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. This area includes four overlapping calderas, many lava domes, raised areas within the calderas, crater lakes, and layers of rock formed from lava and ash, which originally covered about 17,000 square kilometers.
Volcanic activity in this region began 2.15 million years ago and happened in three major stages. Each stage included a large eruption of volcanic material, fast-moving lava and ash, widespread ash falling across continents, and the collapse of the caldera. Smaller lava flows and rock layers formed before and after each major event. The first and largest stage, called the Huckleberry Ridge Tuff eruption, happened about 2.08 million years ago and created the Island Park Caldera. The most recent major eruption, about 630,000 years ago, produced the Lava Creek Tuff and formed the current Yellowstone Caldera. After these major eruptions, smaller lava flows, rock formations, and some explosive events occurred, with the last volcanic eruption about 70,000 years ago. Large steam explosions also happened during the Holocene period.
Between 2004 and 2009, the area experienced noticeable rising ground, which scientists believe was caused by new magma moving underground. A 2005 television movie called Supervolcano, made by the BBC and Discovery Channel, increased public interest in the possibility of future eruptions. The Yellowstone Volcano Observatory monitors the area and does not believe a major eruption is likely soon. Studies of the underground magma reservoir show there is a large amount of melted rock beneath Yellowstone, but it is not currently active enough to cause an eruption.
Geologic setting
The Yellowstone Plateau Volcanic Field is located at the eastern end of the Snake River Plain. It interrupts the continuous structure of the Laramide orogenic belt, a geological formation that developed during the Late Cretaceous period. Between about 53 and 43 million years ago, this area had major volcanic activity involving andesite rock, with a total volume exceeding 29,000 km (7,000 mi). This activity formed the Absaroka Volcanic Supergroup. Notable peaks like Mount Washburn and Eagle Peak are now eroded remains of older 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 direct continuation of the Laramide tectonic processes or the Absaroka volcanic province. Instead, it is the most recent part of a long line of rhyolitic rock formations along the Snake River Plain, stretching at least 16 million years back to the McDermitt caldera complex. Large eruptions of rhyolitic tuff occurred at older volcanic sites. 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. Earlier 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 models suggest processes in the upper mantle, such as the upward push of the mantle 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 theory proposes that a piece of the Farallon slab broke through the 660 km (410 mi) mantle boundary, causing the lower mantle to rise and melting water-rich rock beneath the western United States. A third idea is a long-lasting mantle plume rooted at the core–mantle boundary, which erupted the Columbia River Basalt Group and now supplies magma to the Yellowstone hotspot. Seismic tomography has found a 350 km (220 mi) wide, cylindrical heat source extending from the deepest mantle up 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 the plume, creating the observed pattern of volcanic ages.
Volcanic landforms
The northern and eastern edges of the first-cycle caldera are not fully understood because they are covered by layers of rock. However, it is likely that this caldera 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 deposited inside the Island Park Caldera. The 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 marks 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, meaning they rose again after forming.
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 the northern wall of this caldera. The fault along Big Bend Ridge was reactivated during the formation of the second-cycle caldera, causing it to collapse again. Although basalt rock layers cover the southern and eastern boundaries, a positive gravity anomaly suggests the presence of 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 proposed 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 of the caldera is not clearly defined due to post-caldera rhyolite rock covering it. He suggested that the south flank of Purple Mountain and the Washburn Range, along with the west flank of the Absaroka Range, mark the caldera's 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 believed to mark the southeast and south sides of the Yellowstone caldera rim. However, the proposed Sour Creek ring-fault zone and the location of the eastern caldera boundary have been questioned. More 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 points and is interpreted as a fourth caldera, formed by an explosive eruption during the third cycle.
Eruption history
A total of 6,500 km (1,600 mi) of rhyolite and 250 km (60 mi) of basalt were deposited over three volcanic cycles between about 2.15 million and 0.07 million years ago. Each cycle lasted about 750,000 years. The events in each cycle are similar: a large rhyolitic ash-flow sheet 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 all the volcanic material in the Yellowstone Plateau.
