Ash flows around Aniakchak and Fisher calderas in the Aleutian volcanic arc show evidence of having flowed over formidable topographic barriers at distances of tens of kilometres from their source. Ash flows swept down glaciated valleys on the south side of Aniakchak caldera, crossed a broad lowland with an altitude of less than 35 m, and continued on through passes as much as 260 m high in the Aleutian Range into the Pacific Ocean, a distance of some 50 km. North of Fisher caldera, ash flows flowed over a ridge barrier of 500 m and into the Bering Sea. Knowledge that ash flows have this mobility may help in understanding the distribution of other ash flows and in the discovery of their sources.

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