Mount Hood is Oregon's highest peak and an active volcano of the Cascade Range. It is located about 80 km (50 mi) east of the Portland metropolitan area. Volcanism occurs at Mount Hood and other Cascades arc volcanoes because of the subduction of the Juan de Fuca Plate off the western coast of North America.
Mount Hood is a long-lived volcanic center that has erupted recurrently during the past 500,000 years, and geologic evidence records eruptions from a similar volcano at about the same site back to more than one million years ago. In fact, geologic studies of the Mount Hood region have identified products of numerous local volcanoes that post-date the great floods of basalt lava (the Columbia River Basalts) that flowed down ancestral valleys of the Columbia River between about 16 and 15 million years ago. Local volcanoes have ranged from long- lived andesitic to dacitic volcanoes similar to Mount Hood to relatively short-lived outpourings of basalt and basalticandesite that built shield volcanoes and cinder cones.
Present-day Mount Hood has grown episodically, with decades to centuries of frequent eruptions separated by quiet periods lasting from centuries to more than 10,000 years. In the recent past, the volcano has produced two significant eruptive periods, one about 1,500 years ago and the other during the late 18th century.
Mount Hood eruptions produce andesite and dacite lavas of a much narrower range of composition than does its neighbor to the northwest, Mount St. Helens. The primary eruptive style has alternated between production of lava flows that have traveled as far as 12 km (7 mi) and lava domes that have piled up over vents on the steep upper slopes of the volcano; both types of eruptions were accompanied by modest production of tephra fallout. On the steep upper slopes of Mount Hood, growing lava domes have repeatedly collapsed to form hot, fast-moving pyroclastic flows. The extreme heat from such flows can swiftly melt significant quantities of snow and ice to produce lahars that surge down river valleys, typically far beyond the flanks of the volcano. Over the past 30,000 years, growth and collapse of lava domes and generation of lahars have dominated Mount Hood's eruptive activity.
Throughout Mount Hood's history, swift landslides, called debris avalanches, of various sizes have occurred. The largest ones removed the summit and sizable parts of the volcano's flanks and formed lahars that flowed to the Columbia River. Large debris avalanches occur infrequently and are usually triggered by eruptive activity. Smaller ones, not associated with eruptive activity, occur more frequently. Typically these are triggered by failure of rocks that have been altered and weakened by acidic volcanic fluids or by weathering.