Silverpit crater

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Coordinates: 54 ° 14 ′ 0 ″  N , 1 ° 51 ′ 0 ″  E

Location of the Silverpit crater

The silverpit crater is near the coast of Britain in the North Sea lying crater . It was discovered in 2001 while analyzing seismic data collected while searching for oil wells. Initially it was believed to be an impact crater , but alternative origins have since been suggested. The age of the crater is estimated to be around 55–65 million years.

discovery

A perspective view of the uppermost limestone layer facing northeast. The central crater and the surrounding rings are shown. The coloring was based on the depth (red / yellow = flatter; blue / purple = deeper). Image by Phil Allen (PGL) and Simon Stewart (BP).

During a search for unknown sources of oil in 2001 seismic data in a region 130 kilometers away were the estuary of the River Humber levied. Geologists Simon Stewart (of BP ) and Phillip Allen (Production Geoscience Ltd.) encountered an undocumented anomaly while analyzing this data. Allen noticed a series of concentric rings but was unable to interpret them and posted a picture of the measurements in his office. Stewart, visiting Production Geoscience for other reasons, saw this map and suggested an impact crater as a possible origin. The discovery as well as the preliminary hypothesis about the origin of the crater were published in the journal Nature in 2002 . ( Lit .: Stewart and Allen, 2002)

The name of the crater was chosen based on the name given by local fishermen to the subsidence of the seabed that surrounds the crater. It is believed that this depression is an old river bed from the Ice Age .

Only three years before the Silverpit crater was discovered, statistical calculations showed that there is a high probability of an impact crater in the North Sea, given its size and the frequency of crater formation.

The crater lies under a layer of sediment up to 1500 meters thick , which forms the bottom of the North Sea at a depth of around 40 meters. Studies suggest that the area was 50 to 300 meters below the surface of the water at the time the crater was formed.

origin

Formed by a meteorite impact

After examining the crater, Allen and Stewart concluded that the impact of a meteorite was the best possible explanation for its origin. They checked alternative mechanisms immediately after discovering the crater, but rejected them again. Volcanism was excluded because the crater does not have the magnetic anomalies typical of volcanic eruptions . The erosion of salt deposits - a well-known mechanism for the formation of crater-like structures - was also ruled out, as the Triassic and Permian layers below the crater showed no signs of erosion. The central mountain within the crater, which can hardly form without an impact, is viewed as a strong indicator of a meteorite impact.

Mineralogical and geochemical studies of drill cores from the central region of the crater were examined for signs of a meteorite impact. Signs of changes in the minerals due to shock waves emanating from an impact ( shock metamorphosis ) could not be found, nor were traces of the causing meteorite. ( Lit .: Koeberl and Reimold, 2004)

A meteorite of this size hitting a sea would also generate large tsunamis . The deposits formed in other places as a result would represent definitive evidence of an impact, but could not be detected until 2004. ( Lit .: Smith, 2004)

Alternative theories

However , after analyzing older, larger-scale seismic data, Professor John Underhill, a geologist at the University of Edinburgh , claimed that deep erosion explained the crater better. ( Lit .: Underhill, 2004) Underhill found that all rock strata up to the Permian (about 250 million years old) are folded synclinally - that is, they represent a concave indentation. Sediments from this era are thinned around the crater, suggesting crater formation during the deposition of Permian sediments.

According to Underhill, the Zentralberg is nothing more than an artifact of image processing. However, later seismic surveys confirm the existence of this survey. ( Lit .: Stewart and Allen, 2005)

Ken Thomson from the University of Birmingham also showed in mid-2004 that salts in the southern North Sea collect in large, linear deposits, the leaching of which could well have produced the concentric structures. ( Lit .: Thomson, 2004)

Structure of the crater

Seismic data show the crater and its structure of concentric rings. Image by Phil Allen (PGL) and Simon Stewart (BP).

