The Tamu Massif was formed about 145 million years ago during the Late Jurassic to Early Cretaceous period over a relatively short period of time (a few million years) and then became extinct. Tamu Massif was formed during one, geologically brief eruptive period, which scientists had previously thought was impossible on Earth. If confirmed, the suggestion that it could be a single volcano would make the Tamu Massif the largest known volcano on Earth, dwarfing the current record-holder, Pūhāhonu, in the Hawaiian Islands. The main part of Tamu's rounded dome extends over an area of , totaling more than , many times larger than Mauna Loa, which has an area of , and about half the area of the Martian volcano Olympus Mons. The entire mass of Tamu consists of basalt. Its slopes are very gradual, ranging from less than half a degree to one degree near its summit. The Shatsky Rise oceanic plateau is comparable in size to California or Japan, but Tamu Massif, which is the plateau's oldest and largest edifice, is comparable in size to New Mexico or Britain and Ireland together. A study in 2016 found that Tamu Massif likely encompassed the entire Shatsky Rise, meaning that Tamu Massif has an area of about , comparable to the size of New Mexico or Japan, and surpassing Olympus Mons in surface area, though it has not yet been determined which of the two volcanoes has a greater mass.