Formation of rocks 1235284 223722891 2008-07-05T13:17:36Z Vsmith 84417 /* Rock synthesis */ more name links :''This article discusses how rocks are formed. There are also articles on physical rock formations, also known as [[stratum|strata]], and the formal naming of [[geologic formation]]s.'' The three main ways '''rocks are formed''': * [[Sedimentary rocks]] are formed through the gradual accumulation of [[sediment]]: for example, sand on a beach or mud on a river bed. As the sediment is buried it is compacted as more and more material is deposited on top. Eventually the sediment will become so dense that it is essentially [[rock (geology)|rock]]. This process is known as [[lithification]]. * [[Igneous rocks]] are rocks which have [[crystal]]lized from a [[melt]] or [[magma]]. The melt is made up of various components of pre-existing rocks which have been subjected to melting either at [[subduction zone]]s or within the [[Earth]]'s [[mantle (geology)|mantle]]. The melt is hot and so passes upward through cooler [[country rock (geology)|country rock]]. As it moves it cools and various rock types will form through a process known as [[Fractional crystallization (geology)|fractional crystallization]]. Igneous rocks can be seen at [[mid ocean ridge]]s, areas of [[Volcanic arc|island arc volcanism]] or in [[Hotspot (geology)|intra-plate hotspots]]. * [[Metamorphic rocks]] are rocks which once existed as igneous or sedimentary rocks but have been subjected to varying degrees of pressure and heat within the earth's crust. The processes involved will change the composition and fabric of the rock and their original nature is often hard to distinguish. Metamorphic rocks are typically found in areas of [[mountain building]]. ==Rock synthesis== The synthetic investigation of rocks, which proceeds by experimental work to reproduce different rock types and in this way to elucidate their origin and explain their structures. In many cases no experiment is necessary. Every stage in the origin of clays, sands and gravels can be seen in process around us, but where these have been converted into coherent shales, sandstone and conglomerates, and still more where they have experienced some degree of metamorphism, there are many obscure points about their history upon which experiment may yet throw light. Attempts have been made to reproduce igneous rocks, by fusion of mixtures of crushed minerals or of chemicals in specially contrived furnaces. The earliest researches of this sort are those of [[Barthélemy Faujas de Saint-Fond|Faujas St Fond]] and of [[Horace-Bénédict de Saussure|de Saussure]], but Sir [[Sir James Hall, 4th Baronet|James Hall]] really laid the foundations of this branch of petrology. He showed (1798) that the whinstones ([[diabase]]s) of Edinburgh were fusible and if rapidly cooled yielded black vitreous masses closely resembling natural pitchstones and [[obsidian]]s, if cooled more slowly they consolidated as crystalline rocks not unlike the whinstones themselves and containing [[olivine]], [[augite]] and [[feldspar]] (the essential minerals of these rocks). Many years later [[Gabriel Auguste Daubrée|Daubrée]], [[Achille Ernest Oscar Joseph Delesse|Delesse]] and others carried on similar experiments, but the first notable advance was made in 1878, when [[Ferdinand André Fouqué|Fouqué]] and [[Auguste Michel-Lévy|Lévy]] began their researches. They succeeded in producing such rocks as porphyrite, [[leucite]]-[[tephrite]], [[basalt]] and dolerite, and obtained also various structural modifications well known in igneous rocks, e.g. the [[porphyritic]] and the [[ophitic]]. Incidentally they showed that while many basic rocks (basalts, etc.) could be perfectly imitated in the laboratory, the acid rocks could not, and advanced the explanation that for the crystallization of the latter the gases never absent in natural rock magmas were indispensable mineralizing agents. It has subsequently been proved that steam, or such volatile substances as certain borates, molybdates, chlorides, fluorides, assist in the formation of [[orthoclase]], [[quartz]] and [[mica]] (the minerals of [[granite]]). Sir James Hall also made the first contribution to the experimental study of metamorphic rocks by converting [[chalk]] into [[marble]] by heating it in a closed gun-barrel, which prevented the escape of the [[carbonic acid]] at high temperatures. In 1901 [[Frank Dawson Adams|Adams]] and [[John Thomas Nicolson|Nicholson]] carried this a stage further by subjecting marble to great pressures in hydraulic presses and have shown how the foliated structures, frequent in natural marbles, may be produced artificially.<ref name=EB1911>{{1911|article=Petrology}}</ref> ==References== <references /> [[Category:Petrology]]