By comparison, Mauna Loa on the Big Island of Hawaii, is just two percent 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. On Tamu Massif, which must have had vastly more lava erupting than Kilauea, it 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 for 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 up 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.