J.B. Murphy/
Supercontinent Cycles Through Earth History 2016,
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; M. Yoshida/
Geology 2016

Push or pull
For centuries, geologists,
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, biogeographers and explorers have tried to explain various features of the natural world by invoking lost continents. Some of the wilder concepts included Lemuria,
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, a sunken realm between Madagascar and India that offered an out-there rationale for the presence of lemurs and lemurlike fossils in both places,
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, and Mu, an underwater land supposedly described in ancient Mayan manuscripts. While those fantastic notions have fallen out of favor,
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, scientists are exploring the equally mind-bending story of the supercontinents that actually existed.
Earth’s constantly shifting jigsaw puzzle of continents and oceans traces back to the fundamental forces of plate tectonics. The story begins in the centers of oceans, where hot molten rock wells up from deep inside the Earth along underwater mountain chains. The lava cools and solidifies into newborn ocean crust,
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, which moves continually away from either side of the mountain ridge as if carried outward on a conveyor belt. Eventually, the moving ocean crust bumps into a continent,
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, where it either stalls or begins diving beneath that continental crust in a process called subduction.
Those competing forces — pushing newborn crust away from the mid-ocean mountains and pulling older crust down through subduction — are constantly rearranging Earth’s crustal plates,
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. That’s why North America and Europe are getting farther away from each

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