Mount Hood, located in Cascade Volcanic Arc of northern Oregon, is a stratovolcano, a conical volcano built by layers of hardened lava, volcanic ash and other by-products of volcanic activity. Mount Hood is known to be a potentially active volcano, with the last eruption taking place around 200 years ago in the 1790s (not too long before the Lewis and Clark expedition) and a series of small streams and ash explosions occurring in the mid-1800s. However, this 500,000-year-old mountain rarely showed violent eruptions like Mount St. Helens, with only slow lava flows occurring in the past eruptions. While scientists assure that it is showing no signs of eruptions today, visitors frequently witness stream plume rising from the fumaroles, the opening of the volcano.
Glaciers and perennial snowfields are also important constituents of Mount Hood, covering approximately 13.5 km2. There are 11 major glaciers and one snowfield, with the largest glacier being the Eliot and Coe Glacier on the north flank of the mountain. Interestingly, the past lava flows during the last ice age influenced the distributions of these glaciers, and glaciers, in turn, provided water, the source of mobilization for lahars (destructive mudflows).