What do you see when oceanic lithosphere subducts?
At a convergent plate boundary when one plate is oceanic, there are large volcanoes. These volcanoes are found in lines that outline the subduction zone. Earthquakes also happen in these zones. The Aleutian Islands that border southern Alaska are an island arc. In this winter image from space, the volcanoes are covered with snow.
Convergent Plate Boundaries
A convergent plate boundary forms where two plates collide. That collision can happen between a continent and oceanic crust, between two oceanic plates, or between two continents. Oceanic crust is always destroyed in these collisions.
Oceanic crust may collide with a continent. The oceanic plate is denser, so it undergoes subduction. This means that the oceanic plate sinks beneath the continent. This occurs at an ocean trench (Figure below). Subduction zones are where subduction takes place.
Subduction of an oceanic plate beneath a continental plate forms a line of volcanoes known as a continental arc and causes earthquakes.
As you would expect, where plates collide there are lots of intense earthquakes and volcanic eruptions. The subducting oceanic plate melts as it reenters the mantle. The magma rises and erupts. This creates a volcanic mountain range near the coast of the continent. This range is called a continental arc. The Andes Mountains, along the western edge of South America, are a volcanic arc (Figure below).
A relief map of South America shows the trench west of the continent. The Andes Mountains line the western edge of South America.
Two oceanic plates may collide. In this case, the older plate is denser. This plate subducts beneath the younger plate. As the subducting plate is pushed deeper into the mantle, it melts. The magma this creates rises and erupts. This forms a line of volcanoes, known as an island arc (Figure below). Japan, Indonesia, the Philippine Islands, and the Aleutian Islands of Alaska are examples of island arcs (Figure below).
A convergent plate boundary subduction zone between two plates of oceanic lithosphere. Melting of the subducting plate causes volcanic activity and earthquakes.
The country of Japan is an island arc that lies at the intersection of the North American, Filipino, and Eurasian plates. Colors in this picture indicate elevation.
continental arc: A line of volcanoes on a continental plate. The volcanoes are on a continent above a subducting oceanic plate.
convergent plate boundary: A location where two lithospheric plates come together.
island arc: A line of volcanoes on an oceanic plate. The volcanoes are above a subducting oceanic plate and near a deep sea trench.
subduction: The sinking of one lithospheric plate beneath another.
subduction zone: The area where two lithospheric plates come together and one sinks beneath the other.
- When two plates come towards each other they create a convergent plate boundary.
- If at least one plate is oceanic, there will be subduction.
- Subduction of a plate leads to melting and volcanism.
- An island arc is a line of volcanoes on an oceanic plate. A continental arc is a line of volcanoes on a continental plate.
Use these resources to answer the questions that follow.
- Describe a subduction zone.
- What forms this subduction zone?
- How far does the subducting oceanic plate descend?
- What is formed on the continental plate?
- Where can an example of this plate boundary be found?
[PHOTO PEOPLE: PUT IN THIS VIDEO: http://earthguide.ucsd.edu/eoc/teachers/t_tectonics/p_subduction.html]
- Why do arc volcanoes appear in a line?
- Why are there volcanoes near subduction zones?
- What is the direction of plate motion at a convergent plate boundary?
- What creates an island arc?
- How is a continental arc different from an island arc?