The volcanoes, mostly composite stratocones, run along a narrow axis through the island center. A submarine basin just off the coast, containing up to 4000 m of Cenozoic sediments. An interarc basin containing 6000 m of largely undeformed Cenozoic deposits. The subduction line, south of which the basaltic floor covered by Jurassic pelagic sediments of the North Australian basin encroaches against the Eurasian plate. From the above, the general pattern of sedimentation and structural deformation with age shows a progressive northward shift toward younger events.
Volcanism likewise shifts to the north from more basic to more silicic.
Along the main volcanic belt, most volcanic rocks are pyroxene andesites and high- Al basalts. Off the main belt, Volcano Muria, now inactive, has produced leucite-bearing eruptive rocks of potassic alkalic composition. Some of these volcanoes have proved to be especially dangerous to life and property. Eastward, the same geologic terranes extend through the Lesser Sunda Islands. Mt. Api on Sangeang is still active.