LAND AND SEA FEATURES OF INDONESIA: MOUNTAINS, RIVERS, COASTS

ISLANDS OF INDONESIA

main islands and regions of Indonesia

Indonesia is the world’s largest group of islands and the world's largest country comprised solely of islands. It includes the world’s second-, third- and fifth-largest islands, New Guinea, Borneo and Sumatra. The Indonesian islands are part of the Malay Archipelago, which also encompasses the Philippines. There are five main islands in Indonesia (Sumatra, Java, Kalimantan, Sulawesi, and Papua), two major archipelagos (Nusa Tenggara and the Maluku Islands), and sixty smaller archipelagos. Two of the islands are shared with other nations; Kalimantan (known in the colonial period as Borneo) is shared with Malaysia and Brunei, and Papua shares the island of New Guinea with Papua New Guinea.

Indonesia is a huge archipelagic country extending 5,120 kilometers from east to west and 1,760 kilometers from north to south. According to the Indonesian Naval Hydro-Oceanographic Office, the country encompasses 17,508 islands (some sources say more than 18,000, see above; others say 13,667 islands). Around 900 to 1,700 islands are "permanently inhabited". Another 4,000 or so islands have various kinds of temporary populations. The majority of the islands are uninhabited. Many are only a few hectares (acres) in size. Not all of these islands have been officially named. Some islands have disappeared as a result of the export of sand to Singapore.

Most of the islands of Indonesia rise from the submerged Sunda Shelf, which is considered a continuation of the Asian continent. The western and central islands are known as the Sunda Islands: Sumatra, Java, Borneo (the Indonesian part of which is called Kalimantan) and Sulawesi, along with the surrounding islands, are known as the Greater Sunda Islands. Borneo is the largest of these islands, covering 751,929 square kilometres (290,320 square miles) and ranking as the third largest island on Earth. Smaller islands in this region include Bangka, Belitung and the Mentawai Islands. [Source: Junior Worldmark Encyclopedia of Physical Geography, Gale Group, Inc., 2003]

Further east are the Lesser Sunda Islands. They begin with Bali and extend to Timor. The other large islands in this chain are Lombok, Sumbawa, Flores and Sumba, and it is also known as Nusa Tenggara. Along with the islands of Savu and Roti, they enclose the Savu Sea. Even further east are the Moluccas, also known as the Maluku Islands. Most of the Moluccas are found in groups of small and medium-sized islands, including the Tanimbar Islands, the Aru Islands, the Kai Islands and the Sula Islands. The largest individual islands are Halmahera, Wetar, Buru and Ceram. New Guinea, of which Indonesia's Papua province occupies the western half, is the second largest island in the world (884,824 square kilometres/341,631 square miles).

The largest areas landwise are Sumatra (between Java and peninsular Malaysia. 473,605 square kilometers), Kalimantan (southern two thirds of Borneo, 539,000 square kilometers) and Papua and West Papua (western half of New Guinea, 421,981 square kilometers). These places have large areas of wilderness with marshy coastal plains and rainforest-covered mountains in the interior. They have also suffered from heavy deforestation. The western archipelago embraces the largest islands of Indonesia. Sulawesi (Celebes, 202,000 square kilometers) is crab-shaped islands fringed by coral refs and covered by mountains that are more deforested than those on Borneo. The volcanic Malukas (Moluccas) were formerly known as the Spice Islands. Nusa Tenggara, a string of agricultural volcanic islands east of Bali, includes Komodo home of the Komodo dragon. The eastern archipelago is composed of many islands but they account for only 10 percent of the country’s land area. Many of the small islands are new and volcanic.

Topography and Land Areas of Indonesia

topography of Indonesia Researchgate

Indonesia's terrain is mostly coastal lowlands, with larger islands featuring interior mountains. The landscape along Indonesia's island chain is highly varied, with volcanic mountains standing out in sharp relief on most of the larger islands. [Source: Junior Worldmark Encyclopedia of Physical Geography, Gale Group, Inc., 2003; CIA World Factbook.

Five major islands make up 90 per cent of Indonesia's land area: These are Sumatra, Java, Sulawesi and parts of Borneo and New Guinea. The country also contains around thirty smaller island groups, the largest of which is Nusa Tenggara, comprising the islands of Lombok, Sumba, Sumbawa, Flores and Timor. East Timor gained independence from Indonesia in 1999.

