Mount Churchill

Mount Churchill is a dormant volcano in the Saint Elias Mountains and the Wrangell Volcanic Field (WVF) of eastern Alaska. Churchill and its neighbor Mount Bona are both ice-covered volcanoes with Churchill having a caldera just east of its summit. There are sparse outcrops of lava flows and tephra, mostly dacite.

Subduction of the Pacific Plate beneath southeastern Alaska has largely ceased during the last one million years, causing a decline of the volcanic activity in the WVF. Churchill appears to be fed by melts derived from a stagnant slab in the mantle, left over by the previous subduction.

The volcano erupted several times during the Holocene. The most notable eruptions are the two White River Ash eruptions, deposited during two of the largest volcanic eruptions in North America during the past two millennia. The northern lobe was emplaced about 1,890 years ago, while the larger eastern lobe erupted in winter 852/853. The White River Ash covers vast expanses of Alaska and western Canada and has been found as far as Europe, and there is evidence that the Athabaskan people migrated out of the region and into the present-day United States as a consequence of the eruption.

Geography and geomorphology

The mountain is in the University Mountains sub-rangeTemplate:Sfn of the Saint Elias Mountains of Alaska,Template:Sfn east of McCarthy, Alaska,Template:Sfn and Template:Sfn or from the border with Canada.Template:Sfn The area is part of the Wrangell-St. Elias National Park and Preserve.Template:Sfn It is extremely remoteTemplate:Sfn and there are no roads from which it is visible.Template:Sfn The mountain was first ascended in 20 August 1951 by R. Gates and J. LindbergTemplate:Sfn and named in 1965 after the English politician Winston ChurchillTemplate:Sfn and is also known as Klutlan Glacier, Churchill-Bona, or White River volcano.Template:Sfn

Various measurements have yielded summit heightsTemplate:Efn of ,Template:Sfn ,Template:SfnTemplate:Sfn Template:Sfn or .Template:Sfn It is a mountain in a glaciated,Template:Efn rugged mountain massifTemplate:Sfn that rises sharply above the surrounding land.Template:Sfn It is the tenth-highest peak in the United States.Template:Sfn The mountain is mostly covered byTemplate:Sfn ice hundreds of meters thick,Template:Sfn but lava flows with columnar jointing and tephra deposits form outcrops,Template:Sfn indicating that Mount Churchill may be a stratovolcano.Template:Sfn East of Mount Churchill,Template:Sfn below the summit,Template:Sfn is a caldera,Template:Sfn which forms a flat amphitheater open to the northeast. Numerous outcrops of light-colored pumice with embedded lithicsTemplate:Efn occur around the amphitheater,Template:Sfn which is otherwise entirely ice-covered.Template:Sfn There are further outcrops of tephra in areas protected from erosion around the volcano; the largest such outcrop covers an area exceeding .Template:Sfn Pumice forms terraces above the sides of the Klutlan Glacier, over a length of more than .Template:Sfn Their position above the present-day glacier surface may indicate that at the time of their deposition, the ice was thicker than present-day. Alternatively,Template:Sfn they could have deposited during floods over the ice, perhaps after an eruption or the breach of a pumice-dammed lake.Template:Sfn A pumice mound on the other side of the glacier,Template:Sfn from Mount Churchill,Template:Sfn was formed by tephra building up on a bedrock bench.Template:Sfn It was once considered the vent of the White River Ash.Template:Sfn In some places, volcanic ash covers the pumice.Template:Sfn

The olderTemplate:Sfn Mount Bona is southwest of Mount Churchill.Template:Sfn With a summit height of Template:Efn above sea level,Template:Sfn it is the highest mountain in the Wrangell MountainsTemplate:SfnTemplate:Sfn and the highest volcano in the Wrangell volcanic fieldTemplate:Sfn and the United States in general.Template:Sfn A snow-covered col at elevation separates the two mountains.Template:Sfn Both mountains are covered with about of ice.Template:Sfn The Russell and Klutlan Glaciers run along the northern-western and easternTemplate:Sfn-southern side of Mount Churchill, respectively.Template:SfnTemplate:Sfn The Klutlan Glacier is flanked by moraines and talus deposits.Template:Sfn Both glaciers eventually discharge into the White River.Template:Sfn Glaciers on the southern flank of Mount Bona discharge into the Chitina River.Template:Sfn Ice on Mount Churchill is up to 800, and possibly 1500, years old.Template:Sfn

