Did a Crashing Comet Shake Up the World?
By Steve Kuchera, Duluth News-Tribune, Minn.
Jan. 10–A large meteorite hitting Earth may have created the moon. Another may have killed off the dinosaurs. Others may be responsible for concentrations of valuable minerals. And, according to a new hypothesis by University of Minnesota Duluth geology professor Vicki L. Hansen, one or more may have helped split Earth’s surface into large plates which are still in motion.
According to the theory of plate tectonics, the movements of plates make continents drift, create mountains, cause earthquakes and make oceans larger and smaller. But the theory doesn’t explain how the process began.
Hansen laid out her hypothesis explaining how the impact of a bolide — a large comet or asteroid — could have started plate tectonics in the December issue of Geology, the journal of the Geological Society of America.
“It’s strictly a thought experiment,” Hansen said. “The fun thing about it is that it’s an idea that hopefully will cause us to ask new questions. As soon as we ask new questions it makes us think about things in a different way.”
Since its publication, both New Scientist magazine and Discovery Channel News picked up the story. Hansen, a McKnight Presidential Professor of Earth and Planetary Sciences, said the article has generated more interest than any of her past publications.
“This is an interesting and highly provocative paper that I am sure will further stimulate debate on when and how plate tectonics began,” said Peter Cawood, director of the Tectonics Special Research Centre, School of Earth and Geographical Sciences at the University of Western Australia, who reviewed Hansen’s article for Geology.
Rock solid is a relative term. Earth, which appears so solid and unchanging, is anything but. Our planet’s surface is fractured into several huge plates, moving in different directions at different speeds. Some plates grind along another. Others move away from each other, new crust forming between by lava coming up along the boundary.
To balance the spreading, when an oceanic plate converges with another plate, its lead edge is pushed deep into the Earth and melts, a process called subduction.
“Subduction of oceanic crust is one of the most fundamental processes taking place on the modern-day Earth,” Cawood said. “It is the cause of major earthquakes and explosive volcanic activity and is the major process for recycling of surface material back into the interior of the Earth. However, when subduction began and by what mechanism are questions that we still have to answer.”
Cawood said linking a bolide impact with the starting subduction “is an exciting idea that needs to be considered.”
Geologists believe that plate tectonics began between 4 billion to 2.5 billion years ago. Before that, the Earth’s surface may have been too warm and soft to divide into plates of different strengths and densities. Hansen describes the Earth’s early crust as “Silly Putty” — soft and squishy.
“You just couldn’t get plate tectonics going with that sort of stuff,” she said.
Her hypothesis is that plate tectonics began when a bolide perhaps 18 to 30 miles across slammed into Earth, punching a hole through the crust and causing massive melting in the underlying mantel. Molten rock flowed to the surface, forming a plate stronger and denser than the surrounding crust.
The result, Hansen said, is “that you actually create both a spending center and, ultimately, a subduction zone.”
“I have not heard anyone say this is a crackpot idea,” Hansen said. “I imagine that there are plenty of people out there who think that. I also expect that there are plenty of people out there saying ‘this is so simple it’s embarrassing that we, as a scientific community, haven’t considered it before.’ “
Hansen considered the idea because she has studied Venus for 15 years. While Earth geologists seldom ask what happens when bolides hit Earth, planetary geologists commonly consider what happens when bolides hit other bodies, she said.
“There where a lot of bolides winging around during the solar system’s early days,” she said. “So one should ask ‘What would happen if we take what we think the early Earth looked like and hit it with a big bolide?’ I just asked that question and said ‘Oh my goodness, that’s a way you could actually start plate tectonics.’ “
STEVE KUCHERA can be reached weekdays at (218) 279-5503 or by e-mail at skuchera@duluthnews.com.
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