The Silverpit crater has a diameter of about 2.4 kilometers. Around the crater, concentric rings spread out at a distance of up to 10 kilometers. This rather rare in terrestrial craters appearance makes the Silverpit structure the Valhalla Crater on the ice-covered moon of Jupiter Callisto and some craters in the icy crust of Jupiter's moon Europa appear similar. ( Lit .: Allen and Stewart, 2003) Under normal circumstances, however, such “ringed” craters are much larger than the Silverpit, which, assuming the impact hypothesis, makes the origin of the ring structures appear questionable. The finding is made more complicated, especially since almost all known impact craters are on land and therefore the effects of impacts in water are far less well investigated. Probably the best studied maritime crater is the Chesapeake Bay Crater .

One possibility would be that the impact initially ejected a bowl-shaped depression, then softer material slipped in the direction of the crater, leaving the concentric rings behind. It is assumed that the layer of such soft material for this purpose would have to be quite thin and there would have to be additional, brittle material on top. In contrast to ice-covered moons, a thin layer of mobile material beneath a solid crust is rarely found on the rocky bodies of the solar system. One suspicion is that limestone under the surface under high pressure may have acted as a soft, moving material. ( Lit .: Collins, Turtle and Melosh, 2003)

The impact

The size of the impactor can be estimated from the size of the crater and assumptions about the speed of an impacting object. Such objects usually move at speeds between 20 and 50 kilometers per second in the area of ​​the earth's orbit . To create a crater the size of the Silverpit, an asteroid would have to be around 120 meters in diameter and weigh 2 million tons, while a comet would have to be slightly larger due to its lower density .

As a comparison: The diameter of the object that ejected the Chicxulub crater is estimated to be around 10 kilometers. When it hit, around 500,000 times more energy was released than would have been released by an impact when the Silverpit crater was formed.

Age

The stratigraphy , i.e. the position of the crater within the rock and sediment layers on the sea floor, can be used to estimate its age: sediments that covered the sea floor before the formation of the crater were stirred up in contrast to later sediments. Allen and Stewart discovered that the Silverpit lime in the Cretaceous and shale from the Jura was formed and a continuous layer of sediment from the tertiary was covered. The Cretaceous Period ended about 65 million years ago, but nearby research boreholes suggest that the lowest tertiary layers are missing from the sedimentary stratification. Accordingly, the Silverpit crater is between 55 and 65 million years old. The Chicxulub, the formation of which is believed to be responsible for the extinction of the dinosaurs, was formed at the end of the Cretaceous period, 65 million years ago.

This stratigraphic age determination is crude and is challenged by Underhill's hypothesis. Other dating options include examining ejecta - such as impact glasses or tektites - or deposits from triggered tsunamis. The latter should be found in the entire North Sea basin in the event of an impact, but could have been affected by glaciation several times. In addition to precise radiometric dating , these investigations could also strengthen the impact hypothesis. However, no ejecta material that can be assigned to the Silverpit crater and no deposits from tsunamis have been found, recovered and investigated.

Part of several impacts?

The Silverpit crater looks more similar to the Valhalla crater on Jupiter's moon Callisto than other terrestrial craters.

The estimated age of the Silverpit crater inevitably leads to speculation as to whether there is a connection to the much larger Chicxulub crater and the extinction of the dinosaurs. Other craters of roughly the same age - all between latitudes 20 ° N and 70 ° N - have also been discovered, which could indicate that the Chicxulub impact was just one of several impacts at the end of the Cretaceous Period: Besides Chicxulub and Silverpit, it is also likely Boltysh crater in Ukraine (24 km diameter, age 65.2 ± 0.6 million years) on the K / T boundary, as well as possibly the still insufficiently investigated craters Eagle Butte ( Canada ) and Vista Alegre ( Brazil ) with diameters of around 10 km each.

The collision of the comet Shoemaker-Levy 9 with Jupiter in 1994 proved that a comet can be broken into several parts by tidal forces and thus hit different places on a planet over several days. Comets are exposed to such forces mainly in the vicinity of gas giants , so that there is a high probability that fragmentation would have occurred some time before the impact. In the absence of more precise dating, researchers can currently only speculate whether there actually was a multiple impact on the earth at the end of the Cretaceous period.

See also

literature

Web links

Commons : Silverpit  - album with pictures, videos and audio files
This version was added to the list of articles worth reading on April 17, 2006 .