Volcanoes are an opening, or rupture, in a planet's surface or crust, which allows hot, molten rock, ash, and gases to escape from below the surface. Volcanic activity involving the extrusion of rock tends to form mountains or features like mountains over a period of time.
The Taranis outpost on the planet Taranis used geothermal energy from a formerly dormant supervolcano to power its shield. However, as more energy was drawn, the volcano became extremely active. The Atlantis expedition came to Taranis to help most of the people escape, but the Stargate was buried in lava, and a massive dust cloud emerged from the volcano in a massive eruption, which rendered the planet uninhabitable for many years. (SGA: "Inferno")
Planet Novus is known to have an active supervolcano that erupted in the last thirty years did the planet uninhabitable. The entire population of Novus, which was set at millions managed to leave his plane before the outbreak. Several expedition was sent out to find other suitable worlds, and the rest of the people flew to a colony in their spaceship. (SGU: "Common Descent")
While visiting Emege to determine the status of the shield generator there, Doctor Rodney McKay determined that the valley on Athos upon which Emege sat was in fact the caldera of a supervolcano similar to the one on Taranis. Like on Taranis, the Emege shield had been powered by geothermal energy but after a year of continuous operation due to Queen Death's attack, threatened to cause the supervolcano to erupt. Instead of ignoring the warnings, the people of Emege shut down the generator, preventing the eruption at the cost of the lives of everyone in the city, saving the rest of the planet and those still alive in hiding upon it in return. (SGA: "The Mysteries of Emege")