Yellowstone Caldera

The Yellowstone Plateau Volcanic Field lies at the eastern end of the Snake River Plain and disrupts the continuity of the Laramide orogenic belt, which formed during the Late Cretaceous. From about 53 to 43 million years ago, this area experienced significant andesitic volcanism exceeding in total volume, forming the Absaroka Volcanic Supergroup. Prominent peaks such as Mount Washburn and Eagle Peak are eroded remnants of these earlier stratovolcanoes. Before the formation of the Yellowstone Plateau, the Teton Range and Madison Range were likely structurally continuous, as were the Red Mountains and Gallatin Range. Current Yellowstone volcanism is not a continuation of Laramide tectonism or the Absaroka volcanic province. Instead, it is the most recent part of a linear age-progression of rhyolitic complexes along the Snake River Plain, extending at least 16 million years to the McDermitt caldera complex. Large rhyolitic tuff supereruptions occurred at these older eruptive centers. One is the 12.1 million-year-old Ibex Hollow Tuff from the Bruneau-Jarbidge volcanic field in southern Idaho, burying herds of Nebraska mammals under volcanic ash. Older volcanics proposed to be part of this hotspot track include the 56 million-year-old Siletzia oceanic plateau and the 70 million-year-old Carmacks Group. The cause of the northeastward progression of volcanism is debated. Some models invoke only upper-mantle processes, such as mantle pushed upward by the leading edge of the subducting Farallon plate, slab rollback, a propagating rift, or mantle convection driven by abrupt changes in thermal layer thickness at the continent–ocean boundary. A proposed lower-mantle origin suggests a fragment of the subducting Farallon slab penetrated the discontinuity, pushing up the lower mantle and triggering melting of water-rich transition zone beneath the western United States. Alternatively, a long-lived mantle plume rooted at the core–mantle boundary has been proposed. The plume erupted the Columbia River Basalt Group and is now feeding the Yellowstone hotspot. Seismic tomography has revealed a wide, cylindrical thermal anomaly extending from the deepest mantle to just beneath Yellowstone, supporting the mantle plume origin. In this model, the North American Plate moves southwest at about per year over the relatively stationary plume, creating the observed age-progression of eruptive centers. == Volcanic landforms ==

The northern and eastern extent of the first-cycle caldera are unknown due to burial, although it likely reached into the third-cycle caldera, perhaps east of the Central Plateau. The Huckleberry Ridge Tuff in the Red Mountains is interpreted as thick intracaldera fill of the Island Park Caldera, and Big Bend Ridge at the southwestern edge of the volcanic plateau is inferred to be part of its caldera wall. A fault along the Snake River and Glade Creek, bounding the northern end of Teton Range and Huckleberry Ridge, is also thought to be part of the Island Park ring-fault. It is not known whether any of the first-cycle caldera segments was resurgent. The second-cycle caldera is known as the Henry's Fork Caldera. Thurmon Ridge at the northwestern edge of the volcanic plateau is inferred to be its northern caldera wall. The fault along Big Bend Ridge was reactivated, collapsing again during the second-cycle caldera formation. Although basalt flows bury its southern and eastern boundary, a positive gravity anomaly indicates a circular caldera about in diameter, with its southern boundary in the middle of the Island Park basin. Robert L. Christiansen inferred that the Yellowstone Caldera is a compound caldera comprising two partially overlapping ring-fault zones, centered on the resurgent Mallard Lake dome and Sour Creek dome. The southwest boundary is unconstrained due to post-caldera rhyolite burial, but 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, Flat Mountain Arm of Yellowstone Lake, north foothill of Red Mountains and Lewis Falls mark the southeast and south sides of Yellowstone caldera rim. However, the purported Sour Creek ring-fault zone and the location of the eastern caldera boundary have been challenged. More recent field mappings suggest the eastern ring-fault lies west of Sour Creek dome, closely following the Yellowstone River. The most western portion of Yellowstone Lake is the elliptical West Thumb Basin, which includes one of the lake's deepest areas. It is interpreted as a fourth caldera, formed by a third-cycle post-caldera explosive eruption. ==Eruption history==

