FOUR DAYS THAT SHOOK THE WORLD New Scientist May 8, 2004
Just when you thought the dust had settled on the cause of the demise of the dinosaurs, there's a new type of catastrophe kicking it up again. Forget meteorites and mega-volcanoes, Verneshots are the real culprits, says Kate Ravilious. The Earth exploded under their feet. Noxious gases spouted into the atmosphere and quickly circulated around the globe. The ground shook with the force of a hundred massive earthquakes, and 20 gigatonnes of the Earth's crust and mantle were blasted into the sky before raining back down onto the surface. It was a terrible day for the dinosaurs. They never recovered. Is this, at last, a true description of what happened 66 million years ago? The argument over what killed the dinosaurs has raged for 25 years, and has polarised into two opposing camps: a meteorite impact, or a prolonged bout of mega-volcanism called a continental flood basalt. But now a team from Geomar, an earth sciences institute at Kiel University in Germany, has come up with a completely new type of geological catastrophe to explain the death of the dinosaurs, as well as three previous mass extinctions. If they are right the culprit was neither a meteorite nor a flood basalt, but a colossal underground explosion called a Verneshot. As yet the idea is in its infancy (Earth and Planetary Science Letters, VO1217, p 263). But the Verneshot hypothesis has one big advantage over its rivals. It explains a mystery that haunts the debate over mass extinctions: why the extinctions always seem to coincide with both continental flood basalts and meteorite impacts when the odds of these happening simultaneously are vanishingly slim. In the past 400 million years there have been four major mass extinctions. Between 380 and 364 million years ago the Frasnian-Fammenian extinction pulses wiped out 60 per cent of marine life. That was followed by the biggest extinction of all, when 96 per cent of the world's species disappeared at the end of the Permian, 251 million years ago. There was another huge die-off at the end of the Triassic 201 million years ago. And finally, 66 million years ago, the dinosaurs and numerous other groups met their end at the Cretaceous-Tertiary (K-T) boundary. Each of these events is associated with both a meteorite impact and a continental flood basalt. Yet even one such double whammy is highly unlikely: could it really have happened four times? Meteorite impacts large enough to trigger a mass extinction are believed to occur less than once every 100 million years. Continental flood basalts, meanwhile, happen roughly once every 30 to 50 million years and last for about a million years. "Doing a quick back-of-the-envelope calculation reveals that the odds of a meteorite impact occurring at the same time as a continental flood basalt within the last 400 million years is around 1 in 8," says Jason Phipps Morgan, who led the Geomar team. "Unlikely, but perhaps within the realm of terrestrial bad luck." But the probability of four of these coincidences happening within the last 400 million years is 1 in 3500. As the evidence for this coincidence mounted, so too has the need for a way of explaining how it came about. And so Phipps Morgan and two of his Geomar colleagues, Tim Reston and Cesar Ranero, set out to find one. The result is the Verneshot hypothesis - and the beginning of a new and acrimonious battle in the war over mass extinctions. The first idea the Geomar team considered was whether impacts could somehow cause continental flood basalts. This has been proposed several times, most recently in 2002 when Adrian Jones of University College London suggested that a large impact could pierce the Earth's crust and initiate a mammoth outpouring of lava (New Scientist, 14 December 2002, p 16). But Phipps Morgan's team dismissed this idea, concluding that it was physically impossible for a meteorite to cause the sustained melting associated with flood basalt. What is more, the geology of the K-T boundary does not support the idea that an impact triggered the flood basalts. The crater at Chicxulub is on the opposite side of the world to its contemporaneous flood basalt, the Deccan Traps in India, and geochemical dating suggests the volcanism began long before the meteorite hit. So the Geomar scientists decided to turn the argument on its head. Perhaps the continental flood basalts somehow created the distinctive "signatures" of a meteorite impact. If this could happen then the impacts could be dismissed as spurious. This is not a new line of thought: geologists have been following it since the early 1960S. But until now no one had come up with a convincing mechanism to explain how flood basalts could mimic all the geological signs of an impact, the most obvious and rarest of which is a crater. Other important signatures include shocked quartz crystals criss-crossed with tiny fractures, high concentrations of the rare element iridium, small blobs of melted rock called microspherules, and fullerenes. Continental flood basalts are widely believed to occur when mantle plumes - streams of lava welling up from deep inside the Earth-butt up against continental lithosphere (crust plus a thick layer