Most people know that the Pacific Ring of Fire is related to boundaries between tectonic plates, but there’s a common misconception about where the magma comes from to fuel those volcanoes. At those boundaries, called subduction zones, a plate made of denser oceanic crust dives beneath a continent (or another oceanic plate). It’s not that the diving plate heats up and melts as it sinks downward, though.

Actually, the minerals in the diving plate contain lots of water, and that water migrates upward as the plate slowly warms up. The addition of water to hot mantle rocks lowers the melting point of the rock, and this effect is enough to convert some mantle rock into magma. Since magma is less dense than solid rock, it works its way upward toward the surface, resulting in the arcs of volcanoes we see along subduction zones.

Within this simplified picture, however, there are complexities and open questions. Does the water simply rise directly into the mantle rocks above, or does it take a more tortuous path? Is that water the cause of all the magma production in an area, or does some magma form because the flow of mantle rock brings some up to lower pressures where it can melt?

One way to study what goes on at these formidable depths is to use the energy released by earthquakes like a geologic CT scan. Measure that energy with enough seismographs spaced along the surface, and you can build a rudimentary picture of the structure down there, since some zones will transmit seismic energy more or less easily than others.

In a similar way, geophysicists can make precise measurements of electromagnetic fields in many locations and build images of the region beneath the continental plate. This technique has the advantage of being sensitive to things other than seismic waves, including the subduction zone fluids and magma we’re interested in.

A new study led by R. Shane McGary at the Woods Hole Oceanographic Institution combines these two techniques along a swatch of central Washington, creating an exceptionally detailed image of the subduction zone in the vicinity of Mt. Rainier. It’s detailed enough, and extends deep enough, to show us where the magma is being generated.

The image is a west-to-east cross-section of the subduction zone. The curved, dark blue region is a slab of subducting oceanic crust called the Juan de Fuca plate, which sinks below the Pacific Northwest of the US as it slowly pushes eastward. At a depth of about 100 kilometers, a dark red, low resistivity region can be seen just above the subducting slab, labeled with a letter “A.” The resistivity there is so low that it must be due to the presence of water released from the slab, causing the rock to melt. The calculated temperature for that region isn’t high enough to produce melt any other way.

The fact that the low resistivity zone also extends off to the right in the image hints at motion in the mantle rock, dragged downward by the sinking slab—or just a larger area where water is being released.

A red region indicating the presence of magma rises vertically from there. The resistivity doesn’t change much as it rises, leading researchers to conclude that it must be ascending fairly rapidly rather than stalling and cooling off. That finding points to lava-lamp-like blobs of magma heading upward, or perhaps through well-connected conduit-like pathways.

Finally, there seems to be a pool of magma below Mt. Rainier, labeled with a letter “C," which may be getting an extra contribution of water from point “D.” Features like that pool of magma have been detected elsewhere in the Cascade Range, since they're close to the surface and therefore easier to study, but they aren’t always composed of magma. It may just be hot fluids or metamorphosed rock in other places, but here it seems to be the real magmatic McCoy.

Judging from similar types of research done elsewhere, there seems to be a lot of variability in the amount of magma produced beneath the different peaks of the Cascade Range. The researchers think that actually relates to varying amounts of water in the subducting plate, which could be due to faults in the plate—reopened as it bends downward into the subduction zone—that provide a pathway for water to travel. The more faults that form in a spot, the more water it can soak up. Once the plate sinks deep enough, all that water goes to work making magma.

Basically, the process below Mt. Rainier does appear to be pretty similar to the standard “simplified picture” that appears in textbooks. More Images like this one, showing the entire magma-production system, would enable researchers to see if other areas differ in interesting ways.

Nature, 2014. DOI: 10.1038/nature13493 (About DOIs).