shaped cross section. The valley floors became deeply gouged, producing the rock basins that are today occupied by lakes. The deposition of glacial debris, in the form of clays or moraines has in places formed natural dams behind which lakes have formed. Wastwater sits in one such valley. It is the deepest lake in England at around 78 metres deep, about three miles long and half a mile wide. Since the retreat of the glacier, silting and erosion has created huge fans of scree on the lake’s south shore, originating from a high mountain from which stones and earth continually tumble. On the west of the lake likes the village of Wastdale. The geology of this area is some of the oldest in Europe. It was a centre of volcanic activity when these mountains were formed. Originally the mountains were twice as high as they are today, but weather and erosion have reduced them to a mere 978 metres at their highest (Scafell Pike). Malham Cove, Yorkshire - Limestone Scarps The astonishing limestone escarpment at Malham Cove stands 80 metres high and some 300 long. It is a curved cliff of carboniferous limestone that formed sometime after the last ice age. Its craggy curved surface was formed as meltwater from Malham Tarn cut its way backwards as it fell over the edge as a waterfall. The erosion cut away more material from the lip of the waterfall than from the edges, creating the curved shape. These days water does not flow over the escarpment, what water there is makes its way instead down deep fissures in the limestone. At the top of the scarp lies a magnificent limestone pavement, deeply grooved and cracked by uneven weathering by slightly acidic rain. They formed by the rain etching existing cracks in the pavement, dissolving away the rock until a fissure is formed. The resulting limestone pavement is known as 'clints' or 'grykes', where the naked limestone lumps are the clints and the fissures in between are the grykes. Nant Ffrancon Valley, North Wales. Wales is one of the most southerly points in the UK to have experienced the ice age and subsequent glaciation during the Pleistocene Epoch. The Welsh ice fields were centred on the Migneint plateau and the Arenig mountains, and glaciers from this sheet radiated out from its centre. These glaciers carved out the spectacular glacial trough valleys of Nant Ffrancon and Llanberis/Nant Peris (Snowdonia) and Tal-y-Llyn (Cadair Idris). Nant Ffrancon valley is a classic 'U' -shaped glacial valley. Large boulders on the valley floor were deposited as the glacier receded. The curved basin valley floor is the product of post-glacial sedimentation, created by an ancient valley floor lake. It currently is the home of what is known as a 'misfit stream', a small stream that wanders across the valley floor. A 'misfit stream' is so named because it does not carve the valley itself, but is rather is the drainage from a landform created by other means. Dartmoor - Granite Tors Most of Dartmoor consists of a single type of rock - granite. This granite was formed in the Carboniferous/early Permian period, around 280 million years ago. Mysterious granite tors dot the landscape, on summits, steep valley edges, at the edges of the two great plateaux and close to the main river gorges. They were formed from weathering. The exposed granite usually has both horizontal and vertical jointing, the vertical ones probably the result of the granite cooling during formation, and the horizontal ones usually follow the surrounding landscape, a result of pressure being released as surrounding rocks were released by erosion. The jointing in the surface of the rock allows acid water to seep into the rock. The freezing and thawing of water (helped by four ice ages) jacked open the breaks in the rock. Blocks of granite were levered away by this process, leaving the tor isolated and exposed, littering the ground beneath with boulders and rubble. Rock Types Rocks can be divided into three broad categories - igneous, metamorphic and sedimentary. Igneous Rocks All igneous rocks have solidified from a molten state, either inside the Earth or on the surface. The lavas and molten fragments (pyroclasts) and ash produced by volcanoes are called extrusive igneous rocks, because they are formed by the extrusion of magma on to the Earth's surface. Other igneous rocks such as granite formed deep underground; these are called intrusive igneous rocks formed where magmas have cooled within the Earth. Intrusive Igneous Rocks Intrusive igneous rocks represent batches of magmas that didn't make it to the Earth's surface before cooling and solidifying. They form by the slow crystallisation of magmas, either at depth (several km below the Earth's surface) as large irregular intrusions or plutons, or nearer the surface in cracks and fractures as shallow minor intrusions such as dykes or sills. Extrusive Igneous Rocks Extrusive igneous rocks (sometimes called volcanic rocks) are formed when magma erupts at the surface. When magmas reaches the surface it may spill out as a lava flow, either on land such as in Hawaii, or beneath the sea, such as on mid-oceanic ridges. Alternatively it may erupt explosively producing fragmental debris (pyroclasts) which eventually deposit on the surface, forming a pyroclastic rock. Differences in composition, mineralogy and crystallisation temperature of different magmas influence their eruptive style. Magmas Magmas are complex chemical mixtures, containing many elements that organise themselves into several minerals as the magma crystallises. Each mineral in an igneous rock usually begins to crystallise at a different temperature. The fundamental control on the mineral composition of an igneous rock is the chemistry of the magma from which it crystallises. But chemistry is not the only thing that makes igneous rocks look different. The three rocks in here all have exactly the same chemical composition. They appear different because of the size of the crystals. The number and size of the crystals depends on the amount of time they have to grow. For extrusive rocks this can be seconds, for a small fragment of magma flying through the air, to a few years for the middle of a thick lava flow (e.g. basalt). This kind of rapid cooling results in small crystals because they have very little time to grow. For intrusive igneous rocks, the cooling rate is much slower (probably several thousand years!) and the magma has time to grow larger crystals (e.g. Gabbro). Generally speaking the slower the cooling, the bigger the crystals. Sedimentary Rocks Sedimentary rocks form under a great variety of circumstances, such as glacial environments, deserts, rivers and coral reefs. They are formed by the laying down, or deposition of tiny grains which form layers of sediment. When sediment is transported by water in rivers or seas, it can settle to the bottom like tea leaves settle to the bottom of a cup. You can see sediment being deposited today in rivers and on beaches. Where does all the sediment come from? Sedimentary grains are formed when rocks at the Earth’s surface are weathered or broke up by water, wind, frost and ice. The tiny fragments and individual mineral grains are usually transported from one place to another by wind or water before being deposited as roughly horizontal layers of sediment. With time these layers of sediment may themselves get washed or blown away, or they may be buried by yet more sediment and compacted. When this happens and water is squeezed out of the sediment, new minerals may grow in the spaces between the grains and the loose sediment grains become cemented togheter forming a solid sedimentary rock such as sandstone. Layers of sedimentary rocks are called beds. Fossils Some sedimentary rocks contain fossils of plants or animals which were living at the time the sedimentary grains were deposited. Accumulations of shells and the cacite skeletons of marine organisms like corals make up a rock called limestone Sedimentary rocks are often porous. The individual pore spaces are connected with each other. This means that liquids and gasses can move through the rock, so often reservoirs of water, oil and gas are contained within sedimentary rocks like sandstone. Metamorphic Rocks Metamorphic are existing sedimentary and igneous rocks that have changed form. Any type of rock can undergo a change of form, becoming metamorphic rock, if it is heated to temperatures of several hundreds of degrees Celsius, and/or if subject to high pressure (because of the weight of overlying rocks). An increase in pressure and temperature will come about if a rock becomes more deeply buried in the Earth as a result of earth movements, or if it is covered by a deepening layer of sedimentary deposits. Igneous and metamorphic rocks both have a crystalline texture and both form at high temperatures, but an important distinction is that metamorphism occurs in the solid state, whereas igneous rocks form from liquid (molten) rock. During metamorphism, the atoms in the minerals making up the rock become reorganised, sometimes resulting in the regrowth of existing minerals (crystals), or in the formation of new minerals. As a result, the new rock may look very different from the original rock. The overall chemical composition of the rock normally remains about the same, however, with elements just rearranging themselves into new minerals that are more suited to the new temperature and pressure conditions. This is what happens during recrystallisation and can often result in banding or alignment of crystals in the rock. You may already know of two metamorphic rocks, slate and marble. Although the terms slate and marble are often used to describe construction and ornamental stones, the colloquial use of 'slate' and 'marble' covers a much wider range of materials than is covered by the strict definitions of these metamorphic rocks. Slate is a metamorphic rock with an extremely fine grain size; it is difficult to make out individual crystals even with a hand lens. It was originally laid down as a soft mud, but it has been recrystallised and the result is a hard, water-resistant rock that can be split into thin sheets. Marble is a metamorphic rock formed from limestone, but unlike schist and slate, marble doesn't always have a banded structure. This is because marble usually contains only one mineral, calcite, so there cannot be alternating bands of different minerals. This means that marble doesn't break along preferred directions like metamorphic rocks that contain minerals arranged in parallel bands. It therefore makes a good material for statues, as smooth surfaces can be carved in any direction. Any impurities in the marble tend to result in a mottled appearance. SAFETY TIPS Collecting Rocks We are lucky in the UK because we have some really spectacular geology and a huge range of rock types all packed into quite a small area. If you want to start a rock collection, try to find freshly broken pieces from cliffs, crags and quarries. Rocks that are buried in the soil, and pebbles on beaches, or in lakes and rivers are not good to collect (although they may be very attractive). It is often difficult to see what they are made of and it is very difficult to work out where they originally came from. The colour, texture and hardness of a rock can be greatly altered by weathering and sea washing. Don’t get caught out by a nice, smooth, rounded fragment of brick or even cement! Here’s a few tip on getting your rock collection started: * Collect clean specimens with a least one freshly broken surface. * Paint a tipex rectangle on one corner and assign each rock a number. * Record the number, name and location where you found it. * Make a label with the name of the rock and location on it. Safety when collecting rocks * Always wear safety glasses or goggles when breaking rocks. * Use only a geological hammer for collecting; do not use a household claw hammer. Hard rocks can break splinters of steel off a claw hammer. * Do not climb on crags or quarry walls. * Be very careful when approaching the tops and bases of steep cliffs, and wear a protective ‘hardhat’ if you do. * Never enter mine tunnels, they are extremely dangerous. * Never collect rocks from Sites of Special Scientific Interest (SSSI). Remember: if you're going to break rocks ALWAYS wear safety goggles and make sure you're about 2m away from the nearest person. Crystalline and very hard, fine-grained rocks can produce sharp, flying splinters! Classifying Rocks In some cases, observing a rock in hand specimen may not be enough to classify it completely. You may need to examine it in 'thin section' down a microscope, or work out its relationship to the other rocks around it in the field. But that's a completely different world, and if you are interested in that kind of detail, you might want to check out some of the courses the Open University has to offer.