A glacial lake outburst flood occurs when the dam blocking the glacier's path collapses. The dam may comprise glacier ice or a terminal moraine, eroded material from the sides and bottom of a valley, which the glacier pushes to the front. The collapse may occur due to erosion, build-up of water pressure, an avalanche, heavy snow, earthquake, volcanic eruption under the ice, or if a large portion of a glacier breaks off and massively displaces the waters in a glacial lake at its base. When the outburst occurs, water dammed by a glacier or a moraine is released. Glacial lakes may hold millions to hundreds of millions of cubic meters of water. Catastrophic failure of the containing ice or glacial sediment can release this water over a period of minutes or a few days.