Editing Mount Shasta (section)

== Geology ==
[[File:Mount Shasta ISS068-E-17084.jpg|thumb|Mount Shasta photographed by a crew member during the [[International Space Station]]'s [[Expedition 68|68th expedition]], in October 2022]]

About 593,000 years ago, [[andesite|andesitic]] lavas erupted in what is now Mount Shasta's western flank near McBride Spring. Over time, an ancestral Mount Shasta [[stratovolcano]] was built to a large but unknown height; sometime between 300,000 and 360,000 years ago the entire north side of the [[volcano]] collapsed, creating an enormous [[volcanic landslide|landslide]] or [[debris avalanche]], {{convert|6.5|mi3|km3|0|abbr=on}}<ref name =eov>{{cite book | last = Sigurdsson | first = Haraldur | author-link = Haraldur Sigurdsson | title = Encyclopedia of Volcanoes | publisher = Academic Press | year = 2001 | isbn = 978-0-12-643140-7 }}</ref> in volume. The slide flowed northwestward into [[Shasta Valley]], where the [[Shasta River]] now cuts through the {{convert|28|mi|km|adj=mid|-long}} flow.

What remains of the oldest of Mount Shasta's four cones is exposed at Sargents Ridge on the south side of the mountain. Lavas from the Sargents Ridge vent cover the Everitt Hill shield at Mount Shasta's southern foot. The last lavas to erupt from the vent were [[hornblende]]-[[pyroxene]] andesites with a hornblende [[dacite]] dome at its summit. Glacial erosion has since modified its shape.<ref>{{Cite journal |last1=Christiansen |first1=Robert L. |last2=Calvert |first2=Andrew T. |last3=Champion |first3=Duane E. |last4=Gardner |first4=Cynthia A. |last5=Fierstein |first5=Judith E. |last6=Vazquez |first6=Jorge A. |date=2020-08-10 |title=The remarkable volcanism of Shastina, a stratocone segment of Mount Shasta, California |journal=Geosphere |volume=16 |issue=5 |pages=1153–1178 |doi=10.1130/GES02080.1 |issn=1553-040X|doi-access=free |bibcode=2020Geosp..16.1153C }}</ref>

The next cone to form is exposed south of Mount Shasta's current summit and is called Misery Hill. It was formed 15,000 to 20,000 years ago from pyroxene andesite flows and has since been intruded by a hornblende dacite dome.{{citation needed|date=September 2012}}

[[File:Black Butte from Weed, California-750px.jpg|thumb|Nearby [[Black Butte (Siskiyou County, California)|Black Butte]], seen from Weed, California]]

There are many buried glacial scars on the mountain that were created in recent glacial periods ("ice ages") of the present [[Wisconsin glaciation|Wisconsinian glaciation]]. Most have since been filled in with [[andesite]] [[lava]], [[pyroclastic flow]]s, and [[Scree|talus]] from lava domes. Shastina, by comparison, has a fully intact summit crater indicating Shastina developed after the [[Last glacial period|last ice age]]. Shastina has been built by mostly pyroxene andesite lava flows. Some 9,500 years ago, these flows reached about {{convert|6.8|mi|km|abbr=on}} south and {{convert|3|mi|km|abbr=on}} north of the area now occupied by nearby [[Black Butte (Siskiyou County, California)|Black Butte]]. The last eruptions formed Shastina's present summit about a hundred years later. But before that, Shastina, along with the then forming Black Butte dacite [[plug dome]] complex to the west, created numerous pyroclastic flows that covered {{convert|43|mi2|km2|abbr=on}}, including large parts of what is now [[Mount Shasta, California]] and [[Weed, California]]. Diller Canyon ({{convert|400|ft|m|-1|abbr=on|disp=or}} deep and {{convert|0.25|mi|m|abbr=on|disp=or}} wide) is an avalanche chute that was probably carved into Shastina's western face by these flows.{{citation needed|date=September 2012}}

