The Yellowstone Caldera, also called the Yellowstone Plateau Volcanic Field, is a large volcanic area covering parts of Wyoming, Idaho, and Montana. It is powered by the Yellowstone hotspot and mostly lies within Yellowstone National Park. The area includes four overlapping calderas, lava domes, resurgent domes, crater lakes, and many layers of lava and rock formed from basaltic and rhyolitic materials, covering about 17,000 square kilometers (6,600 square miles).
Volcanic activity in the region began 2.15 million years ago and occurred in three major cycles. Each cycle included a massive volcanic eruption, pyroclastic flows, widespread ash falling across continents, and caldera collapse, with smaller lava flows and rock layers before and after. The first and largest cycle was the Huckleberry Ridge Tuff eruption about 2.08 million years ago, forming the Island Park Caldera. The most recent supereruption, about 630,000 years ago, created the Lava Creek Tuff and the current Yellowstone Caldera. After these eruptions, smaller volcanic events included basalt flows, rhyolite domes and flows, and minor explosive deposits. The last magmatic eruption happened about 70,000 years ago. Large hydrothermal explosions also occurred during the Holocene era.
Between 2004 and 2009, the area experienced noticeable ground rising due to new magma moving underground. A 2005 television show titled Supervolcano, made by the BBC and Discovery Channel, raised public interest in the possibility of future eruptions. The Yellowstone Volcano Observatory monitors volcanic activity and does not believe a major eruption is likely soon. Studies of the magma reservoir show a large amount of partially melted rock exists beneath Yellowstone, but it is not currently capable of erupting.
Geologic setting
The Yellowstone Plateau Volcanic Field is located at the eastern end of the Snake River Plain and interrupts the continuous structure of the Laramide orogenic belt, which formed during the Late Cretaceous. Between about 53 and 43 million years ago, this area had large volcanic eruptions of andesite, covering more than 29,000 kilometers (7,000 miles) in total volume. These eruptions created the Absaroka Volcanic Supergroup. Prominent peaks like Mount Washburn and Eagle Peak are the eroded remains of earlier stratovolcanoes. Before the Yellowstone Plateau formed, the Teton Range and Madison Range were likely connected structurally, as were the Red Mountains and Gallatin Range.
Current volcanic activity in Yellowstone is not a continuation of Laramide tectonics or the Absaroka volcanic province. Instead, it is the most recent part of a line of rhyolitic volcanic areas along the Snake River Plain, extending at least 16 million years to the McDermitt caldera complex. Large eruptions of rhyolitic tuff occurred at older volcanic centers. One example is the 12.1 million-year-old Ibex Hollow Tuff from the Bruneau-Jarbidge volcanic field in southern Idaho, which covered herds of Nebraska mammals in volcanic ash. Older volcanic formations linked to this hotspot track include the 56 million-year-old Siletzia oceanic plateau and the 70 million-year-old Carmacks Group.
Scientists debate the cause of the northeastward movement of volcanic activity. Some theories suggest processes in the upper mantle, such as mantle material pushed upward by the leading edge of the subducting Farallon plate, slab rollback, a spreading rift, or mantle convection caused by sudden changes in thermal layer thickness at the continent-ocean boundary. Another idea proposes that a piece of the Farallon slab broke through the 660 km (410 mi) deep boundary, pushing up the lower mantle and causing melting in the water-rich transition zone beneath the western United States. A third theory suggests a long-lasting mantle plume rooted at the core-mantle boundary, which erupted the Columbia River Basalt Group and now feeds the Yellowstone hotspot. Seismic tomography has identified a 350 km (220 mi) wide, cylindrical thermal anomaly extending from the deepest mantle to just below Yellowstone, supporting the mantle plume theory. In this model, the North American Plate moves southwest at about 2.2 cm (0.87 in) per year over a relatively stationary plume, creating the observed pattern of volcanic activity over time.
