Questions about the Yellowstone Caldera...

[theHandpuppet]

Please bear with me, as it has been many, MANY moons since my college geology classes and even so, there was virtually no discussion on the mechanics of calderas.

First of all, I would like to understand what "powers" a caldera to blow, especially with the kind of force the Yellowstone Caldera is expected to (eventually) display. My understanding is that the caldera exists as a brittle, thin crust over a malleable magma layer, which accounts for the shallowness of its seismic events.

Speaking in the most elementary terms, it would seem to me that the kind of force generated by a singlular, major blow would result from pressures well beneath the surface and with limited space for magma to vent. Rather like the difference in pressure between tipping over a five gallon bucket of water and that of turning on a water hose.

Having looked at the maps of seismic activity in the YC over the past thirty years or so, the quakes do follow recognizable directional patterns but they are, with few exceptions, shallow events. So where/what is the generating force that would cause such a enormous blow over such a wide area?

Forgive my ignorance, but I'm here to learn.

[alittlelark]

1. Good info here..

http://www.yellowstoneparknet.com/area_info/yellowstone_caldera.php

PS -I worked at Lake Hotel for a few seasons during early college.

[theHandpuppet]

6. Thanks for the link!

I now have it bookmarked.

[Psephos]

2. The forces are within the magma itself...

...in the form of dissolved or bound gasses, held in abeyance by pressure. If a chamber of magma is relieved of some its overburden pressure (for example, by a tectonic event moving some of the containment layers), then some of that gas begins to bubble out, creating extraordinary expansion pressure. If there is a weakness in the overburden for the magma to exploit, it will move up, relieving more of the containment pressure, leading to more gas bubbling out - and a chain reaction begins leading to the magma chamber blowing its lid off in a tremendous explosion.

[HeeBGBz]

3. A gigantic earth fart

With most deadly consequences.

I would like to go to Yellowstone this summer.

[theHandpuppet]

7. Ah -- okay!

Thanks for the explanation... and especially for crafting it in layman's terms! :)

So, the next question is, what are the differences between monitoring a caldera's activity with that of monitoring signs of volcanic unrest, ie, is there any real way to gauge whether or not a caldera is getting ready to blow?

[Ediacara]

4. Yellowstone is "powered" by a mantle plume

It's the same sort of process that powers "hot-spot" volcanoes like Hawai'i. The thing is, Hawai'i's mantle plume is blasting magma through a relatively thin crust made entirely of basalt. Basalt has low silica and high metal content, and is very fluid when molten. In Yellowstone (and other continental super-volvanoes) the mantle plume is cutting through tens, if not hundreds, of kilometers of granite and granitic rocks. Granite is high in silica, low in metals, and is super viscous. In molten form, the difference between basalt and granite is like the difference between hershey's syrup and ice cream.

Because magma at Yellowstone is granitic, and very, very viscous, it takes a lot more energy to cause an eruption than it does in Hawai'i, and when they do happen they are much more expolsive. This isn't like twice as much energy, it's on the order of thousands to tens of thousands of times more energy, and as a result, explosiveness.

When Yellowstone does erupt, the magma chanber becomes almost completely vacated, and the roof collapses into the void that once housed the magma chamber. That's what creates a caldera structure. Most of the modern seismicity has to do with very shallow depth faults in the roof of the caldera.

Out of curiousity, what maps are you looking at that have the 30y earthquake activity? I might be able to have some insight for you.

[carlvs]

5. Actually, it is a bit more complicated...

While most discussion about Yellowstone concerns the possibility of another "super eruption" (which is understandable, since an event on the scale of the 640,000 eruption would likely do to global civilization what the iceberg did to the Titanic,) what is often unmentioned is that the area has had quite a few eruptions in the time since the last "big bang."

While some of these have been relatively "quiet" outpouring of basalt (mostly on the edges of the caldera,) there have been at least 20 rhyolitic (the "granitic" magma one person was referring to in his post) eruptions in the area since the 640,000 blast (the last one took place 70,000 yrs ago.) Some of these eruptions have been pretty violent themselves - Pinatubo 1991 scale or larger.

To add insult to injury, Yellowstone just happens to be what vulcanologists refer to as an "restless" caldera. These volcanic structures tend to have many small quakes going on inside their boundaries, as well as periods of land uplift and depression.

In fact, Yellowstone is not the most infamous of these type of calderas - that "honor" would most likely go to the Phlegrean Fields in Southern Italy (a caldera which had produced smaller versions of what Yellowstone had done as early as 12,000 yrs ago, and is far more active.) There had a scare back in the 1980's when the area experience a period of "rapid" ground uplift and moderate-sized earthquakes; fortunately no eruption occurred that time.

They had reason to be nervous - a similar event in the early 16th century lead to a small volcanic eruption in 1538 ("small" is relative term here - while it did not generate much ash, the final blast knocked down trees 5 km away and killed 24 people) and the the area is much more densely populated now than it was at the time of that last outburst.

[Ediacara]

8. Well considering the poster asked for an explanation in the most

elementary of terms, I don't think the lack of complication is a bad thing.... And as an aside, the characterization of the magma under Yellowstone as "granitic" is not incorrect, as granitic refers to an array of high silica, low metal intrusive rocks such as granite, granodiorite, syenite, etc. Granitic magma does not become rhyolitic lava until the moment of an eruption.