The Tectonic of Indonesia

Indonesia is the largest archipelagic state in the world comprising five major islands and about 300 smaller island groups. Altogether there are 13,667 islands and islets. The archipelago is situated on a crossroad between two oceans, the Pacific and Indian oceans, and bridges two continents, the Asian and Australian. Indonesia has a total area of 9.8 million sq km, of which more than 7.9 million sq km are under water.

The tectonics of Indonesia are very complex, as it is a meeting point of several tectonic plates. Indonesia is located between two continental plates: the Eurasian Plate (Sunda Plate) and Australian Plate (Sahul Shelf); and between two oceanic plates: the Philippine Sea Plate and Pacific Plate. The subduction of the Indian oceanic plate beneath the Eurasian continental plate formed the volcanic arc in western Indonesia, one of the most seismically active areas on the planet with a long history of powerful eruptions and earthquakes. This chain of active volcanoes formed Sumatra, Java, Bali, and Nusa Tenggara islands, most of which, particularly Java and Bali, emerged within the last 2-3 million years. The Pacific and Australian plate movements controlled the tectonics of the eastern portion of Indonesia.

Tectonically, Indonesia is highly unstable. As it location that lies on the Pacific where Indo-Australian Plate and the Pacific plate are pushed under the Eurasian Plate where they melt at about 100 km deep. A string of volcanoes runs through Sumatra, Java, bali, Nusa Tenggara and then loops around through to the Banda Islands of Maluku to the northeastern of Sulawesi. Of the 400 volcanoes, approximately 150 are active.

The Australian plate was moved northward and subducted under the Eurasian plate. The subduction zone can be traced from northern tip of Sumatra until the Lesser Islands, that creates deep submarine trench. Most of the earthquake also concentrate in this subduction zone. This subduction also triggers the formation of volcanic range from Sumatra, Java to Lesser Islands. The center part of Indonesia also experienced another subduction of Pacific plate that move southwesterly under the Eurasian plate. This subduction create the formation of volcanoes in the North Sulawesi, Sangihe and Halmahera. The Australian and Pacific plate collides in the eastern part of Indonesia and formed the mountain range in Papua, with Puncak Jaya / Carstenz Pyramide as the highest peak.

The tectonics processes in Indonesia formed major structures in Indonesia. The most prominent fault in the west of Indonesia is the Semangko Fault or the Great Sumatran Fault, a dextral strike-slip fault along Sumatra Island (about 1900 km). The formation of this fault zone is related to the subduction zone in the west of Sumatra. Palu-Koro fault is another major structural feature formed in the central part of Indonesia. This fault runs across the central part of Sulawesi Island and extends offshore to the west across Makassar Strait and ends in the Mangkalihat Peninsula in Borneo. The fault is named after the capital city of Central Sulawesi, Palu, on the west coast of Sulawesi and the Koro River, which is formed by the fault zone. Sorong fault is a significant left lateral fault in the eastern part of Indonesia, named after Sorong City. It has east-west orientation and extends from the northern part of West Papua to East Sulawesi for about 2000 km.