On volcanoes, a fissure is an elongated fracture or crack at the surface from
which lava erupts. Fissure eruptions typically dwindle to a central vent
after a period of hours or days. Occasionally, lava will flow back into the
ground by pouring into a crack or an open eruptive fissure, a process called
drainback; sometimes lava will flow back into the same fissure from which it
Left: A close-up of a crack in the volcanic surface of a
cooled lava flow from the crater of Mt. St. Helens.