Photograph by NOAA, NSF

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A seafloor volcano erupts in a riot of lava, rock, and ash in the Pacific Ocean.

Photograph by NOAA, NSF

Seafloor Eruptions Triggered by Tides, Ice Ages

New research shows that volcanoes on mid-ocean ridges are surprisingly sensitive to changes in sea level.

Earth’s seafloor is born in fiery eruptions along volcanic mid-ocean ridges. According to a new study published this week in Geophysical Research Letters, those volcanoes are surprisingly sensitive to the tides—and they just might have something to do with ice ages as well.

Maya Tolstoy, a marine geophysicist at Columbia University’s Lamont-Doherty Earth Observatory, examined seismic records of ten seafloor eruptions. She found the eruptions tended to occur near “neap” tide, every two weeks, when the amount of seawater over the volcanoes is slightly lower than at other times. The reduced weight on the volcanoes apparently encourages small earthquakes, which can be associated with eruptions. (See “Pictures: Giant Undersea Volcano Revealed.”)

What’s more, all ten eruptions occurred during the first six months of the year, when Earth is moving farther away from the sun on its very slightly elliptical orbit. That’s when the sun’s tidal pull on Earth’s solid crust is diminishing in a way that also favors eruptions.

Those observations led Tolstoy to wonder whether seafloor volcanoes might also respond to the much slower but larger changes associated with ice ages.

During ice ages, massive glaciers overrun the continents. The water that freezes to make all that ice comes out of the oceans, lowering sea level by hundreds of feet. When the Earth warms and glaciers begin to melt, sea level rises.

Over the past two to three million years the ice sheets have come and gone every 100,000 years or so-paced by a cycle in Earth’s orbit, which gets more or less elliptical every 100,000 years.

Tolstoy found evidence of that same 100,000-year cycle in seafloor maps of the Pacific. During periods when sea level was low, large hills of lava formed on the East Pacific Rise—because the water pressure on the ridge was relieved, and so volcanic eruptions increased, Tolstoy explains.

If she’s right, then the same orbital cycle that affects global climate—and that is itself ultimately caused by the gravity of Jupiter and Saturn tugging on Earth—also controls the frequency with which volcanoes erupt on the deep seafloor.

Why It Matters

Tolstoy speculates that underwater eruptions might not just follow the ice-age cycle; they might influence it as well.

The 100,000-year orbital cycle changes the seasonal distribution of sunlight on Earth, but not nearly enough to explain the difference between today’s climate and that of the last ice age, when a mile-thick ice sheet covered an area that now includes Boston and Chicago. The cycle’s climate impact is amplified by large fluctuations in atmospheric carbon dioxide. Antarctic ice cores show that CO2 levels also follow the 100,000-year cycle. But it’s not clear what drives the changes in CO2.

Underwater eruptions pump CO2 into the ocean, and some of it eventually makes its way into the atmosphere. Researchers have neglected this input because it’s believed to be relatively small, Tolstoy says.

“But that assumes [these volcanoes] have been churning away at a steady rate,” she says. “The rate matters as well as the amount.” Her work suggests that the carbon input from seafloor volcanoes comes in bursts when sea level is low at the height of an ice age. In theory, if the bursts are large enough, the warming effect of all that CO2 could help explain why ice ages tend to end abruptly.

The Big Picture

What are the implications for the present? (See “Greenhouse Gases Hit Record High Amid Fears of CO2 Saturation Point.”)

“When we try and understand climate today, we do that in large part by looking at how climate was in the past,” Tolstoy says. “What I’m saying in this paper is that I think mid-ocean ridges may be an important part of that equation.”

“It doesn’t surprise me that there’s some relationship between volcanism and sea level,” says Jim White, a paleoclimatologist at the University of Colorado in Boulder, who was not involved in the study. Similar ideas have floated around the research community before. But this paper isn’t necessarily going to change the way researchers think about our current climate, he adds.

That’s because the amount of greenhouse gases exuded into the atmosphere by volcanic eruptions both on land and underwater is swamped by the amount people emit by burning fossil fuels. “The creatures with by far the biggest hammer are human beings when it comes to carbon,” White says. (See “Earth’s Dashboard Is Flashing Red—Are Enough People Listening?”)

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