COURTESY PHOTO: Sequence showing a process called \"gas-pistoning\" at the Drainhole vent in Kilauea\'s Pu`u `O`o crater. Gas accumulates beneath a column of lava in the vent until it begins to rise, pushing the lava column (the \"piston\") upward. When the gas reaches the surface, it suddenly escapes into the atmosphere, producing fume and throwing some spatter near the vent. The vent is about 10 meters (30 feet) across. U.S. Geological Survey photographs taken by time-lapse camera.