Volcanic landforms
The northern and eastern edges of the first-cycle caldera are not fully known because they are covered by other rock layers. However, it may have extended into the third-cycle caldera, possibly east of the Central Plateau. The Huckleberry Ridge Tuff in the Red Mountains is believed to be thick layers of material inside the Island Park Caldera. Big Bend Ridge, located at the southwestern edge of the volcanic plateau, is thought to be part of the caldera's wall. A fault along the Snake River and Glade Creek, which forms the northern end of the Teton Range and Huckleberry Ridge, is also considered part of the Island Park ring-fault. It is unknown if any parts of the first-cycle caldera were resurgent.
The second-cycle caldera is known as the Henry's Fork Caldera. Thurmon Ridge, at the northwestern edge of the volcanic plateau, is believed to be its northern caldera wall. The fault along Big Bend Ridge was reactivated and collapsed again during the formation of the second-cycle caldera. Although basalt flows cover its southern and eastern boundaries, a gravity anomaly suggests a circular caldera about 19 km (12 mi) in diameter, with its southern edge located in the middle of the Island Park basin.
Robert L. Christiansen suggested that the Yellowstone Caldera is a compound caldera made up of two overlapping ring-fault zones, centered on the resurgent Mallard Lake dome and Sour Creek dome. The southern boundary is unclear due to post-caldera rhyolite layers covering it. He proposed that the south flank of Purple Mountain and the Washburn Range, along with the west flank of the Absaroka Range, mark the caldera boundary on the north and east sides. Lake Butte, the Flat Mountain Arm of Yellowstone Lake, the north foothill of the Red Mountains, and Lewis Falls are thought to mark the southeast and south sides of the Yellowstone caldera rim. However, the Sour Creek ring-fault zone and the location of the eastern caldera boundary have been questioned. Recent field studies suggest the eastern ring-fault lies west of the Sour Creek dome, closely following the Yellowstone River.
The westernmost part of Yellowstone Lake is the elliptical West Thumb Basin, measuring 6 km × 8 km (3.7 mi × 5.0 mi). This area includes one of the lake's deepest spots and is interpreted as a fourth caldera formed by a third-cycle explosive eruption after the main caldera activity.
Eruption history
A total of 6,500 kilometers (1,600 miles) of rhyolite and 250 kilometers (60 miles) of basalt were deposited over three volcanic cycles between about 2.15 million and 0.07 million years ago. Each cycle lasted roughly 750,000 years. The events in each cycle followed a similar pattern: a sudden and large-scale eruption of rhyolitic ash-flow sheets and caldera collapse, with eruptions of rhyolitic lavas and tuffs and basaltic eruptions near the caldera margin before and after. Ash-flow sheets make up more than half of the total volcanic material in the Yellowstone Plateau.
The first cycle occurred from about 2.15 million to 1.95 million years ago, lasting approximately 200,000 years. The only known rhyolitic unit before the caldera collapse is the Rhyolite of Snake River Butte, located near Ashton and dated to about 2.1398 million years ago, roughly 60–70,000 years before the caldera-forming Huckleberry Ridge Tuff. Its vent was near the eventual first-cycle caldera margin close to the Big Bend Bridge. Additional rhyolite flows may have erupted along the incipient ring-fault, but the pre-collapse rhyolite history likely lasted no more than about 70,000 years. Another pre-collapse unit is the 60 to 70 meter (200 to 230 foot) thick Junction Butte Basalt on the northeastern margin of the plateau, dated to about 2.16 million years ago. The Overhanging Cliff basalt is a flow from this unit.
