The geological history of Yellowstone National Park

Yellowstone National Park is known throughout the world for its volcanic and thermal features. Over the past 2 million years, Yellowstone has been the site of three major volcanic eruptions, reflecting its position over a "hot spot" in the earth's crust and upper mantle.

Yellowstone and Its Environs

The first recorded observations of the Yellowstone area were made by fur trappers in the early nineteenth century, and later government-sponsored surveys confirmed the geological wonders of the area. As a result, much of what is now northwestern Wyoming and adjacent portions of Idaho and Montana were set aside in 1872 as the Yellowstone National Park, the United States' first national park. The largest thermal area in the world, Yellowstone is best known for its outstanding assemblage of hot springs, geysers, and mud pots.

Yellowstone National Park is an 899,152-hectare, wooded mountain area in the northern Rocky Mountains. The rugged and geologically youthful terrain is mostly of volcanic and glacial origin, with many waterfalls and lakes. Its eastern border is within the Absaroka Range, and the Teton Range and Jackson Hole lie to the south. The northeastern sector of the park is cut by the Grand Canyon of the Yellowstone River. The park occupies the central part of the Yellowstone Plateau, a 6,500-square-kilometer (2,510-square-miles), high plateau that is three times larger than the park and averages 2,500 meters (8,202 feet) in elevation. Immediately west of Yellowstone, Island Park, Idaho, is a shallow basin that is partly filled with basalt lava flows and sediments. To the southwest, the Yellowstone Plateau drops gradually to the eastern Snake River plain of Idaho, a downwarped region (1,000 to 1,500 meters, or 3,281 to 4,921 feet, in elevation) of low topographic relief that is largely covered by basalt lava flows.

A strong geological relationship exists between Yellowstone and the Snake River plain, an 800-kilometer-long (497-miles-long), low-lying volcanic belt that crosses southern Idaho. Near the Idaho-Oregon border, the oldest volcanism on the western Snake River plain began about 17 million years ago. Since that time, volcanic activity has slowly progressed northeastward toward Yellowstone at an average rate of about 4 centimeters (1 1/2 inches) per year. The youngest basalt eruptions on the eastern Snake River plain occurred about 2,100 years ago, and the youngest rhyolite eruptions at Yellowstone occurred around 50,000 years ago.

The most popular explanation for the progression of volcanism toward Yellowstone is that a stationary plume of upwelling, perhaps partially molten rock, is rooted in the earth's upper mantle beneath the northwestern United States. Although the plume seems to be fixed in place, the tectonic plate of North America has been moving westward for at least 17 million years. Like a candle flame held beneath a moving piece of paper, the volcanoes form an eastward-bound trail, first with explosive rhyolite eruptions and then with basalt lava outpourings. Thus, the old rhyolites of the Snake River plain in southern Idaho are covered almost completely by basalt lava flows, but Yellowstone is the youngest rhyolite volcanic center, and very little basalt has erupted there. The mantle plume, or hot spot, is now considered to lie beneath Yellowstone and is the ultimate source of magma and geothermal heat under that area.

Volcanic Activity

For the past 2 million years, Yellowstone has been the site of huge but infrequent eruptions of rhyolite lava and pumice; the youngest eruptions of rhyolite lava occurred about 50,000 years ago. The products of those eruptions comprise the Yellowstone volcanic field, which formed during three cycles of volcanism. Each cycle began and ended with the eruption of lava flows of rhyolite obsidian, a dense, black, silica-rich volcanic glass. The lava flows issued mostly from curved fractures of the Yellowstone caldera and now cover much of the caldera floor. All three volcanic cycles culminated with explosive eruptions of voluminous rhyolite ash and pumice. The ash and pumice deposits from the three climactic eruptions of the Yellowstone-Island Park area are known as the 2.1-million-year-old Huckleberry Ridge tuff, with an estimated volume of 2,500 cubic kilometers (600 cubic miles); the 1.3-million-year-old Mesa Falls tuff, with an estimated volume of 280 cubic kilometers; and the 0.6-million-year-old Lava Creek tuff, with an estimated volume of 1,000 cubic kilometers (240 cubic miles). For comparison, the total volume of material emitted from Mount St. Helens on May 18, 1980, was only 1 to 2 cubic kilometers (1/4 to 1/2 cubic miles).

