Three Sisters

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  • Country
  • Subregion Name
  • Primary Volcano Type
  • Last Known Eruption
  • 44.17°N
  • 121.77°W

  • 3074 m
    10083 ft

  • 322070
  • Latitude
  • Longitude

  • Summit
    Elevation

  • Volcano
    Number

24 March-30 March 2004

On 23 March at around 1000 a seismic swarm began at South Sister that lasted ~48 hours. Over 300 volcano-tectonic earthquakes up to M 1.9 were recorded. The earthquakes were located in the NE quadrant of the area of on-going uplift.

Sources: US Geological Survey Cascades Volcano Observatory (CVO), Pacific Northwest Seismic Network



 Available Weekly Reports


2004: March
2001: May


24 March-30 March 2004

On 23 March at around 1000 a seismic swarm began at South Sister that lasted ~48 hours. Over 300 volcano-tectonic earthquakes up to M 1.9 were recorded. The earthquakes were located in the NE quadrant of the area of on-going uplift.

Sources: US Geological Survey Cascades Volcano Observatory (CVO), Pacific Northwest Seismic Network


2 May-8 May 2001

The USGS Cascades Volcano Observatory reported on 8 May that between 1996 and 2000 they detected slight uplift of the ground surface over a broad area 5 km W of South Sister volcano. The uplift occurred over an area ~15-20 km in diameter and the maximum amount of uplift at its center is ~10 cm. CVO personnel believe the uplift may reflect intrusion of a small volume of magma at ~7 km depth beneath the ground surface. They stated that if the intrusion of magma continues it could eventually lead to a volcanic eruption, but precursory activity would most likely occur beforehand. There is no precursory activity that suggests an eruption is imminent; seismic activity at the volcano is near or below background levels, gas emissions are low, and no unusual surface changes have been observed.

Sources: US Geological Survey Cascades Volcano Observatory (CVO), Associated Press


Summary of eruption dates and Volcanic Explosivity Indices (VEI).

Start Date Stop Date Eruption Certainty VEI Evidence Activity Area or Unit
[ 1853 Jul ] [ Unknown ] Discredited    
0440 ± 150 years Unknown Confirmed 2 Radiocarbon (corrected) NW of North Sister (Collier Cone)
0040 ± 200 years Unknown Confirmed 2 Radiocarbon (corrected) NW of North Sister (Four-in-One Cone)
0050 BCE (?) Unknown Confirmed   Radiocarbon (uncorrected) N & S flanks of South Sister (Devils Hill)
0350 BCE (?) Unknown Confirmed   Radiocarbon (uncorrected) SW flank of South Sister (Rock Mesa)
0800 BCE (?) Unknown Confirmed 2 Tephrochronology North of North Sister (Yapoah Cone)
7350 BCE ± 2700 years Unknown Confirmed 2 Tephrochronology WNW of North Sister (Sims Butte)

The following references are the sources used for data regarding this volcano. References are linked directly to our volcano data file. Discussion of another volcano or eruption (sometimes far from the one that is the subject of the manuscript) may produce a citation that is not at all apparent from the title. Additional discussion of data sources can be found under Volcano Data Criteria.

Brophy J G, Dreher S T, 2000. The origin of composition gaps at South Sister volcano, central Oregon: implications for fractional crystallization processes beneath active calc-alkaline volcanoes. J Volc Geotherm Res, 102: 287-307.

Fierstein J, Hildreth W, Calvert A T, 2011. Eruptive history of South Sister, Oregon Cascades. J Volc Geotherm Res, 207: 145-179.

Green J, Short N M, 1971. Volcanic Landforms and Surface Features: a Photographic Atlas and Glossary. New York: Springer-Verlag, 519 p.

Hildreth W E, 2007. Quaternary magmatism in the Cascades--geologic perpectives. U S Geol Surv Prof Pap, 1744: 1-125.

