crust under continental crust. But around 400 million years ago, that subduction suddenly stopped. In a major shift,
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, a different,
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, much younger seafloor began to subduct instead beneath the continents. That young ocean crust kept getting sucked up until it all disappeared,
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, and the continents were left merged in the giant mass of Pangaea.
Imagine in today’s world, if the Pacific stopped shrinking and all of a sudden the Atlantic started shrinking instead. “That’s quite a significant problem,
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,” Murphy says. In unpublished work, he has been exploring the physics of how plates of oceanic and continental crust — which have different densities,
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, buoyancies and other physical characteristics — could have interacted with one another in the run-up to Pangaea.
Supercontinent breakups are similarly complicated. Once all the land amasses in a single big chunk,
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, it cannot stay together forever. In one scenario,
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, its sheer bulk acts as an electric blanket, allowing heat from the deep Earth to pond up beneath it until things get too hot and the supercontinent splinters (SN: 1/21/17, p. 14). In another, physical stressors pull the supercontinent apart.
Peter Cawood, a geologist at the University of St. Andrews in Fife, Scotland, likes the second option. He has been studying mountain ranges that arose when the crustal plates that made up Rodinia collided, pushing up soaring peaks where they met. These include the Grenville mountain-building event of about 1 billion years ago,
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, traces of which linger today in the eroded peaks of the Appalachians. Cawood and his

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