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 2.7-by-4.2-kilometre-wide (1.7 mi × 2.6 mi) 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-range of the Saint Elias Mountains of Alaska, 64 kilometres (40 mi) east of McCarthy, Alaska, and 25 kilometres (16 mi) or 40 kilometres (25 mi) from 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 first ascended in 20 August 1951 by R. Gates and J. Lindberg and named in 1965 after the English politician Winston Churchill and is also known as Klutlan Glacier, Churchill-Bona, or White River volcano. Various measurements have yielded summit heights of 4,744 metres (15,564 ft), 4,766 metres (15,636 ft), 4,767 metres (15,640 ft) or 4,768 metres (15,643 ft). It is a mountain in a glaciated, rugged mountain massif that rises sharply above the surrounding land. It is the tenth-highest peak in the United States. The mountain is mostly covered by ice hundreds of meters thick, but lava flows with columnar jointing and tephra deposits form outcrops, indicating that Mount Churchill may be a stratovolcano. East of Mount Churchill, 300 metres (980 ft) below the summit, is a 2.7-by-4.2-kilometre-wide (1.7 mi × 2.6 mi) caldera, which forms a flat amphitheater open to the northeast. Numerous outcrops of light-colored pumice with embedded lithics occur around the amphitheater, which is otherwise entirely ice-covered. There are 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). Pumice forms terraces above the sides of the Klutlan Glacier, over a length of more than 20 kilometres (12 mi). 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, they could have deposited during floods over the ice, perhaps after an eruption or the breach of a pumice-dammed lake. A 90-metre-high (300 ft) pumice mound on the other side of the glacier, 16 kilometres (9.9 mi) from Mount Churchill, was formed by tephra building up on a bedrock bench. It was once considered the vent of the White River Ash. In some places, 1-metre-thick (3.3 ft) volcanic ash covers the pumice. The older Mount Bona is 3.2 kilometres (2 mi) southwest of Mount Churchill. With a summit height of 5,005 metres (16,421 ft) above sea level, it is the highest mountain in the Wrangell Mountains and the highest volcano in the Wrangell volcanic field and the United States in general. A snow-covered col at 4,400 metres (14,400 ft) elevation separates the two mountains. Both mountains are covered with about 5 cubic kilometres (1.2 cu mi) of ice. The Russell and Klutlan Glaciers run along the northern-western and eastern-southern side of Mount Churchill, respectively. The Klutlan Glacier is flanked by moraines and talus deposits. Both glaciers eventually discharge into the White River. Glaciers on the southern flank of Mount Bona discharge into the Chitina River. Ice on Mount Churchill is up to 800, and possibly 1500, years old.
Geology The more than 500-kilometre-long (310 mi) Wrangell volcanic field (WVF) has been active for the past 30 million years in the Wrangell and St. Elias Mountains. The Wrangell volcanic field features numerous large shield volcanoes, which are among the largest arc volcanoes on Earth. Mount Drum and other volcanoes in the WVF during the middle Pleistocene had eruptions even larger than the White River Ash eruptions. Mount Churchill and Mount Wrangell are the only volcanoes in the WVF with Holocene eruptions. With the exception of Mount Churchill, volcanism in the Wrangell volcanic field has migrated northwestwards and declined as plate configuration changed about 200,000 years ago and subduction ceased. 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. The basement under Mount Bona is formed by a plateau consisting of Permian to Pennsylvanian-age rocks and Tertiary granites; most of Mount Bona may be formed by these nonvolcanic rocks. Off the western coast of southeastern Alaska, the Pacific Plate used to subduct under the North American Plate, giving rise to the WVF. Since the Jurassic, seven separate terranes were transported to Alaska by the Pacific Plate and attached to the continent: 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 the cessation of subduction, with plate motion now occurring along strike-slip faults like the Denali and Totschunda Faults while subduction continues farther west in the Aleutian megathrust. The intersection of the Totschunda Fault with the Connector and Duke River fault(s) may be the point where magma ascends into Mount Churchill.
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