Land and Resources, Physical Features
The mainland, which is wide in western Papua New Guinea and narrows in the southeast, has a rugged, mountainous interior. Steep slopes and jagged peaks stretch across the entire island from east to west. In Papua New Guinea, the mountains rise to a maximum elevation of 4,509 m (14,793 ft) at Mount Wilhelm in the Bismarck Range. Among the other ranges is the Owen Stanley in the southeast. Between the mountains are broad valleys that lie more than 1,500 m (4,900 ft) above sea level. These mountains and valleys constitute the central highlands. To the north of the central highlands is a low-lying, swampy plain. It was formed by sediment deposited by large rivers, including the Sepik and Ramu, which flow from the mountains into the Bismarck Sea. North of this plain are other mountain ranges that fringe the island's coast. These run from west to east and continue in scattered peaks offshore, forming the islands of New Britain, New Ireland, and Bougainville. Most of the other large islands are mountainous. A few islands, such as the Trobriands, are low coral formations.
The coastline of mainland Papua New Guinea is mostly low-lying. In the south it is deeply indented by river mouths and by a number of bays, such as Milne Bay at the eastern extremity. Most southward-flowing rivers empty into the Gulf of Papua. Major rivers of the mainland include the Fly, in the southwest; the Purari and Kikori, in the south; and the Sepik and Ramu, in the north. The Fly is navigable for about 800 km (about 500 mi) and the Sepik for about 500 km (about 300 mi).
Papua New Guinea lies along the so-called Ring of Fire, a belt of frequent tectonic activity in the Pacific Ocean caused by the collision of several continental plates. The country often experiences earthquakes and there are about 40 active volcanoes along the north coast of the mainland and on the smaller islands. Dramatic examples of tectonic activity include the 1951 eruption of Mount Lamington on the mainland and volcanic eruptions and earthquakes in 1937 and 1994 near Rabaul in eastern New Britain. The latter event, which the government described as the nation's worst recorded natural disaster, caused the evacuation of about 90,000 people.