Volcanoes in the Three Sisters region have erupted a wide variety of magma types, therefore the shape, size, and impact of future lava flows could vary greatly.
Dacite to rhyolite lava flows erupted from South or Middle Sister may form a bulbous lava dome over a vent and flow for a relatively short distance (few kilometers) down slope. Lava domes that grow on steep slopes are typically unstable and collapse repeatedly as they grow, which can form hot avalanches of rock debris and gases that move rapidly down slope resulting in pyroclastic flows. Such flows would not directly affect communities but could swiftly melt snow and ice and lead to lahars. More fluid lava flows could erupt anywhere in the Sisters region and move down valleys as tongues of lava that could destroy structures in their path and start forest fires, some could reach settle areas. All lava flows advance so slowly that they seldom endanger people directly.