A total of of rhyolite and of basalt were emplaced over three volcanic cycles between about 2.15 million and 0.07 million years ago. Each cycle lasted roughly three-quarters of a million years. The sequence of events in each cycle is similar: a catastrophic rhyolitic ash-flow sheet and caldera collapse, preceded and followed by eruptions of rhyolitic lavas and tuffs and basaltic eruptions near the caldera margin. Ash-flow sheets account for more than half of the total volcanic volume of the Yellowstone Plateau. First-cycle The first-cycle lasted from about 2.15 million to 1.95 million years ago, spanning approximately 200 kyr. The only known pre-collapse rhyolitic unit is the Rhyolite of Snake River Butte, located just north of Ashton and dated at , roughly 60–70 kyr before the caldera-forming Huckleberry Ridge Tuff. Its vent lies 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 spans no more than ~70 kyr. Another pre-collapse unit is the -thick Junction Butte Basalt on the northeastern margin of the plateau, dated at . 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 ago, during transitional magnetic polarity. Its thickness exceeds in the Red Mountains area. The initial Plinian phase deposited up to of fallout ash at Mount Everts before transitioning to ash-flow tuff. Early Plinian activity was intermittent, sourced from multiple vents, probably lasted a few weeks and evacuated about of magma from four magma bodies, triggering caldera collapse at the onset of transition to ash-flow. The ash-flow tuff is a composite sheet consisted of three intermittent members, with a total magma volume of about . Member A likely vented from the plateau's central area and tapped nine magma bodies. After a hiatus of a few weeks or more, the most voluminous Member B erupted from north of Big Bend Ridge. After another extended break of years to decades, part of the Member A magmatic system was rejuvenated to feed Member C. The least voluminous Member C might have source area near the Red Mountains, where it is about thick. Some outcrops of Member A and Member C have been misidentified as Member B, complicating volume estimates of individual ash-flow unit. Glen A. Izett estimated that an additional 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 exceeds . It is widely distributed and has been identified in the Pacific Ocean at Deep Sea Drilling Project Site 36, about 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 of the first-cycle volcanism. The Sheridan Reservoir Rhyolite, dated at , if vented from the Island Park ring-fracture, required a flow distance of at least . Its volume is estimated to exceed . The other two flows, the Blue Creek flow and the overlying Headquarters flow, have a combined volume of and erupted respectively at and ago. Second-cycle After ~500 kyr of quiescence, a new magmatic system formed north of Big Bend Ridge. It erupted the Bishop Mountain Flow at and the Tuff of Lyle Spring at . The Bishop Mountain Flow is a rhyolite with an exposed volume of about and reaches a thickness of along the inner caldera wall. The Tuff of Lyle Spring is a , composite ash-flow sheet consisting of two cooling units. Both eruptions appear to have originated from an isolated, highly evolved local magma chamber distinct from the second-cycle magma source. Tiffany A. Rivera et al. (2017) suggest these two eruptions should not be assigned to the second cycle but instead represent the separate Lyle Spring magmatic system. The next pre-collapse rhyolite eruption is the Green Canyon Flow in the north of Big Bend Ridge, with a mapped volume of about , dated at . Its age is indistinguishable from that of the subsequent Mesa Falls Tuff, but the Henry's Fork Caldera fracture truncates the Green Canyon Flow, indicating it predates the second-cycle caldera. The second-cycle caldera-forming eruption was the Mesa Falls Tuff, dated at . Its exposed thickness exceeds on Thurmon Ridge, though it is likely much thicker within the caldera. During the initial Plinian phase, about of ash and pumice were deposited around the Ashton area, while much of the vitric ash dispersed to more distant regions, as inferred from the high crystal content of the local deposit. This airfall is overlain by a pyroclastic surge layer also enriched in crystals. A single cooling unit of ash-flow tuff followed, covering about with an estimated volume of . The Mesa Falls ash bed (formerly "Pearlette type S") is the distal ash-fall of this eruption, found in Brainard and Hartington in Nebraska, and in the southern Rocky Mountains of Colorado. Post-collapse eruptions included the Moonshine Mountain dome and five rhyolite domes collectively known as the Island Park Rhyolite. The Moonshine Mountain dome, with an estimated volume of , erupted at . While its age is indistinguishable from the Mesa Falls Tuff, field evidence indicates it formed after the collapse of the Henry's Fork Caldera. The dome's magma source is likely the same region that supplied