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.

How come no sandy beaches in Puget Sound?

Dear Dr. Rock,
We live in Colorado. The family loves to go to the beach. Since we live in Colorado, this means a road trip. Any excuse for a family road trip with our motor home is a good one, and a trip to one coast or the other is always a hit. Last summer we traveled to the Northwest and spent a week visiting friends in Seattle. We were surprised to find that sandy beaches were virtually nonexistent. The dang water was too cold, anyway. Everything is rocky or covered in gravel, or even boulders. Why is this? So much saltwater, and no beaches. Our friends didn't know.

Marv and Julie Harris
Denver, CO

Marv and Julie,

A typical cobble beach along Puget Sound. Each and
every one of these rocks is a glacial erratic. mostly carried south
from Canada.

As a native westerner Washingtonian living near one of those shores and who likes rocks, I'll only half-heartedly apologize. It's not my fault. I hope you had a good time in the area anyway!
Why are beaches in Puget Sound so rocky? It is our glacial legacy. Great glaciers have advanced from the interior of British Columbia down into the Puget Lowlands at least 6 times over the past 2 million years. These glaciers carried a lot of rock mixed in with the ice. Much of that rock load was rounded gravel and cobbles scoured out of northern river beds. All that rock got dumped when the climate warmed and the ice melted. It left a thick fill of glacial deposits covering the landscape. Technically, this is composed of glacial erratics- glacially transported rock material that differs from the underlying bedrock (see the January 2013 post about glacial erratics).

As the ice melted off the land surface, global sea level rose to flood the area we now call Puget Sound. At high tides and especially during storm surges, waves erode away at the base of the bluffs that line much of the shoreline. These bluffs consist of the glacial debris- small stuff like sand and silt, and larger stones: pebbles, cobbles, and big boulders.  The steep bluffs provide a ready-made supply of rocks. Wave energy can easily wash the fine sediment into deeper water, but is rarely powerful enough to carry the rocks off the beach. The result is a 'lag deposit'- the fine material has been removed, the heavier rocks are left behind. Voila, a rocky beach that is not much fun to lie around on, unless you find a smooth drift log or a nice big smooth glacial erratic to stretch out on to enjoy the skin-softening northwest drizzle. Beats skin cancer.


A fine sandy beach south of Bellingham.

There are some natural sandy beaches in the inland waters of the Northwest, but they require a special set of circumstances to last through the rough winter storms and waves. There has to be a supply of sand to start with, such as a sandy bluff with few cobbles in it nearby, and just the right combination of tidal energy and current direction to concentrate the sand on the shore above the low tide line.
You are liable to find these same cobble beaches anywhere along the shoreline in areas that have been glaciated: Maine and the rest of New England, and in places in the Great Lakes, too.