Geysers, mud pots, and other relatively benign thermal features in Yellowstone Park hint at the fiery origins of the landscape. Heat from a magma chamber or “hotspot” beneath the surface drives the thermal features in the park, which boasts the largest geothermal field in the world. Visitors stroll among the steaming waters of Yellowstone with little apparent concern for their safety, and wildlife abounds in lush mountain valleys. The landscape differs greatly from the smoking cones people usually associate with volcanic activity. There is none of the towering menace that caused volcanoes to be termed the forges of Volcan, the Roman god of weapons, and later, gateways to hell.
According to geologists, the hotspot sits about three miles beneath the surface of Yellowstone National Park and impacts nearly every aspect of the land above it. When a hotspot erupts and ejects enough of its contents to cause the overhanging crust of the earth to collapse, the hotspot creates a depression in the surface known as a caldera. The Yellowstone caldera, located above the hotspot in Yellowstone Park, is the result of a past eruption of the hotspot.
The landscape, climate, geology, and ecology of the Yellowstone ecosystem have all been affected by periods of explosive violence and millennia of incremental change caused by the hotspot. Previous eruptions of the Yellowstone caldera have been some of the largest in global geological history, blowing apart mountain ranges and propelling their heated remains as far as Louisiana, Southern California, and Saskatchewan. The most recent major eruption occurred about 640,000 years ago, ejecting at least 250 cubic miles of magma, or 1,000 times the amount ejected by the 1980 eruption of Mount St. Helens.
Visitors and scientists enjoyed and studied America’s first national park for decades before anyone discovered the caldera. It was not until the late 1960s that geologist Bob Christiansen identified jutting cliffs around the park as the rim of a large caldera. Observers were slow to recognize the Yellowstone Caldera because of its immense size. Most calderas are approximately one to three miles in diameter, but the Yellowstone caldera is one of the largest calderas on earth, measuring approximately 30 miles by 50 miles.
The effects of the last major eruption in Yellowstone and about 80 other minor eruptions are apparent in the landscape today. The “Lava Creek” eruption of 640,000 years ago is believed to have swallowed a portion of the Washburn Range extending from Mt. Washburn to Mt. Sheridan. From either of these peaks one can look 37 miles across the expanse of the caldera to the other peak. The summits are within 65 feet of each other in elevation and both ranges are dated by geologists to be 50 million years old, while the intervening landscape is composed of younger lava flows which emerged during and after the massive Lava Creek eruption. The sloping land above the caldera composed of “young” lava flows is commonly referred to as the Yellowstone Plateau.
Much of the Yellowstone Plateau is composed of rhyolite, a low-nutrient lava that erodes into fine soil incapable of retaining water or supporting extensive vegetation. Most of the rhyolite soil is covered by uninterrupted forests of lodgepole pine, which are well adapted to the barren rhyolitic conditions. But some areas in the park, such as the Hayden and Pelican valleys, support richer vegetation despite their rhyolitic foundations. Ice-age glacial lakes deposited nutrient-rich silt in these locations, allowing grasslands, sagebrush steppes, and meadows to form habitat for deer, elk, bears, and bison in valleys that were once covered in water.
Many landscape features of the park are recognizable rhyolite formations. Park geologist Hank Heasler compares viscous rhyolite flows seeping out of the earth to toothpaste being squeezed out of a tube. The flows form dome-shaped hills such as Elephant Back Hill on the north rim of Yellowstone Lake. Rhyolite also forms obsidian, and the Obsidian Cliff between Mammoth and Norris Junction provided a nearly limitless supply of the shiny black stone to local Native Americans. Other areas of the park adjacent to the plateau rest on basalt and andesite, which are richer in nutrients. These areas can be recognized by grassland expanses and forests of Engleman spruce, subalpine fir, whitebark pine, and Douglas fir.
Numerous minor eruptions and rhyolite flows over the last 640,000 years have contributed to the rise of the Yellowstone Plateau to an elevation of 8,000 feet.
Streams flowing outward from the plateau cut steep tracks through the rock and form colorful canyons. The Yellowstone River cuts 18 miles into the plateau to form the Grand Canyon of the Yellowstone. Numerous waterfalls throughout the park flow over transition points in the rock, often marking a change in density of the underlying rhyolite.
Shifting gases and magma in the hotspot contribute to an extraordinary amount of seismic activity on the surface. There are more than 1,000 earthquakes on average every year in the park, most of which are very minor, but there have been dozens of moderate to large quakes registering from 5.5 to 7.5 on the Richter scale since 1900. The 1959 Hebgen Lake earthquake in the Madison River area near West Yellowstone caused a landslide that killed 33 people and nearly dammed the Madison River, creating the six-mile-long and 190-foot-deep Quake Lake.
