The geography of Indonesia: an archipelago on the seam between Asia and Australia

Where the Asian and Australian shelves meet

Most of Indonesia's seas are shallow and new. Sumatra, Java, Bali, the Lesser Sunda chain and Borneo stand on the Sunda Shelf, a southward extension of the Asian continental mass. New Guinea and the islands near it, possibly including Halmahera, stand on the Sahul Shelf, the north-western edge of the Australian continent. On both shelves the water averages 300 metres deep or less, and Britannica notes that these seas rest on continental crust, not on true ocean floor. During the Pleistocene, Java, Sumatra and Borneo were joined to mainland Asia and New Guinea to Australia; the Java Sea, the Karimata Strait and the Arafura Sea filled as the ice melted.

Between the two shelves lies the third unit, an extension of the mountain belt that runs down through Japan and the Philippines. Sulawesi, the Moluccas and Nusa Tenggara stand in deep water there: the seas around them go down to 4,500 metres, and Sulawesi alone sits across three plates. The Moluccas involve four. That middle zone has steep islands with very few level coastal plains, and it is where the archipelago's animals change.

Britannica measures the whole spread at about 5,100 kilometres from east to west and 1,800 from north to south; Wikipedia's geography article gives 5,120 and 1,760. Sumatra, Kalimantan and western New Guinea together make up almost three-quarters of the land area. The country has land borders on four islands: with Malaysia on Borneo and on the small island of Sebatik, with Papua New Guinea across New Guinea and with Timor-Leste on Timor.

How many islands Indonesia actually has

There is no single answer, because the surveys counted different things. A geospatial survey run by the national mapping agency, Bakosurtanal, between 2007 and 2010 found 17,500 islands. An earlier survey of 2002 by the space agency LAPAN found 18,307, a difference Wikipedia's geography article attributes to the earlier method counting tidal islands, sandy cays and reefs that surface only at low tide. The CIA World Factbook gives 17,508. Britannica rounds to about 17,500 and adds that more than 7,000 are uninhabited.

What the count hides is how unevenly the land is used. Java is about 1,060 kilometres long and at most 200 wide, less than 7% of the country's land, and it holds about 56% of the population; Wikipedia's article on the island puts its density at about 1,192 people per square kilometre in 2025. Papua and West Papua, by Britannica's measure, cover more than a fifth of Indonesia's area and hold only a small percentage of its people.

The sea counts as territory under the archipelagic principle, which measures the country's waters from baselines drawn around its outer islands. The land figure itself includes 93,000 square kilometres of inland seas, straits and bays, and the exclusive economic zone the government claims is 6,159,032 square kilometres. The CIA figure for the coastline is 54,720 kilometres.

Why the volcanoes run in one long curve

The southern edge of the Sunda Shelf is a subduction zone. Along the Sunda megathrust, a fault about 5,500 kilometres long off the south coasts of Sumatra, Java and the Lesser Sunda Islands, the Indo-Australian Plate moves north-east under the Sunda Plate. The fault made the Sunda Trench, which Britannica gives as about 7,450 metres deep at its lowest in the Java Trench, and it raised the mountain spines of Sumatra, Java and the Lesser Sundas. Wikipedia's geography article places the great earthquake of December 2004 in the vicinity of the fault.

The volcanoes follow the same line, from Sumatra through Java and Nusa Tenggara, round through the Banda Islands and up into north-eastern Sulawesi. Wikipedia counts about 400 volcanoes, of which roughly 150 are active. Britannica counts some 50 active cones in Java's central volcanic belt alone, and nearly 20 that have erupted since 1900.

What the eruptions have cost

The record of eruptions is long and specific:

• Tambora, on Sumbawa, erupted in 1815 and killed about 92,000 people.
• Krakatau, in the Sunda Strait, erupted in 1883 and killed about 36,000.
• Merapi, in central Java, erupted for several weeks from late October 2010; Britannica records at least 350 deaths and some 130,000 people evacuated.

Volcanic ash also makes the soil. Wikipedia's geography article notes that the ash improves the fertility of the land around a volcano while making farming there unpredictable, and Britannica describes the "lava-enriched soil" of Java's river valleys as the base of its wet-rice farming. That is the link between the plate boundary and the population map: Java, with its 50 active cones, is also the island where most Indonesians live.

How each island along the arc is built

Sumatra shows the pattern clearly. The Barisan Mountains run its whole 1,600-kilometre length close to the west coast, with active volcanoes among them; east of them, low hills give way to a broad alluvial lowland no more than 30 metres above sea level, much of it swamp forest, which makes up the eastern half of the island. Kalimantan, the Indonesian three-quarters of Borneo, is mostly undulating lowland, seldom above 1,700 metres and mostly below 300.

Java is built in five bands that run along its length, as Britannica describes them. A limestone platform lines the south coast, worn in places into karst of sinkholes, ridges and caves that makes the land hard to farm or cross. North of it in the west lies a mountain belt of eroded volcanic sediment with heavily cultivated basins around Bandung and Garut. The volcanic belt runs through the centre of the island. North of the volcanoes an alluvial plain spreads out over the Sunda Shelf and grows with every delta the rivers build, and a second limestone platform runs along northern Madura and the facing coast of eastern Java. Because the shelf drops sharply to the Indian Ocean on the south side, the island's divisions are less tidy than Sumatra's.

