The most volcanically active belt on Earth
is known as the Ring of Fire, a region of subduction zone
volcanism surrounding the Pacific Ocean. Subduction zone volcanism
occurs where two plates are converging on one another. One plate
containing oceanic lithosphere descends beneath the adjacent plate,
thus consuming the oceanic lithosphere into the earth's mantle.
This on-going process is called subduction. As the
descending plate bends downward at the surface, it creates a large
linear depression called an oceanic trench. These
trenches are the deepest topographic features on the earth's surface.
The crustal portion of the subducting
slab contains a significant amount of surface water, as well as
water contained in hydrated minerals within the seafloor basalt.
As the subducting slab descends to greater and greater depths,
it progressively encounters greater temperatures and greater pressures
which cause the slab to release water into the mantle wedge overlying
the descending plate. Water has the effect of lowering the melting
temperature of the mantle, thus causing it to melt. The magma
produced by this mechanism varies from basalt to andesite in composition.
It rises upward to produce a linear belt of volcanoes parallel
to the oceanic trench, as exemplified in the above image of the
Aleutian Island chain. The chain of volcanoes is called an island
arc. If the oceanic lithosphere subducts beneath an adjacent
plate of continental lithosphere, then a similar belt of volcanoes
will be generated on continental crust.
