Deschutes & Ochoco National Forests
Crooked River National Grassland

Oregon Volcanoes

South Sister Volcano

SUMMARY

Type: stratovolcano
Activity: dormant
Last Eruption: about 1,900 yrs ago
Rock Type: basaltic andesite to rhyodacite.
Eruptive Volume: ?
Latitude: 44.10 N
Longitude: 121.77 W
Location Maps: from TIGER Mapping Service

• State Level - shows state and communities
• County Level - shows county lines, highways, and communities
• Local Level - shows above plus lakes and local roads

Topo Map from TopoZone

South Sister Photo Menu

GEOLOGIC BACKGROUND:

The Three Sisters area contains 4 large cones of Quaternary age -- North Sister, Middle Sister, South Sister, and Broken Top. 

• North Sister and Broken Top are deeply dissected and probably have been inactive for at least 100,000 years. 
• Middle Sister younger than the North Sister and older than the South Sister. It 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.  Map of Holocene Tephras (22k gif)

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. Excerpt from: Scott, et.al., 2001, Volcano Hazards in the Three Sisters Region, Oregon: USGS Open-File Report 99-437

HISTORIC ACTIVITY:

• None

RECENT ACTIVITY:

• During the period from about 10 am on March 23, 2004 to about 1 am on March 25 a swarm of earthquake occurred in the Three Sisters area. More than 100 small quake up to a magnitude of 1.9 occurred. Seismograms of swarm.

• On May 8, 2001, U.S. Geological Survey scientists reported that by using Satellite Radar Interferometry (or InSAR) they had detected a broad area over which the ground has swelled upward slightly during the past few years.  The area is centered 5 kilometers (3 miles) west of South Sister volcano in the Three Sisters region of the central Oregon Cascade Range. The area of uplift is about 15 to 20 kilometers (9 to 12 miles) in diameter and the maximum amount of uplift at its center is about 10 centimeters (4 inches). 

DATA SOURCE:

• USGS

ADDITIONAL INFORMATION

• Link to U.S.G.S. - CVO
  • Three Sisters Area Info
  • Volcano Hazards in the Three Sisters Region, Oregon
  • Radar Interferogram of Ground Uplift Near South Sister Volcano
  • Three Sisters Information Statements
    • March 24, 2004
    • March 18, 2002
    • September 12, 2001
    • May 8, 2001
• Link to U.S.G.S. Visit a Volcano Info for Three Sisters
• Link to University of Washington Geophysics Program Information
  • Three Sisters Seismicity Information
• Link to VolcanoWorld
  • Three Sisters Area Info