The Lesser Sunda Islands, including Sumba, Lombok, Sumbawa and Timor, have extensive grassland areas, as do parts of Sumatra, Kalimantan, Sulawesi and Papua. Most of these grasslands are former forest areas that have been cleared or burned. Bamboo, both wild and cultivated, grows in many parts of Indonesia, although wild bamboo is also being cleared. [Source: Junior Worldmark Encyclopedia of Physical Geography, Gale Group, Inc., 2003]

Indonesia has a variety of forest types, including rainforests in Sumatra, Kalimantan, and Papua, monsoon forests in the Lesser Sunda Islands, coastal mangrove forests, and alpine forests in the mountains of Papua. It is estimated that Indonesia is home to 12 percent of the world's mammal species, 16 percent of bird species and 11 percent of plant species.

Many hill areas in Bali and Java are covered with rice terraces, which help prevent soil erosion. Tea plantations also occupy numerous hillsides on Java. The volcanic foothills of the Bandung district are the best-known hill region on Java. The islands of Nusa Tenggara, including Lombok and Timor, have grass-covered hills. Much of Sulawesi is highland terrain, including the Toraja region in the south of the island. Kalimantan's north-central region is characterised by its hilly terrain.

Dramatic canyons have been carved into the landscape of some regions of Sumatra and Java by rivers. Notable canyons in Sumatra include Sianok Canyon, a 492-foot-deep (150-metre-deep) limestone gorge stretching 9 miles (15 kilometres); the Harau Valley nature reserve, which is between 150 and 400 metres wide, with walls between 262 and 984 feet (80 and 300 metres) deep; and the Anai Valley gorge. The Green Canyon nature reserve is situated in western Java, close to the coast. Java's Thousand Hills (Gunung Sewu) Plateau is also home to many caves.

loss of glaciers on Puncak Jaya,
the highest mountain in Indonesia, Oceania
and south of the Himalayas in Asia Sumatra has significant plateau areas, including the 5,000-square-kilometre (1,930-square-mile) fertile volcanic soil of Tanah Karo; the Agam Plateau; and the 700-metre (2,296-foot)-high Maninjau Plateau, which rises above Maninjau Lake. Java's landscape is elevated in the Thousand Hills and Dieng plateaus, areas famous for their mineral lakes and ancient Hindu temple ruins.

Mountains of Indonesia

Except for coastal plains and river valleys most of Indonesia is mountainous. Mountains ranging between 3,000 and 3,800 meters above sea level can be found on the islands of Sumatra, Java, Bali, Lombok, Sulawesi, and Seram. The country's tallest mountains, which reach between 4,700 and 5,000 meters, are located in the Jayawijaya Mountains and the Sudirman Mountains in Papua. The highest peak, Puncak Jaya, which reaches 5,039 meters, is located in the Sudirman Mountains. On Papua there are mountains so high they covered with snow throughout the year.

The mountains of Indonesia are underwater mountain ranges that rise above sea level to form islands. Sulawesi is extremely mountainous, with peaks rising to over 8,000 feet (2,438 metres) in places. The Barisan Mountains of Sumatra run along the west coast of the island. The highest peaks reach over 3,600 metres (12,000 feet), with Kerinci (3,805 metres/12,483 feet) being the tallest. Java's mountains also lie close to the Indian Ocean shoreline. The highest peaks are found in the Tengger Mountains in the east. Many of the islands in the Nusa Tenggara and Maluku archipelagos (between Sulawesi and New Guinea) are also mountainous. On Bali, Lombok and Ceram, there are peaks over 3,048 metres (10,000 feet) high. [Source: Junior Worldmark Encyclopedia of Physical Geography, Gale Group, Inc., 2003]

Papua in New Guinea has towering non-volcanic mountains, the highest in Indonesia. The Maoke Mountains extend almost the entire length of the province. Some of these peaks are covered in snow all year round, including Puncak Jaya (16,503 feet/5,030 metres), the country's highest peak. Puncak Jaya is considered one of the "Seven Summits" by mountaineers attempting to climb the highest peak on every continent, as it is counted as part of the continent of Australia/Oceania. The Muller Mountains of Borneo run mainly along Indonesia's northern border with Malaysia. The highest peak is Mount Raya (2,278 metres/7,474 feet).

