Volcanoes are a disaster in the SimCity series.
In SimCity 2000, a volcano appears in the Portland scenario. They can occur anywhere else but you need to spawn the volcano from the Debug menu. Once the volcano is spawned, they rise from the ground, destroying anything underneath. The eruption also spreads lots of fires (caused by either lava or lava bombs, depending on the type of eruption) in a ring around the volcano, along with clouds of toxic volcanic gas (appearing as pollution clouds in-game). In this game, you cannot do anything until the eruption finishes. When the fires are out, a huge mountain is left behind, and it won't erupt anymore.
In SimCity 4 the player can create volcanoes, once they erupt, volcanic bombs eject from them in eruption mode and lava comes out of the main vent and it spreads, destroying anything in its path. Unlike SimCity 2000, you can build while the volcano erupts.
If the player spawns a volcano in the water, the lava column wont come out and the lava will just create a mist around the volcano. Unfortunately when the lava dries out the land during it is not created except the whole volcano remains. If you spawn the volcano deep underwater the only thing visible is a red spot on the water with a few bubbles, if its a near a marina you can drive your boat onto the area without getting affected. This is the only disaster from SimCity 4 that actually creates land, (best for making island towns).
Note: You cannot spawn another volcano when one is in progress, other disasters you can spawn but not another volcano.