ISLAND ARC AND TRENCH
Island arcs and trenches are major structural features, together with oceanic ridges, of ocean basins. As the name implies, island arcs are typically a curving chain of volcanic islands occurring around the margin of ocean basins. The curvature and the volcanic nature are important characteristic features. 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.
Arcs and trenches are hundreds of miles long. Some well-known examples of island arcs are Japan, Aleutian Islands of Alaska, Mariana Islands, all of which are in the Pacific, and the Lesser Antilles in the Caribbean. The abundance of volcanic rocks around the Pacific Ocean has led to the designation of the Pacific margin as a “Ring of Fire”. Most of the world’s active volcanoes are in this belt.
Not all volcanic chains are island arcs, however, and not all island arcs are “islands”. For example, the Hawaiian Islands are an example of a linear chain of volcanoes in the middle of the Pacific Ocean that is not an island arc. Some ‘island" arcs have become parts of the continent; the Cascade Range of Washington and Oregon (remember Mt. St. Helens) and the Andes Mountains of South America are examples of “island” arcs that are part of continents.
The rock type of island arcs is typically andesite (named after the Andes Mountains), 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. Trenches are the sites where old oceanic crust plunges into the mantle to be melted and destroyed. This process is called subduction. The oceanic crust originally formed at the oceanic ridge from molten mantle material. In returning to the mantle at a subduction zone oceanic crust completes the cycle from mantle to ocean crust and back to the mantle.