Volcano Watch — Ground uplift in Oregon Cascades is tipoff to rising magma

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Volcanologists have long known that when molten rock rises to within 2-10 km (1-6 miles) of the Earth's surface, the overlying ground is often pushed upward by at least several centimeters (inches), sometimes more than 1 m (3 feet).

In order to track such uplift in real time and be able to issue an eruption warning, scientists must install networks of continuously recording tiltmeters, GPS receivers, and super-precise strainmeters on a volcano.

A promising new satellite-based method for measuring uplift of the ground has enabled scientists to watch the surface of some volcanoes move during a period of months or years without installing a single instrument on the ground. Through this technique, called Satellite Radar Interferometry, scientists have discovered an increasing number of volcanoes where the ground surface has risen at least 5-10 cm (2-4 inches), some without an accompanying swarm of earthquakes or subsequent eruption.

The implication of this innovative research is that many volcanoes inflate episodically with magma between eruptions without much, if any, sign of activity. Because the new technique measures inflation only over a period of months or years, due to the limitation of satellite orbits, it cannot be used to issue a warning of an impending eruption. Only ground-based instruments can currently track moving magma in real time for eruption warnings. But the technique is extremely useful in identifying volcanoes on which ground-based instruments should perhaps be installed for further study.

The most recent and unexpected example of satellite-detected inflation at a volcano in the United States is the Three Sisters volcanic center in the Oregon Cascade Range, located about 225 km (140 mi) south of Mount St. Helens, Washington.

When testing the new satellite-based technique earlier this year on Cascade volcanoes, Dr. Charles Wicks of the U.S. Geological Survey in Menlo Park, California, discovered a broad area of uplift 15-20 km (9-12 miles) in diameter centered 5 km (3 miles) west of the South Sister volcano. The volcano is located in the Three Sisters Wilderness of the Deschutes and Willamette National Forests. Satellite images showed that the uplift of about 10 cm (4 inches) occurred sometime between 1996 and 2000.

Word of the Three Sisters uplift spread quickly to Wicks' colleagues at the USGS Cascades Volcano Observatory (CVO) and University of Washington (UW) Geophysics Program, and many questions were raised. Is the uplift still occurring and at what rate? Can the uplift be verified by some other method? What caused the uplift-rising magma or increased pressure in the hydrothermal system beneath the area? What needs to be done so that we can provide adequate warning of future volcanic activity if the magma keeps moving toward the surface?

Since the discovery this past spring, USGS and UW scientists have worked with representatives from the Willamette and Deschutes National Forests to develop a skeletal real-time monitoring capability using ground-based instruments in the area. A continuously operating GPS receiver was installed to identify any sudden change in the uplift rate. A new seismometer was installed in the area so that small earthquakes of magnitude 1 or greater can be detected. Since July, however, only a magnitude-1.9 earthquake has occurred in the area.

In mid-September, scientists also resurveyed benchmarks installed on South Sister in the 1980s. The measured changes are consistent with the uplift determined from the satellite images. In May, a small amount of carbon dioxide volcanic gas was detected in the area, but not enough to warrant concern for public safety. The presence of carbon dioxide is consistent with the idea that magma is responsible for the uplift.

Ever since the eruption of Mount St. Helens in 1980, any hint of possible volcanic activity in the Cascades generates considerable interest among residents of the Pacific Northwest. USGS scientists have met several times with residents and officials in nearby communities and the news media to talk about their most recent findings in the Three Sisters Wilderness. For now, there is no reason for alarm, and steps are in place for monitoring the area more closely should magma move closer to the surface.

Volcano Activity Update

Eruptive activity of Kīlauea Volcano continued unabated at the Pu`u `O`o vent during the past week. Lavamoves away from the vent toward the ocean in a network of tubes and descends Pulama pali in several separate tubes. Breakouts from the tube system feed surface flows on the pali and in the coastal flats. Lava continues to enter the ocean at Kamoamoa and the area east of Kupapa`u.

The public is reminded that the benches of the two ocean entries are very hazardous, with possible collapses of the new land. The steam clouds are extremely hot, highly acidic, and laced with glass particles. Swimming at the black sand beaches of the benches can be a blistering or even deadly venture.

There were no earthquakes reported felt during the week ending on October 11.