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USGS/Cascades Volcano Observatory, Vancouver, Washington
How Chemistry Controls Volcanoes
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Volcanic rocks have a wide range of compositions. Most rocks in Hawaii
are basalt. 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.
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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.
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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.
URL for CVO HomePage is: <http://vulcan.wr.usgs.gov/home.html>
12/21/01, Lyn Topinka