Silverpit Crater

Approximate location of the Silverpit crater in the North Sea.

Silverpit Crater is a submarine impact crater in the North Sea, off the coast of Great Britain. It was discovered in 2002 by analyzing seismic data collected in the course of routine oil exploration, listed as the first known impact crater in the British Isles. Other possible origins have since been proposed.

Its age must be in the order of 65 million years, so its formation coincides with the impact that created the Chicxulub crater in the Yucatan peninsula, Mexico (at the time of the extinction of the dinosaurs).. If Silverpit is an impact crater, this could imply that the Earth received collisions from various objects at that time, and it is likely that in an event similar to that of the collision of Comet Shoemaker-Levy 9 with Jupiter in 1994. This theory, known as a theory of multiple impacts, it is also supported by the existence, in various parts of the Earth, of other impact craters from the same period.

Discovery

A view in perspective of the crater surface, in the northeast direction, showing the central crater and its surrounding rings. False colors indicate depth (red/yellow = deep; blue/purple= deeper). (Photo credits: Phil Allen (PGL) and Simon Stewart (BP)).

The crater was discovered by analyzing seismic data collected by petroleum geoscientists Simon Stewart of British Petroleum and Philip Allen of Production Geoscience Ltd. at a point 130km up the Humber estuary during a search routine removal of fossil fuel deposits. Allen noticed a set of concentric rings, but gave no interpretation to this phenomenon and hung an image of them on the wall of his office, hoping someone would help him understand the mystery of his existence. Stewart, visiting Production Geoscience on another unrelated matter, saw the map and suggested it might be an impact crater. The discovery and the impact crater hypothesis were published in the journal Nature in 2002. Silverpit's name originates from local fishing grounds.

Just three years before the discovery of Silverpit crater was announced, it had been suggested that seismic data from the North Sea had a good chance of showing evidence of an impact crater: given the rate of cratering on Earth and the size of the North Sea, the expected number of impact craters was one.

The crater lies under a layer of sediment about 1,500 m deep, which forms the floor of the North Sea at a depth of about 40 m. Studies suggest that at the time of the crater's formation, the area was between 50 and 300 m below sea level.

Origin

It is accepted in scientific circles that the best explanation for the origin of Silverpit is a fireball. However, there are other mechanisms that can cause cratering, and the categorization of Silverpit as an impact crater has been called into question.

Evidence in favor of impact origin

Allen and Stewart considered and ruled out other origin mechanisms for the crater at the time of its discovery. Volcanic origin was excluded due to the absence of magnetic anomalies in the crater, which are always associated with eruptions. The removal of salt deposits under the crater, a mechanism that also produces craters, was ruled out because the Triassic and Permian rock layers behind the crater show no trace of that process. Another strong indication that it was an impact that created the crater was the presence of a central peak, something difficult to form except in meteorite impacts.

Alternative interpretation

Analyzing new seismic data, Professor John Underhill, a geologist at the University of Edinburgh, proposed that the structure is a syncline and not an impact crater. Underhill found that the lower rock layers, up to the Permian layers (about 250 million years old), are folded to form a syncline, and that the sediments from this period in the depression are thinner, suggesting that the depression is deepening. formed while Permian sediments were being deposited. This led him to suggest that the mobilization of saline sediments below the structure (halokinesis) was a better explanation for the formation of this depression. He also found in the same area two other synclines flanked by anticlines.

The existence of the central peak, which seems to give strong support to the impact hypothesis, is for Underhill an incorrect interpretation, and suggests that it could be the product of computer image processing. However, subsequent seismic reflections of the crater by Stewart and Allen appear to confirm its existence, so despite Underhill's work, the scientific consensus remains broadly in favor of an impact origin.

Structure

Seismic data showing the crater and its concentric ring structure (Image Credits: Phil Allen (PGL) and Simon Stewart (BP)).

