By: Marlene Affeld
Buried almost five miles beneath the majestic scenery of Yellowstone National Park is a giant slumbering beast. The “Yellowstone Hotspot”, under incredible pressure, is a massive chamber full of molten rock that is the pulsating magma that powers Yellowstone’s incredible geysers, hot springs, mud pots and fumaroles.
Yellowstone Park, the oldest park in the United States National Park System, is unlike any other place on the planet. Breath-taking scenery, a diversity of wildlife and clear, blue skies attract visitors from around the world. However, the diverse hydrothermal features of the park are its greatest attraction and fascination. No other place in the world has as many active geysers, steam vents and hot springs as Yellowstone.
Although considered one of the most beautiful places on the planet, don’t let the magnificent beauty deceive. Yellowstone has a volatile and violent geologic history. Yellowstone National Park rests on top of numerous layers of welded ash deposits; a rock solid record of an explosive volcanic past. The Lava Creek explosion, the last in a series of full-scale eruptions, happened approximately 640,000 years ago, ejecting over 240 cubic miles of rock and dust into the heavens.
On August 17th, 1959, the most devastating earthquake in the recent history of the Yellowstone region hit with a magnitude 7.1 quake. The massive earthquake was centered near Hebgen Lake, Montana. The quake triggered landslides that killed 28 people and caused more than $11 million in property damage. Active geysers in Yellowstone National Park mysteriously changed eruption times, and new ones began to erupt. The park was awakened again on June 30, 1975 when a magnitude 6.4 tremor shook the land.
As reported by the Yellowstone Volcano Observatory, more than 500 earthquakes have been recorded by seismographs between December 27, 2008 and January 5th, 2009. This dramatic increase in activity is being called an "earthquake swarm" by scientists and geologists studying this recently active volcano. Earthquakes near an active volcano are fairly common. Normally seismographs stationed around Yellowstone Park will record between 1,000 to 3,000 earthquakes each year. However, this latest swarm of 500 earthquakes occurring within about a week, the most in the past twenty years, has the focused attention of the scientific community and emergency preparation teams.
The Yellowstone Volcano Observatory reports that the earthquake swarm is, "the most intense in this area for some years and is centered on the east side of the Yellowstone caldera."
To date, the strongest earthquake recorded was a magnitude 3.9 at the onset of the swarm. This strong quake and several subsequent others were felt by visitors, park rangers and residents of the park. Geologists have been able to calculate from seismic data that the earthquakes are occurring at depths ranging from 1 km to about 10 km beneath the surface; they have not yet been able to attribute any of the earthquakes to fault activity. Although, when magma rises, the subsequent heating and breaking of rock causes earthquake swarms similar to the ones recently recorded at Yellowstone National Park.
Thanks to its location in a forty mile wide ancient caldera, Yellowstone National Park has the distinction of being the location with the highest concentration of active geysers in the world. Mystical and haunting, many of the hydrothermal vents have sinter, geyserite or travertine deposits built up within and around them. As ghostly wisps of steam dance in the moonlight, these deposits seem to stand as sentinels to the Gates of Hades.
Geothermal activity, as seen in Yellowstone Park, requires two main ingredients; an abundance of water and intense heat. Yellowstone National Park has both in large quantities.
The various geyser basins are located in areas where rainwater and melting snow can readily percolate into the ground, become indirectly superheated by the underlying Yellowstone hotspot, then erupt thermal energy at the surface as fumaroles, geysers and hot springs. The majority of the geothermal areas are located in flat-bottomed valleys between the ancient lava flows and glacial moraines.
Several smaller geothermal areas can be found where fault lines reach the surface, in places running along the circular fracture zone around the caldera and at the base of steep slopes that drain and contain excess groundwater. Bob Christiansen is the USGS scientist who delineated the three Yellowstone calderas and told the world about the great eruptions that formed them. Bob reports that “he traced out the caldera boundaries through old fashioned field work... walking around with a hammer and hand lens and looking carefully at the rocks and their distributions”. Most of the key observations were made in the 1960s and 1970s.
Due to the high elevation of the Yellowstone Plateau, the average boiling temperature at Yellowstone's geyser basins is 199 °F (93 °C). When highly confined and close to the surface it can periodically release built-up pressure in eruptions of hot water and steam that can reach up to 390 feet (120 m) into the air. The Steamboat Geyser, is the world’s tallest geyser. The water erupting from Yellowstone's geysers is superheated above that boiling point to an average of 204 °F (95.5 °C) when it leaves the vent.
The water cools considerably while airborne and is no longer scalding hot by the time it strikes the ground, adjacent boardwalks, or even spectators. However, visitors to the park are strongly warned that because of the high temperatures of the water, it is extremely important that spectators remain on the boardwalks and designated trails. Several deaths have occurred in the park as a result of slips and falls into hot springs and mud holes.
The first prehistoric people that inhabited the area were well acquainted with the over 10,000 geothermal features and 200 - 250 geyser eruptions annually in and around the area that is now known as Yellowstone National Park. Ancient artifacts indicate that these first people used the pools for healing, bathing and cooking and that these life giving waters held spiritual significance.
During the 19th Century, Father Pierre-Jean De Smet reported that natives he questioned believed that the frightening geyser eruptions were "the result of combat between the infernal spirits.”
In 1806 the Lewis and Clark Expedition traveled to the north of the caldera. The local natives they came upon told of a valley of spirits where the earth trembled and roared like thunder; the frightening loud noises pronounced that the land was possessed and visitors unwelcome. The Indians seldom dared to venture near and held the area in awe and respect.
John Colter, an adventurer who had left the Lewis and Clark Expedition, is the first known white man to journey into the caldera and view the amazing geothermal features. He described what he saw as “hot spring brimstone.” In the 1850s, the famous trapper, Jim Bridger called it "the place where Hell bubbled up.”
Scientific teams have been closely monitoring the fluctuating up-and-down movement of the Yellowstone Plateau as an indication of changes in magma chamber pressure. Seismic data from mid-summer 2004 through mid-summer 2008, shows that the land surface within the giant caldera has moved upwards, as much as 8 inches at the White Lake GPS station. The caldera floor has risen 3 inches per year for the past three years. This rate-of-rise is considered more than three times greater than ever observed since such geologic measurements commenced in 1923.
Although earthquake swarms, similar to the current activity, have occurred in the past in Yellowstone without triggering steam explosions or volcanic activity, there remains potential for hydrothermal explosions. Earthquakes may continue or increase in magnitude. Scientists believe there is a lower potential for related volcanic activity.
Recent activity peaked in December 2008. As of January 8, 2009, the seismic activity has markedly decreased; scientists continue to monitor closely all changes in Yellowstone National Park. Yellowstone Volcano Observatory partners will continue to analyze the seismic and ground deformation data from the Yellowstone Lake swarm and are evaluating any changes to the thermal areas located near the epicenters.