The Three Sisters

In the High Cascades of Oregon – between Eugene and Bend and about 168 miles east of an offshore oceanic trench where the Juan de Fuca tectonic plate dives under the leading edge of the North American plate – three 10,000-foot-plus volcanoes cluster together between Mount Washington and Mount Bachelor.

European settlers first called them Faith, Hope and Charity, but North Sister, Middle Sister and South Sister are more pragmatic names for volcanoes. One of the Sisters is probably extinct, one probably dormant, and the third definitely is active.

There is an amazing variety of volcanic geology around Oregon’s Three Sisters.

Let’s get introduced to them and a couple of their notable companions via an airplane flight recorded by self-styled airport bum, videographer, and OSU geology professor Chuck Rosenfeld:

The Three Sisters

These are the most recent structures in a volcanic zone where massive tectonic forces are stretching and cracking Earth’s 27-mile-deep crust.

Some 650,000 years before the Sisters formed, andesite erupted explosively from a line of volcanoes here. For many hundreds of thousands of years, pyroclastic flows erupted to cover the east central Cascades from the modern town of Sisters to south of Bend. About 170,000 years ago, things eventually quieted down.

Repeated glaciations have cut into North Sister.  Here, the volcano's east face shows about 100 thin lava flows with layers of scoria capped by thick summit lava flows. Thayer Glacier feeds a small lake.  Because of its heavily eroded rock, climbers call this mountain the "Beast of the Cascades." (USGS)

Repeated glaciations have cut into North Sister. Because of its heavily eroded rock, climbers call this mountain the “Beast of the Cascades.” (Image: USGS)

Then, as localized faulting happened, perhaps in response to movements far below in the planet’s mantle, the lava type changed to basalt. After the region had been quiet for 70,000 years or so, North Sister began to erupt and take shape.

It was probably pretty quiet at first – basalt flows much more easily than andesite. Sometimes, though, North Sister’s eruptions turned explosive when andesite got into the mix. Lava sills also intruded into its edifice as North Sister built itself up over the next 55,000 years or so.

Today its throat is completely plugged by a 980-foot-wide lava dome, and its surface has been heavily eroded by glaciers.

Middle Sister forms the backdrop as a USGS geologist conducts ground deformation studies on the Three Sisters.  (Image: John Ewert, 1985 USGS)

Middle Sister forms the backdrop as a USGS geologist conducts ground deformation studies on the Three Sisters. (Image: John Ewert, 1985. USGS)

Not a lot is known about Middle Sister.

At first, researchers thought that the Three Sisters formed sequentially, with the northernmost volcano the oldest and South Sister the youngest.

However, in the early 21st century USGS researchers learned that Middle and South Sister formed at roughly the same time. Middle Sister got going around 40,000 years ago, but its greatest activity was between 25,000 and 18,000 years ago as South Sister was also taking shape.

Middle Sister has erupted basaltic andesite as well as andesite and the even “stickier” dacite and rhyolite. Obsidian from its flows was used and traded by Native Americans.

Glaciers have eroded away most of the eastern half of Middle Sister. This volcano hasn’t erupted for well over 10,000 years, so it is at least dormant, if not extinct.

However, “active,” “dormant” and “extinct” are general terms, not precise definitions. As noted at the Oregon State University website linked in the last paragraph, there have been a number of eruptions from “extinct” volcanoes. Sometimes scientists only learn about volcanoes after an eruption.

Time alone will tell if Middle Sister is napping or if its fires are gone for good.

South Sister is the only one of the three to have a summit crater.  Here, its crater and Teardrop Lake are covered with snow.

South Sister is the only stratovolcano in Oregon to have a summit crater. Here, its crater and Teardrop Pool (the state’s highest lake) are covered with snow. That’s Middle Sister and North Sister in the background. (Source)

Young South Sister has a complex history, having erupted a variety of different magma types from many different vents. Stephen Harris (see source list) says (links and comment added):

An andesite lava flow underlying the top 1,000 feet of the edifice dates at about 27,000 years, but it overlies silicic flows [andesite, dacite and/or rhyolite] at least 50,000 years old. Much of South Sister’s late Pleistocene activity, between about 35,000 and 15,000 years ago, took place at vents along the lower flanks, where large quantities of silicic magma erupted in the form of both pumiceous tephra and thick flows and domes. One conspicuous flow, a 350-foot-thick rhyolitic lava erupted about 35,000 years ago, issued from a vent low on the northeast flank and poured into Squaw Creek.

South Sister last erupted some 2,000 years ago. In two eruptions a few centuries apart, the pattern was again of an explosive start followed by extrusion of very thick, “sticky” silicic lava.

The bulge

In 2001, scientists realized that a 6- x 12-mile area of land, centered west of South Sister’s summit, was bulging upward at a rate of about 1-2 inches per year, and that this had probably started in 1997.

The bulge is only visible on imagery like InSAR.  (USGS)

The bulge is only visible on imagery like InSAR. (USGS)

According to the USGS:

Modeling of the uplift (inflation) suggests that it was caused either by the intrusion of about 20 million cubic meters (26 million cubic yards) of magma at about 5-km (3-mi) depth or by rise of a hot, buoyant plume of water and gas to a similar level that caused heating and expansion of surrounding rock. In either case, an eruption is unlikely in the near future if current trends continue. Similar inflation episodes have been recognized at many volcanoes around the world, and others probably went unnoticed before the development of modern monitoring techniques such as GPS and InSAR.

The neighborhood

Unlike Rainier or Hood, the Three Sisters don’t sit in splendid isolation. Among nearby volcanic feataures are cinder cones, shields, lava fields, and other volcanoes.

Some of those, like Belknap Shield, Mount Bachelor and Mount Washington, I will probably cover on some future Sunday.

However, it’s only about 60 miles east of here, and I hope to visit that area some time next year, when I’ve gotten settled in. So, also stay tuned for some eventual field reports.

In the meantime, here’s a lovely view of the Three Sisters and their neighbors!



Stephen L. Harris. Fire Mountains of the West: The Cascade and Mono Lake Volcanoes. Mountain Press. Missoula, Montana. 2005.

Wes Hildreth, Judy Fierstein, and Andrew T. Calver. Geologic Map of Three Sisters Volcanic Cluster, Cascade Region, Oregon (PDF).

Three Sisters. United States Geological Survey Volcano Hazards Program.

Three Sisters. Wikipedia.

Categories: Sunday morning volcano

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