Boston
SIMPLE friction was enough to melt a chunk of rock a kilometre thick, says a
Canadian geologist. The glassy melted remains of this chunk lie on a
鈥渟uperfault鈥� in Sudbury, Ontario, where huge blocks of rock in a crater suddenly
lurched by hundreds of metres.
John Spray of the University of New Brunswick came to this conclusion after
looking at rocks in a crater that formed around 1.8 billion years ago, when an
asteroid hit the Earth. The rocks beneath the crater are partly exposed,
revealing a vein of distinctive glassy rock called pseudotachylyte.
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Spray recognised this rock from previous work on earthquakes. As rocks slide
by each other during large earthquakes, friction can melt a layer a centimetre
or two thick, forming pseudotachylyte. But the glassy rock at Sudbury is roughly
a kilometre thick and 45 kilometres wide. 鈥淭hat just blew me away because it鈥檚
so huge,鈥� says Spray.
Ordinary faults slip no more than a few metres at once because they are
pinned at both ends. But Spray calculated that if large rocks slipped by
hundreds of metres along an unpinned 鈥渟uperfault鈥�, they could melt a layer of
rock about a kilometre thick in just a minute or two.
In this month鈥檚 Geology (vol 25, p 579), Spray concludes that the
superfault formed moments after the asteroid impact. The impact would have
excavated a steep-walled bowl in the Earth鈥檚 surface with superfaults in the
unstable walls. 鈥淭he bowl cannot stay like a bowl, and gravity collapses its
sides,鈥� he says. Spray predicts that similar superfaults would have formed in
the walls of craters left by volcanoes.