Wednesday, April 19, 2006

More Mass Extinction Mumblings: Volcanoes

After watching the Nova episode on Global Dimming last night, it made me ponder a bit about why volcanoes are not the monsters on a global scale that are often portrayed in the way that they are portrayed. The ash spewing monster is simply not that dangerous on the global scale. Honestly. It might be horrible for the locals and sometimes even for the world as a whole, but their effects are mostly unimportant on the Terra-wide geological scale.

Wait, you think, does that mean that volcanoes are not dangerous to the world as a whole? At least in mass extinction terms?

Yes. And no.

The popular imagination has been captured by and large by volcanoes like Mount Saint Helens because of its eruption in 1980. Mount Pinutabo's 1991 eruption reinforced this idea. This caused a press hunt for similar volcanoes (Tambora and Krakatoa, frex). Some authors have used it as a plot device in their future evolution and extinction events. The problem is that even though these eruptions are huge and make an vast impact, especially with respect to the Indonesian volcanoes and the massive cold snaps that they caused. However, the events that they caused while horrific and worldwide were extremely short in duration. A year or so of frank suckage, but no longer for the cooling events. This means that the volcanoes as we have seen them at least do not provide enough forcing through particulate injection to have a long term consequence. Consider what was said about the global dimming and how quickly the snap up in temperature that they observed was after a few days of little airline travel, it ought to be obvious why.

Put simply, the duration of the injection event is simply too short and not repetitive enough to make a long term difference. One or two years of impact is not enough to wipe out everything. Refugia are simply too common.

That said, volcanoes can and do appear to cause mass extinctions. They seem to have caused the grandpappy whopper of a mass extinction, the Permian Mass Extinction. The Siberian Traps seem to have been the source of all the CO2 that worked so hard to try to cook the planet (with methane's help). The Siberian Traps erupted over a very long period. This means that it had time to produce and maintain the CO2 levels.

The Deccan Traps, it should be noted, were over a long period, eight times longer than the Siberian Traps, but it should also be noted that the Deccan Traps seem to have been an order of magnitude less in area covered than the Siberian Traps. The total output and possibly energy involved would seem to be, very rough BoE calculation, as being 1/80th of the Siberian Traps. It might be considered then that the Deccan Traps would have a significantly less impact than the Siberian Traps. While the KT Event was a lot lower in the kill off percentages than the PT Event,

It would seem then that vulcanism needs to be of a particular kind, for a long duration, and of a minimum energy & mass output/time for it to produce big take down effects. To my very unexpert eyes, with the evidence favoring the impact hypothesis for the KT Extinction, I'd have to say that the Deccan Traps look a lot less important to that event than some people would like to think.


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