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This is a brief summary of thegeology of Indonesia. Indonesia is located between two major tectonic plates namely, theAustralian Plate and the newly-separatedSunda Plate.
Thetectonics ofIndonesia are very complex, as it is a meeting point of severaltectonic plates. Indonesia is located between two continental plates: theSahul Shelf and theSunda Plate; and between two oceanic plates: thePacific Plate and thePhilippine Sea Plate.
Thesubduction of theIndian Plate beneath the Sunda Plate formed thevolcanic 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 formedSumatra,Java,Bali, and theLesser Sunda Islands, most of which, particularly Java and Bali, emerged within the last 2–3 million years. The Pacific and Sahul plate movements controlled the tectonics of the eastern portion of Indonesia.
Subduction occurs along Southeastern Sumatra and West Java. Sumatra is more active in recent years despite being in the same subduction margin. The reason for the lack of frequent seismic activity over West Java is a problem of time frame and not of tectonic activity. While it may only take a hundred years for a large-scale earthquake to occur off the coast of Sumatra, it may take roughly 500 years off the coast of western Java.[1]
Off the coast of western Java there is potentially a backthrust fault. The existence of the backthrust could increase the height of tsunamis off the coast. There are two megathrust segments, one off of Southeastern Sumatra and the other off of Western Java. These segments contribute to tsunamis that average at 11 m (36 ft) in size but that can reach 34 m (112 ft). The combination of megathrusts and backthrusts are reasons for such massive tsunamis.[1]
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 dextralstrike-slip fault alongSumatra Island (about 1,900 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 acrossMakassar Strait, and ends in theMangkalihat 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 afterSorong City. It has east-west orientation and extends from the northern part of West Papua to East Sulawesi for about 2000 km.
The various formations in theIndonesian Archipelago andNew Guinea'sTorres Strait form geologicchoke points toshipping andaqueous transport, particularly at depths below 100m (where said aqueous transport is largely closed); such routes, whether closed or still extant, collectively comprise theIndonesian Seaway between thePacific andIndian oceans.Tectonic uplift of the region, exacerbated by lowered sea levels duringrecent glaciation, is hypothesized to have altered the regional climate, especially with respect tomonsoons, beginning ~4 million years ago.[2]
Thestratigraphy of the western part of Indonesia is dominated by Cenozoic age formations, ranging fromPaleogene toQuaternary. MinorMesozoic andPaleozoic formations were found in places.Devonianlimestones were found in Telen River,East Kalimantan, as fragments withinPaleogeneclastic sediments.
Eastern Indonesia has generally older stratigraphy compared to the western part. The stratigraphy ranges fromPermian toTertiary.Ichthyosaur fossils were found in themud volcanoes inKai Island, indicating Mesozoic deposition in the sub-surface (Charlton, 1992). Mesozoic macrofossils were studied in Misool Island by Fauzie Hasibuan (1996).
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