DESCRIPTION:
Three Sisters Volcanoes, Oregon

[Image,56K,JPG]
Aerial view, Three Sisters Volcanoes, Oregon, showing (left to right) South Sister, Middle Sister, and North Sister.
-- USGS Photo by Lyn Topinka, September 1985

Location: Oregon

North Sister:
Latitude: 44.17 N
Longitude: 121.77 W
Height: 3,074 Meters (10,085 Feet)

Middle Sister:
Latitude:
Longitude:
Height: 3,062 Meters (10,047 Feet)

South Sister:
Latitude: 44.10 N
Longitude: 121.77 W
Height: 3,157 Meters (10,358 Feet)

Number of eruptions in the past 200 years: 0

Latest Eruptions: 950 (?) ² ... Possibly within the last 1,000 years ³

Present thermal activity: None known

Remarks: Debris flows this century ² ... A cluster of closely grouped, glacier-clad large volcanoes in an area of many smaller young volcanic cones. Lava extrusion summit area of South Sister possibly within the last 1,000 years, flank eruptions of lava and pyroclastic deposits less than 2,500 (?) years ago ³.

The Three Sisters appear as the "Three Sisters" on Preston's map of Oregon of 1856. The name was probably originally applied by members of the Methodist Mission in Salem in the early 1840's, and the individual peaks were given the names "Mount Faith", "Mount Hope", and "Mount Charity", beginning from the north.

The town of Sisters derives its name from the three prominent peaks that grace the southwestern skyline: Faith, Hope and Charity, collectively known as the Three Sisters. A very old story suggests that the mountains were named in the 1840's by members of a Methodist mission based in Salem. The town was officially christened when the post office at Camp Polk, some three miles to the north, closed in the 1880's and was moved to the John Smith store. Local residents were invited to submit names for the new facility. Jacob Quiberg suggested "Three Sisters". Postal authorities dropped the "Three" and the handful of wood frame buildings standing at the junction of the McKenzie and Santiam roads became known as Sisters.

The town of Sisters derives its name from the three prominent Cascade peaks that grace the southwestern skyline: Faith, Hope, and Charity, collectively known as the Three Sisters. A very old story suggests that the mountains were named in the 1840s by members of a Methodist mission based in Salem. Others credit trappers who frequented the region in the early part of the 19th century.

Three Sisters is one of three potentially active volcanic centers that lie close to rapidly growing communities and resort areas in Central Oregon. Two types of volcanoes exist in the Three Sisters region and each poses distinct hazards to people and property. South Sister, Middle Sister, and Broken Top, major composite volcanoes clustered near the center of the region, have erupted repeatedly over tens of thousands of years and may erupt explosively in the future. In contrast, mafic volcanoes, which range from small cinder cones to large shield volcanoes like North Sister and Belknap Crater, are typically short-lived (weeks to centuries) and erupt less explosively than do composite volcanoes. Hundreds of mafic volcanoes scattered through the Three Sisters region are part of a much longer zone along the High Cascades of Oregon in which birth of new mafic volcanoes is possible.

The Three Sisters area contains 5 large cones of Quaternary age-- North Sister, Middle Sister, South Sister, Broken Top, and Mount Bachelor. North Sister and Broken Top are deeply dissected and probably have been inactive for at least 100,000 years. Middle Sister is younger than North Sister, and was active in late Pleistocene but not postglacial time. South Sister is the least dissected; its basaltic andesite summit cone has a well preserved crater. Most of South Sister predates late Wisconsin glaciation and is therefore older than 25,000 years; however, eruptions of rhyolite from flank vents have occurred as recently as 2000 years ago.

The Three Sisters-Broken Top area is a long-lived center of basaltic to rhyolitic volcanism. The clustering of large composite cones sets the area apart from others in the High Cascades, although the Mount Mazama area prior to the formation of Crater Lake caldera was also a cluster of composite cones.

The ages of most volcanoes in the Three Sisters area are not precisely known. North Sister, a basaltic andesite pyroclastic and lava cone that rests on a shield volcano, is the oldest of the Three Sisters and postdates the approximately 0.3-million-year-old Shevlin Park Tuff. Middle Sister is intermediate in age between North and South Sister and, like South Sister, is compositionally diverse. Broken Top volcano is also younger than Shevlin Park Tuff and is older than South Sister, but its age relation to Middle and North Sister is not known. The relative degree of erosion of Broken Top is a complex complex composite cone of dominantly basaltic andesite that intermittently erupted andesite, dacite, and rhyolite as lava flows, pyroclastic flows, and pyroclastic falls. Cayuse Crater, which is located between Broken Top and the Cascades Lakes Highway, and two nearby vents on the southwest flank of Broken Top erupted during the earliest Holocene or lates Pleistocene time, but these events were probably unrelated to the long-inactive Broken Top system.

