Lowered sea levels during ice ages can increase the amount of magma bubbling up at mid-ocean ridges, researchers propose online February 5 in Science. The work suggests that long stretches of thick oceanic crust called abyssal hills, among the most common landforms on the planet, are the result of worldwide climate changes. Gradual variations in Earth's orbit and tilt over tens of thousands of years trigger periodic ice ages. As water amasses in massive ice sheets on land, the global sea level can drop by roughly 100 meters. Geophysicist John Crowley, now at Engineering Seismology Group Canada in Kingston, and colleagues noticed that the abyssal hills that line the ocean floor between Australia and Antarctica appear to have been laid down during these sea level lows. Using computer simulations, the researchers show that low sea levels reduce by about 10 percent the amount of pressure pushing down on mid-ocean ridges where new crust forms. Less pressure makes it easier for rock in Earth's upper mantle to melt, resulting in more magma rising to the surface and therefore thicker newly formed crust. The increase in thickness could form abyssal hills rising up to 300 meters above the surrounding seafloor, the researchers say.
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