for National Geographic News
A giant and unusual underwater volcano lies just offshore of Iceland on the Reykjanes Ridge, volcanologists have announced.
The Reykjanes formation is a section of the Mid-Atlantic Ridge, which bisects the Atlantic Ocean where the North American and Eurasian tectonic plates are pulling apart.
As magma wells up from the rift between the plates, it cools to form ridges.
But it doesn't generally form giant volcanoes, said Ármann Höskuldsson, a University of Iceland volcanologist who was part of the international team that discovered the volcano last summer.
That's because mid-ocean ridges are constantly pulling apart, making it harder for large volcanoes to form without being torn asunder.
"We were doing a normal oceangoing mission, and we found a big edifice" about 90 miles (150 kilometers) south of Iceland, Höskuldsson said.
The structure turned out to be an active volcano that rises about 3,300 feet (1,000 meters) above the surrounding sections of the ridge, coming within 1,300 feet (400 meters) of the surface.
At its base the volcano is approximately 30 miles (50 kilometers) across. The peak contains a depression known as a caldera that is 6 miles (10 kilometers) wide.
That indicates that the mountain is being fed by its own magma chamber, Höskuldsson said.
"It's a higher magma production that generates the edifice."
The underwater mountain resembles Krafla, an active aboveground volcano in northeastern Iceland that contains a similar-size caldera, according to Höskuldsson.
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