Neither North nor Middle Sister is likely to resume volcanic activity. An eruption from South Sister would pose a threat to nearby life, as the proximal danger zone extends from the volcano's summits. During an eruption, tephra could accumulate to in the city of Bend, and mudflows and pyroclastic flows could run down the sides of the mountain, threatening any life in their paths. Eruptions from South Sister could be either explosive or effusive, though an eruption of ash with local volcanic rock accumulation and slow lava flows is considered most likely. The Three Sisters area does not have fumarole activity, although there are hot springs west of South Sister.