The Yellowstone Caldera, also known as the Yellowstone Plateau Volcanic Field, is a Quaternary volcanic field, caldera complex, and volcanic plateau spanning parts of Wyoming, Idaho, and Montana. It is driven by the Yellowstone hotspot and lies largely within Yellowstone National Park. The field comprises several overlapping caldera structures, multiple lava domes and resurgent domes, and numerous bimodal lavas and tuffs of basaltic and rhyolitic composition, originally covering about .
Volcanism began 2.15 million years ago and proceeded through three major volcanic cycles. Each cycle involved a large ignimbrite eruption, widespread pyroclastic flows and ash fall, and caldera collapse, preceded and followed by smaller lava flows and tuffs. The climactic eruption of the first and largest cycle produced the Huckleberry Ridge Tuff about 2.08 million years ago and formed the first-cycle caldera complex. The second cycle culminated in eruption of the Mesa Falls Tuff and formation of the Henry's Fork Caldera about 1.3 million years ago. The most recent supereruption, about 631,000 years ago, produced the Lava Creek Tuff and created the present Yellowstone Caldera. Post-caldera eruptions included basalt flows, rhyolite domes and flows, and minor explosive deposits, with the last known magmatic eruption about 70,000 years ago. Large hydrothermal explosions also occurred during the Holocene.
The volcanic field remains active, with continuing earthquakes, ground deformation, and extensive hydrothermal activity driven by heat from the underlying magmatic system. The Yellowstone Volcano Observatory monitors the region and does not consider an eruption imminent. Imaging of the magmatic system indicates a substantial volume of partial melt beneath Yellowstone, although it is not presently considered eruptible.
Geologic setting
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 youngest part of a northeastward age progression of rhyolitic volcanic complexes along the Snake River Plain, extending at least 16 million years to the McDermitt caldera complex. Large rhyolitic tuff eruptions occurred at several of these older eruptive centers. The age progression is generally attributed to southwestward motion of the North American Plate relative to the Yellowstone hotspot.
The Yellowstone Plateau also lies along the northeastern margin of the extensional Basin and Range Province. Regional extension has accompanied the migration of volcanism toward Yellowstone and contributes to the active fault systems surrounding the plateau, including major normal faults north and south of the caldera. Regional extension persists across the volcanic field. Lidar mapping identified more than 1,000 extensional fault scarps across the Yellowstone Caldera and interpreted a north–south zone as kinematically linking the Teton Fault and East Gallatin fault systems.
The ultimate origin of the Yellowstone hotspot remains debated. Proposed explanations include upper-mantle processes related to the subducted Farallon plate, including slab rollback, rifting, and small-scale mantle convection, as well as deeper-mantle mechanisms. An alternative model invokes a long-lived mantle plume, potentially also related to emplacement of the Columbia River Basalt Group. Seismic tomography has imaged a broad thermal anomaly extending through the mantle beneath Yellowstone, which has been interpreted as supporting a deep-mantle origin.
Volcanic landforms The present landscape of the Yellowstone Plateau reflects overlapping caldera structures, resurgent uplift, thick lava flows, and subsequent modification by faulting, hydrothermal activity, glaciation, and erosion.
Caldera structures The first-cycle caldera complex probably consisted of three partly overlapping collapse areas, termed the Big Bend Ridge, Snake River, and Red Mountains caldera segments. Its northern and eastern extent is uncertain because of burial by younger volcanic rocks, although it probably extended into the area of the third-cycle caldera, perhaps east of the Central Plateau. The Huckleberry Ridge Tuff in the Red Mountains is interpreted as thick intracaldera fill, and Big Bend Ridge at the southwestern edge of the volcanic plateau is inferred to be part of the first-cycle caldera wall. A fault along the Snake River and Glade Creek, bounding the northern end of the Teton Range and Huckleberry Ridge, is also interpreted as part of the first-cycle ring-fault system. It is not known whether any of the first-cycle caldera segments underwent resurgence.
The second-cycle collapse structure is 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 during second-cycle collapse. Although basalt flows bury the southern and eastern boundary, a positive gravity anomaly indicates a nearly circular caldera about in diameter, with its southern boundary beneath the central Island Park basin.