Frothy rhyolite pumice and ash were explosively discharged from curved fractures around the rim of the Yellowstone caldera. Plinian eruptions ejected large quantities of rhyolite to heights of tens of kilometers into the atmosphere. Some of the ejecta then cascaded back to the earth like a water fountain or "boiled over" as hot, ground-hugging flows of ash and pumice, forming three major ash-flow deposits that fill the Yellowstone caldera and blanket the surrounding terrain. The ash flows were sufficiently hot when they came to rest (about 550 degrees Celsius or more) that the particles of pumice and ash were compacted into a dense mass, forming welded ash-flow tuffs. Each of the catastrophic eruptions probably lasted only hours or days. The rapid eruption of voluminous rhyolite pumice from shallow (perhaps 5 to 10 kilometers, or 3 to 6 miles, deep) magma chambers led to the collapse of the roof rocks overlying the magma chambers, forming large rhyolite calderas ringed by 50-kilometer-wide (31-mile-wide) sheets of welded ash-flow tuff. Finer particles of ash and pumice were ejected to great heights and carried away by high-altitude winds. The finest particles probably took months or years to sift out of the atmosphere. Ash-fall deposits from the three major Yellowstone eruptions are found as far away as Louisiana, California, and southern Canada, where they are important marker beds for the dating of Ice Age fossils and glacial deposits.

The Yellowstone caldera—a broad, oval-shaped depression about 40 by 60 kilometers (25 by 37 miles) in size—was formed during the climax of the last cycle of volcanism 600,000 years ago, when the Lava Creek tuff was emplaced. Since that time, voluminous lava flows of rhyolite obsidian—the Plateau Rhyolites of Yellowstone—have obscured the caldera rim and now largely cover the caldera floor. The most voluminous Plateau Rhyolite lava flows compose the Madison and Pitchstone Plateaus in the western and southwestern sectors of the park.

During the Ice Age, large masses of glacial ice formed in northern Yellowstone National Park. Two main periods of ice formation have been identified. An older ice cap formed during the Bull Lake glaciation, about 150,000 years ago. Most of the Bull Lake glaciers flowed to the west, and the West Yellowstone Basin to the west of Yellowstone Park is filled with Bull Lake outwash gravels and other deposits. A later ice cap formed during the Pinedale glaciation, between about 30,000 and 15,000 years ago. Most of the Pinedale ice flowed to the north because rhyolite lava flows of the Madison Plateau blocked westward passage of the ice. Both ice caps were probably 1 kilometer or more thick, and much of Yellowstone Park is covered by a thin mantle of glacial deposits. Glacial deposits play an important role in the thermal activity of Yellowstone, since permeable sands and gravels become capped with mineral deposits but remain permeable to hot fluids at depth. The formation of impermeable caprock over permeable strata is important in the development of the shallow plumbing systems beneath hot springs and geysers.

Seismic Activity

Yellowstone is the most seismically active region of North America. In 1959, the Hebgen Lake earthquake, centered about 10 kilometers (6 miles) west of Yellowstone and estimated at between 7.3 and 7.5 on the Richter scale, was the largest tremor ever recorded in the Rockies; it caused changes in the behavior of hundreds of geysers and hot springs. During the fifteen-year period between 1973 and 1988, about fifteen thousand earthquakes greater than Richter magnitude 2 were recorded. Another surge in seismic activity beginning in 2004 brought considerable media attention, though geologists concluded the likelihood of a major eruption remained extremely small. Improved technology has allowed detection of even the smallest earthquakes, and over 3,200 were recorded in 2010. In 2014, a magnitude 4.8 earthquake was reported in the park, the largest since 1980.

The frequent earthquakes of Yellowstone are probably important in maintaining the thermal activity of the region, since permeable fault zones that channel hot fluids to the surface would quickly be sealed with mineral matter if not for regular disturbances from earthquakes. Beneath the Yellowstone caldera, the velocities of earthquake waves traveling through the earth's crust are measurably decreased, suggesting the presence of a plumelike body of partly molten rock extending downward from a depth of about 8 kilometers (5 miles). The amount of heat flowing from the ground beneath Yellowstone is estimated as thirty to forty times the heat flow elsewhere in North America and accounts for about 5 percent of the total heat flow from all North America.

Resurgent Domes

Since the first precise topographic surveys were done during the 1920s, resurveying has shown that the central caldera floor is slowly warping up at an average rate of about 14 millimeters (1/2 inch) per year. Two broad areas of uplift, known as resurgent domes, have been identified in the southwestern and northeastern Yellowstone caldera. Both resurgent domes are about 15 kilometers (9 miles) across and 200 meters (656 feet) high. Resurgence of the caldera floor, together with the high heat flow and seismicity, suggests the presence of rhyolite magma in the upper crust, either left over from the last cycle of volcanism or in the form of a new magma batch that may appear at some time in the future as a fourth volcanic cycle.

