Indonesia is the world's largest archipelagic country, a chain of about 17,500 islands strung for some 5,120 km along the equator between the Indian and Pacific oceans, with a land area of 1,904,569 km². Physically it falls into three parts: the Greater Sunda Islands of Sumatra, Java and Borneo (Kalimantan), which stand on the shallow Sunda Shelf of Asia; a middle zone of Sulawesi, the Lesser Sunda Islands and the Maluku Islands, ringed by seas up to 4,500 m deep; and western New Guinea, which sits on the Australian Plate. About 400 volcanoes, roughly 150 of them active, run through the chain from Sumatra to Nusa Tenggara and around to northern Sulawesi. The highest point is Puncak Jaya, 4,884 m, in the Sudirman Range of Papua, and the longest river is the Kapuas, 1,143 km, in West Kalimantan. The 2020 census counted 270.2 million people, more than half of them on Java. Five named places, from the far east of the country to its far west and back to Borneo, show how that geography works.
Puncak Jaya: 4,884 m of Uplifted Limestone in Papua
Puncak Jaya, also called the Carstensz Pyramid, is the highest peak in Indonesia and the highest mountain on any island on Earth. It stands in the Sudirman Range in Mimika Regency, Central Papua, on the island of New Guinea, and no summit between the Himalayas and the Andes is taller. Because New Guinea sits on the same continental shelf as Australia, many climbers count it as the highest peak of Oceania and one of the Seven Summits, ahead of Mount Wilhelm in Papua New Guinea and Mount Kosciuszko in Australia.
Its rock tells the story of a collision. The peak is made of middle Miocene limestone, laid down in a shallow sea, then thrust up in the late Miocene by the oblique collision of the Australian and Pacific plates that raised New Guinea's central range, a spine some 650 km long. The Dutch explorer Jan Carstenszoon saw ice on the mountain on a clear day in 1623 and was ridiculed in Europe for reporting snow near the equator. He was right. Glaciers still cling to its slopes, among them the Carstensz Glacier and the East and West Northwall Firns, though the summit itself is now bare rock.
Those glaciers are among the few left in the tropics, and they are going fast. Records show retreat since the 1850s. The Meren Glacier in the valley below vanished between 1994 and 2000; the ice on Puncak Trikora, about 170 km to the east, disappeared between 1939 and 1962. A 2010 expedition led by the paleoclimatologist Lonnie Thompson measured the remaining ice thinning by about 7 m a year, and in 2018 it was predicted to disappear in the 2020s. Four kilometres west of the peak lies Grasberg, the huge open-pit gold and copper mine that ranks as the world's second-largest gold mine.
Lake Toba: The Caldera of Sumatra's Supereruption
At the other end of the country, in North Sumatra, lies Lake Toba, the largest lake in Indonesia and the largest volcanic lake in the world. It is about 100 km long and 30 km wide, up to 505 m deep, and its surface stands at roughly 900 m above sea level. The lake fills the Toba Caldera, which at 100 by 30 km is the largest Quaternary caldera on Earth; it is the fourth and youngest of four overlapping calderas and cuts across the three older ones.
That last collapse followed a supereruption about 74,000 years ago, rated 8 on the Volcanic Explosivity Index and the largest known explosive eruption of the past 25 million years. Researchers estimate it released at least 2,800 km³ of material, about 2,000 km³ as ground-hugging flows of hot ash and around 800 km³ as ash fall carried mostly westward. The flows destroyed at least 20,000 km², ash piled up to 600 m thick near the vent, and a layer about 15 cm thick settled across South Asia; at one site in central India the Toba ash is still up to 6 m thick. Scientists once argued that the eruption brought humanity close to extinction, but more recent studies have found no evidence of a lasting collapse in human or animal populations.
After the eruption, pressure from below pushed the caldera floor back up in a resurgent dome, which now forms Samosir, an island of 630 km² in the middle of the lake and the largest island within an island in the world. Seismic studies have found stacked sheets of magma at least 7 to 30 km beneath Samosir, and a volcano on the south edge of the caldera, Pusubukit, still vents gas. Toba Caldera became a UNESCO Global Geopark in July 2020. The full story of the eruption and its global reach is told in our guide to the Toba supervolcano.
