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.

Mt Hood, Oregon, USA. Volcanic arc volcano and ski resort.

Looking along the arc from Mt. Hood; Mt. St. Helens (left), Mt. Adams (right), Mt Ranier (far distance).

Tea plantation on the flanks of Gunung Gede, Java, Indonesia.

Looking along the arc from the summit of Gunung Gede to the summit of Gunung Salak in the distance.

 

Photo on left.

Ash deposits on the flanks of Gunung Merapi, Java, Indonesia.

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