This list of more than 130 possible impact craters on Earth includes theoretical impact sites that have appeared several times in the literature, or may have been endorsed by the Impact Field Studies Group (IFSG)[1] or Expert Database on Earth Impact Structures (EDEIS),[2] but not yet confirmed by the Earth Impact Database (EID).[3]
The following are officially considered "unconfirmed". Due to stringent requirements regarding evidence and peer-reviewed publication, newly discovered craters—or those where collecting evidence is difficult—are generally known for some time before becoming listed. Recent extensive surveys have been done for Australian (2005),[4] African (2014),[5] and South American (2015)[6] craters, as well as those in the Arab world (2016).[7] A book review by A. Crósta and U. Reimold disputes some of the evidence presented for several of the South American structures.[8]
The more than 20 unconfirmed craters in the first table are younger than one million years old and at least 0.1 km in diameter. For comparison, the largest confirmed crater in the Earth Impact Database within the last 10,000 years is the 0.3 km Macha crater in Siberia.
The Cheko crater is thought by one research group to be the result of the famous Tunguska event, although sediments in the lake have been dated back more than 5000 years. There is highly speculative conjecture about the supposed Sirente impact (c. 320 ± 90 AD) causing the Roman emperor Constantine's vision at Milvian Bridge.[29]
The Burckle crater and Umm al Binni structure are proposed to be behind the floods that affected Sumerian civilization.[30][31] The Kachchh impact may have been witnessed by the Harappan civilization and mentioned as a fireball in Sanskrit texts.[15]
The age of the Bloody Creek crater is disputed, with some evidence suggesting it hit glacier ice 12000 years ago, coeval with the Younger Dryas.[18]
As the trend in the Earth Impact Database for about 26 confirmed craters younger than a million years old show that almost all are less than 2 km in diameter (except the 3 km Agoudal and 4 km Rio Cuarto), the suggestion that two large craters, Mahuika (20 km) and Burckle (30 km), formed just within the last few millennia has been met with skepticism.[32][33][34]
However, the source of the young (less than a million years old) and enormous Australasian strewnfield (c. 790 ka) is suggested to be a crater about 100 km across somewhere in Indochina,[35][36] with Hartung and Koeberl (1994) proposing the elongated 100 km × 35 km Tonlé Sap lake in Cambodia (visible in the map at the side) as a suspect structure.[37]
The more than 110 suspected craters in the table below are either older than 1 Ma, or have an unknown age.
The Decorah crater has been conjectured as being part of the Ordovician meteor event.[138]
Several twin impacts have been proposed such as the Rubielos de la Cérida and Azuara (30–40 Ma),[139] Cerro Jarau and Piratininga (c. 117 Ma),[9] and Warburton East and West (300–360 Ma).[131] However, adjacent craters may not necessarily have formed at the same time such as case of the confirmed Clearwater East and West lakes.
Some confirmed impacts like Sudbury or Chicxulub are also sources of magnetic anomalies[140] and/or gravity anomalies. The magnetic anomalies Bangui and Jackpine Creek,[81] the gravity anomalies Wilkes Land crater and Falkland Islands (Malvinas Islands),[64] and others have been considered as being of impact origin. Bangui apparently has been discredited,[43][141] but appears again in a 2014 table of unconfirmed structures in Africa by Reimold and Koeberl.[5]
Several anomalies in Williston Basin were identified by Swatzky in the 1970s as astroblemes including Viewfield, Red Wing Creek, Eagle Butte, Dumas, and Hartney, of which only the last two are unconfirmed.[60]
The Eltanin impact has been confirmed (via an iridium anomaly and meteoritic material from ocean cores) but, as it fell into the Pacific Ocean, apparently no crater was formed. The age of Silverpit and the confirmed Boltysh crater (65.17 ± 0.64 Ma), as well as their latitude, has led to the speculative hypothesis that there may have been several impacts during the KT boundary.[142][143] Of the five oceans in descending order by area, namely the Pacific, Atlantic, Indian, Antarctic, and Arctic, only the smallest (the Arctic) does not yet have a proposed unconfirmed impact crater.
Craters larger than 100 km in the Phanerozoic (after 541 Ma) are notable for their size as well as for the possible coeval events associated with them especially the major extinction events.
For example, the Ishim impact structure[79] is conjectured to be bounded by the late Ordivician-early Silurian (c. 445 ± 5 Ma),[80] the two Warburton basins has been linked to the Late Devonian extinction (c. 360 Ma),[132] both Bedout and the Wilkes Land crater have been associated with the severe Permian–Triassic extinction event (c. 252 Ma),[144][145] Manicouagan (c. 215 Ma) was once thought to be connected to the Triassic–Jurassic extinction event (c. 201 Ma)[146] but more recent dating has made it unlikely, while the consensus is the Chicxulub impact caused the one for Cretaceous–Paleogene (c. 66 Ma).
However, other extinction theories employ coeval periods of massive volcanism such as the Siberian Traps.
There is geological evidence for impact events having taken place on Earth on certain specific occasions, which should have formed craters, but for which no impact craters have been found. In some cases this is because of erosion and Earth's crust having been recycled through plate tectonics, in others likely because exploration of the Earth's surface is incomplete. Typically the ages are already known and the diameters can be estimated.
Parent Crater of | Expected Crater Diameter (km) | Age |
---|---|---|
Dakhleh glass[147][148] | 0.4 km | 150 ka |
Argentinian tektites[149] | 5 km | 480 ka |
Australasian tektites[36] | 32–114 km | 780 ka |
Central American tektites[150][151] | 14 km | 820 ka |
Skye ejecta deposits[152] | Unknown | 60 Ma |
Stac Fada Member | 40 km | 1.2 Ga |
Barberton Greenstone Belt microtektites[153] | 500 km | 3.2 Ga |
Marble Bar impact spherules[154] | "hundreds of kilometers" | 3.4 Ga |
As of Jan 2017, the Earth Impact Database has risen to 190 confirmed impact craters (from 178 by the end of 2010). The following had their status upgraded from unconfirmed to confirmed in the last few years.
Malingen crater is thought to be a double impact with Lockne crater.[157] The pair, plus Hummeln and several other Ordovician craters in a small region in Europe, have been speculated to be connected to the Ordovician meteor event.[158]
Some geological processes can result in circular or near-circular features that may be mistaken for impact craters. Some examples are calderas, maars, sinkholes, glacial cirques, igneous intrusions, ring dikes, salt domes, geologic domes, ventifacts, tuff rings, forest rings, and others. Conversely, an impact crater may originally be thought as one of these geological features, like Meteor Crater (as a maar) or Upheaval Dome (as a salt dome).
The presence of shock metamorphism and shatter cones are important criteria in favor of an impact interpretation, though massive landslides (such as the Köfels landslide of 7800 BC which was once thought to be impact-related) may produce shock-like fused rocks called "frictionite".[159]