Volcanologists studying the velocity of an aa surge during an early episode of the Puu Oo eruption of Kilauea Volcano, Hawaii.  Most aa flows move rather slowly -- a few meters per hour.  Sometimes they thicken rapidly and then surge forward.  In this experiment, the flow surged down the steep road about 100 meters in 50 seconds; the volcanologists made the 100 meters in about 20 seconds.      U.S. Geological Survey Photo by Jim Griggs