Crater was formed by meteor impact" was held at the Geological Society of London. Simon Stewart gave the case for the motion and John Underhill presented the case against. The outcome was overwhelming support for Underhill's alternative genesis through melt-induced salt withdrawal. Structure Silverpit crater is about wide at the top Cretaceous level. Unusually for a terrestrial crater, it is surrounded by a set of concentric rings, which extend to about radius from the centre. These rings give the crater a somewhat similar appearance to Valhalla crater on Jupiter's moon Callisto, and other craters on Europa. Normally, multi-ringed craters tend to be much larger than Silverpit, and so, if the impact hypothesis is correct, the origin of Silverpit's rings is subject to debate. A complicating factor is that almost all known impact craters are on land, despite the fact that two-thirds of impacting objects will land in oceans and seas, so the results of impacts on water are much less well established than those of impacts on land. Compare the Chesapeake Bay impact crater, probably the most thoroughly studied marine impact zone. One possibility is that after the impact excavated a bowl-shaped depression, soft material surrounding it slumped towards the centre, leaving the concentric rings. It is thought that for this to happen, the soft material would have to be quite a thin layer, with more brittle material on top. A thin layer of mobile material beneath a solid crust is easy to understand in the context of icy moons, but is not a common occurrence on the rocky bodies of the solar system. One suggestion is that overpressured chalk below the surface may have acted as the soft mobile layer. Impact If one assumes the meteor impact theory is right, the size of the crater can be combined with assumptions about the speed of an impacting object to estimate the size of the impactor itself. Impacting objects are generally moving at speeds of the order of , and at these speeds an object about across and with a mass of would be required to form a Silverpit-sized crater, if the object was rocky. If it had been a comet, the crater would have been larger. For comparison, the object which struck the Earth at Chicxulub is estimated to have measured approximately 9.6 km (6 miles) across, while the object responsible for the Tunguska event in 1908 is thought to