Geology

The more than Template:Sfn Wrangell volcanic field (WVF) has been active for the past 30 million yearsTemplate:Sfn in the Wrangell and St. Elias Mountains.Template:Sfn The Wrangell volcanic field features numerous large shield volcanoes,Template:Sfn which are among the largest arc volcanoes on Earth.Template:Sfn Mount DrumTemplate:Sfn and other volcanoes in the WVF during the middle Pleistocene had eruptions even larger than the White River Ash eruptions.Template:Sfn Mount ChurchillTemplate:Sfn and Mount Wrangell are the only volcanoes in the WVF with Holocene eruptions.Template:Sfn With the exception of Mount Churchill, volcanism in the Wrangell volcanic field has migrated northwestwardsTemplate:Sfn and declined asTemplate:Sfn plate configuration changed about 200,000 years agoTemplate:Sfn and subduction ceased.Template:Sfn

Mount Churchill and Mount Bona consist of andesitic lava flows. University Peak is a 8.4 million years old volcanic intrusion, now exposed through erosion.Template:Sfn The basement under Mount Bona is formed by a plateau consisting of Permian to Pennsylvanian-age rocksTemplate:Sfn and Tertiary granites; most of Mount Bona may be formed by these nonvolcanic rocks.Template:Sfn

Off the western coast of southeastern Alaska, the Pacific Plate used to subduct under the North American Plate, giving rise to the WVF.Template:Sfn Since the Jurassic,Template:Sfn seven separate terranes were transported to Alaska by the Pacific Plate and attached to the continent:Template:Sfn Windy terrane, the various Wrangellia terranes,Template:Sfn Chugach, Prince William and most recently the Yakutat Block, which is in the process of being accreted.Template:Sfn The collision with the Yakutat Block caused the cessation of subduction, with plate motion now occurring along strike-slip faults like the Denali and Totschunda FaultsTemplate:Sfn while subduction continues farther west in the Aleutian megathrust.Template:Sfn The intersection of the Totschunda Fault with the Connector and Duke River fault(s) may be the point where magma ascends into Mount Churchill.Template:Sfn

Composition and origin of magmas

Churchill rocks are daciticTemplate:Efn and define a calc-alkalineTemplate:Sfn adakite suite.Template:Sfn There is a moderate quantity of phenocrysts, including biotite, hornblende, ilmenite, hypersthene, magnetite and plagioclase, with little apatite and orthopyroxene.Template:SfnTemplate:SfnTemplate:Sfn Several different rock chemistries contribute to each of the White River Ash lobes,Template:Sfn which are otherwise very similar to each otherTemplate:Sfn and thus difficult to distinguish.Template:Sfn The particles in the eastern lobe are coarser than in the northernTemplate:Sfn and show evidence of two separate chemical trends;Template:Sfn the deposits on Mount Churchill match the composition of the eastern lobe.Template:Sfn Reconstructed magma temperatures are for the northern lobe magma and for the eastern lobe magma.Template:Sfn

The Wrangell slabTemplate:Sfn left over from the subduction may have stalled in the mantle, and was heated by asthenosphere flowing through a slab window until it melted and gave rise to the Mount Churchill magmas,Template:SfnTemplate:Sfn which thus have an adakitic composition typical for melts derived from subducted basalts at high temperatures.Template:Sfn During ascent, the magmas were further modified by interaction with the underlying basementTemplate:Sfn of the Alexander terrane.Template:Sfn Each of the White River Ash eruptions probably involved several different magma batches, rather than one layered magma chamber.Template:SfnTemplate:Sfn

Ice cores and climate

Several ice cores have been taken from the Bona-Churchill massifTemplate:Sfn and are an important source of information on the climate of the Pacific Northwest.Template:Sfn An ice core taken in 2002 from the col between Mount Churchill and Mount BonaTemplate:Sfn is the longest non-polar ice core since 2006Template:Dated maintenance category (articles),Template:Sfn being long.Template:Sfn

