Why Do Volcanoes Form Along Edge of Tectonic Plates?

Plate Interaction

• The surface of the planet is covered in a thin layer of crust. This crust has fractured into large pieces, called tectonic plates. The plates move atop the molten magma of the earth's mantle. This movement causes the plates to interact with one another. In some areas, plates spread apart, creating a divergent boundary. In other areas, plates slide past one another, forming a transform boundary. At other times, plates collide into each other, creating convergent boundaries. It is these convergent boundaries that are ultimately responsible for much of the world's volcanic activity.

Subduction Zones

• When tectonic plates collide, the denser of the two will be forced under the more buoyant plate. This occurs when two oceanic plates collide. The older plate will have had more time to cool, causing its density to rise. The older oceanic plate will be plunged beneath the newer ocean plate. This also occurs when an oceanic plate collides with a continental plate. Oceanic plates are always denser and, therefore, will always subduct under continental plates. These two scenarios result in what is known as a subduction zone. These zones are directly responsible for the creation of volcanoes.

Rising Magma

• When a tectonic plate is forced under another plate, it begins a downward trajectory. This path pushes the plate into the asthenosphere. This layer represents the upper part of the Earth's mantle. It consists of molten magma, with both a high temperatures and pressure. The rising temperatures cause the release of water trapped in the plate. This water lowers the melting point of the overlying material, causing it to melt into magma. The magma rises into the overlying plate, forming magma chambers. As pressure in the chambers increase, the magma is forced to the surface where it erupts from volcanoes. This creates a volcanic arc on the buoyant plate that mirrors the convergent boundary between the plates.

Volcanic Examples

• The Andes volcanic arc in South America is an example of a subduction-induced volcanism. The arc mirrors the Peru-Chili Trench that marks the boundary where the Nazca plate is subducting beneath the South American plate. The Northern Mariana Islands are an example of a volcanic island chain created by a subduction zone. Here, the volcanic arc mirrors the Mariana Trench that marks the boundary where the Pacific plate is subducting beneath the Philippine plate. This convergent boundary is the most volcanically active on the planet. Both of these volcanic arcs lie on what is known as the Pacific Ring of Fire.