Listen to part of a lecture in a geology class.

P: All right. Last class, we talked about the ocean floor, what it looks like and how it was formed. We discussed underwater plateaus, large flat regions that rise above the level of the sea floor, and underwater volcanoes. It's been hard for us to learn much about these structures because they're so far underwater, usually several kilometers below the ocean's surface.

But there's one formation in particular that's shedding new light on these structures. It's called Tamu Massif. Tamu Massif is a structure that's part of a huge plateau in the Pacific Ocean, about 1000 kilometers east of Japan. We've known about Tamu Massif for quite a while, since the early 20th century. We assumed for most of that time that Tamu Massif was formed by output from many volcanoes. This is pretty common on Earth. Several different volcanoes erupt, and their output eventually merges them into one large structure. Take the islands of Hawaii, for instance. We know they were formed by volcanic activity, right? And the largest of Hawaii's islands, also called Hawaii, was formed by output from five volcanoes. So it was easy for us to assume that something similar was true of Tamu Massif.

But researchers recently took several rock samples from the structure, samples of hardened lava, and discovered that all the samples they took have a similar chemical composition. The chemical composition of lava comes in part from the volcanic vent it travels through. So this similarity in the samples suggests that Tamu Massif is actually one large volcano with a single vent. If this evidence holds up, it would mean that Tamu Massif is the largest single volcano on Earth, not necessarily in terms of its height, but of its area, the amount of space it covers. As you can imagine, this raises a lot of questions. For one thing, how did Tamu Massif form? It appears that the sea floor might have had just the right circumstances for the formation of a massive volcano. First of all, Earth's crust was thin and fragile in this spot, and also, Tamu Massif formed at the juncture of three plates in Earth's crust. Because of these weaknesses in the crust, it would have been easy for magma below to push its way out, would have just poured out in massive quantities.

And it's not just the size that's interesting, but also the shape. You see, Tamu Massif has these incredibly low slopes and covers a huge area. So what probably happened was, there were several large eruptions from the center of the volcano that took place over a relatively short period of time. The quantity of magma released was so great it would have spread out really far, cooling gradually, and that's how we get these surprisingly shallow slopes. Now, our current models of volcano formation are based on volcanoes much smaller than this one. We simply didn't know until now that volcanoes of Tamu Massif's size could exist on Earth. We'll need new models to account for volcanoes of this size.

So like, what about other volcanoes in our solar system? I read somewhere that the biggest volcano is Olympus Mons on Mars.

Olympus Mons is, as far as we know, the biggest volcano in our solar system. But while Tamu Massif is shorter than Olympus Mons, it covers approximately the same area. Like I said, this isn't something that we would have guessed. Then again, we actually know more about some volcanoes in outer space than we do about volcanoes on the ocean floor.

So that misunderstanding might just have been because many of our volcanoes have found a good place to hide under the sea, where darkness and pressure from the water make it difficult for us to explore. On the other hand, we have many clear and useful images of the surface of Mars. It's exciting to realize that we still have so much more to discover about our own planet.

So what are the next steps? Perhaps we should start looking for more of these volcanoes, but I don't necessarily think that's our most important goal at the moment. We still need to understand more about how such a huge quantity of magma could spill out from a single point of origin. Learning more about this phenomenon will give us better insight into how underwater structures form.