The ice cores record evidence of volcanic eruptions, including of Katmai, Krakatau, Laki and Tambora, and of climate variations like the Medieval Warm Period and the Little Ice Age. Other processes recorded in the Bona-Churchill ice cores are dust emissions in China,Template:Sfn wildfires in Alaska,Template:Sfn North Pacific sea surface temperatures, position of the Aleutian Low weather systemTemplate:Sfn and Arctic sea ice cover.Template:Sfn Shallow ice and snow has been used to reconstruct dust composition at the St. Elias Mountains.Template:Sfn

The Chugach Mountains block the maritime airmasses, leading to a continental climate in the region.Template:Sfn Mean annual temperature on Mount Churchill is about .Template:Sfn Annually, about of snow water equivalent falls on Mount Churchill.Template:Sfn

Eruption history

The age of the Churchill-Bona massif is unknownTemplate:Sfn but Mount Churchill began erupting during the late Pleistocene. Potassium-argon dating has yielded an age of 119,000±17,000 years for a dacite lava close to the summit.Template:Sfn The 190,000 years oldTemplate:Sfn Sheep Creek tephra sub-unit "F"Template:Sfn in Canada and AlaskaTemplate:Sfn may have originated at Mount Churchill, but more likely at Mount Drum.Template:Sfn The appearance and height of Mount Churchill (and neighbouring Bona) imply that they were constructed in recent time.Template:Sfn The mountain may have looked very different before the White River Ash eruptions.Template:Sfn

There are six Holocene volcanic eruptions that may be attributed to Mount Churchill.Template:Sfn Ash emplaced around 647±55 CE may come either from Mount Churchill or Redoubt volcano,Template:Sfn and European tephras emplaced around 2,350 BCE and Greenland-Europe tephras from around 1100 CE resemble these of Mount Churchill.Template:SfnTemplate:Sfn Two tephra layers in southeastern Alaska, the 300 years old "Lena ash" and the 6,330 years old "MTR-146" ash, resemble the White River AshTemplate:Sfn and may have been produced by eruptions of Mount Churchill;Template:SfnTemplate:Sfn tephra with similar composition to the "Lena tephra" has been found in Europe.Template:Sfn If the 1650 CE "Lena ash" comes from the volcano, it would be its youngest eruption.Template:Sfn Beyond these, volcanic activity was uncommon in the region.Template:Sfn

White River Ash eruptions

Mount Churchill is the sourceTemplate:Efn of two of the largest volcanic eruptions of the past two millennia in North America.Template:Sfn The first eruption about 1,890 years ago emplaced the northern lobe of the White River Ash ("Northern White River Ash"),Template:Sfn the better known second eruptionTemplate:Sfn in winter 852/853Template:EfnTemplate:Sfn emplaced the eastern lobe ("Eastern White River Ash").Template:Sfn Both were very violentTemplate:Sfn Plinian eruptionsTemplate:Sfn with a volcanic explosivity index of 6.Template:Sfn

Deposits from the eruption were first discovered in 1883 along the upper Yukon River. After Mount Wrangell had been ruled out as its source in 1892, Mount Natazhat was proposed instead as the source vent and in 1965 Mount Bona. Only in 1984 and 1995 was Mount Churchill identified as the source.Template:Sfn The eruptions produced about of tephraTemplate:Sfn and covered an area exceeding Template:Sfn in Alaska,Template:Efn Yukon Territory and Northwest Territories.Template:SfnTemplate:Sfn The present-day towns of Dawson City and Whitehorse, Canada, are within the thickness area of the northern and eastern lobe, respectively.Template:Sfn The ash is located at shallow depth in the ground,Template:Sfn unless carried deeper underground by soil processes.Template:EfnTemplate:Sfn It forms conspicuous layers along the Alaska Highway,Template:Sfn in riverbanksTemplate:Sfn of the Yukon, Tanana and their tributaries.Template:Sfn The ash layers affect the properties of the soil they are in;Template:Sfn they contribute to the formation of soilsTemplate:Sfn and sometimes they are the detachment surface of landslides.Template:Sfn Closer to the US-Alaska border at the Klutlan Glacier it thickens to form dune-covered ash fieldsTemplate:Sfn and areas lacking vegetation, as the ash is an unsuitable ground for plant growth.Template:Sfn Stumps of trees killed by the fallout emerge from the ash layers close to Mount Churchill.Template:Sfn Ash is frequently reworked and redeposited,Template:SfnTemplate:Sfn and forms soils in the St. Elias Mountains.Template:Sfn Glaciers such as the Barnard and Klutlan Glaciers have captured and transported pumice and ash,Template:SfnTemplate:Sfn or eroded ash layers when they advanced;Template:Sfn some moraines at the foot of the St. Elias Mountains are formed mainly by White River Ash.Template:Sfn Ash is washed away by the Klutlan and White River, contributing in no small part (together with glacial flour) to its distinctive color that gives the White River its name.Template:Sfn The ash deposits have been used as a time marker in tephrochronology to obtain dates for natural events and archaeological sitesTemplate:Sfn from Alaska and YukonTemplate:Sfn as well as Greenland (correlation of ice cores)Template:Sfn and Ireland in Europe.Template:Sfn

