Mount Rainier Magma Mapping
July 20, 2014 1:32 PM   Subscribe

A team of geologists and geophysicists have imaged the path of a Pacific Northwest volcano's molten rock, from the subducting slab to the upper crust.

The resulting magnetotelluric image shows a 2D side view of the Juan de Fuca plate (dark blue is the subducting plate, the red triangle up top is the volcano, and reddish-orange flowing up and being carried along with the plate is the molten rock. Red circles are small earthquakes.) The magma cross section was mapped using magnetotelluric imaging, a remote sensing technique that measures the conductivity and resistance in rock layers using the earth's electromagnetic field, and seismic reflection. The project was funded largely by Earthscope who also bring us USArray, a 15 year long travelling USA lithosphere mapping project.

A similar magma mapping is currently underway for another volcano in the Pacific Northwest Cascades, Mt. St. Helens.

(Previously on Magnetotelluric mapping of Yellowstone. Previously on Magma.)
posted by Secretariat (18 comments total) 17 users marked this as a favorite


I'm trying to parse this image and determine its "message"...

1. I guess it's good that it looks nice and cool under Rainier?
2. Are all those long thin red splotches up near the top of the image actually molten rock (and why so near the surface), or conductive groundwater?
3. What causes those bunchings of earthquakes in the middle to flatten out at ~12-13 miles down?
posted by Greg_Ace at 2:25 PM on July 20, 2014

Subducting slab, what a marvelous phrase.

NEW from The Subducting Slab!
posted by angerbot at 2:31 PM on July 20, 2014

You face death itself in the form of 3 subducting slabs.
(F)ight or (R)un?
posted by RobotVoodooPower at 3:25 PM on July 20, 2014

I answered question #1 myself, by RTFAs:

In an odd twist, the image appears to show that at least part of Mount Rainier’s partly molten magma reservoir is located about 6 to 10 miles northwest of the 14,410-foot volcano ... But that could be because the 80 electrical sensors used for the experiment were placed in a 190-mile-long, west-to-east line about 12 miles north of Rainier. So the main part of the magma chamber could be directly under the peak, but with a lobe extending northwest under the line of detectors

...So, maybe less good than I'd assumed. I'm still curious about the other two things, though.
posted by Greg_Ace at 3:27 PM on July 20, 2014

> 1. I guess it's good that it looks nice and cool under Rainier?

Yeah, it's a shame the image doesn't really have a "key". The different colors represent the resistivity (in ohm meters, I think?). For this purpose, what that means is each color represents a different type or phase of rock, not necessarily a temperature. I'm not sure what the red at the surface shows- I was assuming water or sediments.

Mostly I thought it was cool how similar the real deal image looks to the textbook cartoon of a subduction zone.

There are a couple more images linked with the original article, some of which I understand and some of which I absolutely don't (and they're all too small, I guess because they want you to subscribe). Figure 1 is a map of where the sensors were placed, which at least explains why this study can only represent a West/East slice of the picture.
posted by Secretariat at 3:39 PM on July 20, 2014

Surely I'm not the only one who's been coming across this headline for the last few days and has been compelled to mutter a Dr. Evil "liquid hot MAGMA" every time.
posted by TwoStride at 3:54 PM on July 20, 2014

And as for #3, this gives me an opportunity to deploy the phrase Wadati-Benioff Zone. Which is to say I don't know what's going on with the flattened out earthquakes. My best guess is that they're quakes with incorrectly recorded or estimated depths- because I would have expected to see these tiny earthquakes follow the angle of the subducting plate. Disappointingly, any information about why quakes are shown on the graphic at all seems to be only in the full article, which I'm not sure I have access to.
posted by Secretariat at 4:02 PM on July 20, 2014

The actual publication is here:
McGary et. al. 2014, for those of you who have access to Nature. (Else, there are some MeFites who have in the past offered to provide personal copies of academic publications upon private request.)

The image reproduced in the newspaper article is panel b of figure 2 in the original, and suffers from lack of the original caption, the description in the text, panel a and temperature contour lines superimposed on the resistivity model. I'm not sure where they get the temperature contours, I think this is modeled.

Red / hotter colours indicate lower resistivity. The authors interpret the variations in resistivity as primarily reflecting the presence of fluids -- either water or melt. The hottest colours are immediately above and at the interface of the (blue, very resistive, no fluids to speak of) downgoing oceanic lithospheric plate or slab and the "mantle wedge", a triangular zone whose sides are defined by the downgoing slab and the Earth's surface. This area is interpreted as having high concentrations of both (1) aqueous fluids fluxed off sediments at the top of the slab as minerals in the slab undergo metamorphic reactions as the material is brought to higher pressures and temperatures as it slides into the mantle; and (2) partial melts of rock in the mantle wedge. Above this, there is a 'channel' of low resistivity material all the way to the surface (but interestingly, slightly off-set from Mt. Rainier). The second 'hot spot' of low resistivity occurs just above the small red circles which more of less mark the base of the crust. Above this is continental material, which is provoked to melting by the melts rising from below.

