In most subduction zones, motion of the subducted plate is nearly perpendicular to the trench axis. In some cases, for example Sumatra, where the motion is oblique to the axis, a strike-slip fault zone is seen, and is lying parallel to the volcanic chain.
In the subduction zone southwest of Sumatra, the Sunda trench axis strikes approximately N 37°W. The Indian Ocean crust is moving in an azimuth of approximately N 23°E relative to Southeast Asia, giving an angle of obliquity of 60°. Eastern Indonesia, forming the southeastern extremity of the Southeast Asian lithospheric plate, crushed between the northward-moving Indo-Australian and the westward-moving Pacific plates, is certainly the most complex active tectonic zone on earth. The rate of subduction is some centimeters per year; for example, it is 6.0 cm per year in the West Java Trench at 0°S 97°E (azimuth 23°); 4.9 cm per year in the East Java Trench at 12°S 120°E (azimuth 19°); and 10.7 cm per year in New Guinea at 3°S 142°E (azimuth 75°).
Frequent volcanic eruptions and frequent earthquake shocks testify to the active tectonic processes which are currently in progress in response to the continued movement of these major plates. The distribution of small ocean basins, continental fragments, remnants of ancient magmatic arcs and numerous subduction complexes which make up the Indonesian region indicate that the past history of the region was equally tectonically active.
Abridged from Southeast Asia Association of Seismology and Earthquake Engineering, Series on Seismology, Volume V - Indonesia, June 1985.
Tectonics of IndonesiaMost of Indonesia's volcanoes are part of the Sunda arc, a 3,000-km-long line of volcanoes extending from northern Sumatra to the Banda Sea. Most of these volcanoes are the result of subduction of the Australia Plate beneath the Eurasia Plate. Volcanoes in the Banda Sea result from subduction of the Pacific Plate under the Eurasia Plate. Black "teeth" are on the overriding plate. Arrows show direction of movement along major transform faults. Simplified from Lee and Lawver, 1995, Tectonophysics, v. 251, p. 85-138.
About one-fourth of Indonesia's volcanoes are north of the Sunda arc in an area with complex tectonics. Several small plates have produced mostly north-south trending subduction zones. The volcanoes of Sulawesi, Halmahera, and Sangihe are the result of these subduction zones. Simplified from Hamilton (1979).
The distribution of earthquakes in the subducted plates can be used to make a cross-section of the Molucca Sea area. Most of the Molucca Sea Plate has been "consumed" (subducted) by the Halmahera subduction zone in the east and by the Sangihe subduction zone in the west. The volcanoes of Sulawesi, Sangihe, and Halmahera are fed by magma generated in the asthenospheric mantle that has been modified by fluids derived from the subducted Molucca Sea Plate. In a few million years, all of the Molucca Sea Plate will be subducted and the Sangihe and Halmahera plates will collide, shutting off volcanism. Simplified from Hamilton (1979).
Indonesia has 76 volcanoes that have erupted in historic time - the largest number for any volcanic region. These volcanoes have had at least 1,171 eruptions, placing Indonesia second (after Japan) for the region with the most dated eruptions.
Indonesia has had the highest number of eruptions that:
- produced fatalities
- caused damage to land used for agriculture
- generated mudflows
- generated tsunami
- grew lava domes
- produced pyroclastic flows
- in 1991, during the eruption of Lokon-Empung, 10,000 people were evacuated and there was only one fatality
- in 1990, during the eruption of Kelut, 60,000 people were evacuated and there were 32 fatalities
- in 1988, during the eruption of Makian, 15,000 people were evacuated and there were no fatalities
- in 1982, during the eruption of Galunggung, 75,000 people were evacuated and there were 68 fatalities
Neumann van Padang, M., 1951, Indonesia. Catalogue of the Active Volcanoes of the World, International Association of Volcanology, 1, Rome, Italy, 271 p.
Neumann van Padang, M., 1983, History of volcanology in the former Netherlands east Indies: Scripta Geol, v. 71, p. 1-76.
Simkin, T., and Siebert, L., 1994, Volcanoes of the World: Geoscience Press, Tucson, Arizona, 349 p.
Indonesia's variations in culture have been shaped--although not specifically determined--by centuries of complex interactions with the physical environment. Although Indonesians are now less vulnerable to the vicissitudes of nature as a result of improved technology and social programs, to some extent their social diversity has emerged from traditionally different patterns of adjustment to their physical circumstances.
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 Irian Jaya (formerly 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.
Tectonically, this region--especially Java--is highly unstable, and although the volcanic ash has resulted in fertile soils, it makes agricultural conditions unpredictable in some areas. The country has numerous mountains and some 400 volcanoes, of which approximately 100 are active. Between 1972 and 1991 alone, twentynine 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 tidal wave. The sound of the explosion was reported as far away as Turkey and Japan. For almost a century following that eruption, Krakatau was quiet, until the late 1970s, when it erupted twice.
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 Irian Jaya. The highest peak, Puncak Jaya, which reaches 5,039 meters, is located in the Sudirman Mountains.
Nusa Tenggara consists of two strings of islands stretching eastward from Bali toward Irian Jaya. The inner arc of Nusa Tenggara is a continuation of the chain of mountains and volcanoes extending from Sumatra through Java, Bali, and Flores, and trailing off in the Banda Islands. The outer arc of Nusa Tenggara is a geological extension of the chain of islands west of Sumatra that includes Nias, Mentawai, and Enggano. This chain resurfaces in Nusa Tenggara in the ruggedly mountainous islands of Sumba and Timor.
The Maluku Islands (or Moluccas) are geologically among the most complex of the Indonesian islands. They are located in the northeast sector of the archipelago, bounded by the Philippines to the north, Irian Jaya to the east, and Nusa Tenggara to the south. The largest of these islands include Halmahera, Seram, and Buru, all of which rise steeply out of very deep seas. This abrupt relief pattern from sea to high mountains means that there are very few level coastal plains.
Geographers believe that the island of New Guinea, of which Irian Jaya is a part, may once have been part of the Australian continent. The breakup and tectonic action created both towering, snowcapped mountain peaks lining its central east-west spine and hot, humid alluvial plains along the coast of New Guinea. Irian Jaya's mountains range some 650 kilometers east to west, dividing the province between north and south.