East of Java the islands shrink. Bali and Lombok repeat the build of eastern Java; then the chain narrows through Sumbawa and Flores into a spine of volcanic islands that loops north-east into the Banda Islands and reappears in northern Sulawesi. A second, outer arc runs parallel to it on the south: Sumba and Timor are not volcanic, and Britannica pairs them with the islands off Sumatra's west coast, Simeulue, Nias and the Mentawai group, as one fringe of non-volcanic land along the outside of the volcanic chain. The Riau islands east of Sumatra are different again, with a granite core that makes them a physical extension of the Malay Peninsula.

What Wallace's Line divides

The animals change where the shelves end. Wallace's Line, as Britannica describes it, runs between Borneo and Sulawesi in the north and between Bali and Lombok in the south. West of it the fauna is Asian: rhinoceroses, orangutans, tapirs, tigers and elephants. East of it the animals are related to Australia's: cockatoos, bowerbirds, birds of paradise and marsupials such as bandicoots and cuscuses. The islands in between, from Sulawesi to the Banda Sea, are called Wallacea and carry a mixture of both.

Several species live on a single island or a single group. The babirusa, a wild pig, and the tamarau, a small wild ox, are found only on Sulawesi. The Javan rhinoceros now survives mostly in Ujung Kulon National Park at the western tip of Java, a World Heritage Site since 1991. The orangutan is native to Borneo and Sumatra only.

Komodo National Park, inscribed by UNESCO in 1991, covers 219,322 hectares between Sumbawa and Flores, including the three main islands of Komodo, Rinca and Padar. UNESCO describes it as the "shatter belt" of Wallacea, at the junction of two continental plates, with a dry climate unusual in the archipelago: savanna and monsoon forest in place of rainforest. Its lizards, Varanus komodoensis, grow to an average of 2 to 3 metres and exist nowhere else.

How the monsoon sets the year

The archipelago lies across the equator, so the year is divided by rain, not by temperature. For most of the country the dry season runs from May to October and the wet season from November to April, with the monsoon winds blowing from the south and east from June to October and from the north-west from November to March.

The contrast between seasons is weak in Kalimantan and Sumatra and strong in Nusa Tenggara, where the dry season brings drought and the wet season floods. Western Sumatra, north-western Kalimantan, western Java and western New Guinea get the heaviest rain; parts of Sulawesi, Sumba and Timor, the islands nearest Australia, are drier. Warm seas make up 81% of the national area and hold land temperatures steady: Wikipedia's geography article gives averages of about 28°C on the coastal plains, 26°C inland and 23°C in the higher mountains, with humidity between 70 and 90%.

Why New Guinea has ice on the equator

The highest ground in Indonesia is in the far east, where the Australian and Pacific plates collide. The central range of New Guinea runs about 650 kilometres east to west across the island, and its highest summit, Puncak Jaya, rises in the Sudirman Range of Papua. Above 3,500 metres the climate is tundra.

Lorentz National Park, declared a national park by the forestry minister in 1997 and inscribed by UNESCO in 1999, covers 2.35 million hectares and runs for more than 150 kilometres from those mountains south to the Arafura Sea. UNESCO describes it as one of only three tropical regions in the world with glaciers and as a continuous transect from snow to tropical sea, through extensive lowland wetlands. The park holds five small remnant glaciers, and UNESCO's description records that all five are retreating rapidly. It also records 123 mammal species, among them two of the world's three kinds of monotreme, and seven ethnic groups living in the park, from the Amungme and Dani of the highlands to the Asmat and Kamoro of the lowlands.

The south and west coasts of western New Guinea are sealed by mangrove swamp; the north has a narrow coastal plain. Much of the region is forested and Britannica calls it remote.

What is happening to the forests and coasts

Below 1,500 metres the natural cover across most of the archipelago is mixed lowland and hill rainforest; above that height it gives way to upland forest of oak, laurel and magnolia. Mangrove forest lines the shallow seas off eastern Sumatra, southern Kalimantan and south-eastern New Guinea.

Wikipedia's geography article puts deforestation, much of it illegal, first among the environmental issues, together with the fires that clear land in the dry season. Haze from fires in South Sumatra, Riau and Kalimantan returns most years; the history article on Wikipedia names 1997 and 2015 as the worst episodes and records that Indonesia ratified the ASEAN Agreement on Transboundary Haze Pollution in September 2014. The same source names the Sumatran orangutan among the mammals the IUCN lists as critically endangered. In 2013 arable land covered 12.97% of the country and permanent crops 12.14%.

The rainforest of Sumatra is on the World Heritage list as the Tropical Rainforest Heritage of Sumatra, inscribed in 2004, and Ujung Kulon, Komodo and Lorentz complete the four natural sites.