One of the most dramatic effects of the hotspot is uplift. Geologists estimate between 1,500 and 3,000 feet of uplift since the Lava Creek eruption. The plateau experiences periods of uplift and subsidence, which are sometimes dramatic. Between 1923 and 1985 scientists measured over two feet of uplift followed by a period of subsidence. Scientists studying indications of past periods of uplift and subsidence have discovered movements of more than 10 feet up and down.
Monitoring the Volcano
The hotspot exerts a constant influence on the activity and appearance of the land above it, and geologists monitor the known effects to assess the risk of a future eruption. Park geologist Hank Heasler expresses confidence in the abilities of Yellowstone’s monitoring system to predict an eruption, but adds, “As a scientist I don’t deal in 100 percents, nature always teaches us humility.”
Heasler compares scientific assessment of geothermal activity in the park to a doctor’s assessment of a patient‘s health. A doctor measures vital signs such as pulse, blood pressure, and breathing rate to form an idea of general health. In the same way, geologists and volcanologists measure surface level “vital signs” of the hotspot to gauge the status of the geothermal system. They monitor three primary vital signs-earthquakes, ground deformation, and hot water and gases coming out of the ground.
The “vital signs” are ongoing phenomena in the park, and they maintain a consistent level of activity. The park “breathes,” says Heasler, releasing heat and tension by means of earthquakes and the other phenomena. In the preface to an eruption all three of the vital signs would show dramatic increase in frequency in a concentrated area. Based on the size of the area where the activity would occur and other factors such as the kinds of gases being released, experts could predict the size of an eruption. Large eruptions give more advance warning than small eruptions, but generally there are weeks to months of prefatory activity before an eruption. A very small eruption might not give any warning, but Heasler says an eruption of this size would be the sort of non-threatening event visitors would photograph by the roadside.
The hazard level of volcanoes is assessed from the standpoint of people on the ground and aircraft in the sky. Currently the volcano alert level is normal and the aviation color code is green. These indicators have never reflected any danger in Yellowstone, but in active volcanic areas like the Hawaiian Islands there is more variance in hazard levels. Real-time statistics of earthquake activity, ground deformation, and gas and geyser activity are available on the website of the Yellowstone Volcano Observatory (http://volcanoes.usgs.gov/yvo).
The Year Without a Summer
An eruption the size of the major Yellowstone eruptions has not occurred during recorded human history, but there have been large eruptions with devastating effects within the last two hundred years. From such an eruption we might paint a picture of what the effects of a major Yellowstone eruption would be.
The largest recorded eruption is the explosion of Mt. Tambora on the island of Sambawa in Indonesia in 1815. Scientific methods at the time were still crude, but the volcano is believed to have propelled about 12 cubic miles of volcanic material as high as 25 miles into the atmosphere. The explosion killed as many as 70,000 people including immediate casualties and victims of starvation and disease following the eruption. The eruption peaked April 10 of 1815, and explosions were heard up to 1,550 miles west of Tambora, in Sumatra. Shock waves shook houses 500 miles away in eastern Java. Observers 25 miles away described three tongues of flame that arched out of the volcano, merging into one column high above the mountain. Soon after the sighting, clouds of volcanic ash and fist-sized rocks began to descend upon the town. Hurricane-force winds swept away nearby villages, and 16-foot-tall tsunamis wreaked havoc on boats and islands. The ash created an atmosphere of almost total darkness within 186 miles of the eruption for three days, and after the eruption Sumbawa was covered in more than three feet of ash.
The effects of the Tambora eruption were global. Worldwide temperatures dropped and patterns of rainfall changed. Crops failed in many countries the following year, bringing famine to some. The year 1816 became known as “the year without a summer” because of snows and killing frosts in the northeast United States in June, July, and August. Some Americans called it “eighteen hundred and froze to death.”
Even the smallest of the three major Yellowstone eruptions ejected about 70 cubic miles of magma into the atmosphere. Another major eruption of Yellowstone could cover much of the continent in darkness, and might cover much of the land west of the Mississippi with molten volcanic material within hours of an eruption. The global climate would undergo severe changes and the human race would scramble to adapt, likely suffering millions of casualties.
Geologists say such an eruption is unlikely to occur without prior warning, and they say Yellowstone National Park is safe to visit. Nonetheless, other volcanic eruptions, including Mount St. Helens and a recent eruption in Alaska have occurred with very little warning. Another consideration is the effect that other geological events can have upon Yellowstone. A recent earthquake in Alaska altered the behavior of geysers and thermal features in the park, which came as a surprise to geologists. Even if scientists had sufficient warning, an eruption on the scale of Yellowstone’s largest would necessitate an evacuation of at least the Western United States and would be a logistical nightmare.
Hotspot and the Path of a Continent
Such are the possibilities of the Yellowstone hotspot. However, people have been living alongside the caldera for thousands of years and continue to enjoy the benefits of the volcanic landscape. While research continues, we now understand more about the caldera than ever before. In order to understand the action of the Yellowstone hotspot, one must slow to geological time and consider the motion of the entire North American continent.
