Describe the processes at transform (conservative) plate boundaries and why earthquakes occur there

What this dot point is asking

SEAB wants you to describe what happens at a transform (also called conservative) plate boundary, where plates slide past each other, and to explain why this causes earthquakes but generally no volcanoes. The central insight is that here crust is neither built nor destroyed; the plates simply grind past one another, and the friction along the fault is what stores up and then releases the energy of earthquakes.

The answer

The process at a transform boundary

At a transform boundary, two plates slide horizontally past each other along a crack in the crust called a fault. They may move in opposite directions, or in the same direction at different speeds. Crucially:

• No crust is created (unlike a divergent boundary).
• No crust is destroyed (unlike a convergent boundary).

The crust is simply moved sideways, which is why the boundary is also called conservative, the crust is conserved.

Why earthquakes occur

Earthquakes are common at transform boundaries because the plates do not slide smoothly:

1. Friction locks the plates together along the fault as they try to move past each other.
2. Stress builds up in the locked rocks over years or decades as the plates keep trying to move.
3. When the stress finally overcomes the friction, the plates suddenly jerk past each other.
4. The stored energy is released in moments as seismic waves, an earthquake.

Because so much energy can be released suddenly, these earthquakes can be powerful and damaging, especially near the fault in populated areas.

Why there are generally no volcanoes

Volcanoes need a source of magma reaching the surface. At a transform boundary:

• No plate sinks and melts (there is no subduction), so no magma is generated that way.
• No gap opens for magma to rise (unlike a divergent boundary).

With no source of magma at the surface, transform boundaries generally have no volcanic activity, only earthquakes.

Examples in context

Example 1. The San Andreas Fault, California. The San Andreas Fault marks where the Pacific Plate and the North American Plate slide past each other in opposite directions. Cities such as San Francisco and Los Angeles lie near it and face the threat of major earthquakes when the locked fault suddenly slips, as in the great 1906 San Francisco earthquake. There is little volcanic activity along the fault, making it the classic example of a transform boundary and its earthquake hazard.

Example 2. The North Anatolian Fault, Turkey. The North Anatolian Fault is a transform boundary where the Anatolian Plate slides westward past the Eurasian Plate. It has produced a series of powerful, destructive earthquakes across northern Turkey, including near densely populated areas. Like the San Andreas, it shows how plates grinding sideways past each other store and release enormous energy as earthquakes, with no accompanying volcanoes.

Try this

Q1. Explain why a transform boundary is described as conservative. [2 marks]

• Cue. The plates only slide horizontally past each other, so no crust is created and none is destroyed; the crust is conserved (kept the same), unlike at constructive or destructive boundaries.

Q2. Explain why earthquakes occur at transform boundaries. [3 marks]

• Cue. Friction locks the plates together as they try to slide past, so stress builds up over time; when the stress finally overcomes the friction, the plates suddenly jerk past each other, releasing the stored energy as seismic waves in an earthquake.

Q3. Explