Disaster theories

Did you know that I collect volcanoes?

I wrote this some years back, but it remains relevant.

One of the hallmarks of popular science is the disaster scenario, because it sells well. Sometime, though, the scenarios are popular, but not science. Let us consider the view that asteroid strikes cause volcanoes to erupt.

The world’s flood basalt provinces are the remnants of the largest eruptions of lava on Earth, with known volumes of individual lava flows exceeding 2000 cubic kilometres. By comparison, the ongoing eruption of Kilauea volcano on Hawaii has produced just 1.5 cubic kilometres in 16 years!

The very largest are the Deccan Traps and the Siberian Traps (‘trap’ in this case is a Sanskrit word meaning ‘step’, because of the way the flows weather and erode later to produce stepped hillsides). The Columbia River flows shown above are rather smaller.

A number of the flood basalts formed at times close to the occurrence of certain extinction events, in particular the Newark outpouring of a million cubic kilometres, some 201 million years ago; the Deccan outpouring of perhaps 2 million cubic kilometres, around 66 million years ago; and the Siberian outpouring, also of some 2 million cubic kilometres, around 249 million years ago.

The Deccan outpour lies close to the Cretaceous-Tertiary, at the time when the dinosaurs all died, and the Siberian event matches closely the Permian-Triassic boundary, while the Newark event matches the end of the Triassic.

The probability of having three major volcanic events that would each typically last about a million years should occur within 1 million years of major extinction events during the last 250 Myr (of which there are about 12) is about one in ten thousand.

This has tempted many in the past to assume that these volcanic outbursts were responsible for the extinction events, and when an asteroid in Mexico was associated with the Cretaceous-Tertiary extinctions, some vulcanologists argued that the impact of the asteroid must have triggered the basaltic flow.

How serious would such an event be? The only flood basalt eruption since written history began was the 1783-84 eruption of Laki in Iceland. This produced a basaltic lava flow of 565 cubic kilometres, which represents only 1% of the volume of a typical large igneous province (or LIP) flow, but the eruption’s environmental impact resulted in the deaths of 75% of Iceland’s livestock and 25% of its population from starvation. If such a relatively small eruption happened today, all air traffic over the North Atlantic would probably be halted for three to six months.

So it seems possible that an eruption bigger than that would be enough to possibly trigger an extinction event, but all the same, the idea that volcanoes can erupt when the Earth is smacked by a large comet or meteorite has become a popular idea in geology. That may be so, but it seems there is no proof to back the claim up.

Not only is there no firm evidence that an impact started a volcanic eruption on Earth or on any other planet, there is no known mechanism by which this can occur. According to Jay Melosh who had studied the matter closely:

This idea probably got its start in pre-Apollo days when early observers of the moon noted the common occurrence of dark material — usually supposed to be lava — filling the nearside impact basins. A logical inference is that this is a genetic association: the impacts caused lava to upwell in the biggest craters after they had formed, eventually filling them.

This view should have collapsed in 1965, when the Russian probe Zond 3 made good photos of the lunar farside that showed that the farside basins are not filled with basalt. Moreover, the samples returned from the moon by the Apollo missions showed that the mare basalts are considerably younger (up to about 1Gyr) than the basins in which they lie.

The main point Melosh makes is that there seems to be no way that the impact of an asteroid could punch a deep enough hole to let all that basalt out: “Even in the 100-km (transient) diameter Chicxulub crater, the Moho beneath it is barely disturbed, with less than a few km uplift beneath the centre. Under these circumstances pressure relief melting seems very unlikely, even in the largest known terrestrial craters.”

So exciting as the scenario may be to movie makers, it seems to be an idea without legs, and that isn’t all the bad news for those who enjoy a bit of doom and gloom.