1. Introduction
The Andaman, Nicobar, Nias and Mentawai islands
represent the subaerial parts of the forearc ridge in the
Indonesian island-arc system (Fig. 1A). An oceanic trench
(Andaman-Java trench) borders the forearc ridge to the west
of the Andaman Islands, extending southwards into
Indonesia. To the east of the forearc the Quaternary
Narcondum and Barren Island volcanoes rise from a
submarine ridge with a parallel arcuate trend. Barren Island
has exploded very recently (Halder et al., 1992). At the
present time the arc and forearc are separated from