Volcanoes and Earthquakes of Indonesia

Lying along the borders of the Eurasian, Australian, and Philippine Tectonic Plates, Indonesia is the most highly volcanic region in the world. More than one hundred peaks either are active or were active until recently. The greatest population density is to be found in the regions where volcanoes have erupted. Thus Java, with the most volcanoes, is by far the most densely populated of the islands. [Source: Junior Worldmark Encyclopedia of Physical Geography, Gale Group, Inc., 2003]

Sangeang Api Island, Indonesia: In 1985 a volcano on this 13-kilometers-wide island off the northeast coast of Sumbawa began to erup; . Within a month, the 1250 inhabitants had evacuated to Sumbawa; The eruption lasted until 1988; The lava and pyroclastic flows—the wide channel running west from the summit—are still easily traced on this image taken by Space Shuttle astronauts (STS-112) in 2002; Today, the island's summit 1949-meter-high crater produces intermittent steam clouds

Many of Indonesia’s islands are rugged remnants of extinct volcanoes. There are around 400 volcanoes in Indonesia, A string of volcanoes runs all the way from Sumatra to Flores with more in Sulawesi and The Moluccas. Of these 127 are active, about a third of all the world's active volcanoes. On the positive side the volcanoes produce rich soil. The superior productivity of volcanic regions apparently justifies the risk of living there. However, large crop areas are sometimes burned out or badly scorched, and villages threatened by lava streams are deserted. Indonesians also coined the word 'lahar', which is now used as a scientific term for mudslides in volcanic areas. Despite these dangers, villagers return to live on the volcanic slopes.

Between 1972 and 1991 alone, twenty-nine volcanic eruptions were recorded, mostly on Java. The most violent volcanic eruptions in modern times occurred in Indonesia. In 1815 a volcano at Gunung Tambora on the north coast of Sumbawa, Nusa Tenggara Barat Province, claimed 92,000 lives and created "the year without a summer" in various parts of the world. In 1883 Krakatau in the Sunda Strait, between Java and Sumatra, erupted and some 36,000 West Javans died from the resulting tsunamis. The sound of the explosion was reported as far away as Turkey and Japan. Smoke and dust from the eruption darkened the skies around the world for months, significantly affecting climate and temperature. Krakatau then lay dormant until 1927, when renewed growth on its slopes threatened an eruption. In 1928, a new cone formed a crater island. The most recent major eruption occurred in 1952. After that Krakatau was quiet, until the late 1970s, when it erupted twice. In 1963, the Gunung Agung volcano (3,141 m) on Bali erupted, killing thousands. [Source: Library of Congress]

Indonesia is one of the most geologically active regions in the world. On average there are three earthquakes a day, measuring 5 or more on the Richter scale. There are about 10 times as many deaths from volcanic eruptions in Indonesia as in there are in any other country. A 1992 earthquake killed more than 2,500 people on the island of Flores; a 1994 earthquake and tsunami in eastern Java killed more than 200 people; and an earthquake measuring 7.9 on the Richter scale in June 2000 caused more than 100 deaths in southern Sumatra.”

Coastlines, Rivers and Lakes of Indonesia

Indonesia has one of the world's longest coastlines (54,716 kilometers, 34,000 miles). The southwestern islands tend to have steep shores with few sandy beaches, whereas the terrain of the northern and eastern coasts is mostly flat. River deltas dominate Sumatra's eastern side in particular. Sulawesi is made up of four peninsulas. The long northernmost peninsula, the Minahasa Peninsula, curves around the Tomini Gulf, while the two southern peninsulas enclose the Bone Gulf. The Gulf of Todo lies between the two. Kalimantan's coastline is jagged and dotted with numerous river deltas that empty into the South China Sea, Java Sea, Makassar Strait and Celebes Sea. At the far end of the archipelago lies the northwest region of Papua, known as the Bird's Head Peninsula. In the late 20th century increased siltation from deforestation throughout Indonesia blocked many harbors, making dredging necessary for shipping access. [Source: Junior Worldmark Encyclopedia of Physical Geography, Gale Group, Inc., 2003]

Lakes: More than 500 lakes are scattered across Indonesia. By far the largest is Lake Toba (Danau Toba) in northern Sumatra, which measures 1,150 square kilometers (440 square miles). It is located in the caldera of a super volcano that erupted more than 70,000 years ago and it is the largest volcanic lake in the world. Towering cliffs rise make up much of the rim of a volcanic crater. Toba is one of the deepest lakes in the world, plunging over 450 meters (1,476 feet). It is also relatively high, at 900 meters (2,953 feet) above sea level. In addition to Toba, notable Sumatran lakes include Manindjau and Singkarak.

