In addition to ashy tephra, Vulcanian eruptions will also launch football-sized pyroclastic bombs into the air. Vulcanian eruptions generally aren't associated with lava flow. Photo courtesy USGS A Vulcanian column erupts from Tavurvur Volcano off the coast of Papua New Guinea. Basically, the hot magma heats the water so that it becomes steam. This rapid change of state causes an explosive type of expansion in the water, which breaks apart the pyroclastic material, creating a fine ash.
Hydrovolcanic eruptions vary considerably.
Some are characterized by short bursts, while others build sustained eruptive columns. Volcanic eruptions can also melt large amounts of snow, causing mudslides and major flooding. Photo courtesy USGS A hydrovolcanic eruption from Ukinrek Volcano, off the coast of Alaska. Fissure eruptions occur when magma flows up through cracks in the ground and leaks out onto the surface. These often occur where plate movement has caused large fractures in the earth's crust, and may also spring up around the base of a volcano with a central vent.