Editing Impact crater (section)

==Identifying impact craters==
[[File:Craterstructure.gif|thumb|[[Impact structure]] of craters: simple and [[complex craters]]]]
[[File:Wells creek shatter cones 2.JPG|thumb|[[Wells Creek crater]] in Tennessee, United States: a close-up of shatter cones developed in fine grained [[Dolomite (rock)|dolomite]]]]
[[File:USGS Decorah crater.jpg|thumbnail|[[Decorah crater]]: aerial electromagnetic resistivity map ([[U.S. Geological Survey|USGS]])]]
[[File:Barringer Crater USGS.jpg|thumb|[[Meteor Crater]] in the U.S. state of Arizona, was the world's first confirmed impact crater.]]
[[File:Shoemaker Impact Structure, Western Australia.JPG|thumb|[[Shoemaker Crater]] in Western Australia was renamed in memory of Gene Shoemaker.]]

Non-explosive volcanic craters can usually be distinguished from impact craters by their irregular shape and the association of volcanic flows and other volcanic materials. Impact craters produce melted rocks as well, but usually in smaller volumes with different characteristics.<ref name="LPI_Traces_of_Catastrophe_Ch7"/>

The distinctive mark of an impact crater is the presence of rock that has undergone shock-metamorphic effects, such as [[shatter cone]]s, melted rocks, and crystal deformations. The problem is that these materials tend to be deeply buried, at least for simple craters. They tend to be revealed in the uplifted center of a complex crater, however.<ref name="LPI_Traces_of_Catastrophe_Ch4">{{cite book |last=French |first=Bevan M |year=1998 |chapter=Chapter 4: Shock-Metamorphic Effects in Rocks and Minerals |pages=31{{ndash}}60 |title=[[Traces of Catastrophe]]: A Handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures |publisher=[[Lunar and Planetary Institute]] |oclc=40770730 }}</ref><ref name="LPI_Traces_of_Catastrophe_Ch5">{{cite book |last=French |first=Bevan M |year=1998 |chapter=Chapter 5: Shock-Metamorphosed Rocks (Impactites) in Impact Structures |pages=61{{ndash}}78 |title=[[Traces of Catastrophe]]: A Handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures |publisher=[[Lunar and Planetary Institute]] |oclc=40770730 }}</ref>

Impacts produce distinctive [[Shock metamorphism|shock-metamorphic]] effects that allow impact sites to be distinctively identified. Such shock-metamorphic effects can include:
* A layer of shattered or "[[breccia]]ted" rock under the floor of the crater. This layer is called a "breccia lens".{{sfn |Randall |2015 |p=157}}
* [[Shatter cone]]s, which are chevron-shaped impressions in rocks.{{sfn |Randall |2015 |pp=154–155}} Such cones are formed most easily in fine-grained rocks.
* High-temperature rock types, including laminated and welded blocks of sand, [[spherulite]]s and [[tektite]]s, or glassy spatters of molten rock. The impact origin of tektites has been questioned by some researchers; they have observed some volcanic features in tektites not found in impactites. Tektites are also drier (contain less water) than typical impactites. While rocks melted by the impact resemble volcanic rocks, they incorporate unmelted fragments of bedrock, form unusually large and unbroken fields, and have a much more mixed chemical composition than volcanic materials spewed up from within the Earth. They also may have relatively large amounts of trace elements that are associated with meteorites, such as nickel, platinum, iridium, and cobalt. Note: scientific literature has reported that some "shock" features, such as small shatter cones, which are often associated only with impact events, have been found also in terrestrial volcanic ejecta.{{sfn |Randall |2015 |p=156}}
* Microscopic pressure deformations of minerals.{{sfn |Randall |2015 |p=155}} These include fracture patterns in crystals of quartz and feldspar, and formation of high-pressure materials such as diamond, derived from graphite and other carbon compounds, or [[stishovite]] and [[coesite]], varieties of [[shocked quartz]].
* Buried craters, such as the [[Decorah crater]], can be identified through drill coring, aerial electromagnetic resistivity imaging, and airborne gravity gradiometry.<ref>{{cite web |author=US Geological Survey |title=Iowa Meteorite Crater Confirmed |url=http://www.usgs.gov/newsroom/article.asp?ID=3521 |access-date=7 March 2013}}</ref>