Yellowstone National Park is the site of a super volcano in times past, as evidenced by the huge caldera that lies just below the surface.
An earthquake of magnitude 7.3 at Hebgen Lake, Montana, on August 18, 1959, made quite an impact on the West Yellowstone part of Yellowstone National Park. New geysers erupted and massive slumping caused large cracks in the ground from which steam emitted. Many hot springs became muddy. The earthquake led to extensive fault scarps, subsidence and uplift, a massive landslide, and a seiche in Hebgen Lake.
It caused twenty-eight fatalities most of them the result of rockslides that covered the Rock Creek public campground on the Madison River, about six miles below Hebgen Dam. Total damage amounted to $11 million, almost all of it related to the damage done to highways and timber. This was Montana’s largest earthquake ever as far as its documented history shows.
The most spectacular and disastrous effect of the earthquake was the huge avalanche of rock, soil, and trees that cascaded from the steep south wall of the Madison River Canyon. This slide formed a barrier that blocked the gorge and stopped the flow of the Madison River and, within a few weeks, created a lake almost 150 feet deep. The volume of material that blocked the Madison River below Hebgen Dam has been estimated at many millions of cubic yards. New fault scarps as high as twenty feet formed near Hebgen Lake.
The major fault scarps formed along pre-existing normal faults northeast of Hebgen Lake. Subsidence occurred over much of an area that was about fifteen miles from north to south and about twice as long from east to west. As a result of the faulting near Hebgen Lake, the bedrock beneath the lake was permanently warped, causing the lake floor to drop and generate a seiche.
Maximum subsidence was twenty feet in the Hebgen Lake Basin. About seventy square miles subsided more than nine feet, and an area of more than two hundred square miles subsided more than one foot. The earth-fill dam sustained significant cracks in its concrete core and spillway, but it continued to be an effective structure. Many summer homes in the Hebgen Lake area were damaged: houses and cabins shifted off their foundations, chimneys fell, and pipelines were broken.
Most small-unit masonry structures and wooden buildings along the major fault scarps survived, with little damage when subjected only to vibratory forces. Roadways were cracked and shifted extensively, and much timber was destroyed. Highway damage near Hebgen Lake was due to landslides slumping vertically and flowing laterally beneath pavements and bridges, causing severe cracks and destruction. Three of the five reinforced bridges in the epicentral area also sustained significant damage.
Yellowstone National Park is often referred to as the nation’s jewel of the park system. It was the first to be established in 1872. It was also the world’s first national park. Yellowstone is perched on a series of volcanic plateaus in the northwest corner of Wyoming. Average elevation in this area is about 4,500 feet. Because of its elevation and northern latitude the climate is cold and humid. Center of interest in the park is the collection of hot emissions from deep inside the earth, geysers, mud pots, hot springs, and fumaroles. They constitute one of the greatest number of geothermal features to be found anywhere. Traditionally, a geyser in Yellowstone, such as Old Faithful, would thrust its jet of boiling water high into the air about once every hour, and this was always a major attraction for visitors.
Then in 1959, the earthquake at Hebgen Lake, about forty miles northwest of Yellowstone Park’s center, changed all that. The patterns of eruption of many of Yellowstone’s geysers were permanently altered. Now they are less frequent. Such are the unpredictable nature of volcanic activities. Over geological time they are even less predictable.
The entire Yellowstone Basin is a volcanic caldera, what is left of a gigantic eruption that occurred in the distant past. Geologists estimate that an eruption of this kind happens approximately every 600,000 years. The last two were 1.2 million and 600,000 years ago, the last one having ejected a volume of gas more than a thousand times the amount of ash thrown up by Mount St. Helens in 1980. By all available evidence, this area is due for another massive explosion like the previous two, sometime within the next few hundred years! At some depth beneath ground level is the magma pool and above it in the rock layers closer to the surface are numerous fault lines.
