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Roger Weller, geology instructor

Yellowstone Caldera
by Jerry Stroupe
Physical Geology
Spring 2007



The Yellowstone Caldera


            Yellowstone National Park is a place unlike any other on earth. Located in the western United States, it encompasses the north-western corner of Wyoming, and spills over into Idaho to the east and Montana to the north. Having been designated as our first National Park, it is also the largest within the continental United States. First designated and dedicated in 1904 by President Theodore Roosevelt, it became the first of many such areas set aside for conservation and preservation.

            Many things set Yellowstone apart from any other place on Earth. The flora and fauna there are astounding. The greatest population of grizzly bears in the lower 48 states thrives there, as well as the largest free roaming herd of American bison. Since it’s re-introduction to the lower 48 the grey wolf has thrived there as well.

Photo Courtesy: United States Park Service, Department of the Interior


The greatest difference that sets Yellowstone apart is that it sits above a highly active geothermal hot spot. With only slightly more that a dozen hotspots exist throughout the entire planet, this feature makes it a top destination for tourists around the world. With 2.3 million visitors last year, it is second only to the Grand Canyon of Arizona in the number of visitors per year. On the average summer weekend more than eighty thousand visitors can be found in the park at any given time.

            The geothermal activity that can be seen there is amazing. From the Steamboat and Old Faithful geysers, to the bubbling cauldrons of mud, Yellowstone is truly a unique place. The colors of the rock formations in and around the geysers and hot springs are truly phenomenal. The park’s name “Yellowstone” is derived from the abundance of yellow sulphur coated rock features prevalent in the area.

            I had the opportunity to re-visit the park last year after a nineteen year absence. Upon arriving at the park, we immediately drove to the campground located at the south end of Lake Yellowstone. This is an out of the way place with a twenty-five mile dirt road between it and civilization. When we arrived there we found the entire campground under water. The lake level appeared normal as it had nineteen years earlier. I soon contacted a Park Ranger, who in turn introduced me to one of the members of the USGS team for an explanation of what had happened to my prized campsite.

            Most of the park and all of Yellowstone Lake sits above an ancient volcanic caldera. The caldera of a volcano is the opening left by the most previous eruption. It can be likened to the mouth of the beast. Many of us remember the eruption of Mt. St Helens in 1980. Few can forget the ash clouds and the destruction caused by the pyroclastic flow and the resulting lahars’ that was wreaked upon western Washington State. The caldera left by the eruption at Mt. St Helens is less than a mile across. In comparison the caldera at Yellowstone measures 1260 square miles. With the Mt. St Helens eruption, appx. 1/3 cubic mile of debris was ejected. Geologists estimate the greatest eruption of Yellowstone ejected over 600 cubic miles of debris into the atmosphere. The illustration s below show the relative size of the ash bed deposits left by the previous eruptions of Yellowstone compared to Mt. St Helens.

                                                                       Graph Courtesy: United States Geological Survey

Approximately 74,000 years ago the Toba eruption occurred on the island of Sumatra. It was so violent that it proved apocalyptic to the human race throughout the world. The volcanic gases and debris ejected into the upper troposphere resulted in a volcanic winter that lasted for many years. Biologists believe this event accounts for the small variability in the genetic make up of the entire human race. All but about 2000 humans could not survive the sudden changes to the environment this event brought. It is estimated the Toba eruption ejected 400-500 cubic miles of debris into the atmosphere.

            No one has ever experienced a volcanic explosion on the scale of the Yellowstone eruptions, but smaller eruptions have been observed and their activity described. Mt. Tambora in the country of Indonesia experienced a violent eruption in April of 1815. From the period beginning in the summer of 1813, the dormant volcano began to shake and groan. On April 15, 1815 the volcano produced thundering explosions heard 870 miles away. The next morning volcanic ash began to fall, and the explosions began to come with less frequency and intensity.

