Impact crater

. *NASA&nbsp;photo.*]]

 from the Azuara impact structure, Spain.]]
 from the Wells Creek crater, USA.]]
**Impact craters** are craters formed by meteorites or comets striking the Earth or other solid body.  There is a system for recording and assessing craters. 

## Largest craters on Earth 
The five largest *confirmed* impact structures are:
#Vredefort crater, South Africa: 160&nbsp;km diameter, 2023 million years ago.
#Chicxulub crater, Mexico: 150&nbsp;km diameter, 65 million years ago.
#Sudbury Basin, Ontario, Canada: 130&nbsp;km diameter, 1850 million years ago.
#Popigai crater, Russia: 90&nbsp;km diameter, 35.7 million years ago.
#Acraman crater, 90&nbsp;km diameter, South Australia, ~590 mya.
#Manicouagan crater, Quebec, Canada: 70&nbsp;km diameter, 214 million years ago.
There are some other suggested impact structures which are larger. It is a feature of the Earth that climate, weathering and plate tectonics removes most of the older features and events. In comparison, the Moon retains a nearly complete record of its past impact events. The implication is that the Earth once suffered a similar bombardment in its early history. This period is known as the Late Heavy Bombardment, because it occurred after the Moon was (presumably) knocked from the Earth.

### Largest unconfirmed craters 
#Australian impact structure: Northern Territory/South Australia, Australia: multiple rings 600 km, 545 million years ago at the Neoproterozoic/Cambrian boundary. Two massive impacts were responsible.
#Shiva crater, Indian Ocean, west of India: ~600&nbsp;km length, 400&nbsp;km width, 65 million years ago. Obviously this is of high interest, because of the date.
#Wilkes Land crater, Antarctica: 485 km. 250–500 mya.
#Nastapoka arc, Nunavut/Quebec, Canada: 450 km. unknown
#Ishim impact structure, Kazakhstan: 300 km. 430–460 mya.
#Bedout, off-shore of Western Australia: 250&nbsp;km, 250 mya.

### Largest named craters in the Solar System
# North Polar Basin/Borealis Basin (disputed) – Mars – Diameter: 10,600&nbsp;km
# South Pole–Aitken basin – Moon – Diameter: 2,500&nbsp;km
# Hellas Basin – Mars – Diameter: 2,100&nbsp;km

# Caloris Basin – Mercury – Diameter: 1,550&nbsp;km
# Imbrium Basin – Moon – Diameter: 1,100&nbsp;km

## Identifying impact craters
 craters, in Estonia]]

The distinctive mark of an impact crater is the presence of rock which has undergone shock-metamorphic effects, shattered or melted rocks, and crystal deformations. Examples:
*A layer of shattered rock under the floor of the crater. This layer is called a 'breccia lens'.
*Shatter cones, which are chevron-shaped impressions in rocks. Such cones are formed most easily in fine-grained rocks.
*High-temperature rock types, including laminated and welded blocks of sand, tektites, or glassy spatters of molten rock. They may have relatively large amounts of trace elements that are associated with meteorites, such as nickel, platinum, iridium, and cobalt.
*Microscopic pressure deformations of minerals. 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 varieties of shocked quartz.

## References