A slightly more contentious clue to a cosmic impact is the presence of tektites . Tektites are smooth glass-like stones that are found in certain sedimentary layers on earth. They range in size from a few centimetres to microscopic. These stones show clear signs of having been heated twice. The first heating process seems to have reduced the entire rock to a molten state, while the second only seems to have effected the outer layers.
Some tektites are teardrop shape, indicating that they have been moulded by aerodynamic forces.
Tektites Tektites are believed to be formed by asteroid and comet impacts. As the comet hits the ground the underlying rock is reduced to a molten state and ejected into the atmosphere. This molten material is smoothed and streamlined by air resistance and eventually cools. The second heating process could be produced as the stones fall back to earth, in much the same way as the outer layers of a small meteorite are ablated by atmospheric friction. Using the methods outlined above, geologists have recognised almost 200 craters over the planet.