Formation of rocks
- This article discusses how rocks are formed. There are also articles on physical geologic formations.
Terrestrial rocks are formed by three main mechanisms:
- Sedimentary rocks are formed through the gradual accumulation of sediments: for example, sand on a beach or mud on a river bed. As the sediments are buried they get compacted as more and more material is deposited on top. Eventually the sediments will become so dense that they would essentially form a rock. This process is known as lithification.
- fractional crystallisation. Igneous rocks can be seen at mid-ocean ridges, areas of island arc volcanism or in intra-plate hotspots.
- Metamorphic rocks 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 can also form in the absence of a substantial pressure gradient as material that condensed from a protoplanetary disk, without ever undergoing any transformations in the interior of a large object such as a planet or moon. Astrophysicists classify this as a fourth type of rock: primitive rock. This type is common in asteroids and meteorites.[1]: 145
Rock formation
19th-century efforts to synthesize rocks
The
de Saussure, but Sir James Hall really laid the foundations of this branch of petrology. He showed (1798) that the whinstones (diabases) of Edinburgh were fusible and if rapidly cooled yielded black vitreous masses closely resembling natural pitchstones and obsidians. 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).[2]
Many years later
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 Adams and 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 marble may be produced artificially.[2]
Extraterrestrial rock
Off-
moons. Astrophysicists classify this as a fourth type of rock: Primitive rock.[1]
Primitive rocks "have never been heated much, although some of their constituents may have been quite hot early in the history of our Solar System. Primitive rocks are common on the surfaces of many asteroids, and the majority of meteorites are primitive rocks."[1]: 145
An example of a primitive rock is the
crystal lattice.[3]
See also
- Chemical gardening– Demonstration of metallic salts crystallization
- List of minerals
- List of rocks
- List of rocks on Mars
- Petrifaction
- Rock cycle
References
- ^ ISBN 9781108411981.
- ^ a b One or more of the preceding sentences incorporates text from a publication now in the public domain: Flett, John Smith (1911). "Petrology". In Chisholm, Hugh (ed.). Encyclopædia Britannica. Vol. 21 (11th ed.). Cambridge University Press. pp. 326–327.
- .
External links
- The rock forming process, on sci-culture.com
