The gigantic Yellowstone volcano: Should we be cautious about visiting?

First, thanks for the comments about my last post, directing people to sandy beaches on the Pacific shore of Washington State. The post had been about the preponderance of cobble-strewn beaches on the shores of Puget Sound. The Pacific shoreline is different- it was not affected by the last glacial advance, so cobbles are less common. In particular, an anonymous comment suggested a visit to Long Beach. It is true, there are miles and miles of sandy beaches [and big surf] just north of the mouth of the Columbia River. These beaches are all very young. New sand is continually added to the beaches by currents that carry fine-grained sediment northward from the river. In my upcoming book, Geology Underfoot in Western Washington, I devote a chapter to sedimentation at the mouth of this great river.

Okay, on to this week's question.

Dear Dave,

We love to visit Yellowstone National Park. The hot springs and geysers are out of this world. This geologic wonderland should be on everybody's 'great American pilgrimages' list. We have heard that the Yellowstone area is really a 'supervolcano', capable of gigantic eruptions. Since you are a volcanologist, I hope you can help with my questions. What I want to know is how likely is one of these huge eruptions, who keeps track of volcanic activity there, how much warning would we have, and what would be the consequences? I have seen a scary movie, 'Supervolcano', but don't know how realistic it is. Should we be leery of visiting Yellowstone?
Amanda Kincaid
Chicago

Amanda,
No, don't be worried, and by all means, visit Yellowstone all you wish. Getting run over by a buffalo [or a snowmobile in winter] is more likely to hurt you than volcanic activity. Geologists use the term 'caldera' [Spanish for cooking pot, or cauldron] for volcanoes capable of these gigantic ash eruptions; the media often uses 'supervolcano'. Volcanic activity at Yellowtone is monitored by the Yellowstone Volcano Observatory, [click the link] a branch of the US Geological Survey. Visit their website to get a ton of information about this volcanic wonderland. Especially check out 'Frequently asked questions'. Much of the hype you may hear ('Yellowstone is overdue for a huge eruption'; 'drilling would release pressure'; 'we are all doomed') is dealt with at the YVO website.

Graphic representing the frequency of hazardous events that occur at Yellowstone. YVO-USGS

How likely is a giant caldera-forming eruption? Not very, at least in the time frame that most people can easily grasp. Three are known at Yellowstone. They occurred 2.1 million, 1.3 million and 640,000 years ago. It is not possible to calculate a meaningful recurrence interval with only two time gaps known from the geologic record (2.1 - 1.3 = 0.8 million years; and 1.3 - 0.64 = 0.66 million years between the three eruptions). Not much data, right? A caldera eruption with the attendant huge volumes of ash could blanket much of the midwest and eastern US and Canada with a thick blanket of ash. Such an eruption is likely to alter the climate world-wide and seriously damage agriculture for decades. Cities buried under meters of ash downwind from Yellowstone would likely be unlivable in the short term. It is difficult to visualize what such an eruption would look like. Probably an orbiting satellite would have the only useful view. Any ground-based observer would be in mortal peril if they were close enough to see anything but a towering wall of erupting ash in the far distance. The Lava Creek caldera 640,000 years ago erupted around 240 cubic miles of ash and rock. Compare that with Mount Saint Helens in 1980: 0.34 cubic miles. Or Krakatau (1883): 4 cubic miles.

Areas affected by ash from the last three Yellowstone
caldera eruptions.

How much warning? Well, geologists believe that premonitory activity would extend over hundreds or even thousands of years. There would be so much warning that some scientists and land managers are concerned that people would grow accustomed to it and no longer take heed. It seems almost impossible to researchers that a gigantic caldera eruption that could harm much of the economy of the US and Canada would occur without lots of prior earthquake activity and smaller eruptions, and so without any warning.

A 50-foot-high hydrothermal explosion at Biscuit Basin,
Yellowstone. Photo by Wade Johnson
http://www.jhnewsandguide.com/article.php?art_id=4624

The most typical natural hazard at Yellowstone (other than those buffalo) is a hydrothermal explosion in one of the several geyser basins (these is not volcanic). These are bursts of hot water and steam, and are powerful enough to blast chunks of rock into the air- and back down to the ground. They are local but could certainly kill people and cause serious injuries. They don't seem to be very predictable. Learn more about them at a YVO webpage dedicated to them. A hydrothermal explosion in 2009 is described here.

The typical volcanic eruption at Yellowstone is an isolated lava flow from a vent somewhere in the region. Even these are extremely rare.

There is a great guidebook that any geophile
should take when they visit Yellowstone. It is  'Geology Underfoot in Yellowstone Country' by Marc Hendrix. You can order it from the publisher, Mountain Press Publishers.