*Sorry, particularly to Stephanie, for being so late with this. The morning went slightly sideways is all. Nothing bad, just got distracted. – SAH*
Kiss Your Ash Goodbye: The Yellowstone Supervolcano, Part III, A Vulcanology Primer – By Stephanie Osborn
Excerpted from Kiss Your Ash Goodbye: The Yellowstone Supervolcano, © 2018
Images in this article are public domain unless otherwise noted.
The Yellowstone Hotspot and Structure
NOTE: I am aware that there is current research claiming that the subducted Farallon Plate is the source of the Yellowstone melt, as well as its long-lived behavior and track. However, based on what I know, I am skeptical.
For now, in the absence of more definitive results, and given the fact that the ancient plate is now pretty much crammed almost as far under the eastern North American plate as it can get and still be under it, and given detailed information on a mantle plume of some substantial size, it is my considered opinion that more than likely, the Farallon Plate had only marginal, if any, effects on Yellowstone. That said, it may possibly be at least part of the reason why said mantle plume is anything but vertical. (I do discuss all this in a bit more detail in the ebook.)
I also note that the current accepted model is “mantle hotspot,” with plenty of data to support it. If that should change, I will add an update.
The Yellowstone supervolcano is a very long-lived system. Unlike most more ordinary volcanoes, which are supplied with magma via such relatively shallow means as tectonic plate subduction and subsequent melting, Yellowstone is apparently produced by a large and powerful mantle plume; the reason for the plume is unknown. As the melt in the plume rises, it pushes on the overlying crust, “puddling” in a weakness in the overlying rock, forming a magma chamber. This pressure first forms a bulge (a “dome”), then the crust of the bulge begins to crack (surface cracks). If these cracks deepen enough to reach the magma chamber, an eruption can occur. They are also responsible for the hydrothermal features seen in the national park.
The mantle plume goes down at least 600mi (~966km), but recent seismic evidence discovered by researchers at the University of Texas indicates it may go as far down as the outer core/mantle boundary, some 1,800mi (~2,900km) down. The plume apparently angles sharply south-southwest from the megacaldera, and the base of the plume can be found under the California/Mexico border. It is very roughly cylindrical; early estimates indicated it was some 215-300mi (346-483km) in diameter, but more recent estimates say it is at least 400mi (644km) wide. It is 2,050mi (3,300km) long, and up to 1,500ºF (816ºC) at the base, near the core.
The Magma Chambers
There are a couple of different reconstructions from seismic & other data that indicate the possible shape & size of the magma chamber. More, seismic tomography indicates there are TWO, a shallow and a deep chamber, with the deep chamber likely directly linked to the mantle plume.
The deep magma chamber, connected to the upper mantle plume.
Note state lines and park/caldera outlines on top and bottom of cube, for scale.
According to the USGS, “The shallower magma storage region is about 90km (56mi) long, [and] extends from 5-17km (3-10.5mi) depth.”
In turn, and from the same source, “The deeper magma storage region extends from 20-50km (12-31mi) depth, contains about 2% melt, and is about 4.5 times larger than the shallow magma body.”
If we assume — based on those measurements — average values for length, width, and height, such that the smaller chamber is a rough prolate ellipsoid of approximate dimension 7.5×7.5x56mi (12x12x90km), then it has a rough volume of ~13,000cu. mi. (~54,000km3). This then gives a volume for the deeper chamber of 58,500cu. mi. (~244,000km3).
However, as it turns out, generally speaking, magma chambers don’t induce eruption until they have surpassed the 50%-full mark. And that’s a whole heck of a lot of magma, AND we’re only at 2%. On general principles, I think we’re good.
The Geysers/Hydrothermal Features
The geysers, hot springs, mud pots, fumaroles, and the like are fed by ancient rain- and meltwaters — the local ground water, in essence, except it is often coming from depth — that seep through the network of cracks and fractures in the rock. They are heated by the chambers and gradually rise, eventually forcing their way to the surface to form geysers and all the other hydrothermal features common to Yellowstone and other such similar volcanic landscapes.
Riverside Geyser. Credit: National Park Service.
A fumarole (steam vent) field in Yellowstone, where dangerously superheated steam emerges in the geyser basins. Credit: National Park Service /Jim Peaco
WARNING: most of the hydrothermal features in Yellowstone, and other active megacalderas, are DANGEROUSLY HOT. Not infrequently, these are superheated waters, meaning their temperatures may well be above the boiling temperature of water; this is especially true for geysers and fumaroles, but many — most — hot springs are also near boiling. More, at least at Yellowstone, they also are prone to being highly acidic. It is NOT AN EXAGGERATION in the least to say that entering one of these features means an instant, horrible death; it has happened many times. Worse, sometimes bodies are not recovered, simply because the water has become strongly acidic thanks to the sulfuric gases dissolved in it; the corpses simply dissolve. When the bodies are recovered, they are often in very poor condition. DO NOT EVEN THINK OF TRYING to use one as your personal hot tub, and DO NOT LEAVE THE TRAILS/WALKWAYS, because the high mineral content can form what LOOKS like solid ground, but is really a skim of mineral deposits floating on the water’s surface.
Is There Any Danger of Eruption?
Yes, because this is an active volcanic feature; there has been some uplift in areas of the park, especially under the lake, but given the nature of the feature, and the high levels of seismic activity that occur there normally, that isn’t necessarily anything to worry about. The “uplift” feature on the lake floor seems to be part of an underwater field of hydrothermal vents, fissures and faults, not unlike those found on ocean floors, so it is likely that the uplift is a result of gases and expanding hot water underneath. Certainly a good deal of the seismic activity in and around the caldera has to do with this same hydrothermal activity; this expansion, when the water is in the natural cracks of a rock stratum, will force the cracks wider until the stone eventually breaks. This fracture does create small quakes.
So-called “quake swarms” in the region are NOT in the correct area to indicate magma on the move. They also don’t have the “long-period harmonic” vibrational component to indicate flowing magma. More, detailed geophysical studies show no evidence of either magma chamber inflating.
To obtain a copy of Kiss Your Ash Goodbye: The Yellowstone Supervolcano by Stephanie Osborn, go to: bit.ly/Kin-KYAGTYSV.