            On the morning of April 10, the mountain went wild. .Those watching from over 20 miles away described how 3 columns of flames from the caldera formed one, and how the entire mountain appeared to be covered with liquid fire. Soon these distant observers were pelted with 8 inch still glowing pumice stones. The village of Tambora was destroyed by the rolling clouds of the pyroclastic outflow. On April 16 booming explosions loud enough to be heard on the island of Sumatra some 1600 miles to the west could be heard. Scientists believe this eruption produced the loudest sound ever heard by a human.

            When the eruption ended, the ash cloud had spread to Lombok, 124 miles to the west was covered in a blanket of ash almost 2 feet thick. Tidal waves crashed on several islands as far as 200 miles away. The waves and ash falls alone killed more than 88,000 people.

            The debris blasted into the upper atmosphere completely encircled the earth in a matter of months. In 1816 mean temperatures in the northern hemisphere dropped by more than 20 degrees. In America this year is referred to as “the year without a summer”. Massive crop failures occurred throughout the world. It is unknown how many deaths occurred from this volcanic winter produced by the eruption of Tambora. Scientists estimate from several hundred thousand to as much as 3 million people may have died from starvation and disease. The chart below demonstrates the relative size of volcanic debris ejected by these eruptions.


             The eruption of Tambora was the largest and deadliest eruption in recorded history. Compared with Mt. St Helens, it was over 100 times more destructive. Modern geoglists estimate that Tambora ejected about 36 cubic miles of debris. The last eruption of Yellowstone yielded more than 600 cubic miles of debris. To help put this in perspective this figure represents 17 times more debris than Tambora, and more than 2400 times the debris ejected by Mt. St Helens.

Mt. St. Helens Photo Courtesy: United States Geological Survey


            The recent increase in seismic and geothermal activity has led geologists to believe a near future eruption of the Yellowstone caldera is imminent. Small seismic shocks occur almost daily. Two large quakes have occurred near the periphery of the caldera within the last 50 years. The chart below shows the location of the quakes, and the size of the caldera. If you have ever been to Yellowstone Lake, you will know that it is huge. Keep that in mind when you see the outline of the caldera in this chart.


            The tremendous pressures exerted on the lava dome of the caldera have forced the entire surface upward by 2 feet from 1923-1985. From 1986-1996 the upward movement of the caldera floor slowed to a crawl. Then it began another period of uplift from 1997-2003. In this short amount of time, the floor averaged an uplift of slightly more than 5 inches. As you can see from the chart above most of Yellowstone Lake is located within the caldera. The uplift of the northern portion caused the flooding of my prized campsite at the southern end of the lake.

            The USGS classifies this type of volcano as an explosive maar. The pressures of eruption are so great that little if any cone is formed. The tremendous pressures eject any debris all the way to the upper atmosphere. It is estimated that the pressure under the Yellowstone dome to be in excess of 50,000 pounds of force per square inch.  The chart below shows the estimated size of the magma chamber under the caldera dome.


Correction from a viewer:

In particular, I find the illustration for the magma chamber beneath Yellowstone very misleading. It shows a magma  chamber that  rises from a depth of about 50 miles below the surface and is about 50 miles wide. If you take the two (?) chambers together, according to the drawing. We know it is one chamber that extends to a depth of 450 miles below the surface. In addition, the chamber is, in places, about 150 miles wide.

Sincerely,  Karen Froiland  (correction added 5/1/13)


            The Yellowstone Caldera represents the greatest natural threat to the extinction of life on earth. Some have suggested drilling into the magma chamber to relieve pressure. That could be like sticking a needle into an inflated balloon. The chance of causing a catyclismic eruption is too great.

            The USGS believes that a major eruption of Yellowstone occurs every 640,000 years, and the last occurred almost 640,000 years ago.

            What would a major eruption of the Yellowstone caldera look like? The image below was taken from the Discovery Channel’s presentation of the TV program entitled “Supervolcano”.


Photo Courtesy http://www.discovery


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