cs 81 Feb 95 said:
Arctic Volcanoes Found Active at Unprecedented Depths
Kimberly Johnson
for National Geographic News
June 26, 2008
Buried under thick ice and frigid water, volcanic explosions are shaking the Arctic Ocean floor at depths previously thought impossible, according to a new study.
Using robot-operated submarines, researchers have found deposits of glassy rock—evidence of eruptions—scattered over more than 5 square miles (15 square kilometers) of the seabed.
Explosive volcanic eruptions were not thought to be possible at depths below the critical pressure for steam formation, or 2 miles (3,000 meters). The deposits, however, were found at seafloor depths greater than 2.5 miles (4 kilometers).
"This kind of implosive seismicity is rare anywhere on Earth," said study author Robert Sohn, a geophysicist at the Massachusetts-based Woods Hole Oceanographic Institution.
The study appears today in the Journal Nature.
Anatomy of a Mid-Ocean Ridge
Seismic activity was previously detected in the same region in 1999, along the Gakkel Ridge—a 1,200-mile-long (2,000-kilometer-long) mid-ocean mountain range north of Greenland.
Hundreds of earthquakes were observed over a nine-month period, with magnitudes between 4 and 6.
This earthquake swarm was the largest in recorded history along a spreading mid-ocean ridge and prompted researchers to return to the area for further investigation.
In 2007 Sohn and his team stumbled across the glassy pyroclastic rock deposits while searching for hydrothermal vent fields in the Gakkel Ridge.
Powerful eruptions sent a plume of carbon dioxide, helium, and liquid lava up into the Arctic waters. When the material cooled, rock debris fell to the ocean floor, he explained.
Mid-ocean ridges are formed as hot molten material from inside Earth seeps into and fills the opening between two spreading tectonic plates. The Gakkel Ridge is considered an ultraslow spreading ridge because its plates spread at a rate of 0.4 inch (1 centimeter) a year.
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