The main island of Papua New Guinea has been formed by long-continued interaction between the Australian Plate in the southwest, and the Pacific Plate in the northeast. Between these two major crustal elements, whose components in Papua New Guinea are referred to as the platform and the oceanic crust and island arcs, is a highly deformed mobile belt about 150 km wide. The platform consists of stable continental crust of Palaeozoic crystalline rocks overlain by Mesozoic and Tertiary sedimentary rocks, which because they have been protected by the crystalline basement from the deforming forces so active in the mobile belt are mostly flat-lying or only gently warped. The mobile belt, in contrast to the platform, has been deformed from time to time since at least the late Mesozoic and so has been an unsettled sedimentary environment. It was the repository of a great variety of geosynclinal sediments markedly different from those of the platform, but probably its most striking feature is its great intensity of faulting. It has also been the site of widespread igneous activity in contrast to the platform, in which there was no igneous activity from the early Mesozoic until the onset of volcanism in the Pliocene. Several fault wedges in the mobile belt contain Mesozoic rocks similar to those of the platform, and may be detached fragments of the platform. The oceanic crust and island arcs provides a third contrasting geological environment. The fundamental fault zones forming the northeastern margin of the mobile belt mark an abrupt change from rocks with continental affinities (metamorphosed geosynclinal sediments and associated plutonic rocks) to rocks with oceanic affinities, which consist of: (i) an ophiolite sequence, probably representing upfaulted Mesozoic and Early Tertiary oceanic crust; and (ii) the products of island-arc volcanism consisting of Tertiary subaerial and submarine lavas, pyroclastics, and volcanolithic sediments. The rocks of the oceanic crust and island arcs have reacted to stress generally by broadly folding, and faulting along major widely separated crustal fractures. During the Mesozoic, sedimentation followed a fairly consistent pattern: shelf-type sediments were deposited over the platform, and geosynclinal sediments and minor volcanics were deposited in a trough established by the Late Cretaceous along its northern and northeastern margins. The oldest rocks in the oceanic crust and island arcs are Late Cretaceous ophiolites and island-arc volcanics. The same pattern persisted into the Eocene, until a major orogeny resulting from increased plate interaction in the late Eocene or more likely, the Oligocene formed a belt of low to moderate grade metamorphics along the length of the mobile belt. This event was reflected in the platform by erosion and non deposition, and in the oceanic crust and island arcs as widespread island-arc volcanism. The early Miocene saw another fundamental change, with the deposition of shelf sediments (mainly limestone) in the oceanic crust and island arcs and the platform, and trough type sediments along the mobile belt. Volcanic activity burst forth along the length of the mobile belt in the middle Miocene, but was not reflected in either the oceanic crust and island arcs or the platform, where limestones continued to be deposited. By the late Pliocene the main landmasses of Papua New Guinea had been formed, and the later history is notable for the widespread volcanism which for the first time affected all three provinces simultaneously.