The central region of Sulawesi is home to two deep lakes: Lake Towuti, which is 30 miles (48 kilometres) wide, and Lake Matana. Lake Poso is located in the north-central part of the region. In northern Sulawesi, there are the Limboto and Tandano lakes. Kalimantan's lakes include the three Mahakam lakes. The Mahakam River basin, an important bird habitat, contains ninety-six lakes altogether. The island of Flores is famous for its trio of colourful lakes at the summit of Mount Keli Mutu: green, maroon and black, due to variations in mineral content. Due to the abundance of rivers, dams are common in Indonesia and provide power for many households. These dams create many artificial lakes, most notably along the Asahan River.

The Indonesian Throughflow (ITF) is an ocean current with importance for global climate as is the low-latitude movement of warm, relative freshwater from the north Pacific to the Indian Ocean; It serves as a main upper branch of the global heat/salt conveyor belt

Major Rivers Indonesia’s waterways total 21,579 kilometers (13,406 square miles). Rivers are found in every part of the islands. Although most rivers are short, they are often important for irrigation. Major rivers can be found on Kalimantan, Java, Papua, and Sumatra. Indonesia's longest river, the Kapuas, which is 1,143 kilometers (710 miles) long, is in Kalimantan, flowing from the north-central mountains to the South China Sea. Other major rivers in Kalimantan are the Barito, Mahakham, and Rajang. Southern Kalimantan is crisscrossed with a network of hundreds of smaller rivers.

Sumatra's rivers include the Batanghari and Musi in the south, and the Indragiri and Kampar in the center of the island. Java's rivers are used for irrigation; they include the Solo, which is Java's longest at 560 kilometers (348 miles), Tarum, Bengawan, Citarum,and Brantas. Many rivers wind through Papua, including the Mamberamo, which runs into the Pacific Ocean, and the Digul. The Sepik (shared with Papua New Guinea) is 1,126 kilometers (700 miles) long; The Fly (also shared with Papua New Guinea) is 1,050 kilometers (652 miles) long. [Source: CIA World Factbook; Library of Congress, 2011]

Seas and Maritime Areas Around Indonesia

Indonesians call their country Tanah Air Kita (“Our Earth and Water”). The country forms a natural barrier between the Indian Ocean to the south and west, the open Pacific Ocean to the north-east, and the South China Sea to the north. To the south of Java lies the Java Trench, the lowest point in the Indian Ocean at around 7,300 meters (24,000 feet) deep. The Timor Trough, which is approximately 3,000 meters (9,842 feet) deep, lies between Timor and Australia. At least 10 percent of the world's coral reefs are found in the waters directly off the islands of Indonesia. However, fishing practices and land erosion are increasingly endangering these important marine ecosystems. [Source: Junior Worldmark Encyclopedia of Physical Geography, Gale Group, Inc., 2003]

Indonesia has a strategic location astride or along major sea lanes from the Indian Ocean to Pacific Ocean between east Asia and west Asia, the Middle East, Africa and Europe. These include the Malacca Strait between Sumatra and Malaysia and the southern part of the South China Sea. Indonesia is surrounded by ocean currents that travel in al directions. Its islands provide a convenient stepping stones between Southeast Asia and Australia. Sea lanes around the islands provide a route through which oil passes from the Middle East to east Asia and goods from Asia are transported to Europe.

Indonesia embraces 93,000 square kilometers (35,907 square miles) of inland seas (straits, bays, and other bodies of water) and 54,716 kilometers (34,000 miles) of coastline. The sea areas surrounding Indonesia bring its generally recognized territory (land and sea) to about 5 million square kilometers (about half the size of the U.S.). The government, however, also claims an exclusive economic zone, which brings the total to about 7.9 million square kilometers—more than four times Indonesia’s total land area.

