its include what is an island arc what is the geology of an island arc and how it is formed

1. GEOLOGY OF ISLAND ARC
BY : IDREES QURESHI
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2. 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

4. How island arc is formed ?
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▪ 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.

5. HOW ISLANDARCWERE FORMED
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6. Ocean-ocean  IslandArc (IA)
Ocean-continent  ContinentalArc or
Active 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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7. Structure Of Island Arc
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8. Geology Of Island Arc
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Island arcs are usually accompanied by
rapid erosion and sedimentation into
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
MarianasTrench, bounds an island arc on the
oceanic side beyond the fore-arc basin.

9. Volcanic Rocks of Island Arcs
Table 16-1. Relative Proportions of Quaternary Volcanic
Locality B B-A A D R
Talasea, Papua 9 23 55 9 4
Little Sitkin, Aleutians 0 78 4 18 0
Mt. Misery, Antilles (lavas) 17 22 49 12 0
Ave. Antilles 17 42 39 2
Ave. Japan (lava, ash falls) 14 85 2 0
After Gill (1981, Table 4.4) B = basalt B-A = basaltic andesite
A = 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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10. KOHISTAN ISLAND ARC
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