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GeologyIn order to know more about your rock you need to know more about geology.Here is a crash course in geology. That your rock may prosper! Contents: Igneous rocks Sedimentary rocks Metamorphic rocks Igneous rocksBasalt:Dark coloured, fine grained rocks most often formed by the rapid cooling of lava. The most common of all igneous rocks.Granite:Granite forms as magma cools far under the earth's surface. Because it hardens deep underground it cools very slowly and becomes very hard. Composed of four minerals: quartz, feldspar, mica, and hornblende which often form clusters in the rock, giving it a patchy look.Pumice:A light coloured rock containing lots of air bubbles. Pumice is formed when lava is ejected and shot through the air during an eruption. As the lava hurtles through the air it cools and the gases escape leaving the rock full of holes. Pumice is so light that it actually floats on water.Obsidian:A very clear, glasslike rock of volcanic origin. Formed when lava cools so quickly by exposure to water that no crystals can form. Stone-age people used obsidian for arrowheads, knives, spearheads, and cutting tools of all kinds.Sedimentary rocksSandstone:Sandstone forms from the cementing together of sand sized grains forming a solid rock. Composed mainly of quartz. Minerals such as Calcium carbonate, silica or iron grow crystals in the spaces around the sand grains. As the crystals fill the gaps the individual sand grains are transformed into a solid rock.Limestone:Limestone is produced from calcium carbonate and sediment, most often formed in the ocean. Mainly made out of calcium carbonate which can be precipitated from ocean water or formed from sea creatures that secrete lime such as algae and coral. Chalk is a type of limestone.Halite:Halite is common table salt. It forms where salty lakes or sea beds dry up. This evaporation of the water causes the salt to precipitate forming the salt crystals.Conglomerate:Conglomerate is a sedimentary rock that forms from the cementing of rounded cobble and pebble sized rock fragments. Conglomerate is formed by river movement or ocean wave action. The cementing agents that fill the spaces to form the solid rock conglomerate are silica, calcite, or iron oxides.Metamorphic rocksSlate:Slate is a dark coloured, fine-grained, metamorphic rock. Slate is produced by relatively low temperatures and pressures. Slate is not very hard.Gneiss:Gneiss is a high grade metamorphic rock. This means that gneiss has been subjected to a lot of heat and pressure. The minerals that compose gneiss are the same as granite. Feldspar is the most important mineral that makes up gneiss along with mica and quartz. Gneiss can be formed from a sedimentary rock such as sandstone or shale, or it can be formed from the metamorphism of the igneous rock granite.Marble:Marble is metamorphosed limestone or dolomite. Both limestone and dolomite have a large concentration of calcium carbonate. Marble has many different sizes of crystals and has many color variances due to impurities present at formation. Marble is much harder than its parent rock.Anthracite coal:Organic sedimentary rocks form from the build up and decay of plant and animal material. This usually forms in swamp regions in which there is an abundant supply of growing vegetation and low amounts of oxygen. The vegetation builds so quickly that new layers of vegetation bury the dead and decaying material very quickly. The fast deposition and low oxygen levels mean that bacteria have little time to decompose the vegetation. This compressed vegetation forms coal. Peat is the first stage of coal formation. Lignite is the next grade of coal followed by bituminous and the highest grade, anthracite. Anthracite is actually a metamorphic rock. It forms during mountain building when compaction and friction are extremely high.We are deeply indebted to the rocks and minerals slide show for most of this information. If you would like to learn more we recommend a visit to the site. |
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