The first cycle lasted from about 2.15 million to 1.95 million years ago, spanning about 200,000 years. The only known rhyolitic unit before the caldera formed is the Rhyolite of Snake River Butte, located near Ashton and dated to about 2.14 million years, roughly 60–70,000 years before the caldera-forming Huckleberry Ridge Tuff. Its vent was near the future caldera margin close to the Big Bend Bridge. Other rhyolite flows may have erupted along the 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 m (200 to 230 ft) thick Junction Butte Basalt on the northeastern margin of the plateau, dated to about 2.16 million years. The Overhanging Cliff basalt is a flow of 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 transitional magnetic polarity. Its thickness exceeds 1 km (0.62 mi) in the Red Mountains area. The eruption started with a Plinian phase, which deposited up to 2.5 m (8.2 ft) of ash at Mount Everts before transitioning to ash-flow tuff. Early Plinian activity was intermittent, from multiple vents, likely lasting a few weeks and removing about 50 km (12 mi) of magma from four magma bodies, causing caldera collapse. The ash-flow tuff is a composite sheet made of three parts, with a total magma volume of about 2,450 km (590 mi). Member A likely erupted from the plateau’s central area and tapped nine magma bodies. After a break of weeks or more, the largest part, Member B, erupted from north of Big Bend Ridge. After another long break, part of the Member A system was reused to feed Member C. Member C, the smallest, may have originated near the Red Mountains, where it is about 430 m (1,410 ft) thick. Some outcrops of Member A and C have been mistaken for Member B, making volume estimates unclear. Glen A. Izett estimated that an additional 2,000 km (480 mi) of ash spread across North America. Tephra from this event is known as the Huckleberry Ridge ash bed. It covered more than 3,400,000 km (1,300,000 sq mi) and has been found in the Pacific Ocean, 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 rhyolites from the first cycle. The Sheridan Reservoir Rhyolite, dated to about 2.07 million years, if erupted from the Island Park ring-fracture, traveled at least 20 km (12 mi). Its volume is estimated to exceed 10 km (2.4 mi). The other two flows, the Blue Creek flow and the Headquarters flow, have a combined volume of 10–20 km (2.4–4.8 mi) and erupted at about 1.9811 million and 1.9476 million years ago.
After about 500,000 years of no activity, a new magma system formed north of Big Bend Ridge. It erupted the Bishop Mountain Flow at about 1.4578 million years and the Tuff of Lyle Spring at about 1.4502 million years. The Bishop Mountain Flow is a rhyolite with an exposed volume of about 23 km (5.5 mi) and reaches a thickness of 375 m (1,230 ft) along the inner caldera wall. The Tuff of Lyle Spring is a 1 km (0.24 mi) thick ash-flow sheet made of two cooling units. Both eruptions likely came from a separate magma chamber distinct from the second-cycle source. Tiffany A. Rivera et al. (2017) suggest these eruptions should not be grouped with the second cycle but instead represent the Lyle Spring magmatic system. The next pre-collapse rhyolite eruption was the Green Canyon Flow, dated to about 1.2989 million years, with a mapped volume of about 5 km (1.2 mi). Its age is similar to the Mesa Falls Tuff, but the Henry’s Fork Caldera fracture cuts through the Green Canyon Flow, showing it formed before the caldera.
The third cycle included eruptions of the Mount Jackson Rhyolite group, such as the Flat Mountain Rhyolite (dated to about 0.929 million years) and the Harlequin Lake flow (dated to about 0.83 million years). The Lewis Canyon Rhyolite group contains lavas dated to about 0.8263 million years.
Hazards
Volcanic and tectonic activity in the region causes between 1,000 and 2,000 measurable earthquakes each year. Most are small, measuring magnitude 3 or less. Sometimes, many earthquakes happen in a short time, 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. The USGS says these swarms are likely caused by movement on old faults, not by magma or hydrothermal fluids.
In December 2008, and continuing into January 2009, more than 500 earthquakes were recorded under the northwest part of Yellowstone Lake over seven days. The largest was magnitude 3.9. Another swarm began in January 2010, after the Haiti earthquake and before the Chile earthquake. This swarm had 1,620 small earthquakes between January 17, 2010, and February 1, 2010. It was the second-largest recorded in the Yellowstone Caldera. The largest earthquake in this swarm was magnitude 3.8 on January 21, 2010. Activity returned to normal levels by February 21. 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 mi³) of rock, dust, and volcanic ash into the atmosphere. It was Yellowstone’s third and most recent caldera-forming eruption.
