Nearing the end of the Cascades Volcanoes theme, I offer a panorama of the Three Sisters and Broken Top in central Oregon. The summits of the mountains are hidden by lenticular clouds, which form as air masses are forced over the summit, and condense to form the strange shapes. Or, they are smoke screens placed by aliens as they land on the peaks. North (10,090 ft; 3,075 m) and Middle Sister (10,050 feet; 3,063 m) appear as a single peak on the left, with South Sister (10,360 feet; 3,158 m) in the middle. North Sister is a shield complex that has not erupted in at least 100,000 years. Middle and South Sister are stratovolcanoes, with the most recent activity on South Sister about 2,000 yrs BP. Rock Mesa is a dacite dome (the barren area in the foreground a little to the right of South Sister), and was one of the last outpourings of lava. The much older complex stratovolcano of Broken Top (9,152 ft.; 2,789 m) is on the right.
Measurements starting in 2001 show that an area near South Sister is rising (around 1-2 cm per year) which may indicate renewed volcanic activity.
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