The first-cycle caldera-forming event was the eruption of the Huckleberry Ridge Tuff at about 2.0773 million years ago, during a period of transitional magnetic polarity. Its thickness exceeded 1 kilometer (0.62 miles) in the Red Mountains area. The initial Plinian phase deposited up to 2.5 meters (8.2 feet) of fallout ash at Mount Everts before transitioning to ash-flow tuff. Early Plinian activity was intermittent, coming from multiple vents, likely lasted a few weeks, and evacuated about 50 kilometers (12 miles) of magma from four magma bodies, triggering caldera collapse at the start of the transition to ash-flow. The ash-flow tuff is a composite sheet made of three intermittent layers, with a total magma volume of about 2,450 kilometers (590 miles). Layer A likely erupted from the central area of the plateau and tapped nine magma bodies. After a pause of a few weeks or more, the most voluminous Layer B erupted from north of Big Bend Ridge. After another long break of years to decades, part of the Layer A magmatic system was reactivated to feed Layer C. Layer C, the least voluminous, may have originated near the Red Mountains, where it is about 430 meters (1,410 feet) thick. Some outcrops of Layer A and Layer C have been mistakenly identified as Layer B, complicating volume estimates for individual ash-flow units. Glen A. Izett estimated that an additional 2,000 kilometers (480 miles) of ash was dispersed as fallout across North America. Tephra fallout from this event is known as the Huckleberry Ridge ash bed (formerly "Pearlette type B"). Its area covered more than 3,400,000 kilometers (1,300,000 square miles). It is widely distributed and has been found in the Pacific Ocean at Deep Sea Drilling Project Site 36, about 1,600 kilometers (990 miles) from Island Park Caldera, as well as in the Humboldt and Ventura basins of coastal California, near Afton in Iowa, Benson in Arizona, and Campo Grande Mountain in Texas.
One lava flow near the Sheridan Reservoir and two flows at the north end of Big Bend Ridge are post-collapse rhyolites from the first-cycle volcanism. The Sheridan Reservoir Rhyolite, dated to about 2.07 million years ago, if erupted from the Island Park ring-fracture, required a flow distance of at least 20 kilometers (12 miles). Its volume is estimated to exceed 10 kilometers (2.4 miles). The other two flows, the Blue Creek flow and the overlying Headquarters flow, have a combined volume of 10–20 kilometers (2.4–4.8 miles) and erupted at about 1.9811 million and 1.9476 million years ago, respectively.
After about 500,000 years of inactivity, a new magmatic system formed north of Big Bend Ridge. It erupted the Bishop Mountain Flow at about 1.4578 million years ago and the Tuff of Lyle Spring at about 1.4502 million years ago. The Bishop Mountain Flow is a rhyolite with an exposed volume of about 23 kilometers (5.5 miles) and reaches a thickness of 375 meters (1,230 feet) along the inner caldera wall. The Tuff of Lyle Spring is a 1 kilometer (0.24 mile) thick composite ash-flow sheet made of two cooling units. Both eruptions appear to have originated from an isolated, highly evolved local magma chamber distinct from other sources. Tiffany A. Rivera et al. proposed that these eruptions were not part of the second cycle.
The pre-collapse rhyolite of the second cycle includes the Flat Mountain Rhyolite, dated to about 0.929 million years ago, and the Harlequin Lake flow, dated to about 0.8300 million years ago. The Lewis Canyon Rhyolite group contains lavas dated to about 0.8263 million years ago.
Hazards
Volcanic and tectonic movements in the region cause between 1,000 and 2,000 measurable earthquakes each year. Most of these are small, measuring magnitude 3 or less. Sometimes, many earthquakes happen in a short time, which is called an earthquake swarm. In 1985, more than 3,000 earthquakes were recorded over several months. Between 1983 and 2008, more than 70 smaller swarms were detected. Scientists believe these swarms are likely caused by movements along old faults, not by magma or hot water.
In December 2008, continuing into January 2009, more than 500 earthquakes were recorded under the northwest part of Yellowstone Lake over seven days. The largest of these was magnitude 3.9. Another swarm began in January 2010, after the Haiti earthquake and before the Chile earthquake. This swarm included 1,620 small earthquakes between January 17 and February 1, 2010. It was the second-largest ever recorded in the Yellowstone Caldera. The largest earthquake during this time was magnitude 3.8, which occurred on January 21, 2010. By February 21, the swarm had returned to normal levels. On March 30, 2014, at 6:34 AM MST, a magnitude 4.8 earthquake struck Yellowstone, the largest recorded there since February 1980. In February 2018, more than 300 earthquakes occurred, with the largest being magnitude 2.9.
The Lava Creek eruption of the Yellowstone Caldera, which happened 640,000 years ago, released about 1,000 cubic kilometers (240 miles) of rock, dust, and volcanic ash into the air. This was Yellowstone’s third and most recent caldera-forming eruption.
Scientists closely monitor the height of the Yellowstone Plateau, which has risen as much as 150 millimeters (about 6 inches) each year. This rise is used to measure changes in the pressure of the magma chamber below the surface.
