Maureen Long — Subduction Zones

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LC: Can you tell me a little about your interest in subduction zones and what those are? 

ML: So you’re from Seattle, right? Seattle is an example of a place that sits right on top of a subduction zone. Subduction zones are key components of the plate tectonic system. The idea is you have the oceanic crust and the mantle, which is called the lithosphere, underneath it. And so it is sort of sitting around on the surface of the earth and cooling off with geologic time, and eventually it gets denser and denser. Because of this, it’s going to want to sink down into the Earth’s mantle. So a subduction zone is essentially a place where the ocean floor sinks into the mantle interior and joins the mantle convection system. In a way that sounds sort of simple. I mean, dense things sink, right? That’s pretty basic, but it’s an enormously complicated part of the plate tectonic system. 

And one reason to care about this is that subduction zones are where we have our largest and most damaging earthquakes, they’re where we have our biggest, most spectacular, most damaging volcanic eruptions, and they’re also where you’re sort of setting the conditions for the rest of the solid earth to evolve. One of the things that happens at a subduction zone is that water gets carried down into the earth’s interior. The plate sits on the bottom of the ocean and, over geologic time, it might be sitting around for 200 million years, it is forming minerals that have water in their crystal structure. And then a lot of that water is breaking down when the minerals get to high temperatures and they aren’t stable anymore so they release their water and that’s what causes melting in the mantle wedge which leads to volcanoes. Also, some of that water might get carried down into the deep earth so it’s sort of setting the whole framework for how water cycles down through the solid earth and then comes back out at volcanoes and into the atmosphere and oceans. It’s kind of the most important type of plate boundary to study. 

So my interest in it was basically that we have this cartoon view that if you open any 101 textbook that’s on my shelf you have a subduction zone where the plate is going down and it’s sort of dragging mantle down with it. That’s a pretty fundamental part of why we think there are volcanoes. It’s dragging new mantle material in, then it gets melted, and then you make volcanoes. But our observations of seismic anisotropy in subduction zones are actually not consistent with that simple 2D model, and in a lot of subduction zones the anisotropy directions are like ninety degrees away from what you would expect. That suggests that that simple 2D cartoon of the plate going down is not right, and that you actually have mantle flow in three dimensions and that’s super important for how we understand material transport, water cycling, and subduction zones. It’s actually really important to understand how these slabs are interacting with the mantle around them. 

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