and Celebes (Sulawesi); the Lesser Sunda Islands (Nusa Tenggara) of Bali and a chain of islands that runs eastward through Timor; the Moluccas (Maluku) between Celebes and the island of New Guinea; and the western extent of New Guinea (generally known as Papua). The capital, Jakarta, is located near the northwestern coast of Java. In the early 21st century Indonesia was the most populous country in Southeast Asia and the fourth most populous in the world.
Indonesia was formerly known as the Dutch East Indies (or Netherlands East Indies). Although Indonesia did not become the country’s official name until the time of independence, the name was used as early as 1884 by a German geographer; it is thought to derive from the Greek indos, meaning “India,” and nesos, meaning “island.” After a period of occupation by the Japanese (1942–45) during World War II, Indonesia declared its independence from the Netherlands in 1945. Its struggle for independence, however, continued until 1949, when the Dutch officially recognized Indonesian sovereignty. It was not until the United Nations(UN) acknowledged the western segment of New Guinea as part of Indonesia in 1969 that the country took on its present form. The former Portuguese territory of East Timor (Timor-Leste) was incorporated into Indonesia in 1976. Following a UN-organized referendum in 1999, however, East Timor declared its independence and became fully sovereign in 2002.
The Indonesian archipelago represents one of the most unusual areas in the world: it encompasses a major juncture of Earth’s tectonic plates, spans two faunal realms, and has for millennia served as a nexus of the peoples and cultures of Oceania and mainland Asia. These factors have created a highly diverse environment and society that sometimes seem united only by susceptibility to seismic and volcanic activity, close proximity to the sea, and a moist, tropical climate. Nevertheless, a centralized government and a common language have provided Indonesia with some sense of unity. Furthermore, in keeping with its role as an economic and cultural crossroads, the country is active in numerous international trade and security organizations, such as ASEAN, OPEC, and the UN.
Indonesia is the largest country in Southeast Asia, with a maximum dimension from east to west of about 3,200 miles (5,100 km) and an extent from north to south of 1,100 miles (1,800 km). It shares a border with Malaysia in the northern part of Borneo and with Papua New Guinea in the centre of New Guinea. Indonesia is composed of some 17,500 islands, of which more than 7,000 are uninhabited. Almost three-fourths of Indonesia’s area is embraced by Sumatra, Kalimantan, and western New Guinea; Celebes, Java, and the Moluccas account for most of the country’s remaining area.
The major Indonesian islands are characterized by densely forested volcanic mountains in the interior that slope downward to coastal plains covered by thick alluvial swamps that, in turn, dissolve into shallow seas and coral reefs. Beneath this surface the unique and complex physical structure of Indonesia encompasses the junction of three major sections of the Earth’s crust and involves a complicated series of shelves, volcanic mountain chains, and deep-sea trenches. The island of Borneo and the island arc that includes Sumatra, Java, Bali, and the Lesser Sunda chain sit on the Sunda Shelf, a southward extension of the continental mass of Asia. The shelf is bounded on the south and west by deep-sea trenches, such as the Java Trench (about 24,440 feet [7,450 metres] deep at its lowest point), which form the true continental boundary. New Guinea and its adjacent islands, possibly including the island of Halmahera, sit on the Sahul Shelf, which is a northwestern extension of the Australian continental mass; the shelf is bounded to the northeast by a series of oceanic troughs and to the northwest by troughs, a chain of coral reefs, and a series of submarine ridges. The third major unit of the Earth’s crust in Indonesia is an extension of the belt of mountains that forms Japanand the Philippines; the mountains run southward between Borneo and New Guinea and include a series of volcanoes and deep-sea trenches on and around Celebes and the Moluccas.
The relation between these three landmasses is not clearly understood. The present land-sea formations are somewhat misleading because the seas that lie on the Sunda and Sahul shelves are shallow and of geologically recent origin; they rest on the continental mass rather than on a true ocean floor. The Sunda Shelf in the vicinity of the Java Sea has relatively low relief, contains several coral reefs, and is not volcanic. The mountain system that stretches along the South China and Celebes seas of this shelf and that marks the outer edge of the continental mass of Asia, however, is an area of strong relief and is one of the most active volcanic zones in the world.
The outer (southern) side of the chain of islands from Sumatra through Java and the Lesser Sundas forms the leading edge of the Southeast Asian landmass. It is characterized by active volcanoes, bounded to the south and west by a series of deep-sea trenches. On the inner (northern) side of the islands the volcanic mountains grade into swamps, lowlands, and the shallow Java Sea. This sheltered sea was formed at the close of the Pleistocene Epoch (about 12,000 years ago), and there is evidence of former land bridges, which facilitated the migration of plants and animals from the Asian continent.
