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Matt Ridley is the author of provocative books on evolution, genetics and society. His books have sold over a million copies, been translated into thirty languages, and have won several awards.

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Double disaster

My latest Mind and Matter column for the Wall Street Journal is about the possibility that big meteorites can trigger volcanic activity:


About 65 million years ago, the dinosaurs and maybe two-thirds of all other species suddenly died out. For three decades, the dominant explanation for this mass extinction has been that it was probably caused by the impact of a large meteorite.


A layer of iridium-rich rock from roughly the right date is the fingerprint that convicted this extraterrestrial killer (iridium is more common in space than in the Earth's crust). Even the bullet hole has apparently been found in the shape of a 110-mile-diameter crater called Chicxulub off the coast of Mexico. The explosion would have been the equivalent of two million hydrogen bombs.


Yet, over the last 30 years, a rival theory has refused to go away. This one implicates volcanoes-specifically, a huge heap of molten rock known as the Deccan Traps that burst through the skin of the Indian subcontinent at about the same time. The birth of this "large igneous province" would have poisoned the atmosphere and ocean for a long time with sulfur and carbon dioxide, killing many forms of life.


Just last week the champions of this theory came up with stronger evidence and better dating, from fossil plankton. "We demonstrate a clear cause-and-effect relationship that these massive volcanic eruptions were far more destructive than previously thought and could have caused the…mass extinction even without the addition of large meteorite impacts," Gerta Keller of Princeton University told Science Daily.


But why choose between the theories? Might meteorites have triggered the eruptions? Computer simulations show that sufficiently big impacts can melt the Earth's crust and even part of the mantle beneath (thanks to changes in the mantle pressure). Sudbury, north of Lake Huron in Canada, sits in the middle of an ancient crater made by a middle-sized meteorite 1.8 billion years ago, which melted much of the crust and left rich nickel deposits. In West Greenland, there is evidence of an impact and a coincident lava eruption, 62 million years ago.
It's even widely agreed that some of the volcanic effects of a meteorite impact might occur some distance from the impact. An especially large igneous province in the Pacific called the Ontong Java plateau, possibly caused by a meteorite impact, may have torn the crust of South America, resulting in a concentration of iron-rich rocks.


More intriguingly, the crust could bulge, buckle and break at exactly the antipodean point on the planet from the impact. This is because seismic waves from the impact would ripple around the Earth and converge at the opposite side as if focused by a lens. There is a crumpled patch on the surface of Mercury known as the Chaotic Terrain exactly opposite one of its largest craters, the Caloris Basin.
Calculations show that the Chicxulub meteorite was certainly big enough to have triggered eruptions at its antipode. But it seems the Deccan Traps probably lay at least 1,000 miles away from Chicxulub's antipode at the time, though it would take just a little error in our assumptions about the speed and direction of Mexico's and India's motion to put India over the antipode.


Another possibility is that Chicxulub was one of a swarm of large meteorites to strike at the same time, all from a fragmented asteroid or comet. There are roughly contemporary craters in the North Sea and Ukraine and a disputed one off the west coast of India. Against this, recent analysis suggests only a single iridium layer, implying a single big impact.


My hunch is that this is a case of scientific polarization, where all the academic tribal energy has gone into emphasizing one theory at the expense of the other, and little into trying to see them as two facets of the same event: an impact and a consequent eruption, both of which contributed to poisoning the atmosphere and the ocean to the point where few species could survive.