Why are subduction zone volcanoes more explosive than hotspot volcanoes or mid-ocean ridge volcanoes?
Subduction zones are primarily places where one plate is moving down beneath the other. The descending plate always carries some water down into the mantle. The basalts in the descending slab of ocean floor are almost always altered and have absorbed water as a result of their interaction (when hot) with the sea. Subduction zone volcanoes typically erupt with explosive force, because the magma is too stiff to allow easy escape of volcanic gases and the steam formed by the water. As a consequence, tremendous internal pressures mount as the trapped gases and steam expand during ascent, before the pent-up pressure is suddenly released in a violent eruption. One of the most puzzling aspects of the magma/water mixing process is its unpredictability. In some cases, the mixing of magma and water results in passive, low-level release of steam. In other cases it produces devastating explosions.
