The evidence at the origin of the hypothesis of an impact is the thin layer of clayish sediment containing unusually high quantities of iridium, which is found all around the world exactly at the K-T boundary. Iridium is rare in the earth' s crust, but common in meteorites.
Moreover, several kinds of minerals have been found whose origin is clearly meteoritic. First, shocked crystals of quartz found at the K-T boundary show evidence for very high pressures. Second, nickeliferous magnetites have been found in several K-T sediments; they are formed during the fall of a meteorite through the atmosphere.
A candidate for the impact crater (or one of the impact craters) has been found in Chicxulub, in the Yucatán peninsula, Mexico. The crater has been covered by sediment and is not visible on the surface. It has been detected through anomalies in the gravitation field and magnetic field. Seismic data confirmed the presence of a big basin in-depth. The age of the crater has been estimated to 65 million years. Under the crater, in-depth breaches have been found, and below them, melted minerals, a combination which is found in other known impact craters.
All around the Gulf of Mexico, glass spherules have been found whose chemical composition is similar to that of the Chicxulub crater, and which were probably ejected there after the impact. Their crystallisation structure is irregular, indicating a short and violent event. The droplets go together with shocked quartz. Above them lies a thick layer of coarse sand, probably left by gigantic waves due to the impact. Still above comes the usual iridium-bearing layer. Under these traces, the sediment corresponding to the late Cretaceous is missing, probably eroded by the tsunami.
In some places, soot has been found above the iridium.
So, the presence of a meteorite impact at the end of the Cretaceous is nearly certain (see also the discussion about the volcanic hypothesis). The size of the meteorite has been evaluated to ten kilometres, its weight to one million of millions of tons, and its speed from twenty to eighty kilometres per second. The dynamics, the geological consequences of the impact and the geographical distribution of its traces are currently under precise study.
It has been hypothesised that most extinction periods in the history of life were due to meteoritic impacts. The data are quite controversial.
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