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Adapted from a BBC online article: Friday 30th March 2007

What is thought by some to be the UK's only space impact crater has been mapped in detail in 3D for the first time (left).

The so-called "Silverpit structure" lies several hundred metres under the floor of the North Sea, about 130km (80 miles) east of the Yorkshire coast. The new pictures show a spectacular set of rings sweeping out around a 3km-wide (1.8 miles) central hole.

Dr Simon Stewart and Phil Allen detail in the Geological Society of America Bulletin how the crater's features would have developed from the cataclysmic fall of an asteroid or comet about 60-65 million years ago.

But some, such as Professor John Underhill of Edingburgh University, have questioned this theory. He believes that it was probably the movement of deep salt rocks rather than a one-off meteorite impact. There is now a lively debate about the origin of Silverpit among geologists.

Impact crater theory

Dr Stewart points to a 300m-high central peak, or nipple, in the centre of the inner bowl, typical of impact craters. "I can't understand why John keeps banging away at an alternative model," he says. Impact expert Dr Gareth Collins from Imperial College London agrees with Dr Stewart, saying that the circular structure is geometrically similar to other craters, particularly those found on other planets. "On balance an impact origin is the simplest and most likely explanation," he says. "But ... it has absolutely not been proven to have an impact origin."

Salt movement theory

In contrast, Prof Underhill and some other geologists say that surveys reveal nine similar vast chasms in the area and that these are the result of movement of a thick layer of salt - formed 250 million years ago - that lies below the whole area. This salt is highly mobile and flows between areas of high and low pressure. Huge blisters of salt can force overlying rocks up into domes, known as anticlines; elsewhere the salt flows entirely away and the overlying layers buckle and subside. Prof Underhill maintains that this is probably what caused the crater-like Silverpit structure.