Nusantara: Earthquakes and Vulcanism

I began the day thinking about the

This set me thinking again about volcanoes and earthquakes in Indonesia.

According to the Smithsonian Institute there are 1,337 active and extinct volcanic features in Indonesia. Andesite volcanoes associated with subduction predominate. Over the past 400 years 80 Indonesian volcanoes have been active and 64 are Andesite and Basaltic Andesite producing. 48 of these are strato-volcanoes the most explosive types.

Mt Agung last erupted in 1964.

Looking for resources on volcanoes and earthquakes I found this brilliant animation showing global volcanic and earthquake activity since 1960.

Subduction, Strato-volcanoes and Earthquakes

To the nortn Australia, ocean floor, the leading edge of the India-Australia earth plate in the region, collides with the Eurasian plate. This happens a little south of the Indonesian archipelago along Indonesia. The India-Australia plate is forced down below the Indonesian archipelago. As the India-Australia plate descends along this collision zone it forms the deep sea Java Trench. This narrow trench includes the deepest point of the Indian Ocean, over 7,700m below sea level.

When one colliding plate slides below the other plate the process is known as subduction. The India-Australia plate has been subducting below the Indonesian front for 70 million years, to the north it collides with the Eurasian plate to cause the uplift of the Himalayas.

Parts of the plate may have descended to depths of 1,200km below Indonesia. The descending plate triggers the many, large and often deep earthquakes felt in Indonesia. Friction, compression and heat from the earth’s mantle promotes melting in the sub ducting plate edge and in overlying rocks. Molten materials rise up as magma and erupt at the surface, where they form Indonesia’s volcanoes. It is the process that provides the molten material feeds Indonesia’s volcanic growth.

Earthquakes can either be the direct result of this tectonic movement or more localised events associated with one particular volcanic system.