If something is volcanic, it's violently explosive, like the volcanic eruption from a mountain or a two-year-old's volcanic temper tantrum after skipping his afternoon nap.
Use this adjective to describe anything related to volcanoes, like volcanic ash and rock left behind after a volcanic eruption, or the volcanic activity picked up by a scientist's equipment. It's also a great way to talk about emotions that are (figuratively, at least) as volatile as a volcano. If your sister's mood is volcanic, you'll want to steer clear of her. The source of both volcanic and volcano is the Roman god of fire, Vulcan.