An island arc is a unique type of island chain, mostly made up of clusters of volcanoes through an arc-shaped arrangement, situated close and parallel to the boundary between two converging tectonic plates.
All chains of volcanoes are not island arcs, and all island arcs are not islands. The Hawaiian Islands have a chain of linear volcanoes at the center of the Pacific Ocean which is not island arc. Various island arcs including Andes Mountains and Mt St Helens have ended up joining the continent. Andes Mountains are volcanic arcs but not an island; therefore, they are not classified as island arc. The Hellenic or Aegean Arc Island in the Mediterranean region has numerous islands like Crete but are not volcanic.
Island arcs form when one tectonic plate subducts another plate resulting in the production of magma directly below the top oceanic tectonic plate. The subduction process occurs at the boundaries of the oceanic tectonic plates when one plate sinks under the other right into the mantle. However, this happens on island arcs which are part of mountain belts, referred to as volcanic arcs. South Aegean Arc and Aleutian Island located in Alaska are volcanic islands.
What Is Subduction?
Subduction is a geological process which occurs on the converging margins of the tectonic plates; one plate sinks under another plate into the mantle of the Earth. Subduction occurs in specific regions referred to as the subduction zones. Subduction zones are the regions of convective down-swelling of the lithosphere. This zone exists at the convergent boundary plates where one oceanic lithosphere plate converges onto another plate. The descending plate is usually over-ridden by the edge of the leading plate. The descending slab sinks at an angle of between 25 to 45 degrees to the surface of the Earth.
The temperature difference between the surrounding mantle asthenosphere and the sinking oceanic lithosphere motivates this process, plus the oceanic lithosphere is denser than the asthenosphere. At about 120km depth the basalt on the oceanic crust is converted to eclogite which increases the density of the crust thus providing an additional negative downward force. The oceanic crust, lithosphere trapped water, and sedimentary layers turn into deep mantle on the subduction zones. Subduction is the primary dynamic force behind the plate tectonics; without this energy, plate tectonics will never form.
What Are Plate Tectonics?
Plate tectonics is a scientific hypothesis describing the movement of numerous small plates and the large-scale motion of the seven massive plates of the lithosphere. Tectonic processes on the Earth crust started over three billion years ago. The lithosphere has 7-8 major tectonic plates and numerous minor plates. When these plates meet, their motion determines the type of boundary formation that will occur along the meeting plates. There are three boundary types on Earth which include convergent boundaries, divergent boundaries, and transform boundaries. The convergent boundaries form the island arcs.
What Is a Convergent Boundary?
A convergent boundary takes place where two tectonic plates slide towards each other, and this result in the formation of either a continental collision or a subduction. At the regions of continent-to-oceanic subduction, the denser lithosphere sinks beneath the lighter plate. Earthquakes follow the course of subducting plate as it falls into the asthenosphere, which creates a trench, and as the subducted plate is heated it releases water with hydrous minerals into the mantle. The water reduces the melting point of the material mantle above the subducted plate causing it to melt; this results in volcanism.
At the ocean-to-ocean subduction zone, the older dense and colder crust slips beneath the lighter crust. The subduction process creates the arc-shaped deep trench. The top mantle of the sunken crust heats up, and magma rises resulting in the creation of a curving volcanic island chain. The basin which develops along the converging boundary is referred to as foreland basin while the subduction zones form deep trenches. Closure of the oceanic basins occurs at the continent-to-continent boundaries collision between the granite continental lithosphere, but neither plate subdues, and the edges tend to fold up, compress, and then push upwards.
In the subduction region, the sinking slab promotes partial melting of the overriding mantle a process referred to as flux melting. Flux melting generates a less dense calc-magma which buoyantly rises and passes through the lithosphere of the over-riding plate. The created volcano chain is arc-shaped and parallel to the boundary of the convergent plate and convex towards the subducting plate.
The narrow, deep oceanic trench, on the sinking part of the island, is the trace of the boundary on the surface of the earth between the overriding and the down-going plates. The gravitational pull of the denser down-going plate pulls the edge of the overriding plate downwards forming the oceanic trench. Numerous earthquakes occur along this boundary, plus the seismic hypocenter is at an ever-increasing depth beneath the island arc. The ocean basins which are reduced by this process are referred to as remnant oceans since they tend to shrink slowly out of existence before being crushed into the orogenic collision.
Examples of Island Arcs
Located along the Pacific, in the United States, the only real island arc is the Aleutian Islands. The overriding plate is the North American plate while the down-riding plate is the Pacific plate.
Japan is the home to two of the most significant island arcs on the earth, the Ryukyu Islands and the Japanese Archipelago. The overriding plate of the Ryukyu Islands is the Eurasian plate, and the down-riding plate is the Philippine Sea plate. The top plates of the Japanese Archipelago are the Eurasian and North American plates, while the down-riding plates are the Philippine Sea plate and the Pacific plate. Other smaller island arcs in Japan include the Bonin Island and the Izu islands.
Greece has two large island arcs, the South Aegean-volcanic arc and the Hellenic arcs which formed along the Aegean Sea. The overriding plate for these two islands is the Hellenic plate, and the down-riding plate is the African plate.
One of the oldest island arcs on Earth is the insular islands arc where giant chains of volcanic islands formed in the Pacific Ocean during the Cretaceous era.