| Three Sisters | |
|---|---|
The Three Sisters as seen from the south. |
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| Elevation | South 10,358 ft (3,157 m) [1] Middle 10,047 ft (3,062 m) [2] North 10,085 ft (3,074 m) [3] |
| Prominence | South 5,598 ft (1,706 m) [1] Middle 1,127 ft (344 m) [2] North 2,725 ft (831 m) [3] |
| Location | |
| Location | Lane / Deschutes counties, Oregon, USA |
| Range | Cascade Range |
| Coordinates | South [4] Middle [5] North [6] |
| Topo map | USGS South Sister and North Sister |
| Geology | |
| Type | Stratovolcanoes and shield volcanoes |
| Age of rock | Quaternary |
| Volcanic arc/belt | Cascade Volcanic Arc |
| Last eruption | about 50 BC[7] |
| Climbing | |
| Easiest route | hiking or scrambling, plus glacier travel on some routes[8] |
The Three Sisters are three volcanic peaks of the Cascade Volcanic Arc and the Cascade Range in Oregon, each of which exceeds 10,000 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,[9] about 15 mi (24 km) southwest from the nearest town of Sisters, Oregon. The three peaks have 15 named glaciers 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".[10][11]
The Three Sisters are the centerpiece of a region of closely grouped volcanic peaks which form an exception to the typical 40-mile (64 km) to 60-mile (97 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.[12] 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.[13]
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North Sister, also known as "Faith," is the oldest and most eroded of the three,[14] with towering rock pinnacles and glaciers. It has not erupted since the late Pleistocene.[14] 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.[15] 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.[16]
Middle Sister also known as "Hope," is a stratovolcano consisting primarily of basalt but also has erupted andesite, dacite and rhyodacite. Last erupting approximately 50,000 years ago, it is considered extinct.[17] 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.[18]
Middle Sister is the smallest and most poorly studied. It is also the middle in age,[14] but only somewhat older than South Sister, with the most recent flows dated to 14,000 years ago.[16]
South Sister, also known as "Charity," is the youngest[14] and tallest volcano of the trio. Its eruptive products range from basaltic andesite to rhyolite and rhyodacite.[14][19] It is a stratovolcano overlying an older shield structure, no more than 50,000 years old,[20] which last erupted about 2000 years ago.[7] 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.[21]
South Sister has an uneroded summit crater about 0.25 mi (0.4 km) in diameter, which holds a small crater lake known as Teardrop Pool, the highest lake in Oregon.[22] South Sister supports two small glaciers, the Lewis and Clark glaciers, near the crater rim.[23] 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.[24]
In 2001 a satellite discovered that there was a deforming uplift 3 miles (4.8 km) west of the South Sister.[25] 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.[26]
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