page_title: Formation of rocks
section_title: Extraterrestrial rock
hierarchical_section_title: Formation of rocks / Extraterrestrial rock
caption_attribution_description: English: Widmanstätten pattern of the Staunton Meteorite, on display at the Smithsonian Museum of Natural History, Washington, DC
context_page_description: The three main ways terrestrial rocks are formed:
  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.
  Igneous rocks are rocks which have crystallised 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 zones or within the Earth's mantle. The melt is hot and so passes upward through cooler country rock. As it moves, it cools and various rock types will form through a process known as fractional crystallisation. Igneous rocks can be seen at mid ocean ridges, areas of island arc volcanism or in 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.
context_section_description: Off-Earth, rock can also form in the absence of a substantial pressure gradient as material that condensed from a protoplanetary disk, without ever undergoing transformations in the interior of a large object such as planets and moons.  Astrophysicists classify this as a fourth type of rock:  Primitive rock.  
  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."
  An example of a primitive rock is the achondritic iron-nickel octahedrite mineral seen in the Widmanstätten pattern that is found in a number of iron-rich meterorites.  Consisting of kamacite and taenite and formed under extremely slow cooling conditions—about 100 to 10,000 °C/Myr, with total cooling times of 10 Myr or less—it will precipitate kamacite and grow kamacite plates along certain crystallographic planes in the taenite crystal lattice.
caption: Widmanstätten pattern in an iron-rich meteorite
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