Another meteor impact coincides with large-scale volcanic eruptions -Planet Earth 27FEB09

http://planetearth.nerc.ac.uk/news/story.aspx?id=332

Another meteor impact coincides with large-scale volcanic eruptions
---Planet Earth online 23 February 2009
Scientists have long debated the cause of the dinosaurs' extinction about 65 million years ago. The remnants of a large volcanic eruption in the Faroe Islands. These eruptions can go on for millions of years. Around this time a giant meteorite struck the Gulf of Mexico. But the extinction also seems to coincide with massive and long-lasting volcanic eruptions in India known as the Deccan Traps. So which event was responsible? And are these phenomena linked?
New research now shows that this combination of meteorite impact andlarge-scale volcanic activity - known as flood basalt eruptions - is not unique. An international team of researchers looked at a 30-million-year-old meteorite crater in Belarus called Logoisk. They found that this too coincided with volcanic eruptions further south which covered Yemen and Ethiopia with basalt rock. These events are similar to those that occurred 65 million years ago, but on a much smaller scale. The scientists suggest such coincidences may be more common than previously thought. Dr Sarah Sherlock from the Open University and lead author of the paper, says, 'If you have a flood basalt then people wonder if there's also an impact. ''There will be, almost certainly,' she added. According to the paper, a meteorite will strike the Earth and leave a crater the size of Logoisk on average once every 1.5 million years. Flood volcanic eruptions occur over several million years, so a Logoisk-sized crater is likely to occur during each of the 16 identified periods of flood volcanism on Earth in the last 360 million years. However, researchers do not think there is a causal link between flood volcanism and meteorite impact.'There is simply no geological evidence to link the two,' says Sherlock. To determine the precise age of the Logoisk crater the researchers used argon dating. 'Argon dating is very versatile.' said Sherlock. 'It's the only technique that can be used to date both [impacts and flood volcanism].' Samples of material from the crater were gradually heated using an infrared laser, causing the release of argon gas. The ratio of two isotopes of argon released in the gas gives an accurate indication ofthe age of the sample. Using this technique, the researchers showed that the two events occurred simultaneously.
One question raised by the results was why the meteorite impact and flood volcanism 65 million years ago wiped out much of life on Earth, including the dinosaurs, but the similar events 30 million years ago did not. According to Sherlock, it was down to the size of the events.'These coincidences in Earth's history are not as rare as people think,but in order to actually do significant damage to the environment they have to be really, really big.' Sherlock added. Together, the 65-million-year-old Chicxulub crater in the Gulf of Mexico and volcanic eruptions that produced the Deccan Traps eruption 65 million years ago released 8000 gigatonnes (Gt) of sulfur dioxide, causing global environmental damage. By comparison the Logoisk and Afro-Arabian events released only 30Gt - insufficient to cause change on a global scale.

The research is published in the Journal of the Geological Society, London.