Underwater Volcano as Big as New Mexico is Largest on Earth, Scientists Confirm

At a size equivalent to the state of New Mexico or British Isles, the underwater shield volcano known as Tamu Massif is the largest individual volcano ever documented on Earth, according to new research to be published in the journal Nature Geoscience.

The underwater behemoth is nearly as big as the giant volcanoes on the surface of Mars, making it not only the largest on Earth, but among the largest in the solar system.

Estimated to be 145 million years old, Tamu Massif's summit is located beneath about 6,500 feet of water in the Pacific Ocean, about 1,000 miles off the east coast of Japan. Parts of its base are believed to be in waters almost four miles deep.

Until now, it was unclear whether Tamu Massif was a single volcano or part of a series of volcanoes.

But the new research, which drew from core samples and data collected from underwater explorations, has confirmed that the mass of basalt that constitutes Tamu Massif was indeed formed by a single volcanic source near its center.

While the volcano's size, about 120,000 square miles, is staggering, its shape is also noteworthy. It is low and broad, which the researchers say means that the eruptive lava flows that formed it must have traveled from long distances compared to Earth's other volcanoes.

"It's not high, but very wide, so the flank slopes are very gradual," said William Sager, a professor in the Department of Earth and Atmospheric Sciences at University of Houston who began researching Tamu Massif two decades ago. "In fact, if you were standing on its flank, you would have trouble telling which way is downhill. We know that it is a single immense volcano constructed from massive lava flows that emanated from the center of the volcano to form a broad, shield-like shape. Before now, we didn't know this because oceanic plateaus are huge features hidden beneath the sea. They have found a good place to hide."

To put Tamu Massif's size into scale, its area is about 60 times greater than that of Earth's most active volcano, Mauna Loa in Hawaii, which takes up about 2,000 square miles.

"Its shape is different from any other sub-marine volcano found on Earth, and it's very possible it can give us some clues about how massive volcanoes can form," Sager said. "An immense amount of magma came from the center, and this magma had to have come from the Earth's mantle. So this is important information for geologists trying to understand how the Earth's interior works."