Inside the Subduction Factory

Published by the American Geophysical Union as part of the
Geophysical Monograph Series, Volume 138.

Subduction zones helped nucleate and grow the continents, they
fertilize and lubricate the earth's interior, they are the site of
most subaerial volcanism and many major earthquakes, and they yield
a large fraction of the earth's precious metals. They are obvious
targets for study--almost anything you learn is likely to
impact important problems--yet arriving at a general
understanding is notoriously difficult: Each subduction zone is
distinct, differing in some important aspect from other subduction
zones; fundamental aspects of their mechanics and igneous processes
differ from those in other, relatively well-understood parts of the
earth; and there are few direct samples of some of their most
important metamorphic and metasomatic processes. As a result, even
first-order features of subduction zones have generated conflict
and apparent paradox. A central question about convergent margins,
for instance--how vigorous magmatism can occur where plates
sink and the mantle cools--has a host of mutually inconsistent
answers: Early suggestions that magmatism resulted from melting
subducted crust have been emphatically disproved and recently just
as emphatically revived; the idea that melting is fluxed by fluid
released from subducted crust is widely held but cannot explain the
temperatures and volatile contents of many arc magmas; generations
of kinematic and dynamic models have told us the mantle sinks at
convergent margins, yet strong evidence suggests that melting there
is often driven by upwelling. In contrast, our understanding ofwhy
volcanoes appear at ocean ridges and "hotspots"--although
still presenting their own chestnuts--are fundamentally solved
problems.