Mount Churchill

This article is about a landform in Alaska. For the mountain in Canada, see Mount Churchill (British Columbia).

Mount Churchill
U.S. Geological Survey climbing party ascending the Klutlan Glacier en route to Mount Churchill
Highest point
Elevation	15,638 ft (4,766 m)
Prominence	1,188 ft (362 m)
Coordinates	61°25′10″N 141°42′53″W
Geography
Mount Churchill Wrangell-St. Elias National Park and Preserve, Alaska, U.S.
Parent range	Saint Elias Mountains
Topo map	USGS McCarthy B-2 Quadrangle
Geology
Mountain type	Stratovolcano with caldera
Volcanic arc/belt	Wrangell Volcanic Field
Last eruption	700 AD ± 200 years
Climbing
First ascent	August 20, 1951 by R. Gates, J. Lindberg
Easiest route	snow climb

Mount Churchill is a dormant volcano in the Saint Elias Mountains and the Wrangell Volcanic Field of eastern Alaska. Churchill and its higher neighbor Mount Bona about 2 mi (3 km) to the southwest are both large ice-covered stratovolcanoes, with Churchill being the fourth highest volcano in the United States and the seventh highest in North America.

Mount Churchill is most noteworthy as the source of the White River Ash, deposited during two of the largest volcanic eruptions in North America during the past two millennia. This twin-lobed tephra deposit covers more than 130,000 square miles (340,000 km²) of eastern Alaska and northwestern Canada, with the northern lobe deposited 1900 years ago and extending over 250 miles (400 km) and the larger eastern lobe about 1,250 years ago and stretching over 500 miles (800 km). The total volume of the ash exceeds 12 cubic miles (50 km³), or roughly 50 times the volume of the 1980 eruption of Mount St. Helens, and ash layers up to 2 feet (61 cm) thick can be seen just below the surface in many roadcuts along the Alaska Highway.

The extensive ash deposits in the lowlands near the White and Yukon Rivers were first recognized in 1883, but their source remained a mystery for the next century. Geologists in the 1960s traced the two lobes of the ash back into the Saint Elias Mountains, and postulated that the ash may have come from a vent now buried under the Klutlan Glacier, which flows east for over 40 mi (64 km) from the Bona-Churchill massif into the Yukon Territory of Canada. More detailed studies in the 1990s by the U.S. Geological Survey finally produced the definitive answer. Aerial photos showed a 2.6 by 1.7 miles (4.2 km × 2.7 km) elliptical, gently sloping, ice-filled depression at 14,500 feet (4,400 m) just east of the present summit of Mount Churchill. This was identified as a caldera, which had formed by the collapse of the volcano's previous summit during the cataclysmic eruptions. The geological field work revealed thick young pumice deposits along the rim of the caldera which are mineralogically and chemically identical to the White River Ash.

U.S. Geological Survey climbing party reaching the east rim of the summit caldera on Mount Churchill. Blocky debris in the photo consists of pumice and lithic fragments ejected 1,250 years ago, during the eruption which formed the eastern lobe of the White River Ash.

Mount Churchill was first climbed in 1951 by R. Gates and J. Lindberg, but the peak was merely an unnamed satellite of Mount Bona at the time. The mountain was named in 1965 by the Alaska State Legislature for English statesman Winston Churchill.

In terms of elevation, it is a major North American peak, at well over 15,000 feet (4,600 m); however, in terms of topographic prominence or isolation, it is less significant, with less than a 1200-foot drop from its summit to the saddle with Bona. Churchill also lies on the northern, gentler side of the Bona massif, making it a less visually spectacular peak than some of the lower outliers of Bona such as University Peak or Aello Peak. The current standard climbing route is the South Ridge, usually as part of a climb of Mount Bona from the east starting from a ski-equipped bush plane landing at around 10,000 feet (3,000 m) on the Klutlan Glacier.

Geography and geomorphology

The mountain is in the University Mountains sub-range of St. Elias Mountains of Alaska, 64 kilometres (40 mi) east of McCarthy, Alaska and close to the border with Canada. The area is part of the Wrangell-St. Elias National Park and Preserve. It is extremely remote and there are no roads from which it is visible. The mountain was named in 1965 after the English politician Winston Churchill and is also known as Klutlan Glacier, Churchill-Bona, or White River volcano.

Churchill is a 4,744 metres (15,564 ft), 4,766 metres (15,636 ft) or 4,767 metres (15,640 ft) high peak in a glaciated, rugged mountain massif. It is located on the western side of and 300 metres (980 ft) above a 2.7 by 4.2 kilometres (1.7 mi × 2.6 mi) wide caldera, which forms a flat amphitheatre open to the northeast. Numerous outcrops of light-coloured pumice with embedded lithics occur around the amphitheatre, which is otherwise entirely ice-covered. There further outcrops of tephra in areas protected from erosion around the volcano; the largest such outcrop covers an area exceeding 3 square kilometres (1.2 sq mi).