South Sister is the youngest composite volcano of the Three Sisters-Broken Top center and has erupted lavas ranging from basaltic andesite through rhyolite. Although not dated directly, most, if not all, of South Sister is probably of late Pleistocene age. ... The cone of basaltic andesite that forms the summit of South Sister is probably of latest Pleistocene age; its crater is still closed and is filled with 60 meters of ice and snow. Le Conte Crater a basaltic andesite scoria cone on the south flank, is between about 15,000 and 6,850 years old. The youngest eruptions recognized on the volcano occurred at a series of vents on the south and northeast flanks that erupted rhyolite tephra and lava flows and domes between about 2,200 and 2,000 years before the present.

Between 1980 and 1984, the U. S. Geological Survey's David A. Johnston Cascades Volcano Observatory (CVO) established baseline geodetic networks at Mount Baker, Mount Rainer, and Mount St. Helens in Washington, Mount Hood and Crater Lake in Oregon, and Mount Shasta and Lassen Peak in California. To this list of potentially active volcanoes, CVO extended its monitoring program in 1985 to include Newberry and South Sister volcanoes in central Oregon. The Newberry and South Sister networks were re-measured in 1986 and will be measured periodically in future years. Improvements since 1984 in the recording of endpoint and flightline temperatures resulted in better overall data than obtained previously. The improvements included: calibration of all the sensors and precision thermistors, installation of a new recording system for flightline data, and recording of endpoint temperatures 6 meters above ground level. The data collected in 1985 and 1986 indicate little or no apparent deformation at either volcano between surveys.

In addition to locating regional earthquakes, the Pacific Northwest Seismograph Network (PNSN), in cooperation with the Cascades Volcano Observatory, is also responsible for monitoring seismic activity at volcanoes in the Pacific Northwest. The PNSN currently operates seismometers on or near Mount Adams, Mount Rainier, Mount St. Helens, Mount Hood, Mount Baker, Three Sisters, and Crater Lake.

Three Sisters Monitoring Menu

[Image,47K,JPG]
South Sister Volcano, from Sparks Lake area.
-- USGS Photo by Lyn Topinka, October 1984

The latest eruptions on South Sister, which occurred in two closely spaced episodes about 2,000 years ago, illustrate a relatively modest scale of eruptive activity. Initial explosive eruptions produced small pyroclastic flows and tephra fallout from several aligned vents low on the south flank. Tephra fallout deposits more than 2 meters (7 feet) thick, composed of pumice, rock fragments, and ash, blanketed areas within 2 kilometers (1 mile) downwind of vents; at 13 kilometers (8 miles) about 10 centimeters (4 inches) fell. Less than one centimeter (0.5 inch) of ash fell at least as far as 40 kilometers (25 miles) south of the vents (at Cultus Lake) and east of the vents (at Bend). Following tephra eruptions, lava emerged from two vent areas, forming a large lava flow, Rock Mesa, and several small lava domes. Decades to a few centuries later, a similar eruptive sequence occurred along a zone of vents that extended from just north of Sparks Lake to high on the southeast flank of South Sister, as well as along a shorter zone on the north flank near Carver Lake. ... Similar-style eruptions, but up to about ten times larger in terms of volume of ejecta, occurred during and just before the last ice age, about 30,000 to 15,000 year ago.

The geologic record shows that even much larger eruptions with much wider impact have occurred in the Three Sisters volcanic center. At least four times in the past 700,000 years, explosive eruptions that were probably sited near the present location of Broken Top and Three Sisters produced pyroclastic flows that swept over a broad area from Sisters to south of Bend. A tephra fallout deposit as thick as 13 meters (42 feet) composed largely of fist-sized and smaller white pumice clasts from one of these eruptions is exposed in numerous pumice quarries. Distal tephra deposits from this event have even been found in northern California and in cores from the northeast Pacific Ocean. Such an event today would be catastrophic for Deschutes County, but, fortunately, events of this magnitude are infrequent. Furthermore, there is no evidence that the large volume of magma necessary to drive such an eruption is present in the Three Sisters region today, nor would such a volume likely be generated in the near future.

