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O'a Caldera
"O'a caldera along the central Ethiopian Rift is the country's largest rift-valley caldera. The caldera forms the eastern portion of the 15 x 25 km dumbbell-shaped Lake O'a (also known as Lake Shalla). Formation of the caldera about 240,000 years ago was accompanied by the eruption of two ignimbrite deposits, the first of which was strongly welded. The only post-caldera activity consists of two pyroclastic cones north of the caldera, one silicic and the other basaltic, and a group of tuff rings, spatter cones, and lava flows of Holocene (perhaps as young as prehistorical) age near the SW shore of the lake. These were erupted along the Corbetti-Shalla segment of the Wonji Fault Belt, which extends north from Corbetti caldera. Fumarolic activity continues on all sides of the lake."  -Smithsonian Volcano Archive
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