The world's most explosive -- and devastating -- volcanic eruptions usually occur in subduction zones, because subducting oceanic plates are soaked with water
, and that water helps the overlying rock melt. Ultimately, the result is a particularly gassy magma.
This andesitic magma, as it is called, is very viscous -- that is, resistant to flow, like maple syrup compared to water. Such is the case in the Cascades Range of the Pacific northwest, the home of Mount St. Helens and 14 other large volcanoes.
is not explosive in and of itself. But it does impede the escape of gases out of the magma chamber
. Their migration inhibited, the gases form bubbles and pockets in the magma. Eventually, the pressure of the collected gases rises so high that they blow through the magma like a cork out of a champagne bottle.
The result is an explosion of gas, ash, and fiery fragments of volcanic rock.