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.
