Pool of lava filling a basin at the summit of Mauna Ulu, July 29, 1970

Pool of lava filling a basin 150 m (500 m) wide at the summit of Mauna Ulu on July 29, 1969. This image and the next two illustrate a process called gas pistoning. The pool is fed by lava rising 50 m (160 ft) in 15 minutes up a fissure 3-7 m (10-25 ft) wide. Bubbling is just starting above the fissure, leading to draining shown in the next image. In the background, a separate vent is spattering.