Large meteorites with the potential to cause major global environmental change strikes the earth on average every 100000 years, according to NASA. Geologically speaking, 100000 years is a short time interval making it plausible that every so often a "biggie" meteorite could hit the earth and cause a global catastrophe on the Permian type scale .
When a large meteorite collides with the earth it leaves much more evidence than the obvious crater. Meteorite craters are often destroyed by burial or plate tectonics so for older impacts where the crater has long since gone the other lines of evidence assume more importance
Shocked quartz (or Stishovite) is a high pressure polymorph of quartz produced at or close to the impact site. It is produced under such high pressurs that the only way it can form is by cosmic impacts. Finding traces of this in any geological strata is a very good indicator of an impact. Shocked quartz grains can only usually be seen with a scanning electron microscope.
Meteorites have become a popular debated cause for mass extinctions ever since the Chixulub crater was identified in the early 90's. The Chicxulub crater is in the Yukatan peninsular in Mexico and is seen as good evidence for a meteorite which could have killed off the dinosaurs in the K-T extinction event. This has been dated accurately to the K-T extinction event by radiometric techniques and also through stratigraphy analysis. Since this discovery much work has been focused on meteorite impacts being possible causes of other mass extinctions with many people focusing on the largest, the Permo-Triassic event .
Click here to see a Bristol website on which concentrates on the K-T impact theoryUnfortunately the evidence for an impact at the time of the Permian / Triassic boundary is much more ambiguous that that for the Crataceous-Tertiary boundary. There are many reasons for this with the best one probably being that the Permian was much longer ago with the evidence being exposed for much longer.
Xu and Yang, 1993 and Yang et al. 1995 all reported Iridium spikes and Stishovite microspherules in non-marine Permian -Triassic boundary sediments in Australia and Antarctica. The Iridium spikes found by these workers were at least an order of magnitude smaller than ones found in Cretaceous-Tertiary boundary sediments. The Stishovite microspherules were also at least one order of magnitude less abundant than in Cretaceous -Tertiary boundary sediments (Retallack et al. 1998) .Retallack et al. showed that the peak Iridiums levels were at least 1m stratigraphically below the isotopically and palaeobotanically defined boundary. The presence of the stishovite has been disputed (Holser et al. 1991) which has cast serious doubts on the plausibility of an impact at all.
(Hank Visscher et al.1996)found fungal cells in P-T boundary terrigenous sediments. These have been interpreted as fungi breaking down massive amounts of vegetation that have been catastrophically killed. These are good evidence of a catastrophic extinction and an impact is the most catastrophic of them all.
Very recently a research team led by Dr Luann Becker has published that they have chemical evidence for a meteorite strike 251 millions years ago at the time of the permian crisis. The evidence is based on complex carbon mollecules called fullerenes which have cosmogenic isotopes of helium and argon trapped inside their "football like" structure. The fullerenes show an unusually high concentration at the time of the extinction in three boundary sections in Japan, China and Hungary.
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