Global Volcanism Program | Image GVP-05344
The Gates is a v-shaped notch in the 1-km-high eastern rim of Aniakchak caldera. Surprise Lake was once much larger but is now restricted to the NW part of the caldera floor. It is thought to have drained catastrophically through The Gates at the time of an eruption at Half Cone about 500 years ago.
Photo by Christina Neal, 1994 (Alaska Volcano Observatory, U.S. Geological Survey).