Volcano the Size of Arizona Discovered Tamu Massif is Earth's biggest such structure By Arden Dier, Newser Staff Posted Sep 6, 2013 6:56 AM CDT Updated Sep 8, 2013 9:56 AM CDT 55 comments Comments A 3D map of the Tamu Massif formation. (IODP) (Newser) – Scientists have made a surprising discovery in the Pacific Ocean about 1,000 miles east of Japan: Earth's largest volcano. Tamu Massif is a monster at 280 miles by 400 miles, or roughly the size of Arizona, and ranks among the largest such structures in our solar system, Nature reports by way of a Nature Geoscience study. The existence of Tamu Massif has been long known, but geologists believed it to be composed of several volcanoes that merged. Research conducted in 2010 and 2012 changed things. That's when researchers sailed over Tamu Massif and sent seismic waves through it using air guns. "We saw what appear to be lava flows going out from the center of the volcano in all directions, with no obvious large secondary source of volcanism," says lead author William Sager—meaning this is "one huge volcano." (Though one that has been inactive for as many as 145 million years.) But Tamu Massif differs from typical seamounts in that it has a nearly indiscernible slope—around 1 degree near the summit (which sits 6,500 feet below the surface), and much less near the base, National Geographic reports. And Sager says other oceanic plateaus could also be volcanoes: "There may be bigger ones out there."