The Indonesian Islands lie where the Pacific Plate, Austrailian plate, Eurasian and the Phillipine plate meet. This is a very unique spot because when different tectonic plates are sliding past each other and causing friction, it causes volcanoes and seismically large earthquakes. This means that the Indonesian islands are hitting 4 plate boundaries, which causes great motion and change to the geologic landscape of the seafloor. The plates would be called convergent plate boundaries with subduction. The Indian Oceanic plate is being subducted under the Eurasian plate because the continental crust is less dense than oceanic crust. It is also interesting that a geologic fault line runs the length of Sumatra, The earthquakes here are mostly caused by the converging Australian plate.

How do subducting plates cause volcanoes? As the plate is being subducted, magma from beneath the surface of the earth is being melted from the friction of the tectonic plates. It  reaches the surface, forming a margin within the oceanic crust, forming a volcano.

Source:http://en.wikipedia.org/wiki/Mount_Talang#mediaviewer/File:Mount_Talang.JPG

Mount Talang is an active stratovolcano in Sumatra. It's most recent eruption occurred in 2004.

Sources:
http://earthguide.ucsd.edu/eoc/teachers/t_tectonics/p_map_plate_layers_bath.html

http://www.livescience.com/43220-subduction-zone-definition.html