Gedamsa

The Gedemsa caldera covers 7 x 9 km along the Main Ethiopian Rift E of Lake Koka and SW of Wonji Sugar Estate Farm. The steep-sided edifice has walls 100-200 m high whose upper part consists primarily of rhyolitic lava flows, formed by the eruption of a series of trachytic ignimbrites, It is also cut by many NNE-SSW-trending regional faults, particularly on the E side. Late-Pleistocene to Holocene volcanics form a chain of rhyolitic lava flows and pumice deposits, known as Ittisa, that rises above the floor of the caldera. A large 1-km-wide crater is located at the eastern part of the chain. A Holocene lava dome or flow is found on the SW flank. Small basaltic spatter cones have formed inside the caldera rim. Weak fumarolic activity was reported at two locations.