A volcano is an opening in a planet's crust, often formed into a mountain, from which molten lava and gases are ejected.
The study of volcanoes is volcanology. (TNG: "Pen Pals")
Ocampa had a range of several extinct volcanoes. (VOY: "Caretaker")
Volcanoes on Penthara IV were activated when subterranean pockets of carbon dioxide were opened by a phaser drill. The volcanoes started to erupt and produced large plumes of volcanic dust threatening to bring another ice age on the planet. (TNG: "A Matter of Time")
Volcanic vents could supply enough heat to sustain life even on Rogue planets. An example of such world was Dakala. (ENT: "Rogue Planet")
Captain Jean-Luc Picard saw an active volcano on Earth in the region of France about 3.5 billion years ago when Q brought him there. (TNG: "All Good Things...")