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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 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.