Islands of the Sunda Shelf
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 cultivatedalluvial 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.
Celebes and the Moluccas
Celebes shows some evidence of being squeezed between the conflicting forces of the more stable surrounding masses of the Sunda and Sahul shelves. Its complex shape somewhat resembles a capital K, with an extremely long peninsula running northeast from its north-south backbone. There are, therefore, three large gulfs: Tomini (or Gorontalo) to the north, Tolo to the east, and Bone to the south. The coastline is long in relation to the size of the island. The land consists of ranges of mountains cut by deep rift valleys, many of which contain lakes. The island is fringed by coral reefs and is bordered by oceanic troughs in the south. Its northeastern arm, the Minahasa Peninsula, is volcanic and structurally different from the rest of the island, which is composed of a complex of igneous and metamorphic rocks.
The Moluccas consist of a group of roughly 1,000 islands with a combined area that is about two-thirds the size of Java. HalmaheraIsland is the largest of the group, followed by Ceram and Buru. The Moluccas lie in the same geologically unstable zone as Celebes, although the northern islands are associated more with the Sahul Shelf. Halmahera Island, in the north, is volcanic, as are the islands of the Banda Sea, which are frequently rocked by earthquakes. Most of the northern and central Moluccas have dense vegetation and rugged mountainous interiors where elevations often exceed 3,000 feet (900 metres). Once commonly known as the “Spice Islands,” the Moluccas—especially Ternate, Tidore, Ambon, and Banda Besar—were a source of cloves, nutmeg, and mace, particularly during the 16th and 17th centuries.
There are over 100 active volcanoes in Indonesia and hundreds more that are considered extinct. They run in a crescent-shaped line along the outer margin of the country, through Sumatra and Java as far as Flores, then north through the Banda Sea to a junction with the volcanoes of northern Celebes. Volcanic eruptions are by no means uncommon. Mount Merapi, which rises to 9,551 feet (2,911 metres) near Yogyakarta(Jogjakarta) in central Java, erupts frequently—often causing extensive destruction to roads, fields, and villages but always greatly benefiting the soil. Mount Kelud (5,679 feet [1,731 metres]), near Kediri in eastern Java, can be particularly devastating, because the water in its large crater lake is thrown out during eruption, causing great mudflows that rush down into the plains and sweep away all that is before them.
Perhaps the best-known volcano is Krakatoa (Krakatau), situated in the Sunda Strait between Sumatra and Java, which erupted disastrously in 1883. All life on the surrounding island group was destroyed. The eruptions caused tidal waves throughout Southeast Asia, killing tens of thousands of people, and ash clouds that circled the Earth decreased solar radiation and produced spectacular sunsets for more than a year. Another major incident occurred in 1963, when Mount Agung on Bali erupted violently after having been dormant for more than 140 years. In 2006 the drilling of an exploratory petroleum well triggered the eruption of an unusual mud volcano in a heavily populated region of eastern Java. Hot mud flowed voluminously from the well for the next several years, ultimately engulfing dozens of villages, obstructing roads and railways, and displacing tens of thousands of residents. In 2010 Mount Sinabung, in northern Sumatra, erupted after more than 400 years of dormancy, forcing tens of thousands to evacuate their homes.
Because of its insularity, Indonesia has no large rivers comparable to those on the Asian mainland. Indonesian rivers generally are relatively short and flow from interior mountains to the sea. The Kapuas (710 miles [1,140 km] long), Barito (560 miles [900 km]), and Mahakam (480 miles [770 km]) rivers of Kalimantan are among the longest, but shifting sandbars across their mouths reduce their importance for large-vessel transportation. Western New Guinea, most of which receives heavy rainfall, is drained by a number of large rivers, including the Baliem, the Mamberamo, and the Digul.
There are a number of notable lakes on Sumatra, the most famous of which is Lake Toba, which lies in the north at an elevation of about 3,000 feet (900 metres) above sea level and covers some 440 square miles (1,140 square km). Celebes also has several large, deep lakes, including Lakes Towuti and Matama in the southern part of the island and Lake Poso in the centre.
The seas surrounding Indonesia must also be viewed as important hydrologic features that serve both as channels of communication and as barriers protecting distinctive cultural and environmental features of the islands. The shallow seas between many of the islands are a significant source of offshore petroleum, natural gas, minerals, and food.
