Island arcs are abundant in the Pacific Ocean and make up a part of the famous Ring of Fire. These islands are volcanoes that have formed on the ocean floor and are accompanied by deep trenches.
Island Arcs and the Ring of Fire
If someone were to ask you what the Ring of Fire was, you'd be forgiven for thinking it was the title of a fantasy novel or a line from a Johnny Cash song. But the evocative term actually refers to a real, physical place. The Ring of Fire is an island arc, or a chain of islands that form as the result of volcanic activity when one tectonic plate slides underneath another. It is located in a region roughly encircling the Pacific Ocean, where there is a lot of tectonic activity that causes numerous volcanoes, earthquakes, and island arcs. Most of the world's island arcs are in this region, like for example, the Aleutian Islands of Alaska.
Tectonic Plates and Subduction
To understand how island arcs form, we first need to understand tectonic plates. The earth's crust is divided into a number of these plates, which move relative to each other because of the motion of the fluid rock, called magma, of the mantle underneath the crust.
The upper mantle has two layers. The 'top-most layer is similar to the crust and together with the crust is called the lithosphere. Below that is the asthenosphere, which is made of magma. It is circular convection cycles in the magma of the asthenosphere that cause the tectonic plates to move. When one tectonic plate meets another and sinks underneath it, we call the phenomenon subduction. There are many subduction zones in the Ring of Fire, and it is in these zones that island arcs can form.
Subduction occurs when oceanic lithosphere meets continental lithosphere. The lithosphere under the oceans is denser and heavier than that under the continent. When the two run into each other, the oceanic lithosphere, therefore, sinks under the continent. When two oceanic plates meet, one will sink under the other. The oceanic lithosphere melts into the asthenosphere and turns into magma. It's like a recycling of the rocks that make up the crust and lithosphere.
Subduction and the Formation of Island Arcs
So what does this have to do with an island arc? When the oceanic rock sinks under continental lithosphere and melts into the magma of the asthenosphere, some of it may leak into the crust and bubble up to the surface.
When magma bubbles up to the surface of the earth's crust, we get volcanoes. As the volcanoes forming at a subduction zone erupt, they build up rock at the surface. Over time enough builds up to create a volcanic island that rises above the surface of the ocean. Because a subduction zone can create several volcanoes in a row, these type of islands tend to form in chains or clusters, which we call island arcs.
Another interesting formation that occurs along subduction zones in the ocean is a trench. Where one plate sinks under another, there is a deep trench. This means that all island arcs have a trench running alongside them. These trenches are the deepest parts of the ocean. The Mariana Trench, along the Mariana Islands Arc, is the deepest place on the planet. At its deepest, the trench reaches down nearly seven miles. In 2012, director and explorer James Cameron broke a record by becoming the first person to make a solo dive to this deepest part of the ocean.
Examples of Island Arcs
Most of the island arcs in the world have formed within the Ring of Fire, where there are numerous subduction zones under the Pacific Ocean. Japan's many small, southern islands together compromise an island arc. Other examples in the Pacific include the Philippine Islands, the Solomon Islands, and the Kuril Islands of Russia. Island arcs in other parts of the world include the Aegean Islands of Greece and the Antilles in the Caribbean.
An island arc is a chain or group of islands that forms from volcanic activity along a subduction zone. Subduction occurs when oceanic lithosphere sinks underneath continental or oceanic lithosphere. The sinking rock melts into the magma in the asthenosphere and some comes to the surface, forming volcanoes. When under the ocean, these volcanoes build up into islands over time. Next to each island arc is a deep trench. Most island arcs are part of the Ring of Fire surrounding the Pacific Ocean, but they can also be found in the Aegean and Caribbean Seas.