Maybe Not So Extinct After All
This one will be short so I can get back to work, but it was too good to pass up a quick summary. (I am literally weeks away from having all the pieces in place to get Marker Bed finally published, and am somewhat distracted…)
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I saw an interesting article this morning on Earth.com titled “Gigantic magma chambers discovered beneath volcanoes thought to be dormant”. Turns out that the volcanoes they are concerned about are right here in the Pacific Northwest: the Cascades.
We’ve already covered the tectonic setting of this seismically and volcanically active (or should I say dormant?) area. Feel free to visit some of my earlier posts that touch on subduction zone volcanism if you want or need to get caught up before I jump into the Earth.com article:
Index of posts on tectonics (inc. earthquakes and volcanoes)
A Brief Introduction to Volcanoes
Pompeii vs. Herculaneum in 79 A.D.
The gist of the article — surprise, surprise — is that there are active and viable magma chambers lurking beneath the volcanic peaks of the High Cascades. The article specifically mentions (from north to south) Mt. Rainier, Mt. St. Helens, Mt. Hood, Newberry Volcano, Crater Lake (Mt. Mazama), and Mt. Lassen.
According to Guanning Pang, the lead author of the study:
“This reveals compelling evidence of low-seismic-velocity bodies indicative of partial melt between 5 and 15 km depth beneath most Cascade Range volcanoes,” wrote Pang. The analysis points to bodies roughly 3 to 6 miles thick and several miles across.
So, why do we care? All of these volcanoes are surely extinct, except for possibly Mt. St. Helens; but even it hasn’t cut loose with a stinky toot for over forty-five years. Must be dead by now!
Or is it?
One of the things we always start with in class as we begin our discussion of volcanic processes is the difference between the terms “active”, “dormant”, and “extinct” when it comes to describing the current readiness of a volcano to erupt.
While the terms themselves are fairly obvious, their applicability to any specific volcanic vent is surprisingly complicated — at least to just about every student I’ve ever talked with.
And, sadly, even some professors. When I took my Intro to Geology course at the university way back in 1972, Dr. Leper told us, regarding the Cascades, that Mt. Lassen was “active” (its last significant eruption was in 1915), Mt. Shasta was dormant (its last activity was in the mid-1800s), and — wait for it — every volcanic peak north of the California/Oregon border was extinct.
Oops
By the time we’re done, I am usually able to get my students to agree that an active volcano is either currently erupting or has the capability to erupt in the near future. Examples could include Kilauea on the Big Island in Hawaii or — to bring it home to the Cascades — Mt. St. Helens, or Mt. Lassen.
(BTW: Fool that I am, I tend to encourage lively discussion in class. The big tangent at this point always turns into the definition of the term “near future”, which comes down to whose frame of reference we are using: a human time scale, or the earth’s. I suggest we table a deeper discussion of this critical distinction for a later post.)
Anyway, a dormant volcano is still in the tectonic setting that can lead to an eruption, but it may be a while (the same temporal question is again key to this). Examples would include that majority of the Cascades (sorry Dr. Leper).
An extinct volcano is one that has moved off the tectonic source of the magma, and is unlikely to erupt ever again. A good example would be Kauai in the Hawaiian chain — movement of the Pacific Plate has moved it away from the mantle plume, and its volcanic activity has surely ended.
The Earth.com article suggests that none of the Cascade volcanoes are extinct, merely dormant. The study uses the movement (and lack thereof) of earthquake waves beneath the peaks to model the existence of the presumed magma chambers. Without going into the details (they are all included in the article), the authors apparently used the concept of “harmonic tremors” to determine the presence and depth of the magma.
Pang et al. actually refers to these — the authors do not use the term harmonic tremors, although these are surely what they are referring to:
The study reports no signs of pre-eruption unrest tied to these bodies, such as swarms of small earthquakes centered within them. Seismicity tends to cluster above and below the slow zone, where the rock is colder and brittle.
See Harmonic Tremors are Really Cool for more information on how these swarms of minor earthquakes can be used to pinpoint the location of a magma chamber.
The short version is clear: As long as there is subduction occurring along the Cascadia Subduction Zone there will be the potential for a major earthquake, along with volcanic activity along the length of the Cascade Range.
No matter what, an exciting place to live…