Robert L. Christiansen reconstructed the Yellowstone Caldera as a compound structure comprising two partly overlapping collapse areas, traditionally associated with the Mallard Lake and Sour Creek structural highs. Its southwestern boundary is poorly constrained because it is buried by younger rhyolite flows. In the traditional reconstruction, the south flank of Purple Mountain and the Washburn Range, together with the western flank of the Absaroka Range, mark the northern and eastern margins, while Lake Butte, Flat Mountain Arm of Yellowstone Lake, the northern foothills of the Red Mountains, and Lewis Falls delineate parts of the southeastern and southern margin. The northeastern part of this reconstruction has subsequently been challenged. Mapping and geochronology initially suggested that the ring-fault lay west of the topographic high known as the Sour Creek dome, approximately along the Yellowstone River. More extensive remapping found that the Sour Creek high consists of several packages of Lava Creek Tuff-age ignimbrite rather than uplifted older rocks, while faults formerly attributed to resurgence also cut the much younger Elephant Back lava flow. The high is therefore interpreted as a domical accumulation of ignimbrite rather than a resurgent dome. Myers et al. consequently proposed that the northeastern Lava Creek collapse margin lies farther west than traditionally mapped, approximately along the Yellowstone River and near the northeastern margin of the older first-cycle caldera. Farther south, this proposed structural trend is approximately continued into northern Yellowstone Lake by the active Eagle Bay–Lake Hotel fault zone, of which the Lake Hotel graben is a segment. The fault zone had previously been interpreted as potentially related to the older Huckleberry Ridge caldera margin.
The western part of Yellowstone Lake contains the elliptical West Thumb basin. West Thumb is a caldera nested within the larger Yellowstone Caldera and is interpreted to have formed during eruption of the tuff of Bluff Point about 160,000 years ago. Younger rhyolite flows partly buried its northeastern and southeastern margins, although the basin remains one of the deepest parts of Yellowstone Lake.
Resurgence and faulting The Mallard Lake area preserves clear evidence of structural resurgence. The elliptical Mallard Lake dome, about , uplifts the Mallard Lake lava flow and part of the older Scaup Lake flow and is cut by a complex northwest-trending axial graben. The principal exposed phase of doming occurred around 160,000 years ago, although earlier post-collapse resurgence was inferred by Christiansen to be buried beneath younger volcanic rocks.
The resurgent origin of the Sour Creek dome has been questioned. Myers et al. inferred that the mapped faulting substantially postdates emplacement of the Sour Creek ignimbrites and interpreted the dome's topographic relief primarily as a low-angle, asymmetric accumulation of ignimbrite rather than structural resurgence. The Sour Creek area nevertheless continues to undergo modern deformation associated with the active Yellowstone magmatic and hydrothermal system.
Lava plateaus Much of the present relief within the Yellowstone Caldera is constructional, formed by thick post-caldera rhyolite lava flows rather than by the caldera rim itself. Flows of the Central Plateau Member of the Plateau Rhyolite form much of the Madison Plateau, Pitchstone Plateau, Central Plateau, and Solfatara Plateau, producing broad, relatively flat or gently domed uplands bounded locally by steep lava-flow fronts. Representative volcanic exposures elsewhere in the field include Obsidian Cliff, an exposure of obsidian-rich rhyolite, and Sheepeater Cliff, where columnar-jointed basalt is exposed.
Landscape evolution The modern landscape of the volcanic field reflects erosion, glaciation, sedimentation, and hydrothermal alteration superimposed on the constructional volcanic terrain. Rhyolitic lava flows underlie much of Yellowstone Lake and exert a major control on its bathymetry. Bathymetric and magnetic data indicate that Stevenson, Dot, and Frank islands are underlain by large-volume rhyolite flows mantled by late Pleistocene glaciolacustrine sediments.
The Yellowstone River has incised the Grand Canyon of the Yellowstone through post-caldera volcanic rocks. Hydrothermal alteration weakened the rhyolite and tuff and produced the characteristic yellow, orange, and red rocks exposed in the canyon walls, while river erosion and episodes of enhanced erosion following glaciation helped develop the modern canyon. The Upper Falls and Lower Falls occur where the river crosses more resistant volcanic rocks, with the Lower Falls marking the upstream end of the modern deep canyon.
Eruption history A total of about of rhyolite and of basalt were emplaced in the Yellowstone Plateau Volcanic Field between about 2.15 million and 0.07 million years ago. The rhyolitic volcanism is conventionally divided into three major volcanic cycles, each centered on a large caldera-forming eruption. In broad outline, each cycle included precaldera rhyolitic lavas and tuffs, a climactic eruption of a voluminous rhyolitic ash-flow tuff accompanied by caldera collapse, and postcaldera rhyolitic volcanism. Basaltic volcanism occurred intermittently, chiefly around the margins of the volcanic plateau, and is conventionally discussed with the three rhyolitic cycles. The three caldera-forming ash-flow sheets account for more than half of the total volcanic volume of the Yellowstone Plateau. +Major caldera-forming eruptions of the Yellowstone Plateau Volcanic FieldVolcanic cycleCaldera-forming tuffAgeAsh-flow volumeCollapse structureFirstHuckleberry Ridge Tuff~2.08 Ma>First-cycle caldera complexSecondMesa Falls Tuff~1.30 Ma>Henry's Fork CalderaThirdLava Creek Tuff~0.63 Ma>Yellowstone Caldera
First volcanic cycle