Back when I was a graduate student, I had the opportunity to be part of the scientific team exploring an undersea volcano. That volcano was the Axial Seamount, which lies about 250 miles off the coast of Oregon. It’s a part of the Juan de Fuca Ridge, and like the Mid-Atlantic Ridge, Juan de Fuca is spreading apart, and new rock is being created as lava erupts on the seafloor. Similar to Iceland, Axial Seamount is also over a hot spot. So there are two types of volcanic eruptions going on, and eruptions occur fairly regularly. In fact, our cruise in 2016 followed on the heels of an eruption in 2015 that happened as predicted. The seamount and nearby area have a bunch of instruments helping detect eruptions, and that’s how scientists know what is going on when.

Now, an investigation of the seamount done just last year using seismic imaging data has revealed something pretty cool about this volcano. They have found evidence of stacked magma chambers beneath the main magma reservoir that feeds this system. So there’s a hot spot with its own magma chamber, and beneath that, more magma chambers. And those magma chambers are also located along the ridge where the hotspot isn’t a factor.

The chambers are basically a crystal mush zone, which is mostly crystallized rock but also has a decent amount of melted rock. It’s still mostly solid. The chambers extend about 1-8 kilometers along the ridge itself, with the shallowest at about 100-1200 meters below what we call the axial melt lens. That lens is the thin, narrow reservoir of magma that feeds the spreading center. It’s well below the seafloor but nowhere near the mantle. Earth is a bit of a layer cake, and it’s hard to imagine the distances involved.

These chambers probably don’t exist for long. They likely come about as magma rises up to replenish the melt lens, melting pockets of existing crustal rock along the way. Then those pockets are tapped like a maple tree to also contribute to the magma that eventually erupts on the seafloor.

More Information

Magma Pockets Lie Stacked Beneath Juan de Fuca Ridge (Eos)

“Stacked Magma Lenses Beneath Mid-Ocean Ridges: Insights From New Seismic Observations and Synthesis With Prior Geophysical and Geologic Findings,” Suzanne M. Carbotte et al., 2021 March 24, JGR: Solid Earth