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About Arc Volcanism

How subducting plates create most of the world's volcanoes

Volcanic arcs are all related to lithospheric plate subduction. This accounts for about 90 percent of the world's volcanoes (the rest are related to continental rifting or hotspots). Arc volcanoes are fed by magma that forms as crustal fluids rise from the subducting plate to join the rocks of the upper plate. Let's spell that out in more detail. This illustration should help.

The subduction factory. Descending slab has sediment (yellow) and altered basaltic rocks (green) on top. As these are carried downward they yield fluids, which enter the upper slab (pink) and underlying asthenosphere (white). Black dots are earthquake locations. Figure from the MARGINS project.

Oceanic plates are topped with marine sediment and oceanic crust, about 5 kilometers thick. The mantle rock beneath is deeply altered by seawater into serpentinite. These materials from the surface are carried into the mantle and slowly released from the subducting plate, which is usually called the slab.

First the water is squeezed out as the slab moves deeper, along with other volatile components like carbon dioxide. Farther down serpentinite is converted by heat and pressure, releasing more water. This fluid is charged with dissolved minerals and metals as it rises. Some of the serpentinite itself is released as mud volcanoes.

These materials enter the lithosphere of the upper plate just behind the trench. Initially where the upper plate is thin, serpentinite mud can rise and trigger mud volcanism on the trench slope. Farther down, fluid and mud enter the wedge of deep lithosphere and asthenosphere beneath the upper plate.

Water has a profound effect on mantle rock, lowering its melting point. In the hot, soft asthenosphere the subducted fluids create strings and tongues of molten rock—magma. Because liquids are lighter than solids, magma naturally rises by buoyancy. Reaching the more rigid lithosphere, the magma bodies collect into larger pools. As the occasion arises, these erupt vigorously as lava, building volcanoes in long, curving rows.

It is now becoming worthwhile to look at the arc-volcanic setting as a quantitative process, with measurable inputs and outputs of materials and energy—a subduction factory. A large group of researchers is coordinating its efforts in this direction as part of the Subduction Factory Science Plan.

Geologists are working to gain a better understanding of arc volcanism because it's important to civilization in many different ways. Subduction causes the largest earthquakes on the planet, and volcanism is a comparable hazard. On the positive side, the deep-seated chemical interactions produce large bodies of metal ores. And arc volcanism is a crucial part of the Earth's greater story.

Back to Plate Tectonics in a Nutshell

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