A ground bulge near Oregon’s Three Sisters has been rising since 1997

A broad patch of ground about 6 kilometers west of South Sister, in Oregon’s Three Sisters volcanic center, has been swelling upward since 1997, and the U.S. Geological Survey says the process is still underway nearly three decades later. Satellites first caught the bulge in 2001, and Cascades Volcano Observatory scientists confirmed a renewed pulse of uplift as recently as January 2022. No eruption has followed, and the agency rates the odds of one starting soon as low, but the ground itself has not stopped moving.

What makes the bulge unusual is not its size, which is modest by volcanic standards, but its persistence. Few uplift signals anywhere in the Cascade Range have been tracked continuously for as long as this one.

Radar Satellites Spotted the Bulge in 2001

Scientists did not notice the deformation as it began. According to the USGS’s Three Sisters hazard page, “a phase of uplift had started in 1997 within a broad area about 6 km west of South Sister,” but the pattern only became visible once researchers applied satellite radar interferometry, known as InSAR, to the region in 2001. That analysis, comparing radar images taken years apart, revealed a roughly 20-kilometer-wide zone of ground that had risen measurably without any accompanying earthquake swarm or surface crack large enough to explain it on its own.

By the time scientists caught up with what had already happened, the ground at the center of the bulge had climbed about 130 millimeters, or roughly 5 inches, according to the USGS’s page on modern deformation in the Sisters region. The agency attributes the rise to magma or magmatic fluids, water and gas trapped with molten rock, accumulating roughly 6 kilometers underground, a depth at which such movement can lift the surface without any lava reaching it.

The Rate Has Slowed, Sped Up, and Slowed Again

The uplift has never moved at a constant pace. It peaked at 3 to 5 centimeters, or 1 to 2 inches, per year around 1999, then eased to about 1.4 centimeters annually by 2006 and to roughly half a centimeter a year by 2017, the USGS deformation page states. That decline suggested the source of pressure underground was gradually stabilizing, until a new pulse arrived. Between the summer of 2020 and August 2021, the bulge rose about 2.2 centimeters, close to an inch, and GPS stations logged another 0.5 centimeters of movement in the months that followed.

Cascades Volcano Observatory scientists formally announced the change in a January 2022 update, describing what they called rejuvenated uplift near South Sister and tying it to earthquake swarms recorded that October, December and the following January. The bulletin attributes the renewed rise to “small pulses of magma accumulating at roughly 4 miles (7 km) below the ground surface,” language that echoes the deeper mechanism scientists first proposed for the 1997 phase.

The Alert Level Has Stayed at Normal

Despite the renewed activity, the observatory kept the volcano’s alert status at Normal and Green, its lowest level, in the same bulletin. USGS geologist Dan Dzurisin, who has led GPS field surveys in the area, has helped establish that any eruption would likely be preceded by more obvious warning signs than the ones on record so far. “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, and geochemical changes,” the observatory’s statement reads, adding plainly that scientists “do not detect any of these signs.”

Chloride levels in nearby springs and streams offer a separate clue about what has been sitting underground all along. Elevated chloride concentrations indicate that magma has been present beneath the region for a long time, the USGS notes, which points toward a long-lived reservoir slowly shifting rather than a fresh body of molten rock intruding for the first time.

The Sisters Have Erupted Before, Just Not Recently

South Sister itself last erupted around 2,000 years ago, producing rhyolite domes near its summit, part of a broader eruptive history that also includes andesite and dacite eruptions from North and Middle Sister dating back more than 120,000 years, according to the USGS’s geology and history page for the cluster and its South Sister science-topics page. Volcanologist Howel Williams, writing about the same terrain in 1944, called it a landscape where “for magnificence of glacial scenery, for wealth of recent lavas, and for graphic examples of dissected volcanoes, no part of this range surpasses the area embracing the Sisters and McKenzie Pass.”

Nearly eight decades after Williams wrote that, the ground he was describing is still moving, just slowly enough that most people living nearby would never notice without a satellite watching from orbit.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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