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Accretionary Wedge: Definition & Formation

Instructor: Amy Lange

Amy has taught university-level earth science courses and has a PhD in Geology.

Where two tectonic plates collide and one dives below the other, you'll always find accretionary wedges. Read this lesson to learn how these deposits form and what they are made of.

Accretionary Wedge and Plate Tectonics

If you live in a climate that gets cold in the winter, you've likely had to shovel snow off the sidewalk. To do this, you have to take a shovel and scrape the snow back in a straight line, revealing the concrete below. (Then you repeat until the sidewalk's clear or your arms are too sore to move, whichever comes first.)

This motion of moving snow with a shovel is similar to what happens when one tectonic plate moves beneath another, in a process called subduction. And the snow you're scraping up into a pile? It's actually very similar to an accretionary wedge.

Scraping snow off your sidewalk is very similar to how accretionary wedges are formed.
shoveling snow

Let's first review plate tectonics and subduction before talking about accretionary wedges. According to plate tectonics, the earth's crust is composed of several rigid plates that move around on the surface of the earth on top of a semi-molten mantle. As these plates move, they can either collide, diverge, or run laterally past one another. When they collide, either one plate will dive below the other or they will crumple up to form a very large mountain range, like the Himalayas.

Subduction zones occur when two tectonic plates collide and the more dense plate dives below the other into the earth. Although we cannot look into the earth, we can find subduction zones because they will have volcanoes on the overriding plate. For instance, the volcanoes in Oregon and Washington in the Cascade Range are due to the subduction off the northwestern coast of America.

Subduction zone showing one plate diving beneath another.
subduction zone cartoon

At a subduction zone, as the lower plate dives into the earth, it scrapes along the bottom of the overriding plate. This action removes material, consisting of rock and sediment, which will accumulate onto the overriding plate in an accretionary wedge. So, accretionary wedges are the accumulation of rock removed from the down-going plate in a subduction zone.

This visualization of a subduction zone shows one plate diving beneath another and an accretionary wedge at the front of the overriding plate.
accretionary wedge

What Does an Accretionary Wedge Look Like?

Going back to our analogy of shoveling snow, if you look down at the snow you are scraping off the concrete, it will look like an un-orderly jumbled mess of snow sitting on top of your shovel. Accretionary wedges look much the same way. They have jumbled deposits of original rock layers and metamorphic rock. Now, unlike our snow shovel example, the forces associated with subduction zones are quite large. So as the rock is being scraped off of the down-going plate and deposited on the overriding plate, they are subjected to a huge amount of pressure that will actually cause the original igneous and sedimentary rocks to become metamorphic rocks.

Metamorphic rocks folded by the intense pressure that they experienced during formation.
folded metamorphic rock

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