Water from rain and meltwater seep down into these cracks. The water expands as it gets superheated and is forced up to the surface. Depending on the shape and size of these underground faults the superheated water takes different forms: it may be a fumarole, that is a jet of steam; or a hot spring, which is a boiling pool of water; or it could be a mixture of steam and hot water, known as a geyser, an eruption that occurs in fits and starts under the influence of different types of pressures from below.
There are many other variations in the activities taking place within the caldera. There are mud pots, which are acidic hot springs that hold large amounts of minerals in suspension underground because they cannot flush away the minerals before they reach the surface. As a result, they appear as muddy waters, with their bubbles making a glop-glop sound. Hence the name mud pot. Mammoth Hot Springs rises up through rocks that contain easily dissolved minerals. One of these minerals is calcium carbonate and as it precipitates out of the hot water on cooling it forms terraces around the mouth of the hot spring.
All of these volcanic activities occur inside the thirty by forty-five mile surface area of the caldera. Yellowstone is a very significant place, unique in the conterminous United States, for quite a different reason than its volcanic history and present state. It represents what has come to be called a hot spot, like the others in different places around the globe: Hawaii is one example where the long chain of former volcanoes that lies across the Pacific provides evidence of a hot spot’s existence. Over the past few decades it became clear to geologists that it is almost impossible to fix permanently, say the latitude and longitude, of a place on the surface of the earth. Everything seems to be in motion and the only thing we seem to be able to say is that such and such a place is fixed relative to some other. Plate tectonics revealed all this.
Hot spots are the one exception to this idea of constant motion. Over millions and millions of years hot spots preserve their locations with respect to the deeper part of the earth’s mantle. Continents and ocean plates move over them and leave them where they are. In the case of Hawaii, which is located over a hot spot, we know from the chain of undersea mountains that were formerly volcanic mountains that the Pacific Plate has been moving over it for the past seventy million years. It was a similar story with Yellowstone but the evidence is not nearly as clear as in Hawaii. Continental movements are far more complex than those that take place in the ocean because the crust is far thicker on land.
The theory behind hot spots is that they are the surface expressions of what are called mantle plumes. Heat is continually radiating out from the earth’s core into the mantle where, though solid because of the overlying weight of rock, some movement is possible. Over long periods of time masses of rock deep in the mantle become so hot that they rise close to the surface. Heat expands them, makes them lighter, and therefore they are able to move higher within the mantle.
One of these masses, it has been estimated, could have a volume of millions of cubic miles, enough to maintain a link with the deeper parts of the mantle and, at the same time, provide magma for a hot spot for millions and millions of years.
Searching for evidence that demonstrates Yellowstone’s hot spot history is difficult though many scholars work on it. About one hundred miles to the southwest of Yellowstone’s present position, lava that was dated as being six million years old was located. This fits what we know about the direction of movement of the North American Plate. It moves a little more than one inch very year toward the southwest. Farther to the southwest in Idaho, lava aged ten million years was located and, still later, more than four hundred miles to the southwest, lava flows thirteen million years old were identified.
These locations and times are in the right direction and very close to what would be expected. Beyond these places, tracings have been few. In Oregon, lava sixty million years old was found. The latest finding is indicative of the whole problem of tracing hot spot origins in continents. A geologist is convinced that the seventy-million-year-old lava found on the borders of Alaska was once over Yellowstone. Yellowstone is still very active. There is a high rate of heat coming from the various vents, much higher than from other parts of the United States.
In total, if it were converted into electricity, there would be enough to supply the electrical power needs of five million people. The question has to be asked: Why has this power supply not been developed? Would it not be good for the environment if it replaced an equivalent amount of coal? What are the down sides to transforming Yellowstone into an electrical generating station? Other countries like New Zealand and Iceland make good use of geothermal heat.
There is another side to the story about large quantities of heat. The heat source is shallow, no more than a few kilometers beneath the surface. About thirty years ago it was found that the floor of the caldera was rising. Then less than ten years later it was found to be falling. These movements were accompanied by swarms of earthquakes just as happened in other similar places. All these are indicators of seismic activity. The history of eruptions here, however, does not suggest imminent volcanic action but no one is taking that for granted. Studies and measurements go on continually.