USGS

Eruptive Style: Powerful but Unsually Benign

 By definition, the adjective eruptive describes any object or phenomenon associated with processes of "bursting forth," "breaking out," or "issuing forth suddenly and violently." Strictly speaking, no eruption is truly nonexplosive, but most Hawaiian eruptions closely approach being such. Indeed, the term "Hawaiian" is used by volcanologists worldwide to characterize similar eruptive style at other volcanoes.

Typical activity: "nonexplosive" or weakly explosive

With infrequent exceptions, eruptions of Hawaiian volcanoes are weakly explosive or nonexplosive and relatively benign. Hawaiian eruptions are typically gentle because their lava is highly fluid and thus tends to flow freely both beneath the surface and upon eruption. In contrast, lava of volcanoes located along plate margins, such as Mount St. Helens, generally is more viscous ("stickier" and "stiffer") and tends to fragment, often very explosively, during eruption. Highly fluid lava favors the nonviolent release of the expanding volcanic gases that drive eruptions. In contrast, viscous magma suppresses easy gas escape, which results in pressure build-up underground and ultimately in explosive gas release and magma fragmentation. 		Lava viscosity ("stiffness" or "resistance to flow") is largely determined by the chemical composition and temperature of the magma, the amount of crystals in the magma, and the gas content. The high fluidity (low viscosity) of Hawaiian lavas derives mainly from its basaltic composition, characterized by more iron (Fe), magnesium (Mg), calcium (Ca), and titanium (Ti), and less silicon (Si), aluminum (Al), sodium (Na), and potassium (K), compared to such viscous lavas as the dacite erupted explosively at Mount St. Helens in 1980. In the graph showing this compositional difference between Hawaiian basalt and Mount St. Helens dacite, the chemical elements are given as oxides [(for example, calcium as calcium oxide (CaO)]. Basalt is dark volcanic rock made up of small crystals and glass, whereas dacite, while also glassy or fine-grained, generally is much lighter in color.

Hawaiian eruptions typically start with lava fountains spouting from a series of nearly continuous fissures, "curtain(s) of fire." As most eruptions progress, lava-fountain activity becomes localized at a single vent (an opening from which lava issues), generally within hours of the initial outbreak. Depending on the shape of the vent and other eruptive conditions, lava fountains can vary widely in form, size, and duration.

Left:Mount St. Helens, a typical steep-sided composite volcano, shortly before its decapitation by the May eruption in 1980. (Photograph courtesy of D.R. Pevear, Western Washington University, Bellingham.) Right: Mauna Loa, an excellent example of a shield volcano, viewed from the Hawaiian Volcano Observatory. (Photograph by Robert I. Tilling.). MSH, a composite volcano Mauna Loa, a shield volcano

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Updated 05.01.97