Hayes Volcano is an ice- and snow-mantled, deeply eroded volcanic massif located in the northern Tordrillo Mountains of the north-central Cook Inlet region about 135 kilometers northwest of Anchorage, Alaska. Hayes Volcano consists of a glacially dissected, poorly exposed cluster of lava domes. No historical eruptions of Hayes Volcano are known, and the last period of major eruptive activity occurred within a time interval of 4,400 to 3,600 years ago. During this period, explosive Plinian-style eruptions occurred that dispersed volcanic ash over large areas of interior, south-central, and southeastern Alaska. Pyroclastic flows produced during these eruptions descended Hayes Glacier and entered the Hayes River drainage. The pyroclastic flows initiated volcanic debris flows or lahars that flowed down the Hayes River, into the Skwentna River, and probably reached the Yentna River about 110 kilometers downstream from the volcano. The last major eruptive period of Hayes Volcano may have spanned about 200 years, and at least one of the eruptions during this interval was possibly the largest eruption of any volcano in the Cook Inlet region in the past 10,000 years.

The distribution and thickness of volcanic-ash deposits from Hayes Volcano in the Cook Inlet region indicate that volcanic-ash clouds from prehistoric eruptions were voluminous. Clouds of volcanic ash generated during a future eruption of Hayes Volcano would be a major hazard to all aircraft using Ted Stevens Anchorage International Airport and other local airports, and, depending on wind direction, ash clouds could drift a considerable distance beyond the volcano. Ash fallout from future eruptions could disrupt many types of economic and social activities, including oil and gas operations, shipping activities, and power generation in the Cook Inlet area. An eruption of Hayes Volcano-even a minor episode- would likely melt a significant amount of ice and snow, and the meltwater could result in the formation of large lahars and downstream flooding. The greatest hazards in order of importance are summarized below and are shown on plate 1.