Three Sisters

Modern Deformation and Uplift in the Sisters Region

In 2001, scientists discovered that a broad (10 by 20 km, or 6 by 12 mi) area focused 5–6 km (3– 4 mi) west of the summit of South Sister had been rising at an average rate of 3–5 cm (1–2 in) per year since late 1997.

Close monitoring of the Three Sisters area by satellite and ground-based methods showed that the rate of uplift decreased to about 1.4 cm (0.5 in.) per year during 2004–2006 and to less than 1.0 cm (0.4 in.) per year by 2013. Modeling of the uplift (inflation) suggests that it was caused either by the intrusion of about 20 million cubic meters (26 million cubic yards) of magma at about 5-km (3-mi) depth or by rise of a hot, buoyant plume of water and gas to a similar level that caused heating and expansion of surrounding rock. In either case, an eruption is unlikely in the near future if current trends continue. Similar inflation episodes have been recognized at many volcanoes around the world, and others probably went unnoticed before the development of modern monitoring techniques such as GPS and InSAR.

Interferogram image made from InSAR monitoring, showing 1995-2001 g...

About 130 mm (5 inches) of peak uplift from 1995 to 2001 is shown in this interferogram in the Three Sisters Wilderness. Magma accumulating at about 7 km (about 4.5 mi) depth is the likely cause of uplift. The area of uplift, about 20 km (12 miles) in diameter, is almost entirely within the wilderness area. The two white "thumbtacks" show the location of continuous GPS instruments installed by Cascades Volcano Observatory personnel to monitor the uplift. The last volcanic eruption in the area was about 1500 years ago. (Public domain.)