Apart from direct physical effects, the Mount Churchill eruption likely had a strong psychological effect on the people in the affected area. The eruption column would have been visible for many hundreds of kilometers. Soon after it began, the sky would have turned dark for days and noise and lightning would have been heard and seen in Yukon.Template:SfnTemplate:Sfn About 500 people might have been living in the directly affected area.Template:Sfn It is probable that there were no direct casualties from the eruption; pyroclastic flows and other direct effects of the eruption were limited to the uninhabited surroundings of Mount Churchill, and the structures humans lived in at the time were unlikely to collapse under ash accumulation.Template:SfnTemplate:Sfn There is disagreement on whether oral tradition referring to the White River Ash eruptions can be identified among the Athapaskans.Template:SfnTemplate:SfnTemplate:Sfn

Northern White River Ash

The northern White River Ash extends along the Alaska–Canada borderTemplate:Sfn and reaches a thickness of west of the volcano, declining to north of Mount Churchill.Template:Sfn The White River Ash is a formal stratigraphic unit in Alaska,Template:Sfn and particles from it have been detected as far as the northern Brooks Range in Alaska.Template:Sfn The widespread "PWS tephra" in Prince William Sound was emplaced between 2,039 and 1,520 years ago and resembles the northern White River Ash.Template:Sfn The eruption may have occurred during summer, when winds blow from the south,Template:SfnTemplate:Sfn and the eruption column might have been high.Template:Sfn While not as well studied as the east lobe eruption,Template:Sfn its impact on human populations was relatively modest, with few signs of population or culture shifts.Template:SfnTemplate:Sfn

Eastern White River Ash

The eastern White River Ash is better studiedTemplate:Sfn and covers a wider area.Template:Sfn Its intensity was intermediate between the Mount Mazama eruption and the 1883 eruption of Krakatau.Template:Sfn It was more than twice the size of the 1912 Novarupta/Katmai eruptionTemplate:Sfn and was ten times larger than the 1991 eruption of Mount Pinatubo.Template:Sfn A eruption column rose over the volcano,Template:Sfn injecting ash into the stratosphere;Template:Sfn ash fell more than a thousand kilometers awayTemplate:Sfn and sulfate and chloride precipitated in the Greenland Ice Sheet.Template:Sfn Strong westerly winds carried the ash cloud eastward,Template:Sfn where it may have mixed with snow as it fell out.Template:Sfn The eastern lobe of the White River Ash is thick at distance from Mount Churchill,Template:Sfn extending to the Great Slave and Great Bear Lakes.Template:Sfn The eastern White River Ash has a color ranging from white to beige.Template:Sfn

Ash deposits from the eastern White River Ash have been detected across North America and into Europe, where it is identical to the "AD860B" ash found in Ireland,Template:Sfn Great Britain, Scandinavia, Germany, PolandTemplate:EfnTemplate:Sfn and Greenland. Other findings are in Nova Scotia,Template:Sfn south-central Alaska,Template:Sfn southeastern Alaska and the adjacent Pacific Ocean,Template:Sfn Newfoundland,Template:Sfn Maine,Template:Sfn potentially as far as Tibet.Template:Sfn These findings from the volcano make the White River Ash one of the most extensive tephra deposits of the past 100,000 years,Template:Sfn and drew attention to the potential for intercontinental spread of volcanic ashTemplate:Sfn even by once-per-century eruptions.Template:Sfn