the Bishop Mountain Flow. The Island Park Rhyolite comprises five bodies: Silver Lake dome, Osborne Butte dome, Elk Butte dome, Lookout Butte dome, and Warm River Butte dome. These domes collectively have a total volume of . All five erupted within a few centuries, around , during a single eruptive episode. While Lookout Butte is located on the rim of Big Bend Ridge caldera wall, the vents for the other four domes align along a northwest-trending, structurally controlled linear vent zone about long and no more than wide. Third-cycle Pre-collapse third-cycle silicic rocks are broadly divided into the Mount Jackson Rhyolite and the Lewis Canyon Rhyolite, which vented along what later became the ring-fracture zone of the third-cycle caldera. The earliest known lava in this cycle is the Wapiti Lake flow of the Mount Jackson group, dated at , exposed near the Grand Canyon of the Yellowstone and likely vented near Wapiti Lake. Another flow, the Moose Creek Butte flow (), also belongs to the Mount Jackson group. Although younger than the Island Park Rhyolite, its geochemical similarity has led some researchers to propose it as a second-cycle post-collapse eruption. Pumice of an unknown tuff unit at Broad Creek has an age range from to . Later Mount Jackson eruptions include the Flat Mountain Rhyolite () and the Harlequin Lake flow (). The Lewis Canyon Rhyolite group contains lavas dated to , though Robert L. Christiansen suggests they could be late-stage first-cycle eruptions. A recently discovered ash-flow unit is dated to . An explosive eruption deposited pumiceous fallout near Harlequin Lake, which is immediately overlain by the Mount Haynes lava (). An ash bed from a Yellowstone eruption was deposited in the Great Salt Lake approximately ago. The age of the Big Bear Lake flow is uncertain, but it lies beneath the third-cycle caldera-forming Lava Creek Tuff. Additional Mount Jackson flows may be buried within the Yellowstone caldera, inferred from intracaldera topography. The climatic ash-flow eruption of the third cycle was the Lava Creek Tuff, dated at , during a glacial–interglacial transition in the Marine Isotope Stage. This composite tuff sheet consists of at least two members, distinguishable by a widely occurring welding intensity decrease between them, and represents a total ash-flow volume of about . Member A likely erupted south of Purple Mountain, where it reaches its greatest thickness of and exhibits maximum welding. The Purple Mountain to Gibbon Canyon segment of caldera wall collapsed after the emplacement of Member A but before it completely cooled. A loose crystal ash unit separates Member A from Member B, indicating a break in the eruption sufficiently long for cooling of thick ash-flows. A thick pumiceous ash-fall deposit underlies Member B and probably marks its initial phase. Member B ash-flows extends radially outward along paleovalleys and more extensive plateau segments. The eruptive center for Member B appears to be situated farther east compared to that of Member A. However, this simplistic eruptive sequence has been challenged. An additional ash-flow unit (informally named unit 2) has been identified, venting from around Bog Creek. Unit 2 erupted some decades after Member A had cooled and overlies tuff fragments from Member A. Two additional rhyolite ash-flow units (unit 3 and unit 4) have been recognized, erupting from a vent near Stonetop Mountain and are previously undocumented parts of the Lava Creek Tuff. An unknown welded tuff underlying Member B at Flagg Ranch, not attributed to Member A, was emplaced shortly before the initial ashfall of Member B and is considered part of the early Lava Creek eruption. Rather than having the simple structure of just two ignimbrite sheets, the Lava Creek Tuff may consist of multiple ash-flow lobes from distinct magma bodies. The ash fallout from the Lava Creek Tuff eruption is known as the Lava Creek ash bed (formerly "Pearlette type O"), covering an area exceeding . Perkins and Nash (2002) estimated that the volume of this ash bed is greater than . It has been identified in the Gulf of Mexico, near Regina, Saskatchewan, in Ventura, California, and in Viola Center, Iowa. Post-collapse rhyolites Post-collapse rhyolites likely erupted shortly after the Lava Creek Tuff. The subaerial post-collapse silicic rocks are collectively referred to as the Plateau Rhyolite, which primarily consists of lava flows. Plateau Rhyolite is divided into three intracaldera members—Upper Basin Member, Mallard Lake Member, and Central Plateau Member—and two extracaldera members—Obsidian Creek Member and Roaring Mountain Member. It is likely that rhyolitic pumice and ash were erupted during the opening of vents for each of these lava flows. The earliest intracaldera