When Mount St. Helens erupted on May 18, 1980, red hot lava did not spew out of the volcano and pour down its flanks. This perception of a volcanic eruption is a common one and is probably due in part to pictures seen on television or in books of the beautiful lava flows and lava fountains in Hawai'i. The type of eruptions in Hawai'i are known as hawaiian volcanism and are far less dangerous than the eruptions produced by Mount St. Helens. It is important to know what type of an eruption a volcano is most likely to produce so that the types of hazards produced by such an eruption can be identified. Knowledge of these types of hazards, will help determine where a person would need to go to be safe during a volcanic eruption.
Volcanic eruptions can be placed into two general categories: those that are explosive, such as at Mount St. Helens, and those that are effusive, such as in Hawai'i. The most active volcano in the world, Kilauea Volcano on the big island of Hawai'i, is generally a nonexplosive volcano (though there have been occasions when it erupted explosively). Eruptions from it normally result in gently flowing lava flows, spatter cones, and lava fountains. Another type of nonexplosive volcanism is flood basalts. Lava flows from this type of eruption are extruded from fissures and cover vast areas. These nonexplosive eruptions are the least dangerous type of volcanic eruption since people rarely get killed by them (Francis, 1993). However, they are devastating and may have global consequences.
Many eruptions are explosive in nature. They produce fragmental rocks from erupting lava and surrounding country rock. Some eruptions are highly explosive and produce fine volcanic ash that rises many kilometers into the atmosphere in enormous eruption columns. Explosive activity also causes widespread ash fall, pyroclastic flows, debris avalanches, landslides, pyroclastic surges, and lahars. Explosivity is usually the result of gases expanding within a viscous lava. Another mechanism for explosions at volcanoes occurs when surface water or ground water enters a magma chamber. These eruptions are likely when a volcano occurs in a wet area or in the sea.