The steaming dome of Mount St. Helens seen on November 11, 2004.
Jim Vallance and Matt Logan / USGS-CVO
You may have noticed a lot of rumbling in the science media over the past week over a study presented at last week's Geological Society of America meeting concerning the state of things underneath Mount St. Helens. Many of those articles declared that "new magma chambers were discovered under St. Helens!" and "magma is on the move!" and "the volcano may erupt again!"---all of it said breathlessly and based on a brief talk (we're talking less than 15 minutes) at the meeting. Well, as with most of these types of things, there is a lot less to it than it seems and no, it doesn't change the chances of a new Mount St. Helens eruption. All of this static comes from the iMUSH study at the Washington volcano. Geologists are examining the structure of the crust under St. Helens to try to image---using seismic waves from distant earthquakes, man-made vibrations, and magnetic surveys---the elusive magma body underneath the volcano. The presentation at GSA included some of their preliminary findings (in full disclosure, I did not see it, but rather read the abstract and tried to make sense of the news)---namely that, yes, under St. Helens and the region around it in the Cascades, there are parts of the crust that contain more "melt" (magma) than others and the melt appears to follow pathways to the volcanic centers like St. Helens or Mount Adams. Now, why would the magma under an active volcano be elusive? That’s because magma is not stored under a volcano like St. Helens as a vast, roiling cauldron of 100 percent liquid magma. Recent studies looking at other Cascade volcanoes like Lassen Peak (full disclosure: my study) and compilations of data from arc volcanoes around the world (the same group St. Helens belongs to) have found that during most of the life of a volcano, the magma is stored not as a liquid but rather a mix of some molten material and lots of crystals---what we like to call a "crystal mush." This might be upwards of 60 to 70 percent crystals, a space where the magma doesn't behave like a liquid anymore because of the network created by all those crystals. The magma is not able to erupt in that state (under most circumstances). Instead it needs a jolt in the form of new magma entering the "mush" to heat it back up, melt the crystals and get it to the point that it starts to behave like a liquid again. You might imagine this is somewhat like a jar of honey that has started to crystallize. You won't be able to get it to come out of the container until you heat it back up, dissolve those honey crystals and let the honey get warm and gooey again. So, even at volcanoes like St. Helens that have erupted recently, there are rarely the clear, tell-tale signs of reservoirs of magma. The iMUSH project is carefully surveying the area to try to see those zones of crystal mush and partially molten crust that lie underneath. Then why all the media noise? On the surface, it may seem shocking that deep magma bodies in the crust might feed St. Helens and its close neighbors, but that is exactly what we would expect based on our current petrologic models of the crust in continental arcs like the Cascades. Our models put magma being stored in the shallow crust only a few kilometers below St. Helens, then in the middle crust somewhere, maybe 10 to 15 kilometers down and then at the base of the crust up to 40 kilometers down. These zones are connected by conduits that bring magma upwards. This model is built from geochemical evidence from the lava erupted at arc volcanoes and the cruder seismic data we have about the location of earthquakes, mainly generated by magma rising from the bottom of the crust to the surface. The magma under St. Helens is generated as the mantle over 100 kilometers beneath the volcano melts. This magma is hot and buoyant, so it rises until it hits the bottom of the North American continental crust, where the lower density of the crust causes the magma to no longer be buoyant. Instead, it sits in a MASH zone (defined in the seminal paper by Hildreth and Moorbath). MASH stands for "Melting Assimilation Storage and Homogenization", where magma that is stalled melts the continental crust, assimilates it (mixes with the melted crust) and then is stored and homogenized. This creates a less dense magma that then continues its trip towards the surface. It might stop along the way to form ephemeral magma bodies or crystal mush---and we seen the evidence for this in plutons (magma that solidified underground) that have been exposed at the surface, like in the Sierra Nevada. So, what we might expect to see when we can examine the crust in detail is multiple places where the crust as more magma---the MASH zone, the stalling points, the crystal mush under the volcano with smaller conduits that feed the next level ... and this is almost exactly what we see from the iMUSH study at St. Helens. We haven't added new bodies of magma, we haven't increased the likelihood of another eruption, we just understand the architecture of the crust better now. There isn't even full agreement on what the data might mean---some of what is being interpreted as "partially melted" crust may actually be metamorphosed sediment. Seismic data is highly interpretative, so trying to deduce exactly what it is might be akin to trying to groping into a bag and trying to figure out everything in it without seeing anything. The most fascinating thing about the iMUSH study is the identification of the potential magma stalling point in the middle of the crust (~10-14 kilometers down) that feeds both St. Helens and Mount Adams. Right now, it seems that examining the similarities and differences in the lava erupted from both volcanoes, especially during periods when they were both erupting, could give us a lot of information about what happens to magma as they make their last push to the surface. Mount St. Helens will erupt again, that there is no doubt. The iMUSH study helps us better understand the depths where magma is stored, so when we start seeing earthquakes increase under the volcano, we can be more confident that it is being caused by magma on the move. We know more, but that doesn't mean that we're any closer to disaster.
