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Sleeping Giant:  The Yellowstone Caldera Super Volcano

By:  Justin Reynolds


Yellowstone National Park is our nation’s oldest, most recognized recreational areas due to its many geysers, thermal pools, mountain views, and a vast array of wildlife, including most of America’s remaining herds of bison.  As a result of its popularity, the park attracts nearly 3 million visitors yearly, (  Yet, what many of these visitors do not realize is that landmarks they came to visit, such as Old Faithful and the park’s other numerous geysers, are fueled by a sleeping giant.  Underneath Yellowstone there rests one of the world’s largest, potentially harmful volcanoes known to man, and if historical records prove true, it is due to blow. 

The question for geologists is what fuels this volcano and the various geothermal features dotted across Yellowstone?  Geologists refer to the phenomenon as the Yellowstone Hot Spot (Francis 197). A hot spot is a location in the Earth’s crust where magma flows up from the magma, forming volcanoes in its wake.  The hotspot has been affecting the area for the last 16 million years, yet scientific understanding of its effect on Yellowstone’s formation has only just begun.  What is known is that the hotspot is relatively stationary within the depths of Earth, and the North American tectonic plate is moving in a southwest direction over the hotspot at a rate of 4.6 centimeters annually, (Direction).  While the North American plate moved southwest over the hotspot, the volcanic formations progressed northeast, beginning in northern Nevada and southeast Oregon 16.5 million years ago and reaching its present day location near Yellowstone National Park 2 million years ago, (Direction). The current location of the Yellowstone hotspot is shown in the figure to the right.  The orange outlines are the outlines of the three calderas which have resulted from eruptions during the past 2 million years. Notice how mountains have formed a parabolic shape around the hotspot as the North American plate moves southwest over the hotspot.



Volumes of Yellowstone's largest eruptions compared to large historic eruptions.


Map showing the area of the US that was once covered by ash from Yellowstone eruptions.

During the past 2 million years, the volcanic region has been subject to 3 definable cycles of activity, all occurring approximately 600,000 years apart, (Fisher 107). The first occurred about 2 million years ago and produced a massive 2,500 cubic kilometers of volcanic ash. The next happened 1.2 million years ago, resulting in 280 cubic kilometers. The most recent took place 600,000 years ago, resulting in 1,000 cubic kilometers of ash, (Fisher 108). In comparison, the eruption of Mt. St. Helens in 1980 created an enormous amount of destruction while only spewing 2.5 cubic kilometers of erupted ash, (Fisher 108). The diagram to the left shows the immense amount of ash associated with the three eruptions compared to some other notable volcanic eruptions.  The first Yellowstone eruption produced 1,000 times the ash alone! To the right is a diagram showing the ash distribution across America as a result of the Yellowstone eruptions compared to the Mt. St. Helens eruption. As the diagram shows, an eruption of such large scale would surely be devastating to the Great Plains and Western states.  The eruption which occurred 2 Million years ago provided enough ash to cover or partially cover 20 states. In contrast the St. Helens volcano only partially covered a few regionalized states in 1980. 


The substances present in the magma chamber underneath Yellowstone prior to eruption were composed of ryholite lava and volcanic ash.  Ryholite lava is very viscous, meaning that it is very thick and flows slowly due to friction, (Francis 141). It is the opposite of Basalt lava which is less viscous and flows freely. In each case, pressure built up to insurmountable levels due to the thick nature of the ryholite lava present, which led to three of Earth’s largest known pyroclastic-flow eruptions.  The eruptions, mentioned previously, were powerful enough to result in a massive blanket of volcanic ash that was deposited as far east as Louisiana, (Ash).

In the wake of these eruptions, three separate calderas were formed in relation to their eruption, as the top of the magma chamber producing the ryholite lava collapsed due to the lack of structural support provided by the lava.  A caldera is a large, more-or-less circular depression surrounded by steep cliffs, and formed when a magma chamber empties out its magma, and the surface above the chamber collapses. (Francis 292).  The Yellowstone Caldera is presently 45 km in diameter and 75 km in length.  However, it will ultimately take a new form when the next massive eruption occurs. 


Map showing the locations of features in the photogallery.


Recently in April of 2004, over 500 earthquakes were recorded in the general Yellowstone region.  The largest was just below 3.0 in magnitude, not incredibly large, yet the sheer numbers are somewhat disturbing.  At one point, according to seismic recording stations near Yellowstone, over an eleven day period, 465 individual earthquakes were recorded near or at the caldera. In addition, a “bulge” has recently been recorded in the center of the caldera.  A recorded rise of 86 centimeters was observed between 1923 and 1984, and then it subsided slightly between 1985 and 1989 until rising again recently, (Fisher 109). The significance of these fluctuations is unknown, but it is believed to be related to the level of the magma present in the underground chambers beneath Yellowstone.


Could the “bulge” be a sign of future volcanic activity? Scientists are unsure, as there is currently no technology present that is capable of predicting earthquakes.  The activity of Yellowstone’s hydrothermal system does not vary much from other similar large calderas, yet historical records of volcanic activity in the region suggest that a potentially cataclysmic eruption should occur in the near future, (Fisher 111).  Each of the eruptions that occurred in the past, were spaced about 600,000 years apart, the same amount of time between the last eruption and now. 


Diagram physical and chemical processes of volcanic gas interactions in atmosphere


When dealing with volcanoes, potential eruptions are always a concern to scientists.  What would another eruption of the Yellowstone Caldera mean to the rest of the World?  As with previous eruptions, the ash expelled into the stratosphere, would blanket the entire Great Plains.  Earth’s most fertile lands would be virtually blanketed in a layer of ash. Crops would fail, and a local and global shortage of grains would develop.  The ash would work its way into the Mississippi-Missouri River system, polluting the water and filling in channels from North Dakota to the Gulf of Mexico, (Ash). This ash would also block out the sun light and result in an overall global cooling of a few degrees as shown in more depth above. Essentially, the effects of another eruption of the Yellowstone Caldera would be devastating to our planet and would surely test the resilience of mankind.  Yet, by all scientific accounts, the activity that is occurring presently beneath our nation’s oldest national park does not pose an imminent threat towards mankind.  But the possibility still remains of a catastrophic eruption in the near future unlike any man has ever seen.



For more information on current seismic activity in the Yellowstone region see…

Current Activity:



For more information on Yellowstone National Park visit…





Ash Distribution. 7 Mar. 2003. U.S. Department of the Interior, U.S. Geological Survey,
          Menlo Park, California, USA. 28 Apr. 2004


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Direction of Plate Movement. 7 Mar. 2003. U.S. Department of the Interior, U.S. Geological Survey,
          Menlo Park, California, USA. 28 Apr. 2004


Fisher, Richard V., Grant Heiken, and Jeffrey B. Hulen. Volcanoes: Crucibles of
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