Aa Flow Surge, Hawaii
Puu Oo eruption, Kilauea Volcano, Hawaii. Most aa lava flows move only a few meters per hour, but sometimes they surge, as seen in this photo. Before the aa surge occurred, the flow front thickened from about 1 meter to 3 meters, and then the flow began to move downslope at speeds up to 100 meters per minute.
U.S. Geological Survey Photo by R.W. Decker.