Eastern Sumatra, northeastern Java and most of Borneo are located on an extension of the Asiatic continental shelf called the Sunda Platform. The seas over the platform are several shallow, warm and have a relatively low salt content because of the fresh water delivered to the area by rivers from mainland Southeast Asia. Similar conditions prevail on the Sahul shelf which extends from Australia to New Guinea. A deep trench runs along the southern coast of the Indonesian chain from northern Sumatra through Nusa Tengarra. The water here is deep, cold and salty.

Geographers have conventionally grouped Sumatra, Java (and Madura), Kalimantan (formerly Borneo), and Sulawesi (formerly Celebes) in the Greater Sunda Islands. These islands, except for Sulawesi, lie on the Sunda Shelf — an extension of the Malay Peninsula and the Southeast Asian mainland. Far to the east is Papua (formerly Irian Jaya, Irian Barat or West New Guinea), which takes up the western half of the world's second largest island — New Guinea — on the Sahul Shelf. Sea depths in the Sunda and Sahul shelves average 200 meters or less. Between these two shelves lie Sulawesi, Nusa Tenggara (also known as the Lesser Sunda Islands), and the Maluku Islands (or the Moluccas), which form a second island group where the surrounding seas in some places reach 4,500 meters in depth. The term Outer Islands is used inconsistently by various writers but it is usually taken to mean those islands other than Java and Madura.

Typhoons and other large storms pose little hazard to mariners in Indonesia’s waters; the primary danger comes from swift currents in channels such as the Lombok, Sape, and Sunda straits.

Straits and Important Navigation Routes of Indonesia

A vast number of straits and passages are found around Indonesia's islands. The Karimata Strait, for example, connects the South China Sea to the Java Sea. The Strait of Malacca, which runs between Sumatra and the Malay Peninsula, connects the South China Sea to the Bay of Bengal and is one of the busiest waterways in the world. Most ships travelling from the west to the east coast of Asia pass through this strait, as does most traffic from East Asia heading west. The Sunda Strait, which lies between Java and Sumatra, is also heavily travelled. The Great Channel lies between the northernmost tip of Sumatra and India's Nicobar Islands. Further east, between the islands, is the Makassar Strait, which lies between Borneo and Sulawesi. This strait connects the Sulawesi (Celebes) Sea in the north with the Java, Bali and Flores Seas in the south. [Source: Junior Worldmark Encyclopedia of Physical Geography, Gale Group, Inc., 2003]

Maritime Claims: Indonesia claims a 12-nautical-mile territorial sea and a 200-nautical-mile exclusive economic zone, measured from claimed archipelagic straight baselines. The total area claimed by the Indonesian government, including Indonesia’s territorial sea and an exclusive economic zone, encompasses 7.9 million square kilometers. [Source: Library of Congress, 2011]

In 1982 during the UN Conference on the Law of the Sea, Indonesia sought to defend its March 1980 claim to a 200-nautical-mile exclusive economic zone. Based on the doctrine of the political and security unity of archipelagic land and sea space (wawasan nusantara), the government asserted its rights to marine and geological resources within this coastal zone.

The Strait of Malacca — one of the most heavily traveled sea-lanes in the world — was considered by Indonesia and Malaysia to be their joint possession, and the two countries requested that other nations notify their governments before moving warships through these waters. The United States and several other nations rejected those claims, considering the strait an international waterway. [Library of Congress]

Image Sources: Wikimedia Commons

Text Sources: “Encyclopedia of World Cultures Volume 5: East/Southeast Asia:” edited by Paul Hockings, 1993; “Culture and Customs of Indonesia” by Jill Forshee, Greenwood Press, 2006; National Geographic, New York Times, Washington Post, Los Angeles Times, Smithsonian magazine, Encyclopedia.com, Library of Congress, Indonesia Tourism website (indonesia.travel), Indonesia government websites, Live Science, The Conversation, The New Yorker, Time, BBC, CNN, Reuters, Associated Press, AFP, Lonely Planet Guides, Google AI, Wikipedia, The Guardian and various websites, books and other publications.

Last updated June 2026