Three Sisters (Oregon)
Encyclopedia
The Three Sisters are three volcanic
peaks of the Cascade Volcanic Arc
and the Cascade Range
in Oregon
, each of which exceeds 10000 ft (3,048 m) in elevation. They are the third, fourth, and fifth highest peaks in the state of Oregon and are located in the Three Sisters Wilderness
, about 15 mi (24 km) southwest from the nearest town of Sisters, Oregon
. The three peaks have 15 named glacier
s among them, nearly half of all the 35 named glaciers in Oregon. The Sisters were named Faith, Hope, and Charity by early settlers, but "these names have not prevailed".
The Three Sisters are the centerpiece of a region of closely grouped volcanic peaks which form an exception to the typical 40 miles (64.4 km) to 60 miles (96.6 km) spacing between volcanoes elsewhere in the Cascades. Nearby peaks include Belknap Crater
, Broken Top
and Mount Bachelor
, with Three Fingered Jack
and Mount Washington
somewhat to the north. The region was a volcanic center in the Pleistocene
epoch, with eruptions from 600,000 - 700,000 to 170,000 years ago, an explosively active complex known as the Tumalo volcanic center. Basaltic lava flows from North sister overlay the newest Tumalo pyroclastic deposits, making the age of North Sister, the oldest of the three, less than 170,000 years.
s. It has not erupted since the late Pleistocene
. It is the most dangerous climb of the Three Sisters, due to its level of erosion, and thus rockfall.
North Sister is a shield volcano
consisting primarily of basaltic andesite
and is estimated to have last erupted over 100,000 years ago and is considered extinct. The North Sister possesses more dikes
than any similar Cascade peak. Many dikes were pushed aside by the intrusion of a 300 meter wide plug dome that now forms the mountain's summits of Prouty Peak and the South Horn. The remainder of the peak is chiefly loose debris held in place by the dike system.
consisting primarily of basalt
but also has erupted andesite, dacite
and rhyodacite
. Last erupting approximately 50,000 years ago, it is considered extinct. The mountain's form is that of a cone which has lost its east side to glaciation. The Hayden and Diller glaciers continue to cut into the east face. The large but retreating Collier Glacier descends along the north side of Middle Sister and cuts into North Sister's west side.
Middle Sister is the smallest and most poorly studied. It is also the middle in age, but only somewhat older than South Sister, with the most recent flows dated to 14,000 years ago.
of the trio. Its eruptive products range from basaltic andesite to rhyolite
and rhyodacite
. It is a stratovolcano overlying an older shield structure, no more than 50,000 years old, which last erupted about 2000 years ago. The first such episode, termed the Rock Mesa eruptive cycle first spread tephra from flank vents from the south and southwest flanks, followed by a thick rhyolite lava flow. The second cycle, the Devils Hill eruptive cycle, was similar in result, but was caused by the intrusion of a dike of new siliic magma which erupted from about twenty vents on the southeast side, with a smaller line on the north side.
South Sister has an uneroded summit crater about 0.25 mi (0.402335 km) in diameter, which holds a small crater lake
known as Teardrop Pool, the highest lake in Oregon. South Sister supports two small glaciers, the Lewis and Clark glaciers, near the crater rim. The standard climbing route up the South Ridge of South Sister is a long, steep, non-technical hike that can be easily completed in a day by reasonably fit hikers. Popular starting points are the Green Lakes or Devil's Lake trailheads.
discovered that there was a deforming uplift
3 miles (4.8 km) west of the South Sister. There was concern that the mountain was awakening. A map at the Lava Lands Visitor Center of the Newberry National Volcanic Monument
south of Bend
shows the extent of the uplift, centered a few miles west of the South Sister and reaching a maximum of 11 inches. In 2004 an earthquake swarm
occurred with the epicenter in the area of uplift. The hundreds of small earthquakes subsided over several days. As of 2007 the uplift had slowed and concern had diminished though the area was still considered potentially active.
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...
peaks of the Cascade Volcanic Arc
Cascade Volcanoes
The Cascade Volcanoes are a number of volcanoes in a volcanic arc in western North America, extending from southwestern British Columbia through Washington and Oregon to Northern California, a distance of well over 700 mi ...
and the Cascade Range
Cascade Range
The Cascade Range is a major mountain range of western North America, extending from southern British Columbia through Washington and Oregon to Northern California. It includes both non-volcanic mountains, such as the North Cascades, and the notable volcanoes known as the High Cascades...
in Oregon
Oregon
Oregon is a state in the Pacific Northwest region of the United States. It is located on the Pacific coast, with Washington to the north, California to the south, Nevada on the southeast and Idaho to the east. The Columbia and Snake rivers delineate much of Oregon's northern and eastern...
