A volcano is a place on Earth's surface or any other plant or moon where molten rock, gases and pyroclastic debris erupts through the crust. Volcanoes vary in their structure, some are just cracks in the surface where lava erupts, and some are domes, shields, or mountain-like structures with a crater at the summit.
Volcanoes are usually found where tectonic plates are diverging or converging. The Pacific Ring of Fire is the perfect example of volcanoes that are caused by convergent tectonic plates.