Slab Pull: Definition & Theory

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  • 0:00 The Basics of Plate Tectonics
  • 1:15 What is Slab Pull?
  • 2:50 Consequences
  • 4:25 Lesson Summary
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Lesson Transcript
Instructor: Amy Lange

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

This lesson covers slab pull, which is one of the forces that affect tectonic plate motion. After this lesson, you will understand the mechanics of slab pull and how it factors into theory of plate tectonics.

The Basics of Plate Tectonics

As inhabitants of Earth, we live on the outmost layer of our planet, also known as the crust. This crust is a thin, rigid layer that sits on top of a partially molten portion of the earth's mantle, which makes up the bulk of the earth's interior. The crust is broken into several pieces, called tectonic plates, in order for the rigid layer to cover the surface of the earth. Imagine trying to wrap a dinner plate around a basketball. You'd have to break the plate into numerous pieces to be able to cover the surface of the sphere -- those pieces are similar to tectonic plates.

The mantle is continuously churning due to convection within the earth. Convection cells form where hot mantle rock rises from near the core due to its lowered density. As the hot rock rises, it is removed from the heat source and cools down, which causes an increase in density. Eventually the rock becomes cool enough to begin sinking back to the core, where the process starts all over.

However, the plates are not passive participants riding along the top of a convecting mantle. Geologists now believe that the tectonic plates are an active driver of the movement of plates and the churning mantle. This lesson examines one of the forces that affect plate motion.

What is Slab Pull?

Slab pull is the pulling force exerted by a cold, dense oceanic plate plunging into the mantle due to its own weight. The theory is that because the oceanic plate is denser than the hotter mantle beneath it, this contrast in density causes the plate to sink into the mantle. The process of a tectonic plate descending into the mantle is termed subduction.

Slab pull occurs when an oceanic plate subducts into the underlying mantle.
Illustration showing slab pull at plate boundary

Subduction zones are just one type of convergent boundary where two tectonic plates are colliding. Convergent boundaries can occur between two oceanic plates, such as in the south Pacific, and between an oceanic and a continental plate, such as on the northwest coast of the United States.

Researchers believe that slab pull is a major driving force in the overall movement of plates. They noted that plates which have significant amounts of subduction around their boundaries move at faster velocities than plates that experience less subjection. For example, the Pacific plate, which experiences subduction around approximately 40% of its boundaries, has a high plate velocity of greater than 5 cm/yr. While the North American plate, which experiences only very minimal subduction, has a much lower velocity at around 1 cm/yr.

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