Phillip Larson
Geology 330
9.11.2005
Yellowstoneand Super-Volcanoes
YellowstoneNational Park, which encompasses 3,472 square miles, is larger then both Delaware and Rhode Island combined (Lowenstern). It is presently home to 10,000 geothermal features, including 300 geysers. The park was established in 1872 by, then President Ulysses S. Grant, who declared, “This Park will be forever dedicated and set apart as a public park or pleasuring ground for the benefit and enjoyment of the people (USGS).” Today, Yellowstone is one of the most visited natural sites in the world with hundreds of thousands of tourists visiting every year. You may ask yourself, as many tourists do, why is this spot so unlike anything else in the world? Why is all this geothermal activity concentrated in one large area such as this? The answer to this lies behind the natural beauty of Yellowstone. Yellowstone is one of the world’s largest volcanoesand at some time in the future could pose a threat not only to the area but to all life on Earth. Although the likelihood of such a large scale eruption is minute, the consideration must be given as to the likelihood of future eruptions. The goal of this paper is to inform not only of this likelihood of future eruptions and its consequences,but to examine the past geologic record in order to better understand the nature of the Yellowstone Super-Volcano.
What is Yellowstone?
The Hotspot:
First an understanding of the driving mechanism must be recognized in order to understand what the nature of the Yellowstone volcanic system truly is. The Yellowstone Super-Volcano originates from a hotspot which lies in the mantle where hot, molten rock rises to the Earth surfaces which in-turn heats the ground underneath thus causing the geothermal activity we see today. A hotspot is a volcanic center that has been persistently active for at least a few tens of millions of years. It causes a break in the Earth’s surface due to a rising mantle plume, not always directly associated with volcanic arcs or oceanic ridges (Bates and Jackson). It is believed that the Yellowstone hotspot is stationary within the Earth. It’s apparent motion is due to the North American Plates southwestwardly movement at a rate of about 4.6 cm per year over the Yellowstone hotspots lifespan of a little over 16 million years.
The Super-Volcano:
This hotspot has been responsible for several eruptive episodes which will be discussed in detail later, but has presently found itself in the Northwest corner of Wyoming and has created an enormous volcanic caldera known as a Super-Volcano. A Super-Volcano is a term used to describe a volcanic area of immense size and capable of enormous eruptions, the largest known on Earth. Using this term implies eruptions of a magnitude 8 on the Volcano Explosivity Index. This would mean that 1,000 plus cubic kilometers of magma and partially molten rock would be erupted. This scale eruption would be over 2 to 3 thousand times the size of the Mount St. Helens eruption of 1980. There are many such Super-Volcanoes across the Earth’s surface. There is evidence to support 7 known Super-Volcanoes including the Yellowstone volcanic system. The United States alone is home to three of these Super-Volcanoes located in Yellowstone, Long ValleyCalifornia, and Valles Caldera in northern New Mexico. Super-Volcanoes also exist in many other places worldwide. The four known Super-Volcanoes outside the United States and last eruptive episodes are; Aira Japan, which last erupted on a very small scale (for a Super-Volcano) 22,000 years ago, Lake Taupo New Zealand, 26,500 years ago, The Siberian Traps of Siberia, 251 million years ago, and Lake Toba in Sumatra, Indonesia which had the last major Super-Volcanic eruption 74,000 years ago. It is interesting to note that the massive Toba eruption is thought to have nearly caused the extinction of the homo-sapien species as well as many others. The Siberian Trap eruptions were also thought to be a primary suspect for the Permian Extinctions 251 million years ago (O’Hanlon).
The Yellowstone Hotspot History
There have been numerous eruptive episodes along the Yellowstone Hotspot track (see figure 2). The Yellowstone Hotspot originated roughly 16.6 million years ago along the Nevada-Oregon border. This started as an outpouring of basaltic lava in the Columbia Plateau and rhyolitic centers developing to the south (Perkins and Nash). Along its track there are thought to have been 6 caldera forming events along what today is known as the Snake River Plain. These episodes have occurred approximately 16.6-15 ma, 15-13 ma, 12-10.5ma, 10.5-8.6ma, 10-7ma, and 6.5-4.3ma until it reached its present day location where many more eruptive episodes have occurred. This apparent path is well marked along the 650 km path of the Snake River Plain. Evidence for this hotspot track has been determined through careful examination of silica ash fall records which seem to provide the best space-time transgression of the hotspot (UND.edu).
Figure 2Yellowstone Hotspot Track.
