Chapter 1 Introduction and previous research

Sumatra, with an area of 473 606 km 2 is the largest island in the Indonesian archipelago and the fifth largest island in the world. The island stretches across the equator for 1760 km from NW to SE, and is up to 400 km across (Fig. 1.1). Administratively, and for the purposes of this Memoir, Sumatra includes the Mentawai islands from Simeulue to Pagai, which with Enggano form a forearc chain to the SW, and the Tin Islands' of Bangka and Billiton and the Riau islands to the east. The backbone of the main island is formed of the Barisan Mountains, which extend the whole length of Sumatra in a narrow belt, parallel to, and generally only a few tens of kilometres, from the SW coast. The main peaks (which are mainly Quaternary or Recent volcanoes) commonly rise 2000 m above sea level, culminating in Mt Kerinci at 3805 m. Short, steep river courses drain the Barisans towards the SW, often cuttting deep gorges, while towards the east the rivers follow long meandering courses across broad coastal plains and swamps to the Malacca Straits, which separate Sumatra from the Malay Peninsula, or to the Java Sea. Eastwards, across the Java Sea, lies the almost equally large island of Borneo (Indonesian Kalimantan), and Java lies immediately to the SE across the narrow Sunda Strait. The Malacca Strait and the Java Sea form the southern parts of the Sunda Shelf (Fig. 1.1). Across the shelf the seafloor is shallow with a depth of