Geologists closely monitor the height of the Yellowstone Plateau, which has risen as fast as 150 millimeters (5.9 inches) per year. This rise is an indirect way to measure changes in magma chamber pressure.
Between 2004 and 2008, the floor of the Yellowstone Caldera rose almost 75 millimeters (3.0 inches) each year. This was more than three times the rate observed since measurements began in 1923. At the White Lake GPS station, the land surface rose as much as 8 inches (20 cm) during this time. In January 2010, the USGS stated that the uplift of the Yellowstone Caldera has slowed significantly, though it continues at a slower pace. Scientists from the USGS, University of Utah, and National Park Service with the Yellowstone Volcano Observatory say there is no evidence that another large eruption will occur at Yellowstone in the foreseeable future. They note that these events are not regular or predictable. This conclusion was repeated in December 2013 after a study by University of Utah scientists found that the magma body beneath Yellowstone is much larger than previously thought. The Yellowstone Volcano Observatory stated that the new findings do not increase the risk of a "super eruption" in the near future. It also clarified that Yellowstone is not "overdue" for a super eruption, despite some media reports.
Media coverage of these findings was more dramatic than the actual scientific conclusions.
A study published in GSA Today, the monthly magazine of the Geological Society of America, identified three fault zones where future eruptions are most likely to occur. Two of these zones are linked to lava flows from 174,000 to 70,000 years ago. The third zone is where current seismic activity is happening.
In 2017, NASA studied whether it might be possible to prevent a Yellowstone eruption. The study suggested that cooling the magma chamber by 35% could stop an eruption. NASA proposed injecting high-pressure water 10 kilometers underground. This water would release heat at the surface, which could be used for geothermal energy. The plan would cost about $3.46 billion. Brian Wilcox of the Jet Propulsion Laboratory noted that drilling into the top of the magma chamber could accidentally trigger an eruption.
According to earthquake data from 2013, the magma chamber is 80 km (50 mi) long and 20 km (12 mi) wide. It has a volume of 4,000 km³ (960 mi³) underground, with 6–8% of that space filled with molten rock. This is about 2.5 times larger than previously estimated. However, scientists believe the amount of molten rock is too low to cause a supereruption.
In October 2017, research from Arizona State University showed that before Yellowstone’s last supereruption, magma entered the chamber in two large waves. Analysis of crystals in Yellowstone’s lava revealed that the magma chamber rapidly increased in temperature and changed in composition before the eruption. This suggests that the magma reservoir could reach eruptive capacity and trigger a supereruption within decades, not centuries as previously thought.
Since the Lava Creek eruption about 640,000 years ago, Yellowstone has remained geologically active, mainly because of the large magma chamber beneath the caldera. This chamber is estimated to contain about 4,000 km³ of partially molten material, making it one of the largest of its kind globally. The periodic rise of the caldera floor—measured at up to 75 mm per year—helps scientists understand underground magma movement and is a key focus of geological monitoring.
Volcanic eruptions and geothermal activity at Yellowstone are caused by a large plume of magma beneath the caldera. This magma holds dissolved gases under high pressure. If pressure is reduced due to geological shifts, such as crustal fracturing, the gases can form bubbles, causing the magma to expand. This process can trigger a chain reaction, leading to an explosive eruption if overlying crustal material is ejected.
Studies suggest that the greater risk comes from hydrothermal activity, which can happen independently of volcanic activity. Over 20 large craters have formed in the past 14,000 years
Cultural significance
Because Yellowstone is well known for its past explosive volcanic eruptions, lava flows, and world-class hydrothermal system, the International Union of Geological Sciences (IUGS) added "The Yellowstone volcanic and hydrothermal system" to its collection of 100 geological heritage sites around the world. This listing was published in October 2022. The IUGS defines a Geological Heritage Site as a special place that has important geological features or processes. These places are used as references and have made important contributions to the study of geology over time.