Territories impacted by the ashfall may have needed decades to recover,Template:Sfn with century-long changes in vegetation, aquatic and peat productivityTemplate:Sfn as forests opened up in some areas with ashfall.Template:Sfn In lakes, volcanic ash can either bury organisms,Template:Sfn or release nutrients such as phosphorus and thus increase productivity; both effects have been noted for the White River Ash.Template:Sfn Burial of food sources and ingestion of ash and fluoride would have impacted caribou, goats, moose and sheep populations,Template:Sfn forcing them to move away; genomic data indicate a large shift in caribou populations after the eastern White River Ash eruption,Template:Sfn although this theory is not uncontested.Template:Sfn Ash fall into rivers and the remobilization of ash fallen on land would have disrupted waterfowl, salmon runs and other fish populations,Template:Sfn although anadromous fish populations would have recovered within a short timeframe.Template:Sfn

Southern Yukon was depopulated by the eruption.Template:Sfn Local hunter-gatherer populations probably left the worst-hit areas and sought refuge in unaffected regions, returning only when conditions had improvedTemplate:Sfn or not at all. Archaeological data indicate that some important trade routes were abandoned and new ones established after the eastern White River Ash eruption, implying that the displacement fostered a re-evaluation of economic activity and that displaced people had set up new trade networks.Template:SfnTemplate:Sfn The use of copperTemplate:EfnTemplate:Sfn and bows and arrows may have arrived in the Yukon territory that way,Template:Sfn and Dene people moved into coastal areas, sometimes coming into conflict with previously established people there and sometimes establishing new kin and commercial networks.Template:Sfn Other Dene people migrated south and eastTemplate:Efn after the eruption, driving the Athabaskan expansion and spreading the Na-Dene languages across the continent. By the arrival of the Europeans,Template:SfnTemplate:SfnTemplate:SfnTemplate:Sfn Athabaskans like the ApacheTemplate:Efn and NavajoTemplate:SfnTemplate:Sfn had spread between subarctic Canada and the Great Basin of the southwestern United States, bringing their languages with them.Template:Sfn

The eruption produced sulfate aerosols,Template:Sfn which can dim the Sun and cause a cooling of Earth's climate, creating a volcanic winter.Template:Sfn The sulfur yield, 2.5 teragrams, was relatively modest, one third of that from the 1991 eruption of Mount Pinatubo.Template:Sfn Climate models imply a maximum cooling of ,Template:Sfn reaching in some models,Template:Sfn with no clear changes in precipitation.Template:Sfn There are widespread reports of bad weather and resulting hardships such as famines during that decade in Europe,Template:Sfn and a clear link to the Mount Churchill eruption is not established;Template:Sfn at worst, it would have aggravated a pre-existent climate disturbance.Template:Sfn A link between the White River Ash and the mid-6th century cooling (Late Antique Little Ice Age) has been ruled out.Template:Sfn

Hazards

Mount Churchill is one of Canada's most dangerous volcanoes,Template:Sfn despite being outside of the country,Template:Sfn owing to the size of its eruptions. Renewed large-scale activity would be extremely hazardous for northwestern Canada and adjoining Alaska.Template:Sfn Smaller eruptions could threaten the White River valley and the Alaska Highway thereTemplate:Sfn with ash fall and floodsTemplate:Sfn caused by blockages in the White River.Template:Sfn Similar flood hazards exist in the Chitina and Copper River valleys south of Mount Churchill.Template:Sfn The United States Geological Service ranks Mount Churchill as a "high threat" volcano.Template:EfnTemplate:Sfn

Ashfall could damage machinery, forests and waterbodies, and cause breathing problems.Template:Sfn Even small eruptions of the high volcano could cause disturbances in air travel.Template:Sfn In addition, the intercontinental spread of ash would cause severe disruption, similar but on a larger scale to the 2010 eruptions of Eyjafjallajökull, with resultant consequences to transportation and the airline industry.Template:Sfn Aircraft routes between Asia, Europe and North America pass through the extent of the White River Ash plume.Template:Sfn

References

Sources

• Template:Cite GVP
• Template:Cite thesis

External links

• Template:Cite bivouac