Viewfinder: Mount Hood’s Eruptive Past Still Shapes Our Valley

Mount Hood’s summit is a familiar sight for everyone who calls The Mountain their home. It’s the backdrop to our lives, a landmark that feels unchanging. Yet in geologic terms, Mount Hood is anything but quiet. Beneath the snow and glaciers lies the record of eruptions that reshaped the mountain and altered the valleys where we live.

The story of Mount Hood stretches back roughly half a million years. Like the other great peaks of the Cascades, it rose where the Juan de Fuca Plate dives beneath North America, melting rock deep underground. Eruptions built the mountain layer by layer, but its growth was never steady. Time after time, lava domes swelled at the summit, then collapsed in violent avalanches that swept down the slopes. These cycles of growth and destruction carved the rugged flanks we know today.

The record of that activity is written not only in the rock but in the remains of ancient forests. Along the Sandy and White Rivers, erosion has exposed the remains of trees that once grew tall before they were buried where they stood. Each preserved forest marks the moment when an eruption ended life in the valley, only to see new growth return after the mud and ash cooled. They are reminders that the effects of Mount Hood’s eruptions have never been confined to the summit; they reach far downstream, into places people call home.

The most recent eruptive period is surprisingly close to our own time. Between about 1781 and 1793, a series of eruptions reshaped the summit and produced Crater Rock, a prominent lava dome that looms on Mount Hood’s south side. These eruptions blasted hot pyroclastic flows into the Sandy and White River valleys, eroding the earth and engulfing forests. The mixture of rock, ash, and water turned into massive mudflows that rushed down the valleys, burying the land beneath thick debris. The broad, flat expanse we now call Old Maid Flats on the Sandy River is the direct result of those eruptions, and is a great place to see the remnants of the trees that were victims of the mudflows. 

For perspective, those eruptions ended only about two centuries ago — a blink of an eye in geologic time. They took place not long before Lewis and Clark’s journey to the West and just as settlers were beginning to filter into the region. When we walk the trails of Old Maid Flats or along the Sandy River, we are literally standing on the deposits of Mount Hood’s most recent outburst.

Today, Mount Hood shows only subtle signs of activity: steaming vents near Crater Rock, occasional earthquakes, and ground heated by the magma far below. Scientists expect its next eruption will look much like those of the late 1700s — dome growth followed by collapse, sending debris into the same valleys that carried mudflows in the past. For those of us living along the Sandy River, that history is worth remembering. A future eruption may or may not be catastrophic, but it would almost certainly send mud, ash, and floods into the very places where people now live, work, and recreate.

Mount Hood is more than a postcard view, a weekend destination or a beautiful place to live. It is an active volcano, still shaping the landscape around it. Its buried forests, its broad river flats, and its steaming summit all tell the same story: this mountain is alive. As residents of the Sandy River Valley, we live not just near Mount Hood but within its reach, inheriting both the beauty it creates and the risks it carries.