Hildreth W, Fierstein J, Calvert A T, 2012. Geologic map of Three Sisters volcanic cluster, Cascade Range, Oregon. U S Geol Surv Sci Invest Map, SIM-3186, 1:50,000 scale, 2 sheets,107 p text.

IAVCEI, 1973-80. Post-Miocene Volcanoes of the World. IAVCEI Data Sheets, Rome: Internatl Assoc Volc Chemistry Earth's Interior..

Peterson N V, Groh E A, 1966. Lunar Geological Field Conference guidebook. Oregon Dept Geol Min Ind, 51 p.

Sarna-Wojcicki A M, Champion D E, Davis J O, 1983. Holocene volcanism in the conterminous United States and the role of silicic volcanic ash layers in correlation of latest Pleistocene and Holocene deposits. In: Wright H E (ed) {Late-Quaternary Environments of the United States}, Minneapolis: Univ Minnesota Press, 2: 52-77.

Schmidt M E, Grunder A L, 2009. The evolution of North Sister: a volcano shaped by extension and ice in the central Oregon Cascade Arc. Geol Soc Amer Bull, 121: 643-662.

Scott W E, 1987. Holocene rhyodacite eruptions on the flanks of South Sister volcano, Oregon. Geol Soc Amer Spec Pap, 212: 35-53.

Scott W E, Gardner C A, 1990. Field trip guide to the central Oregon High Cascades, Part 1: Mount Bachelor-South Sister area. Oregon Geol, 52: 99-140.

Scott W E, Gardner C A, Sarna-Wojcicki A M, 1989. Guidebook for field trip to the Mount Bachelor-South Sister-Bend area, central Oregon High Cascades. U S Geol Surv Open-File Rpt, 89-645: 1-68.

Taylor E M, 1968. Roadside geology, Santiam and McKenzie Pass Highways, Oregon. Oregon Dept Geol Min Ind Bull, 62: 3-34.

Taylor E M, 1981. Roadlog for central High Cascade geology, Bend, Sisters, McKenzie Pass, and Santiam Pass, Oregon. U S Geol Surv Circ, 838: 59-83.

Williams H, 1944. Volcanoes of the Three Sisters region, Oregon Cascades. Univ Calif Pub Geol Sci, 27: 37-84.

Williams H, 1957. A geologic map of the Bend Quadrangle, Oregon and a reconnaissance geologic map of the central portion of the High Cascade Mountains. Oregon Dept Geol Min Ind, 1:125,000 and 1:250,000 scale.

Wozniak K C, Taylor E M, 1981. Late Pleistocene summit construction and Holocene flank eruptions of South Sister volcano, Oregon (abs). Eos, Trans Amer Geophys Union, 62: 61.

The north-south-trending Three Sisters volcano group dominates the landscape of the Central Oregon Cascades. All Three Sisters stratovolcanoes ceased activity during the late Pleistocene, but basaltic-to-rhyolitic flank vents erupted during the Holocene, producing both blocky lava flows north of North Sister and rhyolitic lava domes and flows south of South Sister volcano. Glaciers have deeply eroded the Pleistocene andesitic-dacitic North Sister stratovolcano, exposing the volcano's central plug. Construction of the main edifice ceased at about 55,000 yrs ago, but north-flank vents produced blocky lava flows in the McKenzie Pass area as recently as about 1600 years ago. Middle Sister volcano is located only 2 km to the SW and was active largely contemporaneously with South Sister until about 14,000 years ago. South Sister is the highest of the Three Sisters. It was constructed beginning about 50,000 years ago and was capped by a symmetrical summit cinder cone formed about 22,000 years ago. The late Pleistocene or early Holocene Cayuse Crater on the SW flank of Broken Top volcano and other flank vents such as Le Conte Crater on the SW flank of South Sister mark mafic vents that have erupted at considerable distances from South Sister itself, and a chain of dike-fed rhyolitic lava domes and flows at Rock Mesa and Devils Chain south of South Sister erupted about 2000 years ago.