Yellowstone’s Recent History
Since the Yellowstone Hotspot has been located at its present location there have three major caldera forming, eruptive episodes. These were again determined by careful examination of ash-flow tuffs in the areas surrounding and far reaching from the present calderas. There is thought to be a general sequence of how these caldera forming episodes occur. First the large area above the hotspot is uplifted which depicts the rise of rhyolite below the Earth’s crust. This forms a magma chamber which inflates as the rhyolite beneath continues to rise. This then leads to fracturing and faulting at the surface and occasionally extrusion of lava from the newly formed fractures. After pressure continues to build highly pressurized rhyolite magma erupts from the concentric ring fractures and the top of the magma chamber collapses in on itself. This then produces the large calderas we noticed on the present landscape (USGS). The first of these episodes, which was the largest eruption in Yellowstone’s recent history, occurred 2.1-2.3 ma. This eruption, known as the Huckleberry Ridge eruption, vented 2,450+ km3 of ash and rhyolitic lava, covered an area of approximately 15,500 km2, and formed a caldera that was thought to be 75x95 km in size. This caldera is not as clearly evident as the next two to be noted but is thought to include Big Bend Ridge, Snake River, and Red Mountain Caldera segments. It was determined through the study of the Huckleberry Ridge Tuff. The second eruption, which occurred 1.3 ma, was much smaller than the Huckleberry Ridge eruption. The second major eruption vented 280 km3 of ash and lava, covered an area of 2,700 km2 with ash and rock, and created the Henry’s Fork Caldera which is approximately 16km in diameter. This was determined through the study of the Mesa Falls Tuff. The third eruption occurred 640,000 years ago and was determined through the studies of the Lava Creek Tuff.During this eruption 1,000 km3 of ash and rock were erupted which covered an area about 7,500 km2. This created the modern Yellowstone Caldera which can be seen today and stretches 85x45 km in size (USGS)
Figure 2 Yellowstone Calderas and Post-Caldera Volcanic Rocks.
Most often the eruptive episodes at Yellowstone and along its hotspot track are not eruptive. They most often consist of very thick, sticky, and slow moving rhyolite lava flows. Since the last eruption 40 such lava flows have occurred.
The Likelihood of Future Eruptions and Its Consequences
This volcanic system has shown that it continually cycles threw eruptive caldera forming episodes throughout its 16.5 million year existence. This means that it will most likely erupt again in another explosive event similar to those in the past. The question then remains what the likelihood of such an eruption would be on human time scale and what the consequences of such an eruption would be? It has been theorized that Yellowstone is on a roughly 7-8 hundred thousand year cycle but evidence for this is very sparse. If this were the case we should be looking at the possibility of another eruptive event in the making. Most geologists agree that a build up to eruption would signal itself hundreds, even thousands of years before the event actually occurred (USGS). There clearly has been no evidence of any kind of abnormally increased activity in any aspect in the area. And although another catastrophic eruption may be possible there are some geologists who believe a catastrophic eruption, like those of the past, may never occur again. It is far more likely that eruptions of lava flows will occur as they have many times in the Yellowstone Hotspot history. It has been calculated that the odds of this kind of caldera forming eruption has a likelihood of 1 in 730,000 or .00014% of occurring which is thought to be equivalent to a 1km asteroid striking the Earth. The overall unpredictability of any major geologic event adds to this uncertainty (USGS).
If it were to happen the consequences would be devastating not only the United States but to the world as well. The United States economy would be devastated and the loss of life could be in the millions within a few weeks of the eruptions. Ash would blanket most of the country as it has done in past eruptions causing far reaching devastation to most of the nation. Ash from the last eruption, 640,000 years ago was found as far away as Iowa, Louisiana, and California (see figure 3 for ash coverage). The global climate could change drastically due to the debris and gases which would have been erupted into the atmosphere causing millions to die due to temperature change and widespread famine. This has been clearly shown by much smaller scale eruptions like that of Krakatoa in 1883 and Tambora in 1815 which caused global temperatures to plummet a few degrees C. Both these eruptions were not even close in size to any of Yellowstone’s three episodes. Other evidence of the devastating affects of Super-Volcanoes is believed to be shown by the near extinction of homo-sapiens due to the Toba eruption 74,000 years ago and possible the Permian mass extinction due to the Siberian Traps of 251 million years ago.
Figure 3. UW-Madison. Shows ash fall range of the largest two eruptions.
Conclusion
The Yellowstone Hotspot has existed for around 16.5 million years. It has exhibited the ability to have large caldera forming eruptions consistent with that of a Super-Volcano. The three major eruptions in Yellowstone’s recent history have left ash fall to examine the record of its past and through this examination we can determine the activity that has lead to Yellowstone’s formation. Yellowstone is undoubtedly a volcanically active area with many unique features found no where else on Earth. The likelihood of it ever erupting again is uncertain but it is a very rare occurrence at that. A future eruption is something that must be considered and continual study must be made to make sure that we are aware if it does increase its activity. The most likely outcome of any eruptive episode would be lava flows which have abundantly shown themselves in the past along the hotspot track and even the likelihood of that on human timescale is very rare.
