The archipelago consists predominantly of pre-Tertiary continental islands primarily of quartzites and shales covered by basaltic lava. Geologically, the islands are part of Sundaland. The islands have extensive fringing and patchy reefs coral reefs.
The islands are influenced by the Northwest Monsoon during which winds from the west-northwest predominate and ocean currents are in an easterly direction. During the monsoon, rainfall averages 40 mm/day. During the Southeast Monsoon, dry winds from the east-southeast predominate and the ocean currents are in a westerly direction bringing water masses from the Flores Sea. Upwelled water masses during the Southeast Monsoon from the Flores and Banda Seas provides lower sea surface temperatures than in the Northeast Monsoon. The shallow slopes (5� to 15�) of the island shelves in the Java Sea (which rarely exceeds a depth of 55 m), provide environments for extensive reef development
