Volcanic Islands

Volcanism is of widespread occurrence in the tectonically active zones of Southeast Asia. It is a dominant feature in many (particularly smaller) islands where other landform types are absent or scarce. The geographic distribution, major landform types, exogenous and endogenous processes, resources, and hazards of southeast Asian volcanic environments are discussed, first in general terms, and thereafter by using the examples of two typical volcanic islands, Bali and Lombok (Indonesia), which also illustrate the interaction between tectonism and volcanism in this part of the world. The distribution pattern of volcanism in Southeast Asia is related to plate tectonics, as discussed in Chapter 1. Three major plates dominate the region: the Eurasian, Indo-Australian, and Pacific, each of which is composed of several sub-plates. They meet at a triple point situated south of the Bird’s Head of Papua. Volcanism develops where, at some distance from the deep sea trenches that mark subduction zones, the subducting material melts and the magma rises to the surface. Volcanic geanticlinal belts, known as volcanic arcs and stretching parallel to the subduction zones, are thus formed. The arcs are often affected by transcurrent or compartmental faulting, and their roofs may collapse in places. The activity of individual volcanoes comes to an end when the magma chambers concerned are emptied or become inactive otherwise. Volcanism becomes extinct in (part of ) a volcanic arc when subduction abates. It may shift in position with changes in the configurations of the related subduction zone and plates. The plates, subduction zones, and the location of the volcanoes in Southeast Asia are shown in Figure 1.1. All volcanoes discussed in this chapter are Quaternary volcanoes in the sense that the oldest and most eroded ones ended their activity in the Lower Quaternary. The volcanism is of the intermediate andesite–basaltic Circum-Pacific suite, but locally more acidic rocks (rhyolites, dacites, etc.) occur. Neogene volcanic materials, intercalated with marine strata, are common, particularly in the flanks of the volcanic arcs of the region. Volcanic rocks, dating from Cretaceous and older geological periods and related to Pre-Tertiary subduction patterns, occur in Peninsular Malaysia, Borneo, and other areas outside the present arcs.