, each of which exceeds 10000 ft (3,048 m) in elevation. They are the third, fourth, and fifth highest peaks in the state of Oregon and are located in the Three Sisters Wilderness
Three Sisters Wilderness
The Three Sisters Wilderness is a wilderness area in the Cascade Range, within the Willamette and Deschutes National Forests in Oregon. It comprises 286,708 acres , making it the second largest Wilderness area in Oregon...
, about 15 mi (24 km) southwest from the nearest town of Sisters, Oregon
Sisters, Oregon
Sisters is a city in Deschutes County, Oregon, United States. It is part of the Bend, Oregon Metropolitan Statistical Area. The population was 959 at the 2000 census, but more than doubled to 2038 as of the 2010 census.- History :...
. The three peaks have 15 named glacier
Glacier
A glacier is a large persistent body of ice that forms where the accumulation of snow exceeds its ablation over many years, often centuries. At least 0.1 km² in area and 50 m thick, but often much larger, a glacier slowly deforms and flows due to stresses induced by its weight...
s among them, nearly half of all the 35 named glaciers in Oregon. The Sisters were named Faith, Hope, and Charity by early settlers, but "these names have not prevailed".
The Three Sisters are the centerpiece of a region of closely grouped volcanic peaks which form an exception to the typical 40 miles (64.4 km) to 60 miles (96.6 km) spacing between volcanoes elsewhere in the Cascades. Nearby peaks include Belknap Crater
Belknap Crater
Belknap Crater is a shield volcano in the Cascade Range in the U.S. state of Oregon.Belknap volcano is a Holocene example of the process which built the High Cascade Platform during the Pleistocene. It is a small shield volcano with a capping cinder cone....
, Broken Top
Broken Top
Broken Top is an extinct, glacially eroded stratovolcano in Oregon, part of the extensive Cascade Range. Located south of the Three Sisters peaks, the volcano, residing within the Three Sisters Wilderness, is 20 miles west of Bend, Oregon in Deschutes County...
and Mount Bachelor
Mount Bachelor
Mount Bachelor, formerly named Bachelor Butte, is a stratovolcano atop a shield volcano in the Cascade Volcanic Arc and the Cascade Range of central Oregon.The Mount Bachelor ski area has operated on the mountain since 1958....
, with Three Fingered Jack
Three Fingered Jack
Three Fingered Jack, named for its distinctive shape, is a Pleistocene volcano in the Cascade Range of Oregon. It is a deeply glaciated shield volcano and consists mainly of basaltic andesite lava...
and Mount Washington
Mount Washington (Oregon)
Mount Washington is a deeply eroded shield volcano in the Cascade Range of Oregon. The mountain dates to the Late Pleistocene. However, it does have a line of basaltic andesite spatter cones on its northeast flank which are approximately 1,330 years old according to carbon dating...
somewhat to the north. The region was a volcanic center in the Pleistocene
Pleistocene
The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....
epoch, with eruptions from 600,000 - 700,000 to 170,000 years ago, an explosively active complex known as the Tumalo volcanic center. Basaltic lava flows from North sister overlay the newest Tumalo pyroclastic deposits, making the age of North Sister, the oldest of the three, less than 170,000 years.
North Sister
North Sister, also known as "Faith," is the oldest and most eroded of the three, with towering rock pinnacles and glacierGlacier
A glacier is a large persistent body of ice that forms where the accumulation of snow exceeds its ablation over many years, often centuries. At least 0.1 km² in area and 50 m thick, but often much larger, a glacier slowly deforms and flows due to stresses induced by its weight...
s. It has not erupted since the late Pleistocene
Pleistocene
The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....
. It is the most dangerous climb of the Three Sisters, due to its level of erosion, and thus rockfall.
North Sister is a shield volcano
Shield volcano
A shield volcano is a type of volcano usually built almost entirely of fluid lava flows. They are named for their large size and low profile, resembling a warrior's shield. This is caused by the highly fluid lava they erupt, which travels farther than lava erupted from more explosive volcanoes...
consisting primarily of basaltic andesite
Basaltic andesite
Basaltic andesite is a black volcanic rock containing about 55% silica. Minerals in basaltic andesite include olivine, augite and plagioclase. Basaltic andesite can be found in volcanoes around the world, including in Central America and the Andes of South America. Basaltic andesite is common in...
and is estimated to have last erupted over 100,000 years ago and is considered extinct. The North Sister possesses more dikes
Dike (geology)
A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...
than any similar Cascade peak. Many dikes were pushed aside by the intrusion of a 300 meter wide plug dome that now forms the mountain's summits of Prouty Peak and the South Horn. The remainder of the peak is chiefly loose debris held in place by the dike system.
