Red arrows show the caldera-wide uplift
Image: Robert B. Smith
Yellowstone National Park includes a volcanic caldera that encompasses roughly 240 square kilometers, the remains of a series of spectacular eruptions that occurred hundreds of thousands of years ago. The lure of the area comes from the geologic activity, in the form of geysers and hot springs, that are powered by the same forces that caused the ancient eruptions. That same activity also draws geologists to the area, attracted by the ability to study tectonic forces in a dynamic state.
According to a paper that will appear in Science today, Yellowstone's been giving them quite a show recently. After a long period in which it dropped roughly a centimeter a year, the caldera floor made a sudden reverse in 2004, with all areas shifting to an uplift of several centimeters a year, as shown in the image above. The northeast edge of the caldera shot up at rates above 7 cm a year. As of the start of this year (the cutoff for the data in the paper), the uplift was continuing uninterrupted. Shortly after the uplift started, the Norris Geyser Basin, just outside the northern edge of the caldera, started to drop at a rate of nearly 4 cm a year.
The paper attempts to explain the phenomenon using a combination of the composition of the caldera and computer modeling. Yellowstone is believed to be powered by a large magma plume that comes near the surface through a weak point in the continental plate. The past eruptions left a large plug of solidified magma under the caldera, and the researchers suggest that fresh magma has entered a gap within this plug and acted as a wedge to force the gap to widen, pushing the whole caldera floor upward. That shifting has opened cracks which allowed the pressurized water underlying the Norris Basin to drain downwards, causing the accompanying deflation there.
The authors estimate that just over one-tenth of a cubic kilometer of new magma a year would be sufficient to cause the changes they've observed. That may seem like a small number, but it's worth putting in perspective: it's over an order of magnitude higher than the rate that's generated a rather immense pile of rock in the crater of Mount Saint Helens over the last few years.
It's important to emphasize that, even though this activity is clearly significant, it's not a sign that you should stop saving for retirement. "Although the geodetic observations and models do not imply an impending volcanic eruption or hydrothermal explosion, they are important evidence of ongoing processes of a large caldera that was produced by a supervolcano eruption." For now, it appears that Yellowstone is just reminding us that it's worth paying attention to.
Science, 2007. DOI: 10.1126/science.1146842