South Sister—the third highest volcano in Oregon and one of its youngest

The Three Sisters volcanoes, located in the Three Sisters Wilderness west of Bend, Oregon, are part of the Cascade Range of volcanoes formed above the Cascadia subduction zone. “For magnificence of glacial scenery, for wealth of recent lavas, and for graphic examples of dissected volcanoes, no part of this range surpasses the area embracing the Sisters and McKenzie Pass”, said volcanologist Howel Williams in 1944. After a recent backpacking trip to climb South Sister, I concur!

A northward view of South Sister volcano from our campsite on the moraine above Moraine Lake. The cone was built up starting about 50,000 years ago, with a series of rhyolite to andesite lava flows and domes culminating in an explosive eruption about 22,000 years ago that left a 200-meter-thick pyroclastic deposit. After a ~20,000 year hiatus, explosions sent ash falling over a wide area—the ash mantling slopes in the foreground is about 2,000 years old.
Rock formed by a pyroclastic flow—fast-moving flows of hot volcanic pieces and gas weld together to form layers when the flow of hot material stops and compacts.
This Google Earth image shows the South Sister cone, with small remnant glaciers at the summit and on the upper slopes. During the last glacier maximum, about 18,000 years ago, glaciers would have covered most of this area causing erosion of the cone and leaving deposits of loose material in moraines. The ash (light tan color) and lava flows (gray blobs) formed ~2,000 years ago, after the glaciers had shrunk considerably. The string of flows and domes between South Sister and the Cascade Lakes National Scenic Byway are the “Devil’s Chain” of recent eruptions along a fault line. The Newberry flow is located below South Sister cone, to the west of the Devil’s Chain. Several small cones are visible at the south end of Newberry flow and just south of Broken Top volcano.
We started at the Devil’s Lake trailhead located on the Cascades Lake Byway near Devil’s Lake (southernmost lake on Google Earth image above). A 3-mile-long hike brought us to Moraine Lake, where we camped for the night. Moraine Lake is visible in the center of the Google Earth image, at the base of South Sister volcano between the younger Newberry and Devil’s Chain domes and flows. The elevation gain from trailhead to South Sister Summit is 4900 feet (1500 meters). Moraine Lake is at 6450 feet (2000 meters) elevation and the summit is at 10,358 feet (3,160 meters). Jay is ready to hike!
A view of Moraine Lake, looking east to Broken Top, a much older volcano that was active 300–150,000 years ago. The cone has been deeply eroded by successive glacial episodes, leaving isolated peaks with little of the original volcano surface intact. Between Broken Top and Moraine Lake is lava that is part of the Devil’s Chain flows. Ridges on the right side of the lake are loose material piled up by glaciers as they retreated up the valley (to the left). These ridges of loose material formed by glaciers are called moraines. Our camp site was on the moraine; the lake below us was a lovely spot to swim, especially after climbing South Sister!
Starting the climb up South Sister, with southward view of the young (2,000-year-old) Newberry flows and domes at the base. On the left side of the horizon is snowy Diamond Peak, located north of Diamond Lake and Crater Lake National Park.
Along the path were rock gardens with gorgeous wild flowers. We particularly enjoyed the luminescent penstemon species.
Higher up on the volcano at a small lake (cirque) created by the nearly melted Lewis glacier (visible snow). This view is to the south, toward the popular downhill ski slopes of Mount Bachelor and Sparks Lake, located just south of the Cascades Lakes Byway. The trail at the top is on loose red cinders that are not the firmest surface to walk on.
View in the opposite direction—north toward the South Sister summit, with Lewis glacier on the right. What is visible is a “false summit”—it’s most of the elevation gain, but it’s necessary to cross another small glacier at the top to reach the true summit (see snow patch in the center of South Sister on Google Earth image above). Jay is having fun!
The view northward from the summit. To the north are Middle Sister (40–14,000 years old), North Sister (120–45,000 years old), and Mount Jefferson (snowy peak behind North Sister). Mount Hood is also barely visible just below the clouds (right from Mount Jefferson). At 11,249 feet (3430 meters), Mount Hood is the highest peak in Oregon; at 10,502 feet (3200 m), Mount Jefferson is the second highest peak, and South Sister is the third highest.

Mount Jefferson has not erupted during the past 10,000 years and is not considered an active threat. However, like South Sister, Mount Hood has had eruptions during the past 2,000 years and both are considered to have a high threat potential. Happily for hikers, volcanoes usually start to shake, from the action of moving lava, and thus give a warning prior to erupting.
View from the summit looking south over the summit glacier—note person on the trail for scale.
Looking back to the summit from the trail above Moraine Lake, on the trail back to the Devil’s Lake trailhead.
Leaving the area. View northwest—South Sister, Broken Top, Middle Sister, and North Sister—from Lava Lands Visitor Center, part of Newberry National Volcanic Monument. This flow, with twisted tree trunk, was emitted from Lava Butte Cinder Cone about 7,000 years ago. The Newberry volcanic area is one of the most active in Oregon, but it is part of the High Lava Plains string of east-west volcanoes, not part of the Cascade Range. That’s a story for another post!

Important references

U.S. Geological Survey web site about the Three Sisters volcanoes: https://www.usgs.gov/volcanoes/three-sisters

U.S. Geological Survey web site for the Cascade Volcano Observatory, with information about all Cascade Range volcanoes: https://www.usgs.gov/observatories/cascades-volcano-observatory

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