Middle Sister
Middle Sister also known as "Hope," is a stratovolcanoStratovolcano
A stratovolcano, also known as a composite volcano, is a tall, conical volcano built up by many layers of hardened lava, tephra, pumice, and volcanic ash. Unlike shield volcanoes, stratovolcanoes are characterized by a steep profile and periodic, explosive eruptions...
consisting primarily of basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
but also has erupted andesite, dacite
Dacite
Dacite is an igneous, volcanic rock. It has an aphanitic to porphyritic texture and is intermediate in composition between andesite and rhyolite. The relative proportions of feldspars and quartz in dacite, and in many other volcanic rocks, are illustrated in the QAPF diagram...
and rhyodacite
Rhyodacite
Rhyodacite is an extrusive volcanic rock intermediate in composition between dacite and rhyolite. It is the extrusive equivalent of granodiorite. Phenocrysts of sodium rich plagioclase, sanidine, quartz, and biotite or hornblende are typically set in an aphanitic to glassy light to intermediate...
. Last erupting approximately 50,000 years ago, it is considered extinct. The mountain's form is that of a cone which has lost its east side to glaciation. The Hayden and Diller glaciers continue to cut into the east face. The large but retreating Collier Glacier descends along the north side of Middle Sister and cuts into North Sister's west side.
Middle Sister is the smallest and most poorly studied. It is also the middle in age, but only somewhat older than South Sister, with the most recent flows dated to 14,000 years ago.
South Sister
South Sister, also known as "Charity," is the youngest and tallest volcanoVolcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...
of the trio. Its eruptive products range from basaltic andesite to rhyolite
Rhyolite
This page is about a volcanic rock. For the ghost town see Rhyolite, Nevada, and for the satellite system, see Rhyolite/Aquacade.Rhyolite is an igneous, volcanic rock, of felsic composition . It may have any texture from glassy to aphanitic to porphyritic...
and rhyodacite
Rhyodacite
Rhyodacite is an extrusive volcanic rock intermediate in composition between dacite and rhyolite. It is the extrusive equivalent of granodiorite. Phenocrysts of sodium rich plagioclase, sanidine, quartz, and biotite or hornblende are typically set in an aphanitic to glassy light to intermediate...
. It is a stratovolcano overlying an older shield structure, no more than 50,000 years old, which last erupted about 2000 years ago. The first such episode, termed the Rock Mesa eruptive cycle first spread tephra from flank vents from the south and southwest flanks, followed by a thick rhyolite lava flow. The second cycle, the Devils Hill eruptive cycle, was similar in result, but was caused by the intrusion of a dike of new siliic magma which erupted from about twenty vents on the southeast side, with a smaller line on the north side.
South Sister has an uneroded summit crater about 0.25 mi (0.402335 km) in diameter, which holds a small crater lake
Crater lake
A crater lake is a lake that forms in a volcanic crater or caldera, such as a maar; less commonly and with lower association to the term a lake may form in an impact crater caused by a meteorite. Sometimes lakes which form inside calderas are called caldera lakes, but often this distinction is not...
known as Teardrop Pool, the highest lake in Oregon. South Sister supports two small glaciers, the Lewis and Clark glaciers, near the crater rim. The standard climbing route up the South Ridge of South Sister is a long, steep, non-technical hike that can be easily completed in a day by reasonably fit hikers. Popular starting points are the Green Lakes or Devil's Lake trailheads.
Recent history
In 2001 a satelliteSatellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
discovered that there was a deforming uplift
Tectonic uplift
Tectonic uplift is a geological process most often caused by plate tectonics which increases elevation. The opposite of uplift is subsidence, which results in a decrease in elevation. Uplift may be orogenic or isostatic.-Orogenic uplift:...
3 miles (4.8 km) west of the South Sister. There was concern that the mountain was awakening. A map at the Lava Lands Visitor Center of the Newberry National Volcanic Monument
Newberry National Volcanic Monument
Newberry National Volcanic Monument was designated on November 5, 1990, to protect the area around the Newberry Volcano in the United States. It was created within the boundaries of the Deschutes National Forest and is managed by the U.S. Forest Service...
south of Bend
Bend, Oregon
Bend is a city in and the county seat of Deschutes County, Oregon, United States, and the principal city of the Bend, Oregon Metropolitan Statistical Area. Bend is Central Oregon's largest city, and, despite its modest size, is the de facto metropolis of the region, owing to the low population...
shows the extent of the uplift, centered a few miles west of the South Sister and reaching a maximum of 11 inches. In 2004 an earthquake swarm
Earthquake swarm
Earthquake swarms are events where a local area experiences sequences of many earthquakes striking in a relatively short period of time. The length of time used to define the swarm itself varies, but the United States Geological Survey points out that an event may be on the order of days, weeks, or...
occurred with the epicenter in the area of uplift. The hundreds of small earthquakes subsided over several days. As of 2007 the uplift had slowed and concern had diminished though the area was still considered potentially active.