Geology of island arc
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Transcript of 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
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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.
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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.
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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
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KOHISTAN ISLAND ARC