5.13: Transform Plate Boundaries

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What does a transform plate boundary look like?

In the dry part of central California, it looks like this. On one side is the Pacific Plate. On the other side is the North American Plate. Before plate tectonics theory, people thought the San Andreas was just a fault. Now it's known to be a plate boundary!

Transform Plate Boundaries

Two plates may slide past each other in opposite directions. This is called a transform plate boundary. The plates meet at a transform fault. As you might imagine, plates do not slide past each other easily. These plate boundaries experience massive earthquakes. The world’s best known transform fault is the San Andreas Fault in California (Figure below). At this fault, the Pacific and North American plates grind past each other. Transform plate boundaries are common as offsets along mid-ocean ridges. They are very small compared to transform faults on land.

Transform plate boundaries are different from the other two types of plate boundaries. At divergent plate boundaries, new oceanic crust is formed. At convergent boundaries, old oceanic crust is destroyed. But at transform plate boundaries, crust is neither created nor destroyed.

Map of the San Andreas Fault

The red line is the San Andreas Fault. On the left is the Pacific Plate, which is moving northeast. On the right is the North American Plate, which is moving southwest. The movement of the plates is relative to each other.

Summary

At transform plate boundaries, two plates move in opposite direction.
Transform faults are the site of massive earthquakes.
The San Andreas Fault is the boundary between the Pacific and North American plates. It is the site of massive earthquakes.

Review

What is the direction of plate motion at a transform plate boundary?
Why are transform faults prone to massive earthquakes?
Why are earthquakes at the San Andreas Fault so large?