The largest volcano on Earth is not in Hawaii, but hidden beneath the western Pacific Ocean and covers an area the size of New Mexico, scientists announced on Thursday. The vast lump of lava is called the Tamu Massif and lies about 1,000 miles east of Japan. It is comparable, though somewhat less voluminous, than the enormous volcano Olympus Mons on Mars.

Tamu Massif itself is not a new discovery, but until now it had been considered the product of several undersea volcanoes heaping and clumping up lava on the seafloor. New seismic reflection data showing the structures of the lava flows inside Tamu Massif, along with specimens from the Integrated Ocean Drilling Program, now suggest the entire, very low-relief, 120,000 square-mile feature erupted from a single source. For instance, the seismic data show that some of the layers of lava — representing single eruption events — are 75 feet thick and spread out over huge distances.

"Think about that: it's huge," said William Sager of the University of Houston (until recently of Texas A&M University), lead author on a paper about the discovery which will be Sunday's issue of the journal Nature Geoscience.

By comparison, Mauna Loa on the Big Island of Hawaii is just 2% of the area of Tamu Massif. Yet this is a volcano that can't even manage to show itself above the waves. Why?

One reason is that the lava is very low-viscosity, like that seen flowing at high speeds out of Kilauea in on the Big Island of Hawaii. And also like Kilauea, the lava flows probably formed cooled, hard, upper skins that then allowed the hotter lava to flow out underneath it. In Hawaii, this sort of thing leaves behind lava tubes. Tamu Massif, which must have had vastly more lava erupting than Kilauea, allows the flow to spread out over a huge area of the seafloor — like a gigantic pile of spilled glue. In fact, the slopes of Tamu Massif are so subtle, say the researchers, that if you could stand on them and look around, you'd have a hard time deciding which way was uphill.

Another possible reason why Tamu Massif lies so low is the crust on which it formed.

"Tamu Massif is 4 kilometers high, but 30 kilometers thick," Sager explained. That's probably because the magma that erupted was very high density, heavy stuff that pressed down the thin, weak oceanic crust onto which it erupted between 130 to 145 million years ago and pushed the crust lower — sort of like piling bricks on a thin, soft mattress. In contrast, the eruptions of the five volcanoes that created the Big Island of Hawaii piled lava on thicker, sturdier crust — or lithosphere (a thicker, firmer mattress) — which could perhaps bear the weight better, he explained. Mars' giant volcano is at the far end of the spectrum from Tamu Massif.

"Olympus Mons is on very thick lithosphere," said Sager. Which is why it reaches the edge of space.

Tamu Massif could be more like the Columbia River Plateau, which also formed from eruptions of lava that flowed out far and wide and covered a huge area — but on a continent rather than the seafloor, suggested Mike Garcia, a professor of geology at the University of Hawaii.

"The amount of mantle material [required to erupt and create Tamua Massif] is impressive," Garcia noted. And regardless of whether it was from a single volcanic mouth or several, the eruption of that much lava is not a trivial matter and probably also had huge impacts on marine life at the time, he said.

Image: Integrated Ocean Drilling Program

This article originally published at Discovery News here