Mount Tambora: The Sumbawa Volcano That Lost 1,450 m in 1815
Toba shows what the Sunda Arc can do over geological time; Tambora shows what it did within written history. The volcano stands on its own peninsula, Sanggar, on northern Sumbawa in West Nusa Tenggara, about 340 km north of the Java Trench, where the Australian Plate slides under the Sunda Plate at about 7.8 cm a year. Tambora began building its cone roughly 43,000 to 57,000 years ago and, before 1815, was about 4,300 m high, one of the tallest peaks in the archipelago.
After three years of rising activity, it erupted on 5 April 1815. The explosions were heard more than 2,600 km away on Sumatra, and the eruption column climbed over 43 km into the stratosphere. The volcano ejected up to about 150 km³ of material, making it a VEI 7 and the largest eruption in recorded history, with four to ten times the energy of Krakatoa in 1883. When it was over, the summit had collapsed into a caldera 6 to 7 km across and 600 to 700 m deep, and the highest point had dropped to about 2,851 m. Estimates of the death toll vary widely; a widely cited 2003 estimate puts it at no fewer than 71,000. One reconstruction, by Tanguy and colleagues in 1998, attributes about 11,000 deaths to direct volcanic action and some 49,000 to the famine and epidemics that followed.
The fine ash and sulphur spread around the globe. Londoners watched long, brilliantly coloured sunsets and twilights in late June and again from September into early October 1815, and 1816 became the Year Without a Summer, when crops failed and livestock died across the Northern Hemisphere in the worst famine of the century. Today a dense rainforest of Duabanga trees has grown back between about 1,000 and 2,800 m, and the caldera can be reached on foot from Doro Mboha or, over about 14 hours, from Pancasila village. Indonesia's other headline eruption, the 1883 blast that destroyed most of Krakatau in the Sunda Strait, is covered in our complete guide to Krakatoa.
The Lombok Strait: Where Wallace's Line Splits Bali from Lombok
West of Sumbawa lies Lombok, and beyond it a stretch of water no more than 40 km wide marks one of the most famous boundaries in biology. The Lombok Strait runs about 60 km between Bali and Lombok; it is 40 km wide at its northern opening and about 20 km at its narrowest, in the south between Lombok and Nusa Penida. It is at least 250 m deep, far deeper than the Strait of Malacca, so ships too big for Malacca route this way instead, and it is one of the main channels of the Indonesian Throughflow, the current that carries Pacific water into the Indian Ocean.
Biologists believe that depth explains the biology. During the Pleistocene ice ages, when the sea fell, Sumatra, Java and Bali were joined to mainland Asia across the dry Sunda Shelf, and New Guinea was joined to Australia. The deep water of the Lombok Strait kept Lombok and the islands east of it cut off from the Asian mainland, and they were colonised by Australasian animals instead. In 1859 the British naturalist Alfred Russel Wallace drew his line through it and north through the Makassar Strait between Borneo and Sulawesi, noting that the birds and mammals to the west were Asian and those to the east a mixture with Australian origins. The islands between the two shelves are now called Wallacea, and Lombok's own 3,726 m Mount Rinjani, the second-highest volcano in Indonesia, rises just across the water from Bali.
The Kapuas River: 1,143 km Through West Kalimantan
Borneo's defining force is rain rather than fire, and its greatest river is the Kapuas, the longest river in Indonesia and on Borneo, and one of the longest island rivers in the world. It rises in the Müller Mountains near the centre of the island, just south of the border with Malaysia, and flows west to the South China Sea. After about 165 km of mountain terrain it reaches a marshy plain, and from Putussibau to the delta, a distance of 902 km, it descends only about 50 m. Its basin covers about 99,000 km², more than two-thirds of West Kalimantan province.
The catchment receives an average of 3,666 mm of rain a year, and during the April and November peaks the river can rise 10 to 12 m overnight. About 350 km from the source, a network of lakes on the north bank acts as a natural overflow. When monthly rainfall passes about 300 mm, water pours into them at up to 1,000 m³ per second, merging river and lakes into a single sheet of water and sparing the lower valley from worse floods.
Downstream, the Kapuas discharges an average of roughly 5,600 to 7,800 m³ per second at a marshy five-armed delta whose silt reaches 50 to 60 km out to sea. The northernmost and widest arm, the Kapuas Besar, runs through Pontianak, the provincial capital, which lies almost exactly on the equator. Its largest tributary, the Melawi, joins at Sintang about 465 km from the mouth. The forests of its upper basin still yield new species, such as the Kapuas mud snake, discovered between 2003 and 2005. The rest of the island the river drains, shared with Malaysia and Brunei, is covered in our guide to the geography of Borneo.