The last to form, and the highest cone, the Hotlum Cone, formed about 8,000 years ago. It is named after the Hotlum glacier on its northern face; its longest lava flow, the {{convert|500|ft|m|-thick|abbr=|sp=us|adj=mid|-1}} Military Pass flow, extends {{convert|5.5|mi|km|abbr=on}} down its northeast face. Since the creation of the Hotlum Cone, a dacite dome intruded the cone and now forms the summit. The rock at the {{convert|600|ft|m|-wide|abbr=|sp=us|adj=mid|-1}} summit crater has been extensively hydrothermally altered by sulfurous [[hot spring]]s and [[fumarole]]s there (only a few examples still remain).{{citation needed|date=September 2012}}

In the last 8,000 years, the Hotlum Cone has erupted at least eight or nine times. About 200 years ago, the last significant Mount Shasta eruption came from this cone and created a pyroclastic flow, a hot [[lahar]] (mudflow), and three cold lahars, which streamed {{convert|7.5|mi|km|abbr=on}} down Mount Shasta's east flank via Ash Creek. A separate hot lahar went {{convert|12|mi|km|abbr=on}} down Mud Creek. This eruption was thought to have been observed by the explorer [[Jean-François de Galaup, comte de La Pérouse|La Pérouse]], from his ship off the California coast, in 1786, but this has been disputed.<ref>{{cite web |url=http://www.siskiyous.edu/shasta/his/ |website=Mount Shasta Companion |title=History |access-date=2010-03-31 |url-status=dead |archive-url=https://web.archive.org/web/20100829195745/http://www.siskiyous.edu/shasta/his/ |archive-date=2010-08-29 }}</ref>

=== Volcanic status ===
During the last 10,000 years, Mount Shasta has erupted an average of every 800 years, but in the past 4,500 years the volcano has erupted an average of every 600 years.<ref name="gvp " />
[[File:Diller Canyon.jpg|thumb|left|upright=1.25|Diller Canyon on Shastina from Weed]]

USGS seismometers and GPS receivers operated by UNAVCO form the monitoring network for Mount Shasta. The volcano has been relatively quiet during the 21st century, with only a handful of small magnitude earthquakes and no demonstrable [[Deformation (volcanology)|ground deformation]]. Although geophysically quiet, periodic geochemical surveys indicate that volcanic gas emanates from a fumarole at the summit of Mount Shasta from a deep-seated reservoir of partly molten rock.<ref>https://pubs.er.usgs.gov/publication/sir20185159|California’s Exposure to Volcanic Hazards Scientific Investigations Report 2018-5159 Prepared in cooperation with the California Governor’s Office of Emergency Services and the California Geological Surve</ref>

[[File:Mount Shasta west face.jpg|thumb|right|Mount Shasta's west face as seen from Hidden Valley high on the mountain. The west face gulley is an alternate climbing route to the summit.]]
Mount Shasta can release [[volcanic ash]], pyroclastic flows or [[dacite]] and [[andesite]] [[lava]]. Its deposits can be detected under nearby small towns. Mount Shasta has an explosive, eruptive history. There are [[fumarole]]s on the mountain, which show Mount Shasta is still alive.{{citation needed|date=September 2012}}

The worst-case scenario for an eruption is a large pyroclastic flow, similar to that which occurred in the [[1980 eruption of Mount St. Helens]]. Since there is ice, such as [[Whitney Glacier]] and [[Mud Creek Glacier]], [[lahar]]s would also result.

The [[United States Geological Survey]] monitors Mount Shasta<ref>{{cite web|url=https://volcanoes.usgs.gov/volcanoes/mount_shasta/mount_shasta_monitoring_4.html|title=USGS: Volcano Hazards Program CalVO Mount Shasta|first=Volcano Hazards|last=Program|website=volcanoes.usgs.gov|access-date=23 October 2017|archive-date=11 July 2017|archive-url=https://web.archive.org/web/20170711191818/https://volcanoes.usgs.gov/volcanoes/mount_shasta/mount_shasta_monitoring_4.html|url-status=live}}</ref> and rates it as a very high-threat volcano.<ref>{{cite web|url=http://pubs.usgs.gov/of/2005/1164/2005-1164.pdf|title=An Assessment of Volcanic Threat and Monitoring Capabilities in the United States: NVEWS Framework for a National Volcano Early Warning System|year=2005|publisher=USGS|access-date=2013-07-08|archive-date=2012-07-12|archive-url=https://web.archive.org/web/20120712175227/http://pubs.usgs.gov/of/2005/1164/2005-1164.pdf|url-status=live}}</ref>