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Geologists Discover Signs of Volcanoes Blowing their Tops in the Deep Ocean

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June 26, 2008

NOTE: Many readers have inquired about whether the Arctic
volcanism described here might be a cause of the melting of the
polar ice cap. The answer is “no,” and you can learn more here.

A research team led by the Woods Hole Oceanographic
Institution (WHOI) has uncovered evidence of explosive volcanic eruptions deep
beneath the ice-covered surface of the Arctic Ocean.
Such violent eruptions of splintered, fragmented rock—known as pyroclastic deposits—were
not thought possible at great ocean depths because of the intense weight and pressure
of water and because of the composition of seafloor magma and rock.

Researchers found jagged, glassy rock fragments spread out over
a 10 square kilometer (4 square mile) area around a series of small volcanic
craters about 4,000 meters (2.5 miles) below the sea surface. The volcanoes lie
along the Gakkel Ridge, a remote and mostly unexplored section of the mid-ocean
ridge system that runs through the Arctic Ocean.

“These are the first pyroclastic deposits we’ve ever found in
such deep water, at oppressive pressures that inhibit the formation of steam,
and many people thought this was not possible,” said WHOI geophysicist Rob
Reves-Sohn, lead author and chief scientist for the Arctic Gakkel Vents
Expedition (AGAVE) of July 2007. “This means that a tremendous blast of CO2
was released into the water column during the explosive eruption.”

The paper, which was co-authored by 22 investigators from
nine institutions in four countries, was published in the June 26 issue of the
journal Nature.

Seafloor volcanoes usually emit lobes and sheets of lava
during an eruption, rather than explosive plumes of gas, steam, and rock that
are ejected from land-based volcanoes. Because of the hydrostatic pressure of
seawater, ocean eruptions are more likely to resemble those of Kilauea than
Mount Saint Helens or Mount Pinatubo.

Making just the third expedition ever launched to the Gakkel
Ridge—and the first to visually examine the seafloor–researchers used a
combination of survey instruments, cameras, and a seafloor sampling platform to
collect samples of rock and sediment, as well as dozens of hours of
high-definition video. They saw rough shards and bits of basalt blanketing the
seafloor and spread out in all directions from the volcanic craters they
discovered and named Loké, Oden, and Thor.

They also found deposits on top of relatively new lavas and
high-standing features—such as Duque’s Hill and Jessica’s Hill–indications
that the rock debris had fallen or precipitated out of the water, rather than
being moved as part of a lava flow that erupted from the volcanoes.

Closer analysis has shown that the some of the tiny fragments
are angular bits of quenched glass known to volcanologists as limu o Pele, or “Pele’s seaweed.” These
fragments are formed when lava is stretched thin around expanding gas bubbles during an explosion. Reves-Sohn and colleagues also found larger blocks
of rock—known as talus—that could have been ejected by explosive blasts from
the seafloor.

Much of Earth’s surface is made up of oceanic crust formed
by volcanism along seafloor mid-ocean ridges. These volcanic processes are tied
to the rising of magma from Earth’s mantle and the spreading of Earth’s
tectonic plates. Submerged under several kilometers of cold water, the
volcanism of mid-ocean ridges tends to be relatively subdued compared to
land-based eruptions.

To date, there have been scattered signs of pyroclastic
volcanism in the sea, mostly in shallower water depths. Samples of sediment and
rock collected on other expeditions have hinted at the possibilities at depths
down to 3,000 meters, but the likelihood of explosive eruptions at greater
depths seemed slim.

One reason is the tremendous pressure exerted by the weight
of seawater, known as hydrostatic pressure. More importantly, it is very
difficult to build up the amount of steam and carbon dioxide gas in the magma
that would be required to explode a mass of rock up into the water column. (Far
less energy is needed to do so in air.) In fact, the buildup of CO2 in magma in
the sea crust would have to be ten times higher than anyone has ever observed
in seafloor samples.

The findings from the Gakkel Ridge expedition appear to show
that deep-sea pyroclastic eruptions can and do happen. “The circulation and
plumbing of the Gakkel Ridge might be different,” said Reves-Sohn. “There must
be a lot more volatiles in the system than we thought.” The research team
hypothesizes that excess gas may be building up like foam or froth near the
ceiling of the magma chambers beneath the crust, waiting to pop like champagne
beneath a cork.

“Are pyroclastic eruptions more common than we thought, or
is there something special about the conditions along the Gakkel Ridge?” said
Reves-Sohn. “That is our next question.”

Support for the Arctic Gakkel Vents Expedition and for
vehicle development was provided by the National Science Foundation’s Office of
Polar Programs; the NSF Division of Ocean Sciences; the Gordon Center for
Subsurface Sensing and Imaging Systems, an NSF Engineering Research Center; the
NASA Astrobiology Program; and the WHOI Deep Ocean Exploration Institute.

The Woods Hole Oceanographic Institution is a private,
independent organization in Falmouth, Mass., dedicated to marine research,
engineering, and higher education. Established in 1930 on a recommendation from
the National Academy of Sciences, its primary mission is to understand the
oceans and their interaction with the Earth as a whole, and to communicate a
basic understanding of the oceans’ role in the changing global environment.