Between 2004 and 2008, the floor of the Yellowstone Caldera rose by about 75 millimeters (about 3 inches) each year. This was more than three times faster than any rise measured since 1923. At the White Lake GPS station, the land surface moved upward by as much as 8 inches (20 centimeters) during this time. In January 2010, scientists said the uplift had slowed significantly, though it continues at a slower rate. Scientists from the USGS, University of Utah, and National Park Service, who work with the Yellowstone Volcano Observatory, state there is no evidence that a large eruption will happen at Yellowstone in the near future. They also note that these events do not happen at regular or predictable intervals. This conclusion was repeated in December 2013 after a study found the magma body beneath Yellowstone is much larger than previously thought. A statement from the Yellowstone Volcano Observatory said:
Although interesting, these findings do not mean there are more dangers at Yellowstone. They also do not increase the chance of a "super eruption" in the near future. Yellowstone is not "overdue" for a super eruption, as some media reports claimed.
Some media coverage was more dramatic than the actual scientific findings.
A study published in GSA Today, a science magazine, identified three fault zones where future eruptions are most likely to occur. Two of these areas are linked to lava flows that are 174,000 to 70,000 years old. The third zone is where current earthquakes are happening.
In 2017, NASA studied whether it might be possible to prevent a Yellowstone eruption. The study suggested cooling the magma chamber by 35% could stop an eruption. NASA proposed injecting water at high pressure 10 kilometers underground. The water would release heat at the surface, which could be used for geothermal energy. If done, the plan would cost about $3.46 billion. However, a scientist warned that drilling into the top of the magma chamber might accidentally trigger an eruption.
According to earthquake data from 2013, the magma chamber under Yellowstone is 80 kilometers (50 miles) long and 20 kilometers (12 miles) wide. It has a volume of 4,000 cubic kilometers (960 cubic miles) underground, with 6–8% of that space filled with molten rock. This is about 2.5 times larger than scientists previously thought. However, scientists believe the amount of molten rock is too low to cause another super eruption.
In October 2017, research from Arizona State University found that before Yellowstone’s last super eruption, magma flowed into the chamber in two large waves. Analysis of crystals in Yellowstone’s lava showed the magma chamber heated up quickly and changed in composition before the last super eruption. This suggests a super eruption could happen in decades, not centuries as scientists once thought.
Since the last major eruption about 640,000 years ago (the Lava Creek event), Yellowstone has remained geologically active. This is because of the large magma chamber beneath the caldera. Scientists estimate this chamber contains about 4,000 cubic kilometers of partially molten material, making it one of the largest magma chambers in the world. The upward movement of the caldera floor—measured at rates of up to 75 millimeters (about 3 inches) per year—helps scientists understand how magma moves underground. This movement is closely studied by geologists.
Volcanic eruptions and geothermal activity at Yellowstone are caused by a large plume of magma beneath the caldera. This magma contains gases trapped under high pressure. If the pressure drops, such as from cracks in the Earth’s crust, the gases can form bubbles, causing the magma to expand. This expansion may lead to an eruption if the pressure continues to drop.
Studies suggest that the bigger danger at Yellowstone may come from hydrothermal activity, which happens separately from volcanic activity. Over 20 large craters have formed in the past 14,000 years, creating features like Mary Bay, Turbid Lake, and Indian Pond, which was formed by an eruption around 1300 BC.
A 2003 USGS report suggested that an earthquake might have moved more than 77 million cubic feet (2,200,000 cubic meters) of water in Yellowstone Lake. This could have created huge waves that released a trapped geothermal system, leading to the hydrothermal explosion that formed Mary Bay.
Further research shows that earthquakes far away can affect
Cultural significance
Because of its reputation for past volcanic eruptions and lava flows, as well as its excellent hydrothermal system, the International Union of Geological Sciences (IUGS) added "The Yellowstone volcanic and hydrothermal system" to its list of 100 geological heritage sites worldwide in a report published in October 2022. The organization describes an IUGS Geological Heritage Site as a special location that has important geological features or processes that scientists around the world study. These places are used as examples and have helped advance the field of geology over time.