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Ian Madin Rocks Geology – Day Three: Mt. Hood and More

Ian Madin is the chief scientist at the Oregon Department of Geology, and he will be providing us with information about the areas we’ll be visiting each day.

As we leave Dufur, we will ride up a gentle ramp leading toward the High Cascades. The ramp is more of The Dalles Formation, a thick sheet of volcanic rubble shed off volcanoes that were built and eroded away between 5 million and 10 million years ago. Many of those volcanoes were probably every bit as majestic as Mt. Hood or Mt. Adams today, but all that is left of them is the rough volcanic gravel and sand that we will ride through in the morning.

At about mile 15, we will leave The Dalles Formation, and start riding through lava flows from much younger volcanoes. The rocks get younger as we get closer to the crest of the range, starting at 5–10 million years and reaching 1–2 million years by the time we begin to descend around mile 20. Along the way we will catch views of Mt. Hood, which is even younger still.

This is a good point to talk about why the Cascade Range is home to so many volcanoes. The Cascade Range, which stretches from Mt. Lassen in northern California all the way to Mt. Baker in northern Washington, is what geologists call a volcanic arc. Volcanic arcs are chains of volcanoes that are found around the world wherever there is a subduction zone. Subduction zones are part of the global system of moving tectonic plates, and are places where one plate is forced to slide beneath the other and sinks into the body of the earth. The sinking plates trigger melting of the surrounding rock as they descend, and the molten magma rises to form the volcanoes that make up the arc.

You can find a good illustration of this process at The volcanic arc in Oregon has been active for 40 million years, and today’s High Cascade Range was built by the eruption of hundreds of fairly small volcanoes interspersed with a few much larger ones like Hood, Jefferson and the Three Sisters. Mt. Hood is about 1 million years old, and has been built up over time by dozens of eruptions, alternating between lava flows and more explosive eruptions like the 1980 eruption of Mt. St. Helens. The most recent eruptions were relatively small events in the late 1700s, with other, much larger eruptions about 1,500 years and 20,000 years ago.

Geologists know Mt. Hood will erupt again, but have no idea when. A volcano monitoring program by the U.S. Geological Survey ensures that we will not be caught by surprise with future eruptions. Although we will have views of Mt. Hood along the morning’s ride, there really isn’t much of a view as we drop into the valley of the East Fork of the Hood River, which is a shame because the landscape is quite dramatic, as you can see in the image below.

Pic 1

View from the top of the descent along Road 44 toward Mt. Hood and Bennett Pass. At the bottom of the descent, our route (in purple) follows Highway 35 up the valley of the East Fork of the Hood River. Bluegrass Ridge is formed by yet another fault (in red). The Earth’s crust beneath Mt. Hood is actually subsiding along this fault, but the rate of volcanic eruptions allows Mt. Hood to grow faster than the sinking landscape.

This valley’s landscape tells of a dynamic history, played out over the million years since Mt. Hood began to develop. Most river valleys get narrower and steeper as they near their headwaters, but the valley of the East Fork is unusually wide and flat. This is because there have been several global glaciations during this time, and in each, ice on Mt. Hood fed large glaciers that pushed their way down the valley, widening it and giving it the characteristic “U” shape of a glacial valley, with a flat floor and steep sides.

As the last glacier retreated some 20,000 years ago, a series of lahars, or volcanic debris flows, filled the valley, leaving a wide, smooth surface. A lahar is a flowing mix of water, mud and rock that forms when hot lava erupts onto glaciers or snow fields. Lahars can flow for many miles, reach speeds of 30–40 miles per hour, and bury the landscape in hot bouldery mud that quickly solidifies. About 100,000 years ago, an exceptionally large lahar from the north side of Mt Hood traveled 25 miles down the Hood River Valley to the Columbia, then crossed the river and flowed up the valley of the White Salmon River in Washington. The city of Hood River is built on a thick layer of rock left by this lahar.

The volcanic eruptions that fed these lahars left Mt. Hood covered in a thick blanket of lava rubble and ash. As the glaciers steadily melt away, they expose this loose material, and when warm, heavy rains fall on snow, the water can saturate the rubble and cause debris flows to roar down the slopes and into the valleys. Debris flows are a mixture of water, mud, gravel, trees and boulders, and can flow much faster than a human can run. They are incredibly destructive, and like the lahars, tend to solidify when they come to rest, forming thick, almost cement-like layers on the valley floor.

The image below shows the view down the White River from the summit of Mt. Hood. All the light-gray material in the foreground is loose volcanic rubble, and you can see a light-colored fan of material on the valley floor, just to the right of Bennett Pass, where our route turns to go down the valley.

Pic 2

View from the summit of Mt. Hood down the White Glacier and White River valley. Our route (in purple) climbs out of the valley of the East Fork of Hood River and over Bennett Pass, before descending into the White River valley. The slopes of Mt. Hood are covered here in a thick blanket of loose rock and sand rubble, left over from large eruptions 20,000 years ago.

Highway 35 crosses the White River just to the right of where our route doubles back. In 2006, a major “Pineapple Express” storm caused widespread debris flows on Mt. Hood that buried Highway 35 and the White River Bridge, as shown in the photo below.


Highway 35 bridge over White River after 2006 debris flows, photo credit ODOT.

After we leave Highway 35 at the White River Bridge, we will descend for about 10 miles, then turn east and climb out of the valley. As we make that climb, we will cross through an area of boulders, sand and gravel deposited by past glaciers that extended down the valley from Mt. Hood. Once past the glacial gravel, we will ride on older lava flows from Mt. Hood for most of the remaining distance to Tygh Valley.

As we make our final approach to the community of Tygh Valley, we will see Tygh Ridge forming the skyline on the other side of the valley. The ridge once again comprises lava flows of the Columbia River Basalt, but here they are folded up in a sharp wrinkle that defines the north side of the valley. The lava flows that were originally horizontal are tilted as much as 80 degrees in the slopes of Tygh Ridge.

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