Tambora is located north of the Java Trench system and above the upper surface of the active north-dipping subduction zone. Sumbawa Island is flanked to the north and south by oceanic crust. The convergence rate of the Australian Plate beneath the Sunda Plate is per year. Estimates for the onset of the volcanism at Mount Tambora range from 57, to 43 ka. The latter estimate published in 2012 is based on argon dating of the first pre-caldera lava flows. The formation of Tambora drained a large magma chamber pre-existing under the mountain. The Mojo islet was formed as part of this geological process in which Saleh Bay first appeared as a sea basin about 25,000 years BP.