The geography of Indonesia and the surrounding area shows
tectonic boundaries and present volcanic activity. Active volcanoes along
archipelago arcs are the result of a series of plate collision activities that
form the ring of fire. The red arrows show the vector of subduction plates in
some area.
Sumatra Cross Section Schematic (Mobile Oil Corp)
Sumatra arc cross section - trench system is formed by
subduction of the oceanic plate (Indian-Australian) under the continental plate
(Eurasian). The
tectonic system formed is the oblique subduction zone, the Mentawai fault zone,
and the large Sumatra fault zone which divides the island of Sumatra from the
Semangko bay to Banda Aceh. Continental plates are thick and old consisting of volcanic
arcs with Permian, Cretaceous, and Tertiary ages. Magmatic rocks form above the
Benioff zone which is mostly marked by silicates and intermediates. The
movement of Indian and Australian plates caused the two plates to collide
resulting in forearch island and magmatic arc island such as the Mentawai
islands.
Java Cross Section Schematic (Mobile Oil Corp.)
In
the Java cross section demonstrating that this arc was formed due to subduction
of the oceanic plate beneath the continental plate. The continental crust on
Java is thinner and has a relatively young age consisting of plutonic volcanic
arcs. Magmatic rocks are mostly intermediates type. On the Javanese plateau
there are two plate systems that is South China Sea Plate (Eurasia) in the
north and the Indian Ocean Plate in the South. The
Eurasian Plate has been moving southeast since the oligocene while the Indian
Ocean Plate is moving and dipping down the arc systems of the Java and Sumatra
island.
The process of
forming volcanic arcs (Katili,
1974)
The
oblique extension around the subduction zone that is show the mechanical field effects
of the tectonic plate. Collisions between oceanic crust sinks beneath the
continental crust which causes melting of rock that moves to the surface
through fractures becomes magmatic arc. On the Java and Sumatra islands the
formation of volcanoes occurred due to the collisions of the Indian Ocean crust
with the Eurasian Continent crust. Accretion comes to the surface and becomes an island due to
stronger subduction. Beside
the formation of volcanoes, tectonic ctivity and geological conditions
(volcanology, magmatism, geological structures, etc.) are the main factors as
geothermal and porphyry deposits.
Reference
[1] Hall, Robert. 2009. Indonesia
Island. Universitas London
[2] Katili, A. John. 1974. Geological environment of the Indonesian
mineral deposits:a plate tectonic approach. Geological Survey of Indonesia.
[3] Katili, A. John. 1974. Volcanism and Plate Tectonics
in The Indonesian Island Arc. Elsevier Scientific Publisher Company.
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