Types of Volcanic Eruptions

Volcanic eruptions, especially explosive ones, are very dynamic phenomena. That is the behavior of the eruption is continually changing throughout the course of the eruption. This makes it very difficult to classify volcanic eruptions. Nevertheless they can be classified according to the principal types of behavior that they exhibit. An important point to remember, however, is that during a given eruption the type of eruption may change between several different types.

• Hawaiian - These are eruptions of low viscosity basaltic magma. Gas discharge produces a fire fountain that shoots incandescent lava up to 1 km above the vent. The lava, still molten when it returns to the surface flows away down slope as a lava flow. Hawaiian Eruptions are considered non-explosive eruptions. Very little pyroclastic material is produced.

• Strombolian - These eruptions are characterized by distinct blasts of basaltic to andesitic magma from the vent. These blasts produce incandescent bombs that fall near the vent, eventually building a small cone of tephra. Sometimes lava flows erupt from vents low on the flanks of the small cones. Strombolian eruptions are considered mildly explosive, and produce low elevation eruption columns and tephra fall deposits.

• Vulcanian - These eruptions are characterized by sustained explosions of solidified or highly viscous andesite or rhyolite magma from a the vent. Eruption columns can reach several km above the vent, and often collapse to produce pyroclastic flows. Widespread tephra falls are common. Vulcanian eruptions are considered very explosive.

• Pelean - These eruptions result from the collapse of an andesitic or rhyolitic lava dome, with or without a directed blast, to produce glowing avalanches or nuée ardentes, as a type of pyroclastic flow known as a block-and-ash flow. Pelean eruptions are considered violently explosive.

• Plinian - These eruptions result from a sustained ejection of andesitic to rhyolitic magma into eruption columns that may extend up to 45 km above the vent. Eruption columns produce wide-spread fall deposits with thickness decreasing away from the vent, and exhibit eruption column collapse to produce pyroclastic flows. Plinian ash clouds can circle the Earth in a matter of days. Plinian eruptions are considered violently explosive.

• Phreatomagmatic - These eruptions are produced when magma comes in contact with shallow groundwater causing the groundwater to flash to steam and be ejected along with pre-existing fragments of the rock and tephra from the magma. Because the water expands so rapidly, these eruptions are violently explosive although the distribution of pyroclasts around the vent is much less than in a Plinian eruption.

• Phreatic (also called steam blast eruptions) - result when magma encounters shallow groundwater, flashing the groundwater to steam, which is explosively ejected along with pre-exiting fragments of rock. No new magma reaches the surface.