Editing Yellowstone Caldera (section)

== Geology ==

The Yellowstone Plateau Volcanic Field lies at the eastern end of the [[Snake River Plain]] and disrupts the continuity of the [[Laramide orogeny|Laramide]] [[orogenic belt]], which formed during the Late [[Cretaceous]].{{sfn|Christiansen|2001|p=9}} From about 53 to 43&nbsp;million years ago, this area experienced significant [[andesitic]] volcanism exceeding {{Convert|29,000|km3|mi3|abbr=on}} in total volume, forming the [[Absaroka Range|Absaroka Volcanic
Supergroup]]. Prominent peaks such as [[Mount Washburn]] and [[Eagle Peak (Wyoming)|Eagle Peak]] are eroded remnants of these earlier [[stratovolcano]]es.{{sfn|USGS|2021}} Before the formation of the Yellowstone Plateau, the [[Teton Range]] and [[Madison Range]] were likely structurally continuous, as were the [[Mount Sheridan|Red Mountains]] and [[Gallatin Range]].{{sfn|Christiansen|2001|p=11}} 

Current Yellowstone volcanism is not a continuation of [[Laramide orogeny|Laramide]] tectonism or the [[Absaroka Range|Absaroka]] volcanic province.{{sfn|Christiansen|2001|p=11}} Instead, it is the most recent part of a linear age-progression of [[rhyolite|rhyolitic]] complexes along the [[Snake River Plain]], extending at least 16&nbsp;million years to the [[McDermitt Caldera|McDermitt caldera complex]].{{sfn|Henry|Castor|Starkel|Ellis|2017|p=1066}} Large rhyolitic tuff supereruptions occurred at these older eruptive centers.{{sfn|Christiansen|McCurry|Champion|Bolte|2013}}{{sfn|Perkins|Nash|Brown|Fleck|1995|p=1500}} One is the 12.1&nbsp;million-year-old Ibex Hollow Tuff from the [[Bruneau-Jarbidge volcanic field]] in southern [[Idaho]], [[Ashfall Fossil Beds|burying herds of Nebraska mammals under volcanic ash]].{{sfn|Sarna-Wojcicki|Knott|Westgate|Budahn|2023|p=22}} Older volcanics proposed to be part of this hotspot track include the 56&nbsp;million-year-old [[Siletzia]] oceanic plateau and the 70&nbsp;million-year-old [[Carmacks Group]].{{sfn|Camp|Wells|2021|p=4}}{{sfn|Johnston|Jane Wynne|Francis|Hart|1996|p=997}}

The cause of the northeastward progression of volcanism is debated. Some models invoke only [[Upper mantle|upper-mantle]] processes, such as mantle pushed upward by the leading edge of the [[Subduction|subducting]] [[Farallon plate]],{{sfn|Faccenna|Becker|Lallemand|Lagabrielle|2010|p=58}} slab rollback,{{sfn|Long|Till|Druken|Carlson|2012|p=2}} a propagating [[rift]],{{sfn|Christiansen|Foulger|Evans|2002|p=1247}} or [[mantle convection]] driven by abrupt changes in thermal layer thickness at the continent–ocean boundary.{{sfn|King|2007|p=224}} A proposed [[Lower mantle|lower-mantle]] origin suggests a fragment of the subducting Farallon slab penetrated the [[transition zone (Earth)|{{convert|660|km|mi|abbr=on}} discontinuity]], pushing up the lower mantle and triggering melting of water-rich transition zone beneath the [[western United States]].{{sfn|Zhou|2018|p=449}} 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.{{sfn|Richards|Duncan|Courtillot|1989|p=106}} [[Seismic tomography]] has revealed a {{Convert|350|km|mi|abbr=on}} wide, cylindrical thermal anomaly extending from the deepest mantle to just beneath Yellowstone, supporting the mantle plume origin.{{sfn|Nelson|Grand|2018|p=280}} In this model, the [[North American Plate]] moves southwest at about {{Convert|2.2|cm|in|abbr=on}} per year over the relatively stationary plume, creating the observed age-progression of eruptive centers.{{sfn|Anders|1994}}

=== Structure of calderas ===
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.{{sfn|Christiansen|2001|p=53}} The Huckleberry Ridge Tuff in the Red Mountains is interpreted as thick intracaldera fill of the [[Island Park, Idaho|Island Park]] Caldera,{{sfn|Christiansen|2001|p=61}} and Big Bend Ridge at the southwestern edge of the volcanic plateau is inferred to be part of its caldera wall.{{sfn|Christiansen|2001|p=61}} A [[fault (geology)|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.{{sfn|Christiansen|2001|p=62}} It is not known whether any of the first-cycle caldera segments was resurgent.{{sfn|Christiansen|2001|p=63}} 

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.{{sfn|Christiansen|2001|p=66}} The fault along Big Bend Ridge was reactivated, collapsing again during the second-cycle caldera formation.{{sfn|Christiansen|2001|p=61}} Although basalt flows bury its southern and eastern boundary, a positive gravity anomaly indicates a circular caldera about {{convert|19|km|mi|abbr=on}} in diameter, with its southern boundary in the middle of the Island Park basin.{{sfn|Christiansen|2001|p=66}}

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.{{sfn|Christiansen|2001|p=35}} The southwest boundary is unconstrained due to post-caldera [[rhyolite]] burial, but he proposed that the south flank of [[Purple Mountain (Wyoming)|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.{{sfn|Christiansen|2001|p=36}} [[Lewis Falls]], Lake Butte, and Flat Mountain Arm of Yellowstone Lake are also part of the Yellowstone caldera rim.{{sfn|National Park Service}} 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]].{{sfn|Wilson|Stelten|Lowenstern|2018|p=52}}{{sfn|Yellowstone Volcano Observatory|2023|p=29}} 

The most western portion of [[Yellowstone Lake]] is the elliptical {{convert|6|x|8|km|mi|abbr=on}} 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.{{sfn|Christiansen|2001|p=45}}

[[Image:Yellowstone Major Calderas Map.jpg|thumb|upright=1.7|Yellowstone sits on top of four overlapping calderas (U.S. National Park Service).]]