zones are the fiery boundaries where the earth's vast crustal plates
converge one against another. In the immense furnace of a subduction
zone, the denser plate plunges beneath its neighbor, and portions
of it may be melted by the intense heat generated within the mantle,
turning the solid rock of the crust into molten magma. How to determine
just what has gone into the magma mix and ultimately where it came
from is a compelling challenge to geochemists.