Editing Long Valley Caldera (section)

===Caldera===

The tectonic causes of the Long Valley volcanism are still largely unexplained and are therefore a matter of ongoing research. Long Valley is not above a [[hotspot (geology)|hotspot]], such as those which fuel [[Yellowstone Caldera]] or the volcanoes of [[Hawaii]], nor is it the result of [[subduction]] such as that which produces the volcanism of the [[Cascade Range|Cascades]].

[[File:Bishop tuff.jpg|thumb|left|Layers of the Bishop tuff, in a rock quarry in Chalfant Valley, about {{cvt|25|km|abbr=on}} southwest of the Long Valley Caldera, laid down in phases of a major eruption 760,000 years ago.]]

The known volcanic history of Long Valley Caldera area started a few million years ago when magma began to collect several miles below the surface. Volcanic activity became concentrated in the vicinity of the present site of Long Valley Caldera 3.1 to 2.5 million years ago with eruptions of [[rhyodacite]] followed by high-silica rhyolite from 2.1 to 0.8 million years ago. After some time, a cluster of mostly [[rhyolite|rhyolitic]] volcanoes formed in the area. All told, about {{convert|1500|sqmi}} were covered by lava.

All but one of these volcanoes, 1–2-million-year-old [[Glass Mountain (California)|Glass Mountain]] (made of [[obsidian]]),<ref>{{cite book | title=Geology Underfoot in Death Valley and Owens Valley | last=Sharp | first=Robert P. | author2=Allen F. Glazner | publisher=Mountain Press Publishing Company | location=Missoula, Montana | year=1997 | isbn=978-0-87842-362-0}}</ref>{{rp|264}} were destroyed by the major ([[volcanic explosivity index|VEI-7]]) eruption of the area 760,000 years ago, which released {{convert|600.|km3|spell=us}} of material from vents just inside the margin of the caldera.<ref>{{Cite journal|last1=Holohan|first1=Eoghan P.|last2=Troll|first2=Valentin R.|last3=Vries|first3=Benjamin van Wyk de|last4=Walsh|first4=John J.|last5=Walter|first5=Thomas R.|date=2008-04-01|title=Unzipping Long Valley: An explanation for vent migration patterns during an elliptical ring fracture eruption|url=https://pubs.geoscienceworld.org/gsa/geology/article-abstract/36/4/323/103861/Unzipping-Long-Valley-An-explanation-for-vent|journal=Geology|language=en|volume=36|issue=4|pages=323–326|doi=10.1130/G24329A.1|bibcode=2008Geo....36..323H|issn=0091-7613|url-access=subscription}}</ref> (The [[1980 Mount St. Helens eruption]] was a VEI-5 eruption releasing {{convert|1.2|km3|abbr=on}}.) About half of this material was ejected in a series of [[pyroclastic flow]]s of a very hot ({{convert|1500|F|abbr=on|adj=on}}) mixture of gases, [[pumice]], and [[volcanic ash]] that covered the surrounding area hundreds of feet deep. One lobe of this material moved south into [[Owens Valley]], past present-day [[Big Pine, California|Big Pine]]. Another lobe moved west over the crest of the [[Sierra Nevada (U.S.)|Sierra Nevada]] and into the drainage of the [[San Joaquin River]]. The rest of the pyroclastic material, along with {{convert|300|km3|abbr=on}} of other matter, was blown as far as {{convert|25|mi}} into the air where winds distributed it as far away as eastern [[Nebraska]] and [[Kansas]].

The eruption initially produced a caldera {{cvt|2|-|3|km|abbr=on}} deep. However, much of the ejecta went straight up, fell down, and filled the initial caldera about two-thirds full.