Borneo is the third largest island in the world and the main island on the Sunda Shelf. Mount Kinabalu, the highest peak in the Southeast Asian archipelago, is not actually in Indonesia. It rises to 13,455 feet (4,101 metres) in the northeastern corner of the island, in the Malaysian state of Sabah. Otherwise, the island’s relief seldom exceeds an elevation of 5,600 feet (1,700 metres), and most of the island lies below 1,000 feet (300 metres). Structural trends are not as well-defined as on adjacent islands, although a broad mountain system (which includes Mount Kinabalu) runs roughly from northeast to southwest. Kalimantan, which constitutes about three-fourths of the island, consists mostly of undulating lowlands, with alluvial swamps near the coast and forest-covered mountains in the deep interior.

The Riau archipelago lies to the east of Sumatra, near the southern outlet of the Strait of Malacca. These islands have a granite core and can be considered a physical extension of the Malay Peninsula. With the exception of some highlands in the western and southern regions, the islands of the Riau group generally consist of low-lying swampy terrain.

Sumatra spans the Equator, stretching from northwest to southeast for more than 1,000 miles (1,600 km), with a maximum width (including offshore islands) of some 325 miles (525 km). It is flanked on its outer (western) edge by a string of nonvolcanic islands, including Simeulue, Nias, and the Mentawai group, none of which is densely populated. The Sumatran mainland divides into four main physical regions: the narrow coastal plain along the west; the Barisan Mountains, which extend the length of the island close to its western edge and include a number of active volcanoes; an inner nonvolcanic zone of low hills grading down toward the stable platform of the Asian mainland; and the broad alluvial lowland, lying no more than 100 feet (30 metres) above sea level, that constitutes the eastern half of the island. Much of the eastern lowland is a swampy forest that is difficult to penetrate.

Java is some 660 miles (1,060 km) long and has a maximum width of about 125 miles (200 km). Its physical divisions are not as distinct as those of Sumatra, because the continental shelf drops sharply to the Indian Ocean in the southern part of the island. Java can be divided into five latitudinal physiographic regions. The first region, a series of limestone platforms, extends along the southern coast; in some areas the platforms form an eroded karst region (i.e., marked by sinks interspersed with abrupt ridges, irregular rocks, caverns, and underground streams) that makes travel and habitation difficult. A mountain belt just to the north, in the western segment of the island, forms the second region; it is partially composed of sediments derived from eroded volcanoes and includes a number of heavily cultivated alluvial basins, especially around the cities of Bandung and Garut. The belt of volcanoes that runs through the centre of the island constitutes the third region; it contains some 50 active cones and nearly 20 volcanoes that have erupted since the turn of the 20th century. A northern alluvial belt, the fourth region, spreads across the Sunda Shelf toward the sea and is extended by delta formations, particularly during volcanic activity. There are deep inland extensions of this alluvial region, which in central Java cut through to the southern coast. Finally, there is a second limestone platform area along the northern coast of Madura (an island off the northeastern coast of Java) and the adjacent section of eastern Java.

The many islands of the Lesser Sundas to the east of Java are much smaller, less densely populated, and less developed than Java. The physiography of Bali and Lombok is similar to that of eastern Java. The Lesser Sunda Islands continue through Sumbawa and Flores, narrowing progressively until they appear on a map as a spine of volcanic islands that loops northeast into the Banda Islands. The same volcanic system reappears in northern Celebes. Sumba and Timor form an outer (southern) fringe of nonvolcanic islands that resembles the chain off the western edge of the Sunda Shelf near Sumatra.

Islands of the Sahul Shelf

The islands of the Sahul Shelf appear to have a physiographic structure similar to those of the Sunda Shelf. They include the northern Moluccas and New Guinea. The western portion of New Guinea consists of the Indonesian provinces of Papua and West Papua (Papua Barat), which together account for more than one-fifth of the total area of Indonesia but are home to only a tiny percentage of the country’s population. The two provinces cover a remote region with a spectacular and varied landscape. Mangrove swamps seal much of the southern and western coastline, while the Maoke Mountains—including Jaya Peak, which at 16,024 feet (4,884 metres) is the highest point in Indonesia—form a natural barrier across the central area. There is a narrow coastal plain in the north. Much of the region is heavily forested.