The hot rock, or magma, beneath Yellowstone is the source of heat for nearly ten thousand hot springs and several hundred active geysers. Rain and snowmelt soak into the ground and percolate through fractured volcanic rocks to depths of several kilometers, where the fluids become heated. The hot water then buoyantly rises to the surface along north-trending faults of the northern Rocky Mountains, the ringlike fractures of the Yellowstone caldera, and the fractured rocks around the edges of the two resurgent domes. Hence, the thermal features of Yellowstone are not randomly distributed throughout the park but tend to occur where fluid pathways are available, especially along the rim of the Yellowstone caldera and near the two resurgent domes.

Hydrothermal Activity

The Norris Geyser Basin is on the northern edge of the Yellowstone caldera, where circular fractures of the caldera rim are intersected by a major, north-trending fault that extends northward from the caldera. The intersection of two major fault zones enhances the upward migration of deep, hot fluids, and Norris is considered to be the hottest and most active geyser basin in Yellowstone. The basin contains a great variety of thermal features, including geysers, clear boiling springs, mud pots, and fumaroles. Porcelain Basin is the world's largest sinter-covered area (sinter is a variety of hydrated silica that forms around certain hot springs). The geothermal waters at Norris are neutral-chloride to slightly acid-sulfate in composition. The content of dissolved sulfates may be a result of the hot waters traveling through underground sedimentary rock formations that are intersected by the faults at depth.

Old Faithful is a famous geyser in the Upper Geyser Basin, one of several major thermal areas along the Firehole River in the western part of Yellowstone Park. The Upper Geyser Basin is situated on the edge of a resurgent dome within the Yellowstone caldera, where fractured volcanic rocks allow the upward passage of hot fluids. The basin occurs in a topographically low-lying area between three lobes of rhyolite lava, and the valley is underlain by permeable sand and gravel deposits from glaciers and rivers; those deposits allow the hot fluids to disperse over a wide area. As a result of such ideal conditions, the Upper Geyser Basin contains the largest concentration of geysers in the world.

The Mud Volcano area is to the north of Yellowstone Lake, on the southwestern edge of a resurgent dome within the Yellowstone caldera. The water table is boiling underground, and fumaroles are a major feature of this vapor-dominated thermal area. The muddy pools and mud volcanoes are composed of clay, formed by the chemical alteration of near-surface rocks and glacial deposits by acid vapors. The source of the surface water is rainfall and steam condensation. Mammoth Hot Springs is one of the few major thermal areas that occur outside the Yellowstone caldera. Hot, underground water flows northward along the same fault system as Norris, cooling to subboiling temperatures and becoming alkaline in composition during its passage through sedimentary rocks. Calcium carbonate is dissolved from the sedimentary rocks and is later deposited on the surface as mounds and rimmed terraces of travertine, a variety of hot-spring limestone. Travertine grows much more rapidly than sinter, and substantial changes in the configuration of Mammoth Hot Springs are observed from year to year.

Principal Terms

ash-flow tuff: a sheetlike pyroclastic deposit that is laid down as a hot mixture of gas, crystals, pumice, and volcanic ash

caldera: a large, flat-floored volcanic depression that is formed on top of a large, shallow magma chamber during the eruption or withdrawal of magma

hot spot or mantle plume: a zone of hot, upwelling rock that is rooted in the earth's upper mantle; as plates of the earth's crust and lithosphere glide over a mantle plume, a trail of hot spot volcanoes is formed and the earth's surface bulges upward

obsidian: a dense variety of silica-rich volcanic glass; the same material as pumice but does not contain gas bubbles

Plinian eruption: a violent, explosive type of volcanic eruption named either for Pliny the Elder, a Roman naturalist who died while observing the eruption of Mount Vesuvius in 79 CE, or for Pliny the Younger, his nephew, who chronicled the eruption

pumice: frothy, silica-rich volcanic glass that is commonly produced during explosive eruptions of rhyolite magma

resurgent dome: a broad, oval area of uplift within a volcanic caldera that is marked by the upwarping and fracturing of caldera-filling deposits

rhyolite: a viscous, gas-rich type of magma that contains greater than about 70 percent silica; rhyolite erupts nonexplosively as thick, slow-moving lava flows and explosively as widespread sheets of frothy pumice

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