Geology of Island arc

Geology of island arc

1. GEOLOGY OF ISLAND ARC BY : IDREES QURESHI made by : idrees qureshi 1

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. made by : idrees qureshi 2

3. made by : idrees qureshi 3 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 ? made by : idrees qureshi 4 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 ISLAND ARC WERE FORMED made by : idrees qureshi 5

6. Ocean-ocean  Island Arc (IA) Ocean-continent  Continental Arc 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 6

7. Structure Of Island Arc made by : idrees qureshi 7

8. Geology Of Island Arc made by : idrees qureshi 8 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 Marianas Trench, bounds an island arc on the oceanic side beyond the fore-arc basin.

9. Volcanic Rocks of Island Arcs Complex tectonic situation and broad spectrum High proportion of basaltic andesite and andesite Most andesites occur in subduction zone settings made by : idrees qureshi 9

10. KOHISTAN ISLAND ARC made by : idrees qureshi 10