John Crowley from GMRT Synthesis data

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ScienceShot: Earth's Orbit Reshapes Sea Floor

SAN FRANCISCO, CALIFORNIA—Talk about a long-distance connection. Earth’s orbital variations—the wobbling and nodding of the planet on its rotational axis and the rhythmic elongation of the shape of its orbit—can affect the shape of the sea floor, according to a talk presented here earlier this month at the fall meeting of the American Geophysical Union. Scientists already knew that orbital variations, which are driven by gravitational interactions among solar system bodies, pace the comings and goings of the ice ages by shifting where sunlight falls on Earth. During ice age cycles, much water moves back and forth between the ocean and ice piled on land as ice sheets, lowering and raising sea level by upward of 200 meters. That cycles ocean water’s pressure on ocean crust up and down. Now, researchers led by Harvard University geophysicist John Crowley have shown in a computer model that those pressure variations should vary the amount of mantle rock that melts kilometers beneath midocean ridges. That, in turn, would vary the amount of ocean crust that solidifies from the melted rock, changing the thickness of new crust by as much as a kilometer as it slides down either side of a midocean ridge. And the group found that indeed, on the Juan de Fuca Ridge (gray) offshore of the Pacific Northwest, the ocean floor is grooved like a vinyl LP record in time with Earth’s orbital variations of the past million years.

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