Kilauea and Mount St. Helens: How Chemistry Controls Volcanoes

Volcanic rocks in Hawaii have a wide range of compositions. The composition of the rocks corresponds with eruptive stage, eruption rate, and the total volume of the volcano. Kilauea and Mauna Loa are in the shield building stage and erupt only tholeiitic basalt (Clague and others, 1989). Smaller volumes of more diverse compositions are erupted during the preshield, postshield, and rejuvenated stages. The different compositions are the result of different mineralogy and bulk composition of the mantle source, different depths at which melting occurs, different percentages of melting at the source, and changes in the magma composition on the way to the surface, by processes such as settling of olivine crystals (Wright and Helz, 1987). In general, basic physical features, like color and crystal content, are used to identify rocks. Chemical analysis is needed to determine the content of silica and other oxides and for more detailed classification.

Most rocks in Hawaii are basalt (rock on left). Basalts are characterized by a relatively low abundance of silica and high abundances of iron and magnesium. In contrast, most volcanic rocks from stratovolcanoes along continental margins are andesite or dacite. Andesite or dacite are characterized by a relatively high abundance of silica and low abundances of iron and magnesium. The rock on the right is a dacite from Mount St. Helens. Photograph by J.D. Griggs, U.S. Geological Survey, August 26, 1986.

Gentle eruption of lava from a fissure, Mauna Loa, Hawaii. Photograph by Robin Holcomb, U.S. Geological Survey, July 5, 1975.

Violent eruption of ash from a central vent, Mount St. Helens, Washington. Photograph courtesy of U.S. Geological Survey.

The dramatic differences in eruptions of Kilauea and Mount St. Helens are a result of their compositions. The different abundances of elements in a magma, especially silica, exert the primary control on the type of eruption, either non-explosive or explosive. The viscosity (resistance to flow) of a magma is greatly influenced by its silica content. Magmas (and lavas) which are low in silica, like basalt, tend to be very fluid. Magmas (and lavas) which are high in silica, like andesite and dacite, tend to be very sticky. Because Hawaiian magma is fluid, gas dissolved in the magma can escape prior to the eruption, resulting in the gentle effusion of lava onto the surface. In contrast, gas is trapped inside of more viscous andesitic or dacitic magmas. The gas cannot escape until the magma enters the throat of the volcano. Then, due to the reduction in pressure, the gas bubbles nucleate and grow. The outward pressure exerted by the bubbles is greater than the strength of the magma/lava. The lava is fragmented to produce ash that is ejected violently at high velocity.

Lava composition also influences the ultimate shape of a volcano. Fluid Hawaiian lava flows travel great distances from their vent, producing broad, shield-shaped volcanoes. Sticky lava flows from continental volcanoes travel only a short distance from their vent. The lava flows accumulate near the vent, producing a steep-sided stratovolcano.


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