The red at the surface may represent water, but I suspect there might be an edge effect in the inversion method. I'm not sure. I'm quite curious about the blue, low-resistivity region right underneath Rainier.

The circles are earthquake locations.

Missing is panel A, which is seismic tomographic image (this is an imaging technique similar to CAT scan, but which uses the acoustic waves of earthquakes to produce an image). It's not central to the paper; it helps co-locate regions in resistivity space to acoustic space (density, temperature and material properties).

Interesting that you say that the image resembles a textbook cartoon. That's because that textbook cartoon is basically right, and we've basically understood the processes at subduction zones for decades. I'm not in the field, but my impression is that the big deal of the paper is that it is essentially confirmatory. There are changes in resistivity that correspond very well to the expected locations of metamorphic reactions -- expected because of the predicted distribution of pressures and temperatures. It's also provides constraints to all sorts of modeling work regarding convection in the mantle wedge, seismic and mineral properties, quantities and origins of melts and fluids and so on.
posted by bumpkin at 4:09 PM on July 20, 2014 [1 favorite]

Me, I'm Totally Psyched for the Full Rip Nine.

Great article / nightmare fuel! I'm always surprised by how many people in the Seattle area are totally unconcerned about earthquakes. The Cascadia Subduction Zone isn't even the only worry -- the Seattle Fault throws 7's. And there are thousands of people who live in Mt. Rainier's lahar zones...

I'm going to cheer when the Viaduct comes down, regardless of the whole tunnel business. That thing is freaking terrifying.
posted by Blue Jello Elf at 4:11 PM on July 20, 2014 [1 favorite]

There's not much on the earthquakes in the article, Secretariat, you are not missing out.

The shallow, horizontally arrayed earthquake locations are not mis-located. I'm mist-typed above: the shallow quakes are between 10 to 25ish km in depth, they reflect the brittle, seismically active part of the crust. Base of the crust -- as shown in their seismic tomography line -- is ~40km.
posted by bumpkin at 4:13 PM on July 20, 2014 [1 favorite]

Aw, shoot, I should have known that. Thanks, bumpkin.
posted by Secretariat at 4:54 PM on July 20, 2014

Blue Jello Elf: "And there are thousands of people who live in Mt. Rainier's lahar zones"

That's why we have such rich soil! And we'll totally have 45 minutes to evacuate!
posted by stet at 8:08 PM on July 20, 2014

That's why we have such rich soil! And we'll totally have 45 minutes to evacuate!

It's fascinating to fly up to the PNW from California in a window seat -- it seems like every volcano on the way has towns sitting on its old lahars. You just know that hundred-odd years, ago, someone went "Hey, look at this nice flat plane! And such rich soil! What a lovely place to have a town! And what a beautiful view of this very tall mountain standing all by its lonesome!"
posted by Blue Jello Elf at 8:31 PM on July 20, 2014 [1 favorite]

I'm going to cheer when the Viaduct comes down, regardless of the whole tunnel business. That thing is freaking terrifying.

I remember the first time i watched this, i actually burst out laughing when everything catches fire.

Just at like, the sheer ridiculousness of it all. "Wow, not only is everything completely collapsed and fucked, but then it all bursts into flames".

Knowing how long it's taking them though, i bet the next MegaQuake will come before they actually tear the damn thing down.
posted by emptythought at 9:03 PM on July 20, 2014 [1 favorite]

I dunno...psychologically speaking, at least, the last place I'd want to be during an earthquake (and attending tsunami) is an underground tunnel! That just seems like a really seriously bad idea.
posted by Greg_Ace at 9:14 PM on July 20, 2014

I remember the first time i watched this, i actually burst out laughing when everything catches fire.

Definitely! It almost looks like a parody.

the last place I'd want to be during an earthquake (and attending tsunami) is an underground tunnel!

Oh yeah, I'm not going to drive through that thing any more than I drive on the Viaduct, which is never. (And the Viaduct at least has a gorgeous view!) In case an underground tunnel along the waterfront isn't scary enough on its own, the retaining seawall is being nibbled into oblivion by gribbles. There is a project in the works to fix the seawall, at least. And the construction photos of the tunnel are pretty sweet. OK, so I'll probably drive through it once!
posted by Blue Jello Elf at 10:05 PM on July 20, 2014

OK, so I'll probably drive through it once!

If they ever manage to complete it...
posted by Pudhoho at 10:19 PM on July 20, 2014 [3 favorites]

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