The Dynamic Dance of Lava and Glaciers that Shaped Mount Hood’s Landscape

By Robert Matsumura

Among the Klickitat and Multnomah peoples, Mount Hood was often depicted as a powerful and benevolent deity, a sacred and revered landmark. Other stories spoke of supernatural beings or powerful spirits who inhabited the mountain, making it a site of pilgrimage, inspiration and spiritual guidance. For settlers arriving on the Oregon Trail, the mountain was a source of awe, but also trepidation, as it represented a daunting physical obstacle standing between them and their final destination – the Willamette Valley. One thing is for certain, the volcano we know as Mount Hood has played a significant role in both the cultural and geologic history of Oregon.
A Long-Lived Volcano
Mount Hood has been an active volcano for around 500,000 years, creating a series of additional volcanic formations over the past 1.5 million years. The mountain’s current structure is primarily composed of ancient lava flows. Over the past 30,000 years, subsequent eruptions have occurred, mostly involving the formation and collapse of lava domes near its summit, resulting in large fans of volcanic debris. Closer to the present, a number of eruptions occurred around 1,500 years ago, followed by activity as late as the 1700s.
500,000 to 100,000 Years Ago: Building Mount Hood
Mount Hood’s broad slopes, which stretch about 20 km in diameter, were formed from numerous lava flows occurring between 500,000 and 100,000 years ago. These flows varied in thickness and were often shaped by the presence of large glaciers. Famous landmarks like Gnarl Ridge, Lamberson Butte and Illumination Rock were formed during this time. Meanwhile, regional volcanoes like Mount Defiance and Lost Lake Butte also erupted, creating shield volcanoes.
About 100,000 years ago, a massive landslide removed the summit and north flank of Mount Hood, creating a large lahar (volcanic mudflow) that traveled down the Hood River Valley and into the Columbia River. This tremendous landslide significantly altered and reshaped the valley, temporarily blocking waterways, filling in sections of the valley, and actually widening it in some places. Much of the current landscape we see today in the Hood River Valley was a result of this major geologic event.
100,000 to 30,000 Years Ago: Rebuilding
In the wake of the landslide, Mount Hood experienced new lava flows that filled in the scar left by the lahar. These flows, some as old as 40,000 to 50,000 years, formed areas like Langille Crags and Stranahan Ridge. Despite being covered by glaciers during the last ice age, these lava flows still exhibit well-preserved features. Much of the Mount Hood Meadows Ski Resort and the popular Paradise Park area lie on the ancient lava flows generated during this period.
30,000 to 12,000 Years Ago: The Polallie Eruptive Period
About 20,000 years ago, during the last major ice age, Mount Hood was covered by enormous glaciers that shaped its landscape. Over time, these glaciers carved out deep valleys and left piles of rocky debris in their wake. As the ice moved, it eroded older volcanic deposits and interacted with newer lava flows, creating many of the fabulous valleys and ridges visible today. As the climate warmed and the ice melted and receded, it left behind a rugged legacy, a testament to how the combination of volcanic eruptions and glaciers have worked in tandem to shape Mount Hood over time. Many of the deposits from this period can be found along the Sandy, Hood, and White River valleys. Notable features from this time include Steel Cliff and the Eliot Glacier headwall.
Although not directly a result of volcanic activity from Mt. Hood itself, but from one of the volcanic events in the vicinity of the mountain, is the Parkdale flow which occurred approximately 7000 years ago. Evidence of the flow is clearly evident around the town of Parkdale today.
Around 1,500 Years Ago: The Timberline Eruptive Period
After a long dormant period, Mount Hood erupted again around 1,500 years ago. This period once again saw the formation of new lava domes, pyroclastic flows (a fast-moving current of hot gas, ash, and volcanic rock that travels down the slopes of a volcano during an explosive eruption) and lahars. Of particular significance to today’s landscape was a large landslide that rumbled down the southwest flank of the mountain and the subsequent lahar which pushed down through the Zigzag and Sandy River valleys. The areas where Timberline Lodge and Government Camp currently reside were shaped by this activity, along with the Sandy River delta, also formed by the same events.
1781 C.E.: The Old Maid Eruptive Period
The Old Maid eruptive period (named after Old Maid Flat—based on the legend of a reclusive woman who once dwelt in the area), the most recent major activity at Mount Hood, continued for approximately a decade in the late 1700s. This period involved the repeated collapse of a lava dome resulting in additional pyroclastic flows and lahars. Shortly after this activity, in the early 1800s, explorers Lewis and Clark visited the area and noted the Sandy River’s resemblance to the sediment-heavy Platte River—a comparison which would have indicated recent volcanic activity.
Mid-19th Century Eruptions
As recently as 1859 and 1865, early settlers reported seeing eruptions at Mount Hood, describing fire, smoke, flying rocks, and steam. These reports likely referred to minor explosive activity from Crater Rock, which was active during the Old Maid period, although no definitive deposits have been found to corroborate these sightings. Crater Rock, located on the south side of Mount Hood, is a key landmark for climbers and hikers today, and frequently still emits steam, indicative of the geothermal activity still transpiring beneath the surface.
A View From the Top
So the next time you’re gazing up at Mount Hood and marveling at its majestic snow-capped peak, remember the geologic history of this iconic mountain — a testament to the dynamic and powerful forces of nature. From ancient lava flows to explosive eruptions and massive landslides, Oregon’s most famous mountain has continually reshaped itself over the centuries. Whether you’re skiing at Mount Hood Meadows or hiking around Timberline Lodge, you’re standing atop a landscape forged by fiery volcanic activity and the movements of mighty glaciers.