Phillip Larson
Geology 330
November 9, 2005
The Yellowstone Super-volcano
In the northwest corner of Wyoming lies one of the most outstanding areas of nature and beauty in the world. Home to 10,000 geothermal features, including hot springs and over 300 geysers, YellowstoneNational Park, which encompasses 3,472 square miles, is larger then both Delaware and Rhode Island combined (Lowenstern). The park was established in 1872 by, then President Ulysses S. Grant, who declared, “this park will be forever dedicated and set apart as a public park or pleasuring ground for the benefit and enjoyment of the people (USGS)." Today, Yellowstone is one of the most visited natural sites in the world with tens of thousands of tourists visiting every year. You may ask yourself, why is this spot so unlike anything else in the world? Why is all this geothermal activity concentrated in one large area such as this? The answer to this is what lies behind the natural beauty of Yellowstone. What lies behind is one of the world’s largest super-volcanoes, which poses a threat not only to the area, but to all life on Earth. Although the likelihood of such a large eruption is minute, the consideration must be given as to what could happen and what evidence is there that it will happen again.
The Past Record
To understand the volcanic system we are looking at and to understand the likelihood of a future eruption we must first look at the evidence of past volcanic activity. The park is known to have three calderas which were created by three eruptions in geologic history. It is important to note at this point that two of these eruptions were “among the largest individual eruptions known on Earth.” These three eruptions are believed to have occurred roughly 2.1 million, 1.3 million, and 640,000 years ago (believed now to have an approximate 700,000 year cycle). The first eruption, known as the Huckleberry Ridge eruption, “vented more than 2,500 cubic kilometers of volcanic debris (enough to bury the state of Texas 12 feet deep).” This eruption in itself was 6,000 times greater than that of the 1980 eruption of Mount St. Helens (Lowenstern). The eruption which occurred 640,000 years ago created the youngest massive caldera in the park, which measures 45 by 30 miles. This “blasted” more that 1,000 cubic kilometers of volcanic material into the atmosphere. In relation to Mt.St. Helens, it ejected over 8,000 times more ash and lava (discovery).
So why is this happening here?
It is believed that YellowstoneNational Park lies directly above what is known as a hotspot. The Yellowstone hotspot is believed to be driven by the “upwelling of a deep seated mantle plume.” This hotspot has been tracked through examination silica ash fall records and shows and east, north east movement, which now places it in the northwest corner of Wyoming. It has “over the past 17 million years or so, had successive eruptions which have flooded lava over wide stretches of Washington, Oregon, California, Nevada, and Idaho, forming a string of comparatively flat calderas linked like beads, as the North American plate moves across the stationary hot spot.” The apparent motion of the hotspot is actually due to the movement of the North American plate moving to the south-southwest. This is seen through the existence of the Snake River Plain and it being caused by the Yellowstone hotspot. “The Snake River Plain extends 400 miles (650 km) westward from northwest Wyoming to the Idaho-Oregon border. The Snake River Plain is a broad, flat arcuate depression which is concave to the north and covers one quarter of the state of Idaho. Elevations on the Snake River Plain decrease from the east (4,400-5,000 feet; 1,350-1,525 m) to the west (2,950-3,900 feet; 900-1,200 m). Some features within the Snake River Plain become younger to the east. For example, calderas become progressively younger from west to east. The Yellowstone calderas are the youngest and mark the approximate hotspot location.” (UND.edu)
Whatis the Likelihood of a Future Eruption and its consequences?
It is without a doubt that this volcanic system will erupt again at some point. The likelihood of it happening on our timescale is almost non-existent. As Lowenstern pointed out it is far more likely that a small eruption or lava flowwould take place but nothing as catastrophic as many might think. If, which is highly improbable, the Yellowstone Caldera were to erupt it would have extraordinarily devastating consequences. The immediate impact would be thousands, if not millions dead. The United States economy would be in shambles and much of the North American species would be wiped out. Evidence of the devastating affects of a super-volcano eruption has been studied in the most recent eruption of the super-volcano Toba eruption 74,000 years ago. It was believed to have dropped human populations down to a few thousand people, nearly pushing the human race to extinction. The resulting ash cloud from an eruption such as this would be blasted into the high atmosphere and carried around the world by various atmospheric winds. This would cause global temperatures to plummet and could possibly cause millions to die due to temperature change and hunger. Evidence of this is seen when studying the ash fall records of the most recent eruption, where ash has been found as far away as Iowa, Louisiana, and California (USGS). As mentioned earlier the United States would find itself in a dire situation with millions dead in the immediate aftermath of the eruption. The economy would most like collapse and in turn would devastate much of the global economy. So, understandably there is some reason to be concerned, but as mentioned earlier the likelihood of such a blast is highly improbable in any human time scale. Plus, it is believed that a super-volcano eruption would give warning signs up to a thousand years before the event actually occurred.