Mount Hood formed as a stratovolcano from layers of lava flows, ash from the air and volcanic mudflows. Its primary construction is from a family of rocks known as andesite. The initial volcano formed more than 500,000 years ago, and it has seen long periods of inactivity followed by short periods of activity that formed the mountain.
The National Science Foundation reports that two types of magma mixed a few weeks before each of Mount Hood's historic eruptions. This mixing is unlike what occurs in many other stratovolcanos, even those within the same Cascade Range as Mount Hood, like Mount St. Helens, only 60 miles away. Mixing the two lava types causes the pressure to build, causing an eruption, but it is usually less violent than the eruptions of other volcanoes. This same mixing provides the ingredients required for formation of andesite, leading scientists to believe Mount Hood formed in the same way as other andesite volcanoes.
While eruptions built the base of Mount Hood, weathering and erosion have formed the visible mountain. The movement of glaciers from the top of the mountain helps to break down the rock that forms its shape. These movements break through lava domes to allow new flow to escape.
While scientists consider Mount Hood an active volcano, they expect its next eruptions to include small lava flows. Models indicate an eruption is not likely to cause major damage in Portland, Ore., only 45 miles away.