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Mount Churchill
Mount Churchill is a volcano in the Saint Elias Mountains and the Wrangell Volcanic Field of eastern Alaska. Churchill and its higher neighbor Mount Bona about 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.
==Volcanic eruptions==
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 mi² (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 1250 years ago and stretching over 500 miles (800 km). The total volume of the ash exceeds 12 mi³ (50 km³), or roughly 50 times the volume of the 1980 eruption of Mount St. Helens, and ash layers up to 2 ft (60 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 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 x 1.7 miles (4.2 x 2.7 km) elliptical, gently sloping, ice-filled depression at 14,500 ft (4400 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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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