Fissures, vents, and plugs are all associated with a volcanic feature called a volcano. A fissure is a long, narrow crack in the Earth's surface from which lava can erupt, while a vent is an opening in the Earth's surface from which volcanic materials such as lava, ash and gases escape. A plug is a solidified mass of volcanic material that fills a vent or fissure in a volcano. All of these features are indicative of volcanic activity and play a role in the eruption process.