Three Sisters

USGS scientists have detected an increase in the rate of uplift of the ground surface in the Three Sisters volcanic region of the central Oregon Cascade Range. The volcanoes’ status remains normal/green, and there is no sign of an imminent eruption. Episodes of increased uplift have been observed in this area before and are attributed to small pulses of magma moving deep into the volcanic region.Satellite radar images detected an increased rate of uplift of a 12-mile (20-km) diameter area centered about 3 miles (5 km) west of South Sister volcano. The data indicate uplift of up to 0.9 inches (2.2 cm) between June 2020 and August 2021. GPS data from a nearby continuously recording station show that the deformation has continued to the present. Additionally, seismologists observed brief bursts of small earthquakes in October 2021, December 2021, and January 2022. Most of these shallow earthquakes are too small to locate; those located are inside the uplifted area.The present uplift area is in the same general location where uplift was first seen in the mid-1990s and has continued at a generally slowing rate through 2020. This increased inflation appears to be a continuation of an extended period of uplift that is attributed to small pulses of magma accumulating at roughly 4 miles (7 kilometers) below the ground surface. From 1995 to 2020, the area rose approximately 12 inches (30 centimeters) at its center. Although the current uplift rate is slower than the maximum rate of about 2 inches per year (5 cm per year) measured in 1999-2000, it is distinctly faster than the rate observed for several years before 2020.The specific cause of the uplift is uncertain. Because the Three Sisters region is a volcanic area (the most recent eruption was about 2,000 years ago near South Sister), the uplift may reflect a small amount of magma emplaced at around 4 miles (7 km) deep. The idea of repeated intrusions at Three Sisters is supported by a USGS study in 1990 that found evidence that heat and gases from magma had influenced water temperature and chemistry of springs located in the uplifted area. This research was published before satellite volcano monitoring began and well before the current uplift episode. In that context, we view periods of increased uplift like the current one as a continuation of episodic, deep magmatic intrusions that have likely been occurring for centuries or millennia in the Three Sisters area. While any magmatic intrusion could eventually lead to a volcanic eruption, an eruption would likely be preceded by detectable and more vigorous earthquakes, ground movement (deformation), and geochemical changes. In general, as magma moves upward during an intrusion, it causes continued or accelerated uplift, fractures rock to generate swarms of earthquakes, and releases significant amounts of volcanic gases, such as carbon dioxide. We do not detect any of these signs currently. Scientists at Cascades Volcano Observatory (CVO) and the Pacific Northwest Seismic Network (PNSN) will closely monitor data in the coming months and issue further updates as warranted.

CASCADES VOLCANO OBSERVATORY INFORMATION STATEMENT
U.S. Geological Survey
Monday, January 31, 2022, 12:45 PM PST (Monday, January 31, 2022, 20:45 UTC)

THREE SISTERS VOLCANO(VNUM #322070)
44°6'11" N 121°46'5" W, Summit Elevation 10358 ft (3157 m)
Current Volcano Alert Level: NORMAL
Current Aviation Color Code: GREEN

USGS scientists have detected an increase in the rate of uplift of the ground surface in the Three Sisters volcanic region of the central Oregon Cascade Range. The volcanoes’ status remains normal/green, and there is no sign of an imminent eruption. Episodes of increased uplift have been observed in this area before and are attributed to small pulses of magma moving deep into the volcanic region.

Satellite radar images detected an increased rate of uplift of a 12-mile (20-km) diameter area centered about 3 miles (5 km) west of South Sister volcano. The data indicate uplift of up to 0.9 inches (2.2 cm) between June 2020 and August 2021. GPS data from a nearby continuously recording station show that the deformation has continued to the present. Additionally, seismologists observed brief bursts of small earthquakes in October 2021, December 2021, and January 2022. Most of these shallow earthquakes are too small to locate; those located are inside the uplifted area.

The present uplift area is in the same general location where uplift was first seen in the mid-1990s and has continued at a generally slowing rate through 2020. This increased inflation appears to be a continuation of an extended period of uplift that is attributed to small pulses of magma accumulating at roughly 4 miles (7 kilometers) below the ground surface. From 1995 to 2020, the area rose approximately 12 inches (30 centimeters) at its center. Although the current uplift rate is slower than the maximum rate of about 2 inches per year (5 cm per year) measured in 1999-2000, it is distinctly faster than the rate observed for several years before 2020.

The specific cause of the uplift is uncertain. Because the Three Sisters region is a volcanic area (the most recent eruption was about 2,000 years ago near South Sister), the uplift may reflect a small amount of magma emplaced at around 4 miles (7 km) deep. The idea of repeated intrusions at Three Sisters is supported by a USGS study in 1990 that found evidence that heat and gases from magma had influenced water temperature and chemistry of springs located in the uplifted area. This research was published before satellite volcano monitoring began and well before the current uplift episode. In that context, we view periods of increased uplift like the current one as a continuation of episodic, deep magmatic intrusions that have likely been occurring for centuries or millennia in the Three Sisters area.

While any magmatic intrusion could eventually lead to a volcanic eruption, an eruption would likely be preceded by detectable and more vigorous earthquakes, ground movement (deformation), and geochemical changes. In general, as magma moves upward during an intrusion, it causes continued or accelerated uplift, fractures rock to generate swarms of earthquakes, and releases significant amounts of volcanic gases, such as carbon dioxide. We do not detect any of these signs currently.

Scientists at Cascades Volcano Observatory (CVO) and the Pacific Northwest Seismic Network (PNSN) will closely monitor data in the coming months and issue further updates as warranted.

GEOLOGIC CONTEXT:
The Three Sisters volcanic cluster contains three stratovolcanoes constructed between 120,000 and 50,000 years ago (North Sister) and 50,000 and 15,000 years ago (Middle and South Sister). The only eruptions younger than 15,000 years occurred at/near South Sister - the Rock Mesa rhyolite (about 2,200 years ago) southwest of the summit and the Devils Chain rhyolites (about 2,000 years ago) on the volcano’s eastern flank. Both episodes included lava flows and dome-building eruptions accompanied by minor to moderate explosions. These explosions created short-traveled pyroclastic flows, pumice, and ashfall up to 18.6 miles (30 km) away to the east and south.

For more information:
Hildreth, W., Fierstein, J. & Calvert, A. T. (2012). Geologic Map of Three Sisters Volcanic Cluster, Cascade Range, Oregon. U.S. Geological Survey Scientific Investigations Map, 3186, pamphlet 107 p., 2 sheets. https://pubs.usgs.gov/sim/3186/

Lisowski, M., McCaffrey, R., Wicks, C.W., and Dzurisin, D., 2021, Geodetic constraints on a 25-year magmatic inflation episode near Three Sisters, Central Oregon. Journal of Geophysical Research—Solid Earth, 126, e2021JB022360. https://doi.org/10.1029/2021JB022360

Website Resources
CVO Three Sisters Webpage: https://www.usgs.gov/volcanoes/three-sisters

CVO Three Sisters Uplift Webpage: https://www.usgs.gov/volcanoes/three-sisters/modern-deformation-and-uplift-sisters-region

PNSN Three Sisters webpage: https://pnsn.org/volcanoes/three-sisters

Satellite volcano monitoring information: https://www.usgs.gov/programs/VHP/insar-satellite-based-technique-captures-overall-deformation-picture

CONTACT INFORMATION:

Jon Major, Scientist-in-Charge, Cascades Volcano Observatory, [email protected]

General inquiries: [email protected]
Media: Ryan McClymont, PIO, USGS Office of Communications and Publishing [email protected]