The central Oregon Cascade Range peaks that presently sustain glaciers or permanent ice masses are, from north to south, Mount Jefferson, Three Fingered Jack, North Sister, Middle Sister, South Sister, and Broken Top. In addition, Mount Bachelor, Diamond Peak, and Mount Thielsen all had small glaciers that persisted until the end of the Little Ice Age in the early 20th century. The Three Sisters Wilderness Area is the most extensively glacierized region of the central Oregon Cascade Range, with 17 named glaciers that presently cover about 7.5 square kilometers. ...

In the absence of historical records, periods of glacier retreat and advance are difficult to date accurately. According to the summary of Davis (1988), however, there were at least three periods of advanced ice positions during late Holocene time in the North American Cordillera: (1) a poorly dated early Neoglacial phase believed to date between 5,000 and 2,500 years ago; (2) a middle Neoglacial phase, which is recognized only in the Rocky Mountains of Colorado and Wyoming, where moraines date between 2,000 and 1,000 years ago, and (3) a late Neoglacial, or Little Ice Age readvance (Davis, 1988).

(The Neoglacial period was defined by Porter and Denton (1976) as encompassing the last 5,000 to 6,000 carbon-14 years, when alpine glaciers reformed and advanced. The "Little Ice Age" (Matthes, 1939) is generally regarded as the culmination of the Neoglacial period, and is a term used by climatologists, geologists, and glaciologists to describe a period of worldwide lower temperatures and advanced glacier positions from the 16th century through the late 19th century (Grove, 1988, p.3-5).)

Ages of early Neoglacial and Little Ice Age moraines in the Cascade Range have been determined by tephrochronolgy, and lichenometry. Early Neoglacial advances, all dated by radiocarbon dating of stratigraphically linked deposits, occurred between 5,500 years and 3,000 years (based on radiocarbon dates and not calibrated to a calendar year reference) at Glacier Peak (Beget, 1984); between 4,000 and 2,000 years at Mount Rainier (Crandell and Miller, 1964); younger than 4,000 years at Mount Adams (Hopkins, 1976); older than 2,500 to 1,800 years at Mount Hood (Lundstrom, 1992, p.143); and between 6,800 and 2,100 years at Broken Top and Mount Bachelor (Scott, 1989). These dates are consistent with results of recent studies in the Canadian Rockies that indicate a period of glacier advance between 3,100 to 2,500 years (Luckman and others, 1993).

In the Three Sisters and Mount Jefferson Wilderness Areas, most early Neoglacial deposits were removed or buried by Little Ice Age glacier advances during the last few centuries. This is consistent with many observations throughout the world that the Little Ice Age was, in general, the period of most advanced glacier positions of the Holocene (Grove, 1988). Evidence from lichenometric and dendrochronologic studies in Oregon and Washington indicates that glaciers reached maximum downvalley positions during the 17th, 18th, and 19th centuries. At Mount Rainier, which has had the most thoroughly developed Little Ice Age chronology, the outermost terminal moraines of most major glaciers stabilized between 1750 and 1850 AD, and there was general and substantial retreat between 1830 AD and 1950 AD (Crandell and Miller, 1964; Sigafoos and Hendricks, 1972; Burbank, 1981, 1982). The maximum advance of Eliot Glacier on Mount Hood culminated about 1740 AD (Lawrence, 1948; Lundstrom 1992, p.118-126).

Late Neoglacial moraines formed by the glaciers of the central Oregon Cascade Range may have stabilized somewhat later than those constructed by the larger glaciers at Mount Rainier and Mount Hood. On Three-Fingered Jack, the oldest tree cored on the Neoglacial moraine crest germinated about 1884 (Scott, 1974, p.81). Similarly, the oldest trees growing on the left lateral moraine of Skinner Glacier, on the north flank of South Sister, germinated about 1865. This evidence indicates that the maximum late Neoglacial advance in the Central Oregon Cascade Range probably culminated in the 1850's and 1860's. A substantially older moraine, however, was formed by a post-2,300 years advance of Lewis Glacier. The moraine was not covered by a tephra erupted 2,300 to 2,000 years ago (Scott and Gardner, 1992) but does have large mountain hemlocks and whitebark pines growing on it, including one that germinated more than 500 years ago. Although they had thinned substantially, most glaciers in the Three Sisters area remained in contact with Neoglacial-age moraines through the first two decades of the 20th century.

Carver Glacier

Carver Glacier, South Sister 7.5 minute quad, lake names: Chambers Lakes.

Clark Glacier

Clark Glacier, South Sister.

