Philip England 氏 (University of Oxford)

「Why are the Arc Volcanoes Where They Are?」　

2006年10月4日

There is a consensus that the fundamental cause of melting in subduction zones is
the infiltration into the mantle wedge of aqueous fluids released by dehydration
reactions in the slab. This view can account for much of the chemical complexity
observed in the volcanic products of the arcs; it has, however, been unable to
account for a fundamental geometrical simplicity in the arcs themselves.

In nearly all of the arcs, the volcanic activity begins abruptly, across a zone
about 10 km wide running parallel to the neighboring trench. The sharpness of the
fronts to the volcanic arcs implies the operation of a single process that triggers
or focuses melting in the slab or mantle wedge immediately below the volcanoes.
Some authors attribute the sharpness of the volcanic fronts to a localized release
of fluid from the slab immediately beneath the fronts, while others argue that fluid
must be released from the slab over a large range of depths and that sharpness of
the volcanic fronts results from channeling, or focusing, of the melt within the
mantle wedge.

The recent discovery of systematic dependence of the location of the volcanic arcs
on the descent speed of the oceanic slabs, combined with scaling arguments derived
from simple physical models, rule out localized release of fluid from the slabs as
the explanation for the position of the arcs. However, the observations are
consistent with focusing of melt in the mantle wedge, with channelling of melt to
the surface being mediated by a strongly temperature-dependent process in the wedge.