A volcanic arc is a chain of volcanoes, hundreds to thousands of miles long, that forms above a subduction zone.
An island volcanic arc forms via ocean-ocean subduction or Continent-Ocean subduction
The Example Of Ocean-Ocean subduction is- Philippine Island Arc
The example of Continent-Ocean subduction is-The Cascade Volcanoes.
In both cases, the volcanic arc or island arc is an active landform.
Formation of island Arc-
During the convergence of two plates, the denser plate subducts, and the less dense plate is either up thrust or folded.
In Ocean-Ocean Convergence, a denser oceanic plate subducts below while a less dense oceanic plate forming a trench along the boundary.
As the ocean floor loaded with sediments and subducts into the softer asthenosphere, the rocks in the subduction zone become metamorphosed under high pressure and temperature.
After reaching a depth of upper mantle, the plates melt. Magma has lower density and at high pressure, It rises upwards due to the buoyant force offered by surrounding denser medium.
The magma flows out to the surface. A continuous upward movement of magma for a millions of years creates volcanic arc above the subduction zone
Such volcanic landforms all along the boundary form a chain of volcanic islands which are collectively known as Island Arcs
Indonesian Island Arc, Philippine Island Arc and Japanese Island Arc is the best example of Island Arc
Some facts about Island arc-
The arc is convex toward the ocean and concave toward the continent with a deep trench running parallel to the arc along the convex (ocean) side.
Island-arc may associated with porphyry copper deposits, Other types of deposits found in island-arc include epigenetic mercury-gold veins and syngenetic massive sulfides deposit.
The rock type of island arcs is typically andesite, rather than the basalt of oceanic crust. Andesite forms by partial melting of basaltic crust and oceanic sediments as both are subducted into the trench. Thus, the volcano is composed partly of melted basalt and partly of melted sediments, a combination that has the mineral composition of andesite rock.
The Ring of Fire is an island arc, or a chain of islands that form as the result of volcanic activity
Orogenesis or Mountain building is much later stage of Island arc.
Accretionary Wedge is a zone of deformed sediment made up of thrust slices scraped off a subducting oceanic plate and added onto the over-riding plate.
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