The older Mount Bona is 3.2 kilometres (2 mi) southwest of Mount Churchill and rises 5,005 metres (16,421 ft) above sea level. The Russell Glacier and the Klutlan Glacier run along the northern-western and eastern-southern side of Mount Churchill, respectively. A 90 metres (300 ft) high pumice mound on the other side of the glacier, 16 kilometres (9.9 mi) from Mount Churchill, was once considered the vent of the White River Ash. Both glaciers eventually discharge into the White River.

Geology

Off the western coast of southeastern Alaska, the Pacific Plate used to be subducted under the North American Plate. Beginning 26 million years ago, this subduction gave rise to the Wrangell volcanic field that extends through the Wrangell and St. Elias Mountains. Seven separate terranes were transported to Alaska by the Pacific Plate and were attached to the continent since the Jurassic: The Windy terrane, the various Wrangellia terranes, Chugach, Prince William and most recently the Yakutat Block, which is in the process of being accreted. The collision with the Yakutat Block caused subduction to cease, with plate motion now occurring along strike-slip faults like the Denali and Totschunda Faults.

The volcano is part of the Wrangell volcanic field and together with Mount Wrangell the only volcano there with Holocene eruptions. Mount Churchill and the neighbouring Mount Bona consist of andesitic lava flows. University Peak is a 8.4 million years old volcanic intrusion, now exposed through erosion.

Composition

Volcanic rocks are dacitic and define a calc-alkaline rock suite. There is a moderate quantity of phenocrysts, including biotite, hornblende and plagioclase. There are variations in rock chemistry between the various White River Ash samples. Both lobes of the White River Ash have very similar composition; the deposits on Mount Churchill resemble the eastern lobe.

Eruption history

The age of the Churchill-Bona massif is unknown but 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.

Mount Churchill is the source of two of the largest volcanic eruptions of the past two millennia in North America, both reaching a volcanic explosivity index of 6. The White River Ash consists of two lobes, a northern one emplaced 1,890 years ago and an eastern one emplaced 1,250 years ago. The second eruption occurred in 847, with an uncertainty of one year.

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 the correct location, Mount Churchill, identified. The eruptions produced about 25–50 cubic kilometres (6.0–12.0 cu mi) of tephra and covered an area exceeding 340,000 square kilometres (130,000 sq mi) in Alaska, Yukon Territory and Northwest Territories. The present-day towns of Dawson, Alaska and Whitehorse, Canada are within the 25 millimetres (1 in) thickness area of the northern and eastern lobe, respectively. The ash forms conspicuous layers along the Alaska Highway, in riverbanks of the Yukon, Tanana and their tributaries, which are often exploited by ground squirrels who dig their burrows into the ash. Closer to the US-Alaska border at the Klutlan Glacier it thickens to form dune-covered ash fields. The glacier has transported the ash that fell on it to the terminal moraine.

They had a noticeable impact on the Athapaskans. According to early researchers, the eruptions must have darkened the sun for days.

Eastern White River Ash

The eastern White River Ash is the larger of the two.

Hazards

Renewed activity would be extremely hazardous for northwestern Canada and adjoining Alaska.

References

Sources

• Capps, S.R. (1916). An ancient volcanic eruption in the upper Yukon Basin (Report). Professional Paper 95-D. Washington, D.C.: United States Government Printing Office. pp. 59–72. doi:10.3133/pp95D.
• "Churchill". Global Volcanism Program. Smithsonian Institution. Retrieved 7 January 2024.
• "Churchill". USGS National Map. 31 December 1981. Retrieved 7 January 2024.
• McGimsey, Robert G.; Richter, Donald H.; DuBois, Gregory D.; Miller, T. P. (1992). A postulated new source for the White River Ash, Alaska: A section in Geologic studies in Alaska by the US. Geological Survey, 1990 (Report). Bulletin 1999. Denver, CO: U.S. Geological Survey. pp. 212–218. doi:10.3133/70180193.
• Richter, D. H.; Preece, S. J.; Mcgimsey, R. G.; Westgate, J. A. (1 June 1995). "Mount Churchill, Alaska: source of the late Holocene White River Ash". Canadian Journal of Earth Sciences. 32 (6): 741–748. doi:10.1139/e95-063. ISSN 0008-4077 – via ResearchGate.
• Richter, D. H.; Rosenkrans, D. S.; Steigerwald, M. J. (1995). Guide to the Volcanoes of the Western Wrangell Mountains, Alaska: Wrangell-St. Elias National Park and Preserve (No. 2072) (PDF) (Report). US Government Printing Office.
• Winkler, Gary R. A Geologic Guide to Wrangell—Saint Elias National Park and Preserve, Alaska: A Tectonic Collage of Northbound Terranes (PDF). USGS (Report). Retrieved 7 January 2024.
• Wilson, Frederic H.; Labay, Keith A.; Shew, Nora B.; Preller, Cindi C.; Mohadjer, Solmaz; Richter, Donald H. (2005). Preliminary integrated geologic map databases for the United States: digital data for the geology of Wrangell-Saint Elias National Park and Preserve, Alaska (Report). U.S. Geological Survey Open-File Report 2005-1342.
• Wood, Michael; Coombs, Colby. Alaska: A Climbing Guide. The Mountaineers Books – via Google Books.

External links

• "Mount Churchill". Bivouac.com. Retrieved 2014-01-01.