The Dynamic Earth @ National Museum of Natural History

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[Photo: Spreading Ridge and Subducting plate]

Why do so many of the world’s most explosive volcanoes occur along subduction zones, where one plate descends beneath another? Deep below the surface, the descending plate recycles seawater, some of which ultimately powers volcanic eruptions that blast the water — as steam — back into the atmosphere.

Water Goes In

At spreading ridges, heated seawater circulates through new oceanic lithosphere and is incorporated into water-bearing minerals. When this lithosphere is subducted at a deep-sea trench, these minerals are carried to great depths.

Water Goes Out

At great depths, under high pressures and temperatures, water-rich minerals release their water. It rises into overlying mantle rocks, triggering the formation of water-rich magmas that feed explosive, steam-driven volcanic eruptions.

[Photo: Water-rich magma and Released Water]

Water Goes In

This actinolite-talc schist formed deep in a subduction zone when the altered oceanic lithosphere began to release water.

Water Comes Out

The black crystals in this andesite are the water-bearing minerals amphibole and biotite evidence of the magma's high water content. The abundant large white crystals of plagioclase helped make the magma highly viscous.