Island Arc: Definition & Formation

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  • 0:01 Island Arcs & the Ring of Fire
  • 0:44 Tectonic Plates & Subduction
  • 2:05 Subduction & the…
  • 3:22 Examples of Island Arcs
  • 3:49 Lesson Summary
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Lesson Transcript
Instructor: Mary Ellen Ellis
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.

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