Geologists Discover Signs Of Volcanoes Blowing Their Tops In Deep Ocean
Pastor Dale Morgan
Geologists Discover Signs Of Volcanoes Blowing Their Tops In Deep Ocean*
The findings from the Gakkel Ridge expedition appear to show that
deep-sea pyroclastic eruptions can and do happen.
by Staff Writers
Woods Hole MA (SPX) Jun 30, 2008
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 Loke, 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.