When, of course, is the question.

The Yellowstone Cauldera floor goes up and down a lot. Even with primitive transit surveys, this fluctuation in elevation has been observed for decades. Our resolution of observation is getting better all the time. Question is: when do those changes become significant enough to trigger alarm?

There is a big pool of molten (well, part molten, part crystallized....more of a stew, really, phenocrysts of feldspars and quartz floating around an increasingly volatile rich melt....) rock under Yellowstone, with a constant mantle heat source cooking away; and the continent moving slowly over the top.

What makes a magma chamber blow its top? A release of overburden pressure (the cap on a beer bottle) by a significant earthquake could do it. Or, a change in the chemistry of the magma chamber by the injection of new magma disturbs the equilibrium established by tens of thousands of years of just sitting there. New information about this mechanism is in the latest Geology issue, or press releases here and here.

The results of this research show that a pulse of new magma can result in an eruption, but there is some hang time, like a hundred years or so. Therefore, changes in the volume of a magma chamber would represent new magma, but the eruption may not happen for decades. From what I've seen over the years is that the floor of the Yellowstone cauldera is going up in some places and down in others. What does that mean? As magma crystallizes, it shrinks in volume. Maybe it is just cooling, and the topographic changes are a result of non-homogenous cooling, concentration of remaining melt into localized pockets, dikes, and sills....? </armwave>

What is also interesting is the imposition of this essentially vertical stress upon an extensional tectonic setting, which itself is the relaxation of a compressional tectonic event, and the reversal of normal faulting in the Grand Teton Valley. A careful review of the recent basin fill deposits would be a next step in evaluating how long this reversal of the local stress has been occurring. The Teton Valley is a hinged graben...a valley with a dominate fault on one side of the valley and a hinge on the other side. Rivers in these valleys tend to move towards the fault. If the motion on the fault is reversed, the river would begin to move in the opposite direction, towards the hinge.

(Damn. I just read that over, and it needs a visual aid, and it's late, and I've been making visual aids all day, and I'm tired.)

These big volcanos are almost beyond comprehension. Modern humans have not experienced anything near a Yellowstone eruption. That recent movie, Supervolcano, was, in my opinion, a pretty good representation of what would happen.

A very interesting article, REB, thanks for posting it.