Sea topography in Australasia. In this image, the colours represent sea depth, from red (shallowest) through yellow and green to blue (deepest). Land areas are shown in grey, with topography represented by shaded relief. The deep blue linear feature at upper left is the Java Trench, a destructive boundary between the Indo- Australian and Eurasian tectonic plates. At top right is the New Guinea Trench, the border with the Pacific plate. These destructive plate margins give rise to extensive volcanism, and account for the large number of islands in the region. The data for the sea topography was gathered by the Seasat radar satellite.