rhyolite, the East Biscuit Basin Flow of the Upper Basin Member, is dated to , followed by felsic lithic clasts of an unknown unit () in Yellowstone Lake, and the North Biscuit Basin Flow (). The earliest extracaldera rhyolite is the Riverside Flow () of the Roaring Mountain Member, broadly contemporaneous with the Middle Biscuit Basin Flow (). Two ash-flow tuff units of the Upper Basin Member include the -thick Tuff of Uncle Tom's Trail and the -thick Tuff of Sulphur Creek, the latter dated at . Tuff of Sulphur Creek is at least . These tuffs were deposited on the north flank of the Sour Creek dome. The Canyon lava flows of the Upper Basin Member erupted immediately after the Tuff of Sulphur Creek, as the ash-flow was still hot at the time of emplacement. Both the Tuff of Sulphur Creek and Canyon flows originated from a vent near Fern Lake. The two tuffs and Canyon flows have a combined magma volume of . The Dunraven Road Flow () of the Upper Basin Member overlies the Canyon flows and may have had an extracaldera vent. The Cougar Creek lava dome of the Roaring Mountain Member erupted north of the caldera. Four additional lava flows of the Obsidian Creek Member—Willow Park dome, Apollinaris Spring dome, Gardner River complex, and Grizzly Lake complex—erupted between and , in the vicinity of Norris Geyser Basin northward toward Mammoth Hot Springs. The South Biscuit Basin Flow of the Upper Basin Member erupted ago. The Scaup Lake Flow of the Upper Basin Member is dated to , while the Landmark dome of the Obsidian Creek Member is . Non-explosive eruptions of lava and less-violent explosive eruptions have occurred in and near the Yellowstone caldera since the last supereruption. The most recent lava flow occurred about 70,000 years ago, while a violent eruption excavated the West Thumb of Lake Yellowstone 174,000 years ago. Smaller steam explosions occur as well. An explosion 13,800 years ago left a diameter crater at Mary Bay on the edge of Yellowstone Lake (located in the center of the caldera). Currently, volcanic activity is exhibited via numerous geothermal vents scattered throughout the region, including the famous Old Faithful Geyser, plus recorded ground-swelling indicating ongoing inflation of the underlying magma chamber. ==Hazards==

Earthquakes Volcanic and tectonic actions in the region cause between 1,000 and 2,000 measurable earthquakes annually. Most are relatively minor, measuring magnitude 3 or weaker. Occasionally, numerous earthquakes are detected in a relatively short period of time, an event known as an earthquake swarm. In 1985, more than 3,000 earthquakes were measured over a period of several months. More than 70 smaller swarms were detected between 1983 and 2008. The USGS states these swarms are likely caused by slips on pre-existing faults rather than by movements of magma or hydrothermal fluids. In December 2008, continuing into January 2009, more than 500 earthquakes were detected under the northwest end of Yellowstone Lake over a seven-day span, with the largest registering a magnitude of 3.9. Another swarm started in January 2010, after the Haiti earthquake and before the Chile earthquake. With 1,620 small earthquakes between January 17, 2010, and February 1, 2010, this swarm was the second-largest ever recorded in the Yellowstone Caldera. The largest of these shocks was a magnitude 3.8 that occurred on January 21, 2010. This swarm subsided to background 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 a magnitude 2.9. Volcanoes The Lava Creek eruption of the Yellowstone Caldera, which occurred 640,000 years ago, ejected approximately of rock, dust and volcanic ash into the atmosphere. It was Yellowstone's third and most recent caldera-forming eruption. Geologists closely monitor the elevation of the Yellowstone Plateau, which has been rising as quickly as per year, as an indirect measurement of changes in magma chamber pressure. The upward movement of the Yellowstone caldera floor between 2004 and 2008—almost each year—was more than three times greater than ever observed since such measurements began in 1923. From 2004 to 2008, the land surface within the caldera moved upward as much as at the White Lake GPS station. In January 2010, the USGS stated that "uplift of the Yellowstone Caldera has slowed significantly" and that uplift continues but at a slower pace. USGS, University of Utah and National Park Service scientists with the Yellowstone Volcano Observatory maintain that they "see no evidence that another such cataclysmic eruption will occur at Yellowstone in the foreseeable future. Recurrence intervals of these events are neither regular nor predictable." This conclusion was reiterated in December 2013 in the aftermath of the publication of a study by University of Utah