Collier Glacier

The most complete record of historic glacier retreat is for Collier Glacier, which has its source on the northwest flank of Middle Sister. Collier Glacier is the largest glacier in the central Oregon Cascade Range, presently covering about 0.85 square kilometers. At its maximum advance during the Little Ice Age, Collier Glacier covered nearly 2.4 square kilometers and had an ELA (equilibrium line altitudes) about 195 meters lower than present. The pattern of shrinkage of Collier Glacier was apparently common to most central Oregon Cascade Range glaciers. Substantial reduction in glacier mass during the period between the late Neoglacial maximum (assumed to be about 1850 locally) and about 1910 was accommodated by glacier thinning, whereas termini positions did not retreat substantially from Little Ice Age moraines until early in the 20th century. The response lag probably was due to the large volume of ice stored behind the tall terminal and lateral moraines. Between 1910 and 1950 there was rapid retreat of the Collier Glacier terminus and reduction of glacierized area and ice volume (Hopson, 1960; McDonald, 1995).

Diller Glacier

During maximum Neoglacial advances, Diller Glacier covered 1.03 square kilometers on the eastern slope of Middle Sister and formed high lateral and terminal moraines before retreating to a present area less than half its former size.

Eugene Glacier

The first recorded debris flow from a morain-dammed lake in the Cascade Range was from the failure of a Neoglacial moraine dam near the terminus of Eugene Glacier, a small ice body clinging to the steep, 600-meter-high north-northwest facing slope on the north side of South Sister. During maximum Neoglacial advances and until the early 1930's, Eugene Glacier covered 0.21 square kilometers and terminated in three separate lobes against large moraines at an altitude of about 2,475 meters. The present ice-covered area is about 0.09 square kilometers. Two debris flows resulted from morain-dammed-lake releases at Eugene Glacier. The first, 1n 1933, was the largest, and involved the central and eastern lobes of ice. The second resulted from a small release between 1933 and 1949 from a lake dammed behind the terminal moraine formed by the western ice lobe.

Hayden Glacier

Hayden Glacier, Middle Sister

Irving Glacier

Irving Glacier, South Sister

Lewis Glacier

Lewis Glacier, South Sister 7.5 minute quad, 1.3 square kilometers maximum Neoglacial glacier area, 0.3 square kilometers present glacier area.

The oldest trees growing on the left lateral moraine of Skinner Glacier, on the north flank of South Sister, germinated about 1865. This evidence indicates that the maximum late Neoglacial advance in the Central Oregon Cascade Range probably culminated in the 1850's and 1860's. A substantially older moraine, however, was formed by a post-2,300 years advance of Lewis Glacier. The moraine was not covered by a tephra erupted 2,300 to 2,000 years ago (Scott and Gardner, 1992) but does have large mountain hemlocks and whitebark pines growing on it, including one that germinated more than 500 years ago. Although they had thinned substantially, most glaciers in the Three Sisters area remained in contact with Neoglacial-age moraines through the first two decades of the 20th century.

Linn Glacier

Linn Glacier, North Sister.

Lost Creek Glacier

Lost Creek Glacier, South Sister

Prouty Glacier

Prouty Glacier, South Sister 7.5 minute quad, 1.9 square kilometers maxmimum Neoglacial glacier area, 1.1 square kilometers present glacier area, lake name: Carver Lake. ... Carver Lake is the largest of the moraine-dammed lakes in the Oregon Cascade Range, with a surface area of 700,000 cubic meters and a volume of 900,000 cubic meters.

Renfrew Glacier

Renfrew Glacier, Middle Sister

Skinner Glacier

The oldest trees growing on the left lateral moraine of Skinner Glacier, on the north flank of South Sister, germinated about 1865. This evidence indicates that the maximum late Neoglacial advance in the Central Oregon Cascade Range probably culminated in the 1850's and 1860's. A substantially older moraine, however, was formed by a post-2,300 years advance of Lewis Glacier. The moraine was not covered by a tephra erupted 2,300 to 2,000 years ago (Scott and Gardner, 1992) but does have large mountain hemlocks and whitebark pines growing on it, including one that germinated more than 500 years ago. Although they had thinned substantially, most glaciers in the Three Sisters area remained in contact with Neoglacial-age moraines through the first two decades of the 20th century.

Thayer Glacier

Thayer Glacier, North Sister 7.5 minute quad, 0.29 square kilometers maximum Neoglacial glacier area, 0.1 square kilometers present glacier area.

Villard Glacier

Villard Glacier, North Sister

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