Lombok is one of the Lesser Sunda Islands in the Sunda Arc of Indonesia, a subduction zone where the Australian plate subducts beneath the Eurasian plate at a rate of . The magmas feeding Mount Samalas and Mount Rinjani are likely derived from peridotite rocks beneath Lombok, in the mantle wedge. Before the eruption, Mount Samalas may have been as tall as , based on reconstructions that extrapolate upwards from the surviving lower slopes; its current height is less than that of the neighbouring Mount Rinjani, which reaches .