scientists finding that the "size of the magma body beneath Yellowstone is significantly larger than had been thought". The Yellowstone Volcano Observatory issued a statement on its website stating: Although fascinating, the new findings do not imply increased geologic hazards at Yellowstone, and certainly do not increase the chances of a "super eruption" in the near future. Contrary to some media reports, Yellowstone is not "overdue" for a super eruption. Media reports were more hyperbolic in their coverage. A study published in GSA Today, the monthly news and science magazine of the Geological Society of America, identified three fault zones where future eruptions are most likely to be centered. Two of those areas are associated with lava flows aged 174,000–70,000 years ago, and the third is a focus of present-day seismicity. According to analysis of earthquake data in 2013, the magma chamber is long and wide. It also has underground volume, of which 6–8% is filled with molten rock. This is about 2.5 times bigger than scientists had previously imagined; however, scientists believe that the proportion of molten rock in the chamber is too low to allow for another supereruption. In October 2017, research from Arizona State University indicated prior to Yellowstone's last supereruption, magma surged into the magma chamber in two large influxes. An analysis of crystals from Yellowstone's lava showed that prior to the last supereruption, the magma chamber underwent a rapid increase in temperature and change in composition. The analysis indicated that Yellowstone's magma reservoir can reach eruptive capacity and trigger a super-eruption within just decades, not centuries as volcanologists had originally thought. Since its most recent major eruption approximately 640,000 years ago (the Lava Creek event), Yellowstone has remained geologically active, primarily due to the vast magma chamber beneath the caldera. This chamber is estimated to contain around 4,000 km³ of partially molten material, making it one of the largest of its kind globally. Periodic uplift of the caldera floor—measured at rates of up to 75 mm per year—provides valuable insights into the dynamics of subterranean magma movement and is a key focus of ongoing geological monitoring efforts. Hydrothermal explosions Volcanic eruptions and ongoing geothermal activity at Yellowstone are attributed to a large plume of magma located beneath the caldera. This magma contains dissolved gases held under immense pressure. If the pressure is sufficiently reduced—due to geological shifts such as crustal fracturing—these gases can exsolve, forming bubbles and causing the magma to expand. This process can trigger a chain reaction, where further pressure release leads to increased gas expansion. In extreme cases, this may culminate in an explosive eruption if overlying crustal material is forcefully ejected. Studies and analysis may indicate that the greater hazard comes from hydrothermal activity which occurs independently of volcanic activity. Over 20 large craters have been produced in the past 14,000 years, resulting in such features as Mary Bay, Turbid Lake, and Indian Pond, which was created in an eruption about 1300 BC. In a 2003 report, USGS researchers proposed that an earthquake may have displaced more than of water in Yellowstone Lake, creating colossal waves that unsealed a capped geothermal system and led to the hydrothermal explosion that formed Mary Bay. Further research shows that very distant earthquakes reach and have effects upon the activities at Yellowstone, such as the 1992 7.3 magnitude Landers earthquake in California's Mojave Desert that triggered a swarm of quakes from more than away, and the 2002 7.9 magnitude Denali fault earthquake away in Alaska that altered the activity of many geysers and hot springs for several months afterward. In 2016, the USGS announced plans to map the subterranean systems responsible for feeding the area's hydrothermal activity. According to the researchers, these maps could help predict when another eruption occurs. == Cultural significance ==

IUGS geological heritage site In respect of it being "well-known for its past explosive volcanic eruptions and lava flows as well for its world class hydrothermal system", the International Union of Geological Sciences (IUGS) included "The Yellowstone volcanic and hydrothermal system" in its assemblage of 100 geological heritage sites around the world in a listing published in October 2022. The organization defines an IUGS Geological Heritage Site as "a key place with geological elements and/or processes of international scientific relevance, used as a reference, and/or with a substantial contribution to the development of geological sciences through history". ==See also==