Images

Temporary Global Positioning Systems (GPS) stations are deployed to determine if uplift continues in the Three Sisters area. Since surface changes were discovered in 2001, the rate of uplift has decreased to less than 1.0 cm (0.4 in.) per year.

Three Sisters, Oregon simplified hazards map showing potential impact area for ground-based hazards during a volcanic event.

In left foreground, ice-ravaged mafic edifice Little Brother is separated from North Sister by Little Ice Age trough of Collier Glacier. Both North Sister and Little Brother expose numerous oxidized scoria falls, whereas smooth black Middle Sister cone is cloaked by mafic lava flows.

Map showing one-year probability of accumulation of 1 centimeter (0.4 inch) or more of tephra from eruptions of volcanoes in the Cascade Range.

The east face of Middle Sister is 350 m (1150 ft) high. Ice-sculpted pile of lavas at right, variously called "Black Hump," "Prouty Point," or "Step Sister," has 190 m of relief and consists of at least five flows.

The "Devils chain" is a string of nearly contiguous, virtually uneroded rhyolite lava domes and flows aligned north-south along a 5 km (3 mi) stretch on the southeast apron of South Sister. The largest of these is the Newberry flow (right center, below South Sister), only 2.5 km (1.5 mi) from the summit of South Sister.

The photo includes five rhyolitic units of different ages: Unforested 2-ka rhyolite of "Devils chain" rests on forested 35,000 year old flow lobe from adjacent Devils Hill dome. Three pale-gray spurs form sharp salients that extend to elevations only 550 m (1640 ft) below the darker andesitic summit of South Sister.

Eruptions in the Cascade Range during the past 4000 years. USGS GIP 64

South, Middle, and North Sister volcanoes viewed aerially from the south, Three Sisters Oregon.

East face of North Sister, 750 m high. Consists of about 100 thin mafic lava flows and intercalated layers of red scoria, capped by thick summit lava flows. Remnant of Thayer Glacier feeds cirque lake. Pervasively altered yellow-orange buttress on right, 250 m thick, consists of east-dipping, palagonitized, ash-rich fragmental flow deposits.

Prouty Glacier headwall on the northeast face of South Sister. Dipping stacks of lava and scoria were excavated by Prouty Glacier, exposing cross-sectional views of the last 33,000 years of summit activity. The 350-m-thick (11,500-ft) lava stacks are truncated at about 3000 m (9,800 ft) elevation, forming the ragged rim of a broad paleocrater.

Lost Creek Glacier and its cirque are in the center, and Eugene Glacier and its cirque are toward the bottom left. Fingerlike lobes of dacite lava dribble down northwest ridge atop fragmental deposit and beneath capping agglutinate. The entire edifice visible here was built between 30 and 20 ka.

Radially dipping strata of the cone include subequal proportions of lava flows and flow breccias and of proximal agglutinate and scoria falls, nearly all having 54 to 56 percent SiO2. Numerous mafic dikes and sills intrude the cone, and an apron of compositionally similar mafic lavas extends far outward from the cone.

Seismic station at Three Sisters volcano in Central Oregon. Seismometer in box on the ground with data transmission antenna mounted on pole.

Mount Jefferson (pointed peak in middle) and Three Sisters (to the right) in Oregon, south-facing aerial view as seen from Mount St. Helens.

Global Positioning System receiver (dome on pole) and seismometer (box on ground in distance) at a volcano monitoring station for Three Sisters in Central Oregon. Solar panel mounted to pole.

About 130 mm (5 inches) of peak uplift from 1995 to 2001 is shown in this interferogram in the Three Sisters Wilderness. Magma accumulating at about 7 km (about 4.5 mi) depth is the likely cause of uplift. The area of uplift, about 20 km (12 miles) in diameter, is almost entirely within the wilderness area.

Three Sisters volcanoes (left to right - South, Middle, and North) viewed from the south, Oregon. Foreground rhyolite Newberry flow is one of most recent eruptive units (2.2 ka).

Remnant of Villard Glacier is in cirque below. Thick summit lava flow makes up "Glisan Pinnacle," which overlies hundreds of thin rubbly lava flows that are intercalated with red-oxidized agglutinate and scoria falls exposed on 800-m- (2600-ft-) high face below. Flows that form ridge at upper right are thicker than those on most of edifice.

Three Sisters annotated photograph viewed from south to North from Broken Top, Oregon.

Geologic Map of Three Sisters Volcanic Cluster, Cascade Range, Oregon