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Abstract and photos provided by Colin Macpherson,
related to his lecture.
Two Ripping Yarns: Construction of Island Arc Volcanic Chains in
SE Asia.
Krakatau, Tambora, Mount St. Helens, Mount
Pelée, Pinatubo: Volcanoes in subduction zones have generated
some of the most spectacular and most destructive eruptions in
history. No part of the globe has such a concentration of this style
of volcanism as the archipelagos of Indonesia and the Philippines.
November's eruption of Merapi, in Indonesia, was a reminder of just
how much damage can be inflicted by even a small eruption when an
island arc or active continental margin volcano is located close to a
dense population cluster. Cities often develop near volcanoes because
the richness of their soil provides fertile agricultural lands,
particularly in tropical regions where soil quality is,
predominantly, poor. Furthermore, both modern and ancient volcanic
arcs host mineral deposits, with notable enrichments in copper, gold,
silver and zinc. On a global scale, subduction zones - and the magma
they produce - appear to be fundamental in constructing the
continental crust upon which we live. But the power and
destructiveness of eruptions from these volcanoes make their presence
of substantial concern to local populations. Therefore, understanding
where and how such volcanoes occur is of importance to societies and
economies, as well as to science.
Subduction occurs when two tectonic plates start to
converge after the surface of one is pushed beneath the other. The
earliest interpretation of the volcanic chains found on upper plates
was that the lower (subducted) plates melted when they encountered
the hot mantle. The petrology (as well as the physics) of the rocks
involved quickly showed that this was unlikely to be the case.
Instead, the last 40 years have been dominated by models in which
water carried down by the subducted plate is squeezed out into the
mantle trapped between the two plates, lowering its melting point and
making this the source of the magma. Both of these interpretations
link the distribution of volcanoes to processes occurring in the
subducting plate. Indonesia offers the opportunity to examine both
models in detail with some, potentially, surprising results.
Globally, subduction provides the fundamental balance
to sea-floor spreading by destroying lithosphere created at divergent
margins. But how any single subduction zone starts remains one of the
great unknowns of plate tectonics. This is, in large part, due to the
few places where the birth of a subduction zone can be caught in the
act. The east margin of the Philippine provides one such location and
is beginning to yield important new clues as to how a volcanic arc
develops during the earliest stages of subduction.
The volcanic rocks of Indonesia and the Philippines
both confirm that squeezing fluid from a subducted plate generates
the distinctive composition, and hence explosivity, of the magma that
is produced by island arcs. However, the distribution of volcanoes in
both cases indicate that the upper plates play an important role in
where this magma can reach the surface and build the volcanoes that
bring sustenance, and occasionally destruction, to the communities
that live along side them. |
