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We have performed simulations of a meteor impact into crystalline rock overlain by water of different depths to understand how crater characteristics are affected by the presence of water. The application in the first instance is to the Gardnos crater in the Hallingdal Valley of Norway, created some 400-600 million years ago when the area was probably covered by a shallow sea. Water vaporizes explosively under impact conditions, so is very good at dissipating the energy of an impact. The projectile energy required to produce a given crater diameter is higher for a greater water depth. But for the same size crater, the sustained overpressure is greater at greater water depths, and the maximum-pressure depth is also greater. The crater morphology produced under deep water differs from that produced under shallow water. Comparisons of pressure-induced transformations in deep-core samples near the crater centre with similar samples farther from the centre may serve as a diagnostic of the presence and extent of volatiles overlying crystalline rock at crater cites.
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