Yellowstone III: One Hot Spot   Leave a comment

Perhaps you’ve heard this cheerful little nugget of information: Yellowstone National Park conceals a huge volcano that will soon explode and destroy most of America.  Well, I’m not overly worried about this admittedly enormous volcanic system.  And it’s not because the prevailing winds would likely blow the ash toward the east, away from my home in Oregon (even though this does help me sleep at night).

No, the real truth of the situation at Yellowstone lies somewhere between Discovery Channel’s scare tactics and the blissful ignorance that we lived with in the pre-super volcano days.  By the way, just this past summer there were two people killed (one partially eaten) by grizzly bears.  “Jellystone”,  America’s  iconic family camping destination, with friendly Yogi & Booboo hiding behind a pine tree hoping to steal your “picinic” basket, will never again seem so innocent, so charming.

One of Yellowstone’s iconic thermal features (and one huge hot spring) is Grand Prismatic Spring, as viewed from a nearby hill.

This post will concentrate on the geology of the Yellowstone volcano, so if you’re not too much into science, feel free to view the pictures of thermal features instead.  After all, it is the geysers and other features that are a direct surface reflection of the sleeping giant beneath.  If you click on any of these images, which are copyrighted and require permission to download and use, you will be taken to my website, where purchase for either download or print ordering is very easy to do.  Thanks a bunch for your cooperation and interest.

The moon creates a surreal scene in Lower Geyser Basin in Yellowstone National Park.

So-called super volcanoes are known by volcanologists as caldera systems.  Essentially, these are raised, roughly circular areas underlain by large underground chambers of magma (melted rock).  The magma in caldera systems tends to be particularly charged with gases.  This is in large part because the magma is not buried very deeply, thus receiving rainwater from the surface.

When the volcano eventually erupts, these dissolved gases flash to the vapor phase and drive an eruption so violent and complete that most or all of the magma chamber is evacuated.  And since the overlying area is large you get a catastrophic collapse of the volcano back into the emptied magma chamber.  The collapse itself can drive continued violent eruption, like a giant piston driving the remainder of the magma forcefully out.

The eruption produces, for the most part, deadly hot ash flows (called pyroclastic flows) that race across the landscape, along with truly enormous clouds of lighter ash which can bury landscapes hundreds, even thousands, of miles away.

Caldera volcanoes range in size from those that erupt with not much more violence than average-sized volcanoes, the ones we have experienced in historical times, to those that can change the course of life on Earth.  They rarely resemble most people’s idea of a typical volcano.  They’re not usually steep-sided, but instead form large areas of irregular topography, often with one or more large lakes.  The lakes occupy depressed areas in the interiors of the calderas. Large deposits of volcanic ash and the rock equivalent (called tuff) mantle and partly fill the caldera.  So they definitely don’t look like the volcanoes you might have scribbled as a  kid in school.

Luckily for all of us living things, the truly apocalyptic caldera eruptions, those that have been documented in Yellowstone and a handful of other systems around the world, happen very, very infrequently.  Not so seldom as far as Earth is concerned, but on a human timescale they could be regarded as being below a minimum level of probability that we should worry about.

Of course, some living things, even people, must experience a caldera eruption.  It’s believed, for example, that the last really large caldera eruption, that of Toba in Sumatra some 70,000 years ago, led to a collapse of the small human population in Africa to a level that caused a “genetic bottleneck”.  In other words, there was some interbreeding going on among the few thousand survivors, who then went on to reproduce and spread out of Africa, carrying a set of genes that even today still reflects an unusually limited genetic diversity.

Yellowstone’s caldera, one of the world’s largest, is in the middle of the Rocky Mountains.  This might seem a strange place for a big volcano, since the Rockies are not a volcanic range.  But the story of why it’s there is a very interesting one.  Yellowstone lies over a hot spot.  You might have heard that Hawaii has formed over a hot spot.  Most active hot spots have, indeed, been identified in ocean basins (Iceland lies over another).  Hot spots are formed when huge columns of heat rise from deep within the Earth, causing melting of rocks in the upper mantle and lower crust.  This melted rock, since it is lighter, follows fissures upwards, eventually creating volcanoes at the surface.

A frigid morning breaks over a thermally heated meadow in Yellowstone Park.

The neatest thing about hot spots is that they remain in one place while the crustal plates  lying above them move horizontally.  Actually, hot spots may move, but either VERY slowly, or only after long periods of being stationary (not much is known about if and how they might move).  In the case of the Hawaiian hot spot, the movement of the Pacific Plate northwestward has created a string of volcanic islands, with the most recent, still-active one (Hawaii, the Big Island) on the southeast end.  It is here where the hot spot happily pumps away.  Actually, there is a newer volcano, called Loihi, still a few thousand feet below the waves, off the east coast of the Big Island. It’s coming.

Now to the Yellowstone hot spot.  Along the southeastern Oregon – northern Nevada border, there are a series of quite large, caldera systems that last erupted about 17 million years ago.  There are more calderas to the east, slightly younger, and even younger lava fields in southern Idaho.  In fact, there is a line of volcanic fields that becomes younger as you go east, marching in a broad arc and pointing right at Yellowstone.  The Snake River Plain was created by this volcanic activity.

Geologists took too long to recognize this for what it was, but now it is well recognized as the track of one of those rare beasts, a continental hot spot.  Again, the track results not from the hot spot’s eastward movement, but from the movement of the North American plate westward, over the stationary hot spot.  Yellowstone, just like Hawaii, is sitting right over the hot spot.

Hawaii’s Kilauea volcano is the most active volcano on Earth, erupting almost continuously but not too dangerously.  Yellowstone erupts catastrophically, but only once every half million years or so.  That difference is because of the totally different type of crust that the two sit upon.  The crust under Yellowstone is made of granitic rocks, and is quite thick (30 miles or so).  Under Hawaii, the crust is basaltic and very thin (5 or 6 miles).

Yellowstone has not erupted for more than 600,000 years, but the eruptions before this were separated roughly by that same 600,000 time interval.  So does that mean it is due for one now?  Yes and no.  Yes one might assume that quite soon, in geological terms, Yellowstone will erupt again.  But this is a volcano with very long intervals between eruptions, and it could easily be 5000, 10,000, or more years before it erupts again.

So…I would relax if I were you when you visit this park.  Enjoy its living, breathing geology, its wildlife.  Yellowstone is one of the world’s unique places, and unless you’re immune to wonder, you will most certainly spend much of your time here  with wide eyes.

In a remote area of Yellowstone National Park in Wyoming, a large and very hot thermal pool (which occasionally erupts) resembles a scene from another planet.

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