“A sporadic record of subduction is not the same
as sporadic subduction.” — HUGH ROLLINSON
Plate tectonics got a bumpy start
By Alexandra Witze
Plate tectonics might have gotten a fitful
start on the early Earth.
Two researchers propose that plate
tectonics started and stopped over and
over billions of years ago, before running
continuously. The work, published in the
May Geology, could explain how plate
tectonics evolved into the style geologists see today.
Plate tectonics sometimes pushes
one plate beneath another in a process
known as subduction. When that diving
plate gets deep enough, high pressure
and temperature chemically changes
the plate’s rocks. If those rocks are later
uplifted to the surface, geologists can
recognize the chemical alteration and
interleaved with more pristine rocks.
These repetitions may represent subduction turning on and off over time.
Next the researchers simulated how
plates might subduct at various mantle
temperatures. At 200 degrees Celsius
hotter than today, the calculations
showed, the plates make it only partway
down before breaking and foundering.
Plates would then have to cool at the
surface and become dense enough to
sink back into the mantle, starting the
process over. Only when the mantle
cooled sufficiently — perhaps by around
2. 7 billion years ago — could permanent,
modern-style subduction take hold.
“A sporadic record of subduction is
not the same as sporadic subduction,”
says Hugh Rollinson, a geologist at the
University of Derby in England who has
studied Zimbabwe’s Archean rocks. But
he says the basic concept is “a good idea,
and one to test.”
show that plate tectonics has occurred.
June 2, 2012 | SCIENCE NEWS | 11