http://www.sciencedaily.com/releases/2009/01/090107085320.htm
Catastrophic Coincidence: Second Ever Example Of Contemporaneous Meteorite Impact And Flood Volcanism Discovered ScienceDaily (Jan. 7, 2009) Scientists have discovered only the second example of a meteorite impact that occurred at the same time as massive volcanic activity, in research published in the Journal of the Geological Society the week of Jan 12. The first time such a coincidence was observed, at the Cretaceous-Tertiary boundary, was the catastrophic event thought to be responsible for the extinction of the dinosaurs, 65 million years ago. This new event, uncovered after the 17 km diameter Logoisk impact structure in Belarus was precisely dated, is thought to have taken place around 30 million years ago. The crater was dated using argon isotopes, and found to have occurred at a similar time to a period of massive volcanism known as the Afro-Arabian flood volcanism, which started in NW Yemen at around 30.9 Mya, and SW Yemen at around 29.0 Mya. The impact also coincides broadly with a period of sudden global cooling and sea level fluctuation. The researchers, led by Sarah Sherlock at the Open University, argue that massive volcanic eruptions and meteorite impacts are likely to have coincided much more frequently than has previously been thought, but because the preservation of impact craters on Earth is poor much of the evidence for these coincidences is lost. The relationships between meteorite impact craters, volcanism and changes in climate is a subject of much debate among scientists. Prior to the study, only one example of an impact coinciding with volcanism had been found: the Chicxulub and Boltysh impacts and the Deccan Traps flood volcanism, all of which occurred at the Cretaceous-Tertiary boundary. In 2002, the discovery of their coincidence with a global mass extinction led to debate over the causative links between meteorite impacts, volcanism and mass extinction events, and fuelled the search for more impacts at stratigraphic boundaries. Unlike the Cretaceous-Tertiary event, the combination of the Logoisk impact and the Afro-Arabain flood volcanism does not seem to have caused an extinction event. The researchers suggest that the reason for this may be that the magnitude of the event was not sufficiently large in comparison. Whilst the Chicxulub crater associated with the extinction of the dinosaurs measures 170km in diameter and the Deccan Traps released around (2-4) x106 km3 of lava, in comparison the Logoisk impact structure measures 17km across and the Afro-Arabian flood volcanism is around 1.2 x106 km3 in volume. As a result, the effects of each event were likely to have been very different. At the Cretaceous-Tertiary boundary, it is thought that one of the deadly effects was the release of sulphur dioxide, either as acid rain or in the stratosphere, where it would have prevented heat from reaching the earth and caused massive global cooling. Around 8000 billion tons of SO2 are thought to have been released by the volcanism and meteorite impact. In comparison, the Logoisk impact and the Afro-Arabian volcanism are thought to have contributed only 30 billion tons of SO2. Meteorite impact craters are extremely difficult to date, but an understanding of their age and frequency is crucial to attempts to control the number of future impacts, as well as understanding the links between impacts and other catastrophic events such as large volcanic eruptions and mass extinctions. Around 90% of the Earths record of meteorite impacts is lost, and the researchers argue that coincidences between impacts and flood volcanism are far from rare. They suggest that, for every incidence of flood volcanism, at least one crater the size of Logoisk is likely to form, although few such coincidences are likely to be on a scale grand enough to bring about an extinction event comparable with that which destroyed the dinosaurs. ______________________________________________ http://www.meteoritecentral.com Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list
