The location and characteristics of most of the world’s volcanoes can be explained with just two recipes for magma production. The melting point of rock depends on pressure, so hot mantle rock flowing up toward the surface can melt as the pressure drops. The addition of water lowers the melting point, too, so water-laden seafloor plates can trigger melt as they sink down into the Earth at subduction zones. These two facts generally explain both volcanoes along plate boundaries—like the Pacific Ring of Fire or the mid-ocean ridges—and those at hot spots like Hawaii and Yellowstone.

But when looking back through Earth's history, there are plenty of volcanic weirdos that don’t seem to line up with the figures in a textbook. There are volcanoes in the interior of the Western US, for example, far from any relevant plate boundary or hotspot. A new study by Jianfeng Yang and Manuele Faccenda of the University of Padua examines another difficult-to-explain set of past eruptions, both east and west of Japan.

Oddities near Japan

Japan sits on a subduction zone, with the Pacific seafloor sinking downward beneath the island. That’s the cause of both Japan’s dangerous earthquakes and its volcanic peaks like Mt. Fuji. But a thousand kilometers to the west, in northeast China, there are remnants of old volcanism. And 600 kilometers to the east, there are more recent basalt seamounts at the bottom of the ocean.

Those are much too far from Japan’s plate boundary to be explained in the usual way. Instead, a number of hypotheses have been proposed, including renewed water release from the subducting plate after it sank even further into the mantle or peculiar processes related to the bending of that oceanic plate as it turns to begin its descent. Yang and Faccenda have proposed a new explanation that could handle both areas of volcanism at the same time—and apply in other places around the world.

Let’s start with what others had already observed. Between a depth of 410 and 660 kilometers (yes, kilometers), the properties of the mantle change, and it transitions to a different set of minerals that are more stable at that higher pressure. This “mantle transition zone” can be imaged using things like seismic energy released by earthquakes, which produces something akin to a CT scan of your body. The transition zone looks unusual beneath these volcanoes, possibly because it hosts an anomalous amount of water or molten rock.

That’s true beneath some other weird volcanic locations, too. But what causes it, and how is it related to the eruptions above? In the new study, the researchers find that it could actually be explained by the sinking oceanic plate. But in this case, it’s not that the oceanic plate is releasing water. Instead, the idea is that it’s sort of squeezing water out of mantle rock.

Water from stone

In the conditions of the mantle transition zone, mantle rock is capable of holding more water than it can when it's shallower or deeper. That means that if you move some of this mantle rock up or down, it should release some of its water. Releasing water below the transition zone isn’t that interesting—it just rises upward and gets trapped in the transition zone sponge again. But if water gets released above the transition zone, it could help create magma with a free path to the surface.

As the oceanic plate subducts downward beneath Japan, it runs into the mantle transition zone. That can slow its descent, but it also disturbs the transition zone, pressing on the sponge. Using a physics simulation, the researchers found that both in front of and behind the collision point, transition zone rock gets sucked upward rather than pushed downward. And that will melt some rock in those regions.

If you compare that pattern to the setting around Japan, you get areas of magma production that can line up with the eruptions to the west, in China, and on the seafloor to the east. The timeline even seems to make sense, with magma produced on the western side significantly earlier in the process.

Where water accumulates in the mantle transition zone—delivered by subduction or other sinking processes—you wouldn’t expect the amount to be constant everywhere. That variable introduces some “patchiness” that might help explain why volcanoes appear in one specific spot rather than another. But this process should work about the same in other places around the world. The researchers point to areas of past volcanism in the Mediterranean and in Turkey and Iran as examples that might fit. (There’s no subduction there today, but there was in the past.)

This can’t explain every volcano in the middle of a continent—there are other ways to make magma—but it is a pretty tidy hypothesis with testable predictions. And as a pleasant bonus, it’s simple enough that you could even illustrate it in a textbook.

Nature, 2020. DOI: 10.1038/s41586-020-2045-y (About DOIs).