geology of island arc

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GEOLOGY OF ISLAND ARC

BY : IDREES QURESHI

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WHAT IS ISLAND ARC ?

▪A curved chain of volcanic islands located at a tectonic plate margin, typically with a deep ocean trench on the convex side.

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Igneous activity is related to convergent plate situations that result in the subduction of one plate beneath another The initial petrologic model:

• Oceanic crust is partially melted• Melts rise through the overriding plate to

form volcanoes just behind the leading plate edge

• Unlimited supply of oceanic crust to melt


How island arc is formed ?▪ An island arc is a curving series of volcanic islands that are created through the collision of tectonic plates in an

ocean setting.

▪ The particular type plate boundary that yields island arcs is called a subduction zone .

▪ In a subduction zone, one lithospheric (crustal) plate is forced downward under an upper plate

▪ . Continual tectonic movement pushes the lower plate deeper until it reaches a depth where temperatures are sufficient to begin to melt the subducted plate and form magmas.

▪ These magmas then rise through fractures and melt their way through the overlying crust to be extruded in the form of volcanoes.

▪ The volcanoes are generally andesitic in composition.

▪ If the overriding plate is oceanic, then volcanoes are extruded underwater and may eventually rise high enough to become islands.

▪ The volcanoes form in a line because the angle and rate of subduction, and hence the distance to the depth where melting occurs is consistent.

▪ Because the surface of Earth is curved, the line of volcanoes forms in an arcuate pattern in much the same manner as an arc is produced when a planar surface intersects a sphere.


HOW ISLAND ARC WERE FORMED

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Ocean-ocean Island Arc (IA)Ocean-continent Continental Arc orActive Continental Margin (ACM)

Principal subduction zones associated with orogenic volcanism and plutonism. Triangles are on the overriding plate. PBS = Papuan-Bismarck-Solomon-New Hebrides arc. After Wilson (1989) Igneous Petrogenesis, Allen Unwin/Kluwer

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Structure Of Island Arc

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Geology Of Island ArcIsland arcs are usually accompanied by rapid erosion and sedimentation int

o accompanying basins. A back-arc basin occurs on the side of the

overriding plate and a fore-arc basin forms toward the subducted plate side. Typically, a deep oceanic trench, such

as the Marianas Trench, bounds an island arc on the oceanic side beyond

the fore-arc basin.


Volcanic Rocks of Island Arcs

Table 16-1. Relative Proportions of Quaternary Volcanic

Locality B B-A A D RTalasea, Papua 9 23 55 9 4Little Sitkin, Aleutians 0 78 4 18 0Mt. Misery, Antilles (lavas) 17 22 49 12 0Ave. Antilles 17 42 39 2Ave. Japan (lava, ash falls) 14 85 2 0After Gill (1981, Table 4.4) B = basalt B-A = basaltic andesiteA = andesite, D = dacite, R = rhyolite

Island Arc Rock Types▪ Complex tectonic

situation and broad spectrum

▪ High proportion of basaltic andesite and andesite▪ Most andesites

occur in subduction zone settings


KOHISTAN ISLAND ARC

geology of island arc

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