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Death From Below
16 January 2004 (All day)
When it comes to mass extinctions, few suspects can pack the needed punch. Many scientists suspect large asteroid impacts, which would have disturbed climates. But now, three geophysicists have suggested that lethal explosions from deep within Earth's crust may have what it takes to wreak that kind of havoc.
In the 1980s, researchers first proposed a link between impacts and one striking extinction: the dinosaur's demise. The evidence included layers of quartz grains that had been shocked by great pressure and elements rarely found in Earth's crust, such as iridium. This idea went head to head with another scenario, which argued that widespread volcanic eruptions could release enough gas to alter climate and cause widespread extinctions. But it wasn't clear that volcanoes had enough oomph to create the layer of shocked quartz put in place around the time of the death of the dinosaurs. For many followers of the debate, the discovery of a giant crater in the Yucatán about 10 years ago clinched the case that an asteroid killed off the dinosaurs. The hunt for more examples of impacts and mass extinctions gained momentum.
Now volcanic action may get a comeback. Jason Phipps Morgan and colleagues at the GEOMAR research center in Kiel, Germany, argue that you don't always need an asteroid to create shocked quartz and iridium layers. In the 15 January issue of Earth and Planetary Science Letters, the team suggests that a deep volcanic explosion could do the job.
Here's the scenario: When a plume of hot rock rises beneath a continent and begins to split it apart, carbon-rich magma may reach to 80 kilometers of the surface, where it coughs up its gasses. If enough gas builds up, the resulting explosion could catapult a column of overlying rock into the atmosphere. After that, the researchers argue, the surrounding rock would squeeze the hole shut, generating a shock wave that spews shocked minerals and plume rock elements, including iridium, into the atmosphere. The lethal amount of gas released would knock out some species even before the climate change effects kicked in and finished off the job. The team suggests this may have happened when the dinosaurs were wiped out and during at least two other mass extinctions as well--all times when continents were splitting and engulfed with volcanism. The fact that an asteroid struck the Earth at the same time as the dinosaur's demise could be merely coincidence, they say, or have reinforced events already in motion.
"It's a neat idea, and it addresses a lot of enigmatic aspects of extinctions and volcanism," says geologist Paul Wignall of Leeds University in the United Kingdom. "But whether it's feasible or viable is hard to tell." In particular, he says, it may be very difficult to build up the ultrahigh pressures needed to eject the rock column into the atmosphere.