In geology, a fissure is a fracture or crack in rock along which there is a distinct separation; fissures are often filled with mineral-bearing materials. On volcanoes, a fissure is an elongate 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 erupted.