The prevailing theory among geologists claims the hotspot pushed from the earth’s core through soft layers of mantle, bubbling up as a molten “plume” 250 miles in diameter. The plume of magma lurks beneath the solid upper mantle, or lithosphere, three miles beneath the earth’s surface. From this depth the hotspot affects surface volcanism, faulting, and uplift.
The first indication of volcanism caused by the hotspot is a 16 million-year-old caldera formation on the Nevada-Oregon border. There the plume reached the lithosphere and erupted several times as the North American Continental Plate moved across it in a southwest direction. The plate moves about one inch southwest every year giving the Yellowstone hotspot the appearance of migrating northeast across the continent. Evidence of widespread volcanic activity attributed to the hotspot lies in its inferred track in Nevada, Oregon, and Idaho.
About ten million years ago the hotspot is believed to have contributed to the Twin Falls and Picabo volcanic fields in Idaho. At this time the hotspot track began to trace a linear course northeast. This course left in its wake the eastern Snake River Plain. The hotspot arrived in the greater Yellowstone area about two million years ago. Three major eruptions have occurred in the greater Yellowstone area over the last 2.1 million years, and they have produced an unfathomable total of more than 1,500 cubic miles of volcanic material which blanketed the western half of what is now the United States.
The first eruption, “Huckleberry Ridge,” is one of the largest known eruptions ever to have occurred on earth. The eruption took place 2.1 million years ago, ejecting more than 620 cubic miles of fresh magma-the equivalent of six mountains the size of California’s 14,000-foot Mount Shasta. Ash flows from the explosion traveled thousands of miles so quickly that the molten rock barely cooled in transit, and fell glowing to the earth as far away as southern California and northern Louisiana.
More than a million years of minor volcanic activity followed the first eruption, during which rhyolite flows seeped out of the earth and filled the depression left by the first caldera. The second eruption was the smallest of the three and occurred near the head of the Snake River Plain about 1.3 million years go, releasing about 70 cubic miles of magma. After the eruption, rhyolite again seeped out of the hotspot, ramping up to the Yellowstone Plateau.
The third eruption occurred 640,000 years ago beneath what is now the Yellowstone Plateau, ejecting 250 square miles of magma into the atmosphere. After the eruption the roof of the chamber collapsed, forming the present Yellowstone Caldera. Since the last major eruption another 250 square miles of magma have seeped out in extensive rhyolite flows, the most recent of which erupted about 70,000 years ago. These flows have contributed to the rise of the Yellowstone Plateau.
The hotspot beneath the Yellowstone Plateau is also believed to be responsible for extensive faulting in the area, contributing to the creation of the area known as the Greater Yellowstone Ecosystem, or GYE. The GYE includes the mountain ranges, foothills, and valleys surrounding the plateau. Faulting in the GYE created characteristic landscape features of rugged forested ranges alongside grassy valleys. The Teton Range and Jackson Hole as well as the Madison Range and Madison Valley are good examples of GYE faulting. Substantial heat and the mingling of layers of the earth’s crust combined with the motion of the continental plate causes stress along fault lines. This stress eventually causes earthquakes like the one in 1959 that created Quake Lake.
The hotspot has also caused extensive uplift in the GYE and is responsible for many current climatic conditions in the ecosystem. Uplift pushed the plateau approximately 3,000 feet above the Snake River Plain. The high altitudes created by uplift define the environment of the ecosystem by affecting the weather patterns moving across the continent from the Pacific Ocean. High altitudes create snows that remain throughout much of the year. The high altitudes surrounding Yellowstone Plateau allowed for formation of glaciers that had enormous effects on the landscape.
Headwaters of 25 major drainages originate in the GYE, and the Continental Divide passes through it. Waters flowing from atop the Yellowstone Plateau create deep canyons. If uplift had not occurred in the GYE as it had, the Beartooth Range might look more like the Black Hills, and the Absaroka Range might look like the high prairies covering much of Montana and Wyoming.
The hotspot formation of Yellowstone is relatively rare. Volcanoes are more common along continental faults where magma finds an easier route to the surface, as in the case of the famed “Ring of Fire” around the rim of the Pacific Ocean. When hotspots do occur they usually form under the ocean.
The Yellowstone hotspot began to carve out a geological history 16 million years ago, far before mankind had any interaction with it. The span of mankind’s history of interaction with the caldera, spanning approximately 11,000 years, is a blink in geological time.
An Uncertain Future
The Greek philosopher Heracleitus said, “The world…was created by neither gods nor men, but was, is, and will be eternally living fire, regularly becoming ignited and regularly becoming extinguished.” Life on earth is thought by some scientists to have begun with volcanoes, where the elements of water churned and then melded during eruptions. The same forces could end life on earth, but there is currently no indication of any imminent apocalyptic eruption.
—Wes Venteicher
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