


"People have ignored seafloor volcanoes on the idea that their influence is small -- but that's because they are assumed to be in a steady state, which they're not," said the study's author, marine geophysicist Maya Tolstoy of Columbia University's Lamont-Doherty Earth Observatory. "They respond to both very large forces, and to very small ones, and that tells us that we need to look at them much more closely." A related study by a separate team this week in the journalScience bolsters Tolstoy's case by showing similar long-term patterns of submarine volcanism in an Antarctic region Tolstoy did not study.
Volcanically active mid-ocean ridges crisscross earth's seafloors like stitching on a baseball, stretching some 37,000 miles. They are the growing edges of giant tectonic plates; as lavas push out, they form new areas of seafloor, which comprise some 80 percent of the planet's crust. Conventional wisdom holds that they erupt at a fairly constant rate--but Tolstoy finds that the ridges are actually now in a languid phase. Even at that, they produce maybe eight times more lava annually than land volcanoes. Due to the chemistry of their magmas, the carbon dioxide they are thought to emit is currently about the same as, or perhaps a little less than, from land volcanoes -- about 88 million metric tons a year. But were the undersea chains to stir even a little bit more, their CO2 output would shoot up, says Tolstoy.
Some scientists think volcanoes may act in concert with Milankovitch cycles--repeating changes in the shape of earth's solar orbit, and the tilt and direction of its axis -- to produce suddenly seesawing hot and cold periods. The major one is a 100,000-year cycle in which the planet's orbit around the sun changes from more or less an annual circle into an ellipse that annually brings it closer or farther from the sun. Recent ice ages seem to build up through most of the cycle; but then things suddenly warm back up near the orbit's peak eccentricity. The causes are not clear.
The long-term eruption data, spread over more than 700,000 years, showed that during the coldest times, when sea levels are low, undersea volcanism surges, producing visible bands of hills. When things warm up and sea levels rise to levels similar to the present, lava erupts more slowly, creating bands of lower topography. Tolstoy attributes this not only to the varying sea level, but to closely related changes in earth's orbit. When the orbit is more elliptical, Earth gets squeezed and unsqueezed by the sun's gravitational pull at a rapidly varying rate as it spins daily -- a process that she thinks tends to massage undersea magma upward, and help open the tectonic cracks that let it out. When the orbit is fairly (though not completely) circular, as it is now, the squeezing/unsqueezing effect is minimized, and there are fewer eruptions.
Daniel Fornari, a senior scientist at Woods Hole Oceanographic Institution not involved in the research, called the study "a very important contribution." He said it was unclear whether the contemporary seismic measurements signal actual lava flows or just seafloor rumbles and cracking. But, he said, the study "clearly could have important implications for better quantifying and characterizing our assessment of climate variations over decadal to tens to hundreds of thousands of years cycles."
Edward Baker, a senior ocean scientist at the National Oceanic and Atmospheric Administration, said, "The most interesting takeaway from this paper is that it provides further evidence that the solid Earth, and the air and water all operate as a single system."
