A volcano is an opening on the surface of a planet or moon that allows material warmer than its surroundings to escape from its interior. When this material escapes, it causes an eruption. An eruption can be explosive, sending material high into the sky. Or it can be calmer, with gentle flows of material. (source #1)

Any hydrocarbon deposit that can be burned for heat or power, such as petroleum, coal, and natural gas. (source #2)