of solid mantle). At around 240 kilometres thick, this presents a serious obstacle to the mantle plume's upward progress. But if the plume breaks through, it causes prolonged and deadly volcanic eruptions. Violent as they are, however, geologists have never been able to show that these eruptions could create all the signs of a meteorite impact. But Phipps Morgan thinks there is a neglected type of continental flood basalt that could create all the signatures of an impact. What if, he says, a mantle plume sprang up beneath a craton? These ancient chunks of continent stabilised around 2.5 billion years ago, in the Archaean, and have not been deformed by plate tectonics since then. Consequently, they are extremely tough, stable and deep, sometimes extending 300 kilometres below the Earth's surface. Phipps Morgan believes a mantle plume would not be able to punch through the thick lithosphere of a craton but would cause an almighty build-up of explosive gases within it. "Carbon dioxide and sulphur dioxide tend to outgas from rising magma and pool at around 80 kilometres," he says. And the heat from the plume would melt carbon-rich rocks, stockpiling even more gas. Meanwhile, the plume would slink off sideways and pool under the thinnest bit of nearby lithosphere. Eventually some of this would spill out onto the surface as a continental flood basalt. But down below, the mantle plume would still be incubating the craton and stockpiling gases. The final requirement to make it all blow is for the craton to start pulling apart and rifting. This kind of continental splitting occurs roughly every 100 million years or so as part of the Wilson cycle that creates and destroys the world's oceans. Today it is happening along the rift valley in east Africa; crucially there is evidence that rifting was going on during the three most recent mass extinctions. This rifting would be just enough to release the pressure inside the craton and enable a catastrophic gas explosion. Phipps Morgan envisages that the gases would whoosh up through a thin fracture in the crust and burst out at the surface, poisoning the atmosphere and causing severe environmental stress around the world. The blast itself would be devastating. Phipps Morgan has calculated that it would trigger a magnitude 11 earthquake, 100 times more powerful than the magnitude 9 quake that hit Chile in 1960, the biggest ever recorded. But there is worse to come. Immediately after the explosion, the pressure would plummet in the pipe that carried the gases, causing it to cave in from the bottom upwards. "This collapse will travel upwards at hypersonic speeds and could blast out rocks from the top of the pipe, throwing them to any other point on the Earth's surface," Phipps Morgan says. The energy released would be equivalent to 120 billion tonnes of TNT, or 7 million Hiroshimas, the Geomar team estimates. If this release was sudden enough, it could eject as much as 20 gigatonnes of rock into a "super-stratospheric" trajectory. Jules Verne's space-gun This mechanism reminded Phipps Morgan of a book he had read as a child, Jules Verne's From the Earth to the Moon which is about a huge gun that shoots objects into space. "We decided to name our mechanism after Jules Verne's space-gun," he says. Hence the Verneshot. The beauty of the idea is that it can account for all the impact signatures associated with mass extinctions. Deep mantle volcanism would bring iridium to the surface, while the explosion and impacts would produce shocked quartz and craters. During the collapse of the Verneshot pipe, blobs of melt would be flung out, some preserved as microspherules. Meanwhile, the gas decompression and oxygen-gobbling explosion would encourage the formation offullerenes. The Verneshot mechanism might even explain why the object that crashed into Chicxulub left such a lopsided crater. Modellers have concluded that the impactor came in from the south-east at an angle of around 20 degrees. This doesn't rule a meteorite out, but it also fits with Verneshot debris ejected from the Deccan Traps. Compelling the idea may be, but is there any proof? "It is a very difficult theory to prove because it is hard to differentiate between a meteorite impact and a Verneshot," says Phipps Morgan. "We need to find signs of the gas release pipe." He believes the remains of any pipe will be buried under many kilometres of rock spewed out by the flood basalt, but they should show up on seismic images and gravity surveys. "Between 80 kilometres and the surface I would expect the seismic survey to reveal a disturbed area oflithosphere, filled with basaltic melts. There would also be a circular gravity anomaly relating to the basaltic rocks of differing density." Though no one has yet carried out detailed seismic surveys of the relevant areas of flood basalt, gravity survey results are tantalising. Large, near-circular gravity anomalies have been recorded under the Deccan Traps and just off the coast of Freetown, Sierra Leone. This is part of the Central Atlantic Magmatic Province, a flood basalt associated with the mass extinction at the end of the Triassic. Meanwhile, geologists have found part of a crater known as the Great Tunguska Depression