Tamu Massif — a volcano the size of the British Isles — was recently discovered by researchers from the University of Houston. The enormous volcano is not only the largest yet discovered on the Earth, but is also one of the largest volcanoes in the whole of the known solar system — in the same size-range as the giant volcanoes of Mars.

The country-sized undersea volcano is located about 1,000 miles east of Japan, and compromises the largest feature of Shatsky Rise — an underwater mountain range that formed sometime between 130-145 million years ago, as a result of the eruption of several large underwater volcanoes.

While there has of course been some awareness of the general area/geology, until now it’s been unclear if Tamu Massif was in fact a single volcano, of if it was a composite of many separate eruption points. But that point of contention has now been resolved with the aid of core samples and data collected on board the JOIDES Resolution research ship — “the mass of basalt that constitutes Tamu Massif did indeed erupt from a single source near the center”.

“Tamu Massif is the biggest single shield volcano ever discovered on Earth,” stated lead researcher William Sager, a professor in the Department of Earth and Atmospheric Sciences at UH. “There may be larger volcanoes, because there are bigger igneous features out there such as the Ontong Java Plateau, but we don’t know if these features are one volcano or complexes of volcanoes.”

The researchers note that Tamu Massif stands out for reasons other than just its size as well — the shape of the volcano is distinctly low and broad, which implies that erupted lava flows would have traveled over very long-distances, as compared to most other volcanoes on the Earth. “The seafloor is dotted with thousands of underwater volcanoes, or seamounts, most of which are small and steep compared to the low, broad expanse of Tamu Massif.”

“It’s not high, but very wide, so the flank slopes are very gradual,” explained Sager. “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.”

The press release from the University of Houston provides details:

Tamu Massif covers an area of about 120,000 square miles. By comparison, Hawaii’s Mauna Loa — the largest active volcano on Earth — is approximately 2,000 square miles, or roughly 2% the size of Tamu Massif. To find a worthy comparison, one must look skyward to the planet Mars, home to Olympus Mons. That giant volcano, which is visible on a clear night with a good backyard telescope, is only about 25% larger by volume than Tamu Massif.

The study relies on two distinct, yet complementary, sources of evidence — core samples collected on Integrated Ocean Drilling Program (IODP) Expedition 324 (Shatsky Rise Formation) in 2009, and seismic reflection data gathered on two separate expeditions of the R/V Marcus G. Langseth in 2010 and 2012. The core samples, drilled from several locations on Tamu Massif, showed that thick lava flows (up to 75 feet thick), characterize this volcano. Seismic data from the R/V Langseth cruises revealed the structure of the volcano, confirming that the lava flows emanated from its summit and flowed hundreds of miles downhill into the adjacent basins. Tamu Massif is believed to be about 145 million years old, and it became inactive within a few million years after it was formed. Its top lies about 6,500 feet below the ocean surface, while much of its base is believed to be in waters that are almost four miles deep.

“It’s 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,” stated Sager. “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.”

The new findings have been published in the journal Nature Geoscience.