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Extinct for millions of years, Tamu Massif — displayed here as a massive topographic map — is only a few miles tall, but it has a footprint the size of New Mexico. This image shows the volcano's surface area, with the huge-but-not-as-huge Martian volcano Olympus Mons shown as a gray blob at the same scale. Images courtesy William Sager / University of Houston
Big but extinct

The largest single volcano found on Earth so far lay quietly hiding in the depths of the Pacific Ocean, about a thousand miles east of Japan, having been extinct for millions of years. Scientists have now discovered the dome-shaped behemoth, which has a footprint the size of New Mexico.

The discovery topples the previous world record holder for largest volcano — Mauna Loa, one of the five that form the Big Island of Hawaii. The area covered by the newly discovered volcano rivals the biggest volcano in the solar system, Olympus Mons on Mars.

"Olympus Mons is the 800-pound gorilla of the solar system," said geophysicist William Sager of the University of Houston, the study's lead author. "We didn't know these massive volcanoes were here on Earth."

The team named it Tamu Massif. TAMU is the abbreviation for Texas A&M University, Sager's home institution when he and colleagues first studied the undersea mountain range that contains the giant.

Tamu Massif formed layer by layer as fast-moving lava flowed from a central area at the peak and ran down its flank, cooling in place. The volcano is completely submerged, and Sager doesn't believe that its peak ever rose above sea level over its 145-million-year lifetime.

There is no danger of an eruption. "That's probably a good thing, since we've been able to correlate mass extinction with some of these beasts," said Clive Neal, a volcanologist at the University of Notre Dame.

Neal, who was not involved in the study, said the discovery of such an oversize volcano on Earth is groundbreaking for volcanology.

"This finding is paving the way to really rewriting some of the textbooks," he said. "The term 'supervolcano' might be a reality."

Although Tamu Massif has a gigantic footprint, it is relatively short compared with Olympus Mons. The newly discovered volcano rises only a few miles above the sea floor, while the gargantuan Martian mountain rises 16 miles at its peak.

Gigantic volcanoes can form more easily on Mars because unlike Earth, whose crust is made up of many separate plates constantly in flux, the Red Planet has a thick, rigid outer shell made of a single plate. If a volcano forms over a hot spot under the Martian crust, it can keep growing bigger and bigger because the plate stays stationary.

For an Olympus Mons-size volcano to form on Earth, magma must have flowed out extremely quickly in order to keep adding layers before the plates moved away from the hot spot.

Sager compares Tamu Massif to an ice cube in a glass of water — most of its bulk is below the surface — while Olympus Mons is like an ice cube on a two-by-four.

The study was published online Thursday in the journal Nature Geoscience.

Sager and his colleagues first studied Tamu Massif on a research cruise almost 20 years ago without knowing it was a single volcano. At the time, they were gathering data on a larger undersea mountain range called Shatsky Rise, which contains the giant within its features.

Sager was testing two competing theories of how these undersea volcanoes form. One holds that they are like continental volcanoes — the product of a rising plume of magma. Another, however, holds that the upper mantle melts without such a "hot spot" when there is a fracture or spread of the ocean lithosphere.

The question remains unresolved while geochronologists study the rock samples.

Two decades, with over a year total at sea, passed by before the team managed to gather enough evidence through core samples and seismic-reflection data to confirm that much of Shatsky Rise is made up of one central volcano.

"It's nice to be able to find something that's new and exciting and makes people look up from their cup of coffee," said Sager.

Neal said the discovery proves how little we know of our own planet. He said he is eager for other large volcanoes to be uncovered. Both Sager and Neal suspect that the Ontong Java Plateau in the southwestern Pacific Ocean — which is the size of Alaska, even larger than Shatsky Rise — contains a massive volcano that could dethrone Tamu Massif.

Tens of thousands of seamounts pock the ocean floors around the world. But thus far, none appears to be a single basalt shield volcano, but rather a composite of many such volcanoes, Sager said.

Still, if you could stand on Tamu, you would not be able to discern its full profile, and might not even know which way is up. Its slopes are less than a degree — most volcanoes have slopes greater than 5 degrees. It's unlikely that Tamu ever was an island. Fossils of shallow-water creatures in Tamu's sediments indicate it came close to broaching the ocean surface before the ocean lithosphere sank to its present depth, Sager said.