Editing Yellowstone Caldera (section)

=== First-cycle ===
[[File:Map Volcanic Ashes Yellwostone Eruptions color.png|thumb|Map of the known ash-fall boundaries for major Pleistocene eruptions in Southwest US. By Volcano Hazards Program]]
The first-cycle lasted from about 2.15&nbsp;million to 1.95&nbsp;million years ago, spanning approximately 200&nbsp;kyr.{{sfn|Rivera|Darata|Lippert|Jicha|2017|p=384}} The only known pre-collapse rhyolitic unit is the Rhyolite of Snake River Butte, located just north of [[Ashton, Idaho|Ashton]] and dated at {{Value|2.1398|0.0035|u=million years}},{{sfn|Rivera|Darata|Lippert|Jicha|2017|p=380}} roughly 60–70&nbsp;kyr before the caldera-forming [[Huckleberry Ridge Tuff]].{{sfn|Wotzlaw|Bindeman|Stern|D’Abzac|2015|p=4}} Its vent lies near the eventual first-cycle caldera margin close to the Big Bend Bridge.{{sfn|Christiansen|2001|p=53}} Additional rhyolite flows may have erupted along the incipient ring-fault,{{sfn|Christiansen|2001|p=53}} but the pre-collapse rhyolite history likely spans no more than ~70&nbsp;kyr.{{sfn|Wotzlaw|Bindeman|Stern|D’Abzac|2015|p=4}} Another pre-collapse unit is the {{convert|60|to|70|m|ft|abbr=on}}-thick Junction Butte Basalt on the northeastern margin of the plateau,{{sfn|Christiansen|2001|p=53}} dated at {{Value|2.16|0.04|u=million years}}.{{sfn|Christiansen|2001|p=22}} The [[Overhanging Cliff]] basalt is a flow of this unit.{{sfn|Christiansen|2001|p=53}}

The first-cycle caldera-forming event was the eruption of the [[Huckleberry Ridge Tuff]] at {{Value|2.0773|0.0034|u=million years}} ago, during transitional magnetic polarity.{{sfn|Singer|Jicha|Condon|Macho|2014|p=35}} Its thickness exceeds {{convert|1|km|mi|abbr=on}} in the Red Mountains area.{{sfn|Wilson|2017|p=45}} The initial Plinian phase deposited up to {{convert|2.5|m|ft|abbr=on}} of fallout ash at [[Mount Everts]] before transitioning to ash-flow tuff.{{sfn|Christiansen|2001|p=55}}{{sfn|Wilson|2009}} Early Plinian activity was intermittent, sourced from multiple vents, probably lasted a few weeks and evacuated about {{convert|50|km3|mi3|abbr=on}} of magma from four magma bodies,{{sfn|Swallow|Wilson|Myers|Wallace|2018|p=32}} triggering caldera collapse at the onset of transition to ash-flow.{{sfn|Swallow|Wilson|Charlier|Gamble|2019|p=1374}}{{sfn|Swallow|Wilson|Myers|Wallace|2018|p=32}}
The ash-flow tuff is a composite sheet consisted of three intermittent members, with a total magma volume of about {{convert|2450|km3|mi3|abbr=on}}.{{sfn|Christiansen|2001|p=55}} Member A likely vented from the plateau's central area{{sfn|Christiansen|2001|p=55}} and tapped nine magma bodies.{{sfn|Swallow|Wilson|Myers|Wallace|2018|p=32}} After a hiatus of a few weeks or more,{{sfn|Swallow|Wilson|Charlier|Gamble|2019|p=1374}} the most voluminous Member B erupted from north of Big Bend Ridge.{{sfn|Christiansen|2001|p=57}} After another extended break of years to decades,{{sfn|Swallow|Wilson|Charlier|Gamble|2019|p=1374}} part of the Member A magmatic system was rejuvenated to feed Member C.{{sfn|Swallow|Wilson|Charlier|Gamble|2019|p=1374}} The least voluminous Member C might have source area near the Red Mountains, where it is about {{convert|430|m|ft|abbr=on}} thick.{{sfn|Christiansen|2001|p=59}} Some outcrops of Member A and Member C have been misidentified as Member B, complicating volume estimates of individual ash-flow unit.{{sfn|Phillips|Garwood|Feeney|2014}} Glen&nbsp;A. Izett estimated that an additional {{convert|2000|km3|mi3|abbr=on}} of ash was dispersed as fallout across North America.{{sfn|Izett|1981|p=10201}} Tephra fallout from this event is known as the Huckleberry Ridge ash bed (formerly "Pearlette type&nbsp;B"). Its area covered exceeds {{convert|3400000|km2|sqmi|abbr=on}}.{{sfn|Sarna-Wojcicki|Knott|Westgate|Budahn|2023|p=24}}. It is widely distributed and has been identified in the [[Pacific Ocean]] at [[Deep Sea Drilling Project]] Site&nbsp;36, about {{convert|1600|km|mi|abbr=on}} from Island Park Caldera,{{sfn|Sarna-Wojcicki|Morrison|Meyer|Hillhouse|1987|p=215}} as well as in the [[Humboldt County, California|Humboldt]] and [[Ventura County, California|Ventura]] basins of coastal California,{{sfn|Sarna-Wojcicki|Morrison|Meyer|Hillhouse|1987|p=207}} near [[Afton, Iowa|Afton]] in [[Iowa]], [[Benson, Arizona|Benson]] in [[Arizona]], and Campo Grande Mountain in [[Texas]].{{sfn|Izett|Wilcox|1982}}

One lava flow near the Sheridan Reservoir{{sfn|Watts|Bindeman|Schmitt|2011|p=862}} and two flows at the north end of Big Bend Ridge{{sfn|Christiansen|2001|p=64}} are post-collapse rhyolites of the first-cycle volcanism. The Sheridan Reservoir Rhyolite, dated at {{Value|2.07|0.19|u=million years}},{{sfn|Watts|Bindeman|Schmitt|2011|p=862}} if vented from the Island Park ring-fracture, required a flow distance of at least {{convert|20|km|mi|abbr=on}}.{{sfn|Watts|Bindeman|Schmitt|2011|p=863}} Its volume is estimated to exceed {{convert|10|km3|mi3|abbr=on}}.{{sfn|Watts|Bindeman|Schmitt|2011|p=860}} The other two flows, the Blue Creek flow and the overlying Headquarters flow, have a combined volume of {{convert|10-20|km3|mi3|abbr=on}}{{sfn|Balsley|Gregory|1998|p=130}} and erupted respectively at {{Value|1.9811|0.0035|u=million years}} and {{Value|1.9476|0.0037|u=million years}} ago.{{sfn|Rivera|Darata|Lippert|Jicha|2017|p=380}}