Silverpit is about 1.5 miles wide. It is surrounded by concentric rings, which is unusual for craters on the surface, which extend for about 10 km from the center. These rings give the crater an appearance somewhat similar to Valhalla crater on Jupiter's moon Callisto and other similar craters on Europa. Ringed craters tend to be much larger than Silverpit, so if the impact hypothesis is is correct, the origin of the Silverpit rings would still remain to be clarified. A complicating factor is that most of the known impacts are on land, while two-thirds of the objects that impact fall into the sea, so the results of underwater impacts have been much less studied than those. impacts on the continents.

One possibility is that after the impact formed a bowl-shaped depression, the soft material around it slid toward the center, leaving behind the concentric rings. It is thought that for this to have occurred, the soft material would have to form a thin layer, with harder material on top of it. A thin layer of mobile material on top of a hard shell is a common configuration on icy moons, but not common on rocky bodies in the solar system. It has been suggested that a limestone layer strongly pressed below the surface may have acted as the soft, mobile layer.

The Impact

From the size of the crater, and from the assumption about the velocity of the impacting object, the size of the colliding object can be estimated. The objects that impact move at speeds of the order of 20-50 km/s; at this speed, it would take a rocky object about 120 m in diameter and a mass of 2.0×10⁹ kg to form a crater the size of Silverpit. If it was a comet, an object less rocky than a meteorite, then its size should be somewhat larger.

As a comparative data, it is estimated that the object that collided with the Earth in Chicxulub must have measured 9.6 km in diameter. The object responsible for the 1908 Tunguska event is thought to have been a comet or asteroid about 60 m in diameter, with a mass of 4×10⁸ kg.

An object 120 m in diameter falling into the sea at the speed of several kilometers per second must have produced a huge tsunami. Scientists look for evidence of large tsunamis in the surrounding areas that occurred at the estimated time of impact, however no such evidence has been found.

Age

The position of the crater within the layers of rock and sediment on the seabed can be used to establish an age range: sediments deposited before the formation of the crater will have undergone modifications, while those deposited after its formation formation will not have characteristics similar to those of other nearby areas. Allen and Stewart found that the formation of Silverpit modified the Cretaceous and Jurassic sediment layers, leaving the Tertiary period sediment layers intact. The Cretaceous period ended about 65 million years ago, so Silverpit must have formed 60 to 65 million years ago. The Chicxulub impact, which may have been a major factor in the demise of the dinosaurs, occurred 65 million years ago.

This method of estimating the age of formation is inaccurate, and its results are challenged by the hypotheses of other origins of Underhill crater. Other possible ways of dating the impact include looking for evidence of ejected material such as tektites and putative tsunami deposits, which could be found anywhere around the North Sea basin, but these evidences would have undergone repeated glaciations. In addition to allowing a better determination of the age of the crater, these lines of investigation would also confirm the impact hypothesis. Two oil drillings have been carried out in the area of the crater ring system, the study of which could shed light on the origin and age of the crater.

Part of a multiple impact

Silverpit is more like the Valhalla crater of the Jupiter Calisto moon than other Earth craters.

The estimated age of Silverpit leads to the inevitable speculation as to whether it is related to the much larger Chicxulub crater and the extinction of the dinosaurs. The age is not yet known exactly, so it is only possible to speculate. However, other large impact craters of the same age have been discovered, all between latitude 20º and 70º N, suggesting that the impact of Chicxulub may have been one of a series of impacts that all occurred at the same time, known as the multiple impact theory.

The collision of Comet Shoemaker-Levy 9 with Jupiter in 1994 demonstrated that gravitational interactions can fragment a comet, giving rise to several that occurred over a period of several days when colliding with a planet. Comets often show the effects of gravitational interaction in their long tails of gas, and it is likely that similar effects and collisions have occurred in the past. This scenario could have occurred on Earth 65 million years ago.

However, the evidence for this hypothesis is still not very strong, since the approximation of the creation dates of the Silverpit crater and the other hypothetically associated craters are accurate to within a few million years.