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Explosive volcanism

Part of Hall of Planet Earth.

Subduction Zone Volcanism AMNH/R.Mickens

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Subduction Zone Volcanism
Most explosive eruptions occur in volcanoes above subduction zones, where one tectonic plate dives beneath the other. Eighty to 120 kilometers below the surface, magma forms when the rocks of the mantle melt just above the subducting plate. The magma rises through the mantle, and erupts to form great composite volcanoes such as Vesuvius. Explosive eruptions have also built arc-shaped island chains such as Indonesia, and continental mountains like the Cascades. In this zone, you will see rocks from all these areas.

Why some volcanoes explode
A volcano’s explosiveness depends on the composition of the magma (molten rock) and how readily gas can escape from it. As magma rises and pressure is released, gas bubbles (mainly of water vapor and carbon dioxide) form and expand rapidly, causing explosions. Magmas with high silica content tend to erupt violently, because they are viscious. They form volcanoes like Krakatau and Tambora in Indonesia, and Mount Rainier and Mount St. Helens in Washington State. In contrast, basaltic magma (which forms the volcanoes of Hawaii and other oceanic islands) contains less silica, is more fluid, and does not erupt in gain explosions.