About our “certain doom” from the Yellowstone supervolcano

Larry Kummer, Editor

15 years ago

Earth was a benign mother during the 20th century, so that we forgot that changes in the physical world are among the most powerful drivers shaping history. Now we seem to have gone to the other extreme, seeing every change in the physical world as a harbinger of doom.

Here are some antidotes to the nonsense circulating about the Yellowstone supervolcano, from the always reliable USGS. Odd how so much dreck is circulated on the Internet, when there are these well-written and authoritative reports so easily available.

“Steam Explosions, Earthquakes, and Volcanic Eruptions – What’s in Yellowstone’s Future?“, USGS Fact Sheet, 2005 — The best briefing I on the subject I have found; excellent graphics.
“Preliminary Assessment of Volcanic and Hydrothermal Hazards in Yellowstone National Park and Vicinity“, USGS report, 2007
“Yellowstone Lake Earthquake Swarm Summary as of 8 January 2009“, USGS

Excerpt from #3:

The recent swarm is well above typical activity at Yellowstone. Nevertheless it is not unprecedented during the last 40 years of monitoring. Earthquake swarms within the Yellowstone caldera are typical, with magnitudes occasionally ranging above 4.0. The 1985 swarm on the northwest rim of the caldera lasted for three months, with earthquakes up to M4.9 and over 3000 total events recorded.

Excerpt from #2, page 28:

Although the probability of a large caldera-forming eruption at Yellowstone is exceedingly small, it is exceedingly difficult to make a defensible quantitative estimate of that probability. As there have been three such eruptions in about the past 2,100,000 years, there are only two intereruptive periods from which to gauge any additional possible interval between the third and a potential fourth such event.

The first interval, between the Huckleberry Ridge (2.059±0.004 Ma) and Mesa Falls (1.285±0.004 Ma) caldera-forming events, was 774,000±5700 years.

The second interval, between the Mesa Falls and Lava Creek (0.639±0.002 Ma) events, was 646,000±4400 years.

A statement, widely repeated in popular media, regards such eruptions as occurring at Yellowstone “every 600,000 years” with the latest eruption having been “600,000 years ago”. This is commonly taken to imply that another such eruption is “overdue”. Such a statement is statistically indefensible on the basis of the extrapolation of two intervals. (Even the simple arithmetic average of the two intervals is 710,000 years, not 600,000 years).

From the line of reasoning outlined here, the probability of a fourth large caldera-forming event at Yellowstone can be considered to be less than 1 in a million, below the threshold of hazards interest unless future premonitory phenomena, probably more severe than those recorded historically in caldera systems around the world (Newhall and Dzurisin, 1988), were to be recognized.

Premonitory indications of an impending major caldera-forming eruption at Yellowstone would include intense swarm seismicity, perhaps localized near the site of an impending outbreak, but initial indications might not be greatly different from those for a smaller eruption.

A magma body large enough to sustain a major caldera-forming eruption would, however, be expected eventually to respond as a whole. By the time seismicity and ground fracturing spread to encompass a larger area, equivalent to a potential caldera and perhaps to outline a ring-fracture system, major eruption might already be well underway.

Based upon the geologic record of the Lava Creek eruption, magma rising to shallow levels almost certainly would produce significant uplift of both a locus of possible outbreak and also a larger area of shallow rising magma. Quite possibly ground fracturing would accompany intrusion to shallow crustal levels and might even begin to outline a ring-fracture system (compare to figure 6). Magmatic gases venting to the atmosphere before any ash or lava were to erupt, including CO2, various sulfur species (but especially SO2), and halogens, might well be more evident and more copious from such a large shallow magma body as it ascends into the brittle upper-crustal zone than would be expected for a smaller body that might lead to a single central-vent rhyolitic eruption.

Although many of the specific premonitory events for such an eruption might resemble precursors to a smaller rhyolitic eruption, the magnitudes would be expected to be correspondingly greater, there might be geophysical activity over an areally large source, and the course of events would be expected to be more complex and of longer duration than for a smaller eruption.

Afterword

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Posts about Science, Nature, and Geopolitics– this lists not only posts on the FM site, but also a wide range of other online sources.

The key post in the series about shockwaves — low probability, high impact events:

We are so vulnerable to so many things. What is the best response?, 30 December 2008

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