under the Siberian Traps, the flood basalt associated with the Permian mass extinction. This crater has all the hallmarks of an impact, including shocked quartz. Could it be the location of a Verneshot launch pipe? Intriguingly, this crater was also the site of the 1908 Tunguska event, where about 2000 square kilometres of forest was flattened by a mysterious explosion. Eyewitnesses reported seeing a fireball and this led scientists to classify it as a meteorite impact. Now Phipps Morgan has another explanation: a micro-Verneshot. "If the Great Tunguska Depression is a relic of a Verneshot launch site, then it is likely that the original pipe was not completely erased and that it will be reused by much smaller gas release events," he says. Phipps Morgan is now teaming up with other scientists to test the Verneshot theory. Along with geologists at the National Geophysical lnstitute in Hyderabad, India, he is investigating the circular anomalies in the Deccan Traps, and is hoping to explore the Great Tunguska Depression for evidence of a deep, vertical pipe. He would also like to hunt for two other pieces of evidence: shattercones and remnants of Verneshot impactors. Shattercones are pieces of rock with a distinctive pattern of radial fractures created when the rock shatters under the force of an impact. They tend to point towards the centre of an impact site and Phipps Morgan hopes that more detailed surveys of their orientations would reveal which direction the impact came from. "Shattercones might be able to show us whether the shock source came from inside the Earth - a Verneshot event - or from up above- a meteorite," he says. The other obvious clue to a Verneshot event would be to find remains of the impactor inside a crater. Usually the high-speed approach of a meteorite ensures that it vaporises when it hits the Earth. Verneshot debris, on the other hand, would not travel quite so fast and should leave some remnants at the crash site. If scientists were able to geochemical1y fingerprint fragments of rock at the Chicxulub crater, for example, and show that they originated from the Deccan area of India, this would be strong evidence for the Verneshot mechanism. So far the Verneshot theory has received a mixed reception. Supporters include Paul Hoffman of Harvard University, the expohent of the Snowball Earth theory. "I think the hypothesis is a serious proposal!" he says. "It's a creative approach to a real problem." Meanwhile, Vincent Courtillot from the Denis Diderot University in Paris, one of the editors of Earth and Planetary Science Letters and a prominent supporter of volcanism as the cause of mass extinctions, says the idea is making people reassess the evidence. "I believe it is an ingenious idea and I was happy to publish it, but I am not absolutely sure that it is physically viable," he says. Claude Jeaupart from the Denis Diderot University is also sceptical about the mechanism. "It is not established by rigorous physical principles," he says. "But as yet it is not possible to dismiss the idea outright because the argument is based on hypotheses." The strongest criticism comes from geologists who favour the idea that a meteorite killed the dinosaurs. Jan Smit of the Free University of Amsterdam, the Netherlands, argues that the dating of impacts and flood basalts is not precise enough to be sure they actually coincided. And anyway "there is not a scrap of evidence for Verneshot events", he says. Philippe Claeys from the Free University in Brussels (VUB), Belgium, points out that of the four mass extinctions, only the K-T event has clear evidence of an impact. He suggests that Phipps Morgan should consider the simpler option that the impact signatures at boundaries other than the K-T are spurious. "If that is the case, we don't need any mystic and un-testable mega-volcanic hypothesis to solve the problem." Phipps Morgan accepts the logic of Claeys's argument, but points out that there is "near consensus" that the impact signals are "genuine indicators of a major shock". If Claeys is right, it would also mean that impact supporters would have to admit that continental flood basalts were the cause of at least three of the four most recent mass extinctions. However, Phipps Morgan is happy to concede that the K- T mass extinction was probably caused by an unlucky coincidence. "Chicxulub is so far from the Deccan Traps that it is extremely unfavourable for a Verneshot event to eject a piece of crust for that distance," he says, "especially without it fragmenting and leading to simultaneous smaller impacts around the globe at the same time.." But if the Verneshot hypothesis is right then we need to keep an eye on where the next event may be brewing. "This is complete speculation, but I think that the Siberian craton is the most likely place," says Phipps Morgan. "Northern Eurasia is just starting to rift and appears to have the right pre-conditions for an explosive plume to burst through." Hold on to your hats. We could be in for a bumpy ride. Kate Ravilious is a science writer based in Oxford ______________________________________________ Meteorite-list mailing list [EMAIL PROTECTED] http://six.pairlist.net/mailman/listinfo/meteorite-list