Indonesia illustrates the relation between climate and source rock in the formation of soils. The rocks on Java are primarily andesitic volcanics (dark gray rocks consisting essentially of the minerals oligoclase or feldspar), while rhyolites (the acidic lava form of granite) are dominant on Sumatra, granites in the Riau archipelago, granites and sediments in Kalimantan, and sediments in western New Guinea. The resulting soils in humid regions are mainly lateritic (containing iron oxides and aluminum hydroxide) and of varying fertility depending on the source rock; they include heavy black or gray-black margalite soils and limestone soils. Black soils occur in regions with a distinct dry season.
Among the most fertile soils are the ando soils, which developed on the andesitic volcanic sediments of the northeastern coast of Sumatra. Highly fertile soils, also derived from or enriched by basic andesitic volcanic material, occur on Java and Celebes as well. Valuable volcanic ash is transported by wind and deposited as a layer of homogeneous, fresh inorganic material over wide areas; it is also carried as suspended material in streams and irrigation channels. Minerals that are leached from the soil are replaced by alluvial deposition from rivers, as in some parts of Kalimantan, or by deposition in impounded water or riceterraces.
In general, the perpetual high temperatures and heavy precipitation throughout much of Indonesia have caused rapid erosion and deep chemical weathering and leaching, which usually produce impoverished soil. In areas covered with tropical rainforests, such as Kalimantan, the soils are protected by the forest cycle; as plants die, they decompose rapidly, releasing nutrients that are reabsorbed by new vegetation growth. Although such soils support a luxuriant growth, they cannot support a large agricultural population, because clearing the forest breaks the cycle and can lead to accelerated soil deterioration.
The climate of Indonesia is determined partly by its island structure and its position astride the Equator, which assure high, even temperatures. In addition, its location between the two landmasses of Asia and Australiaexposes it to seasonal patterns of precipitation brought by monsoonwinds.
Regional temperature variation is a function of elevation rather than latitude. Temperatures are highest along the coast, where mean annual readings range from the mid-70s to the upper 80s °F (low 20s to low 30s °C). Regions above 2,000 feet (600 metres) are significantly cooler, but only the Maoke Mountains of Papua are high enough to receive snow. The diurnal difference of temperature in Jakarta is at least five times as great as the difference between the high and low temperatures of January and July; on an exceptionally hot day in Jakarta the temperature may reach nearly 100 °F (38 °C), while on an especially cool one it may drop to about 65 °F (18 °C).
Precipitation is more varied in extremes and distribution. Most of Indonesia receives heavy rainfall throughout the year, the greatest amounts occurring from December to March. From central Java eastward toward Australia, however, the dry season (June to October) is progressively more pronounced; the islands of Timor and Sumba receive little rain during these months. The highest amount of precipitation occurs in the mountainous regions of Sumatra, Kalimantan, Celebes, and western New Guinea, where annual rainfall totals more than 120 inches (3,000 mm). The rest of Kalimantan, Sumatra, western New Guinea, western and central Java, and much of Celebes and the Moluccas average at least 80 inches (2,000 mm) of rainfall per year. Eastern Java, Bali, southern and central Celebes, and Timor generally receive between 60 and 80 inches (1,500 and 2,000 mm), while the Lesser Sunda Islandsthat are closest to Australia get only 40 to 60 inches (1,000 to 1,500 mm).
The absolute daily maximum of precipitation can be extremely high, with a number of stations recording between 20 and 28 inches (500 and 700 mm). Local variations, caused in large part by geographic features, are great. For example, Jakarta, which is near sea level, has a mean annual rainfall of 70 inches (1,750 mm), while just 30 miles (50 km) to the south, at an elevation of about 790 feet (240 metres), Bogor records nearly 170 inches (4,300 mm).
Seasonal variations are caused by monsoonal Asian air drifts and the convergence of tropical air masses from both north and south of the Equator along an intertropical front of low pressure. The monsoon pattern in any given part of the archipelago depends on location either north or south of the Equator, proximity to Australia or mainland Asia, and the position of the intertropical front. During December, January, and February, the west monsoon from the Asian mainland brings heavy rain to southern Sumatra, Java, and the Lesser Sunda Islands. In June, July, and August, these areas are affected by the east monsoon, which brings dry air from Australia. Only the Lesser Sunda Islands and eastern Java have a well-developed dry season, which increases in length toward Australia. By the time the east monsoon has crossed the Equator—becoming the southwest monsoon of the Northern Hemisphere—its winds have become humid and a source of rain. Sumatra and Kalimantan, which are located close to the Equator and far from Australia, have no dry season, although precipitation tends to be slightly lower during July and August. Strong cyclones and typhoons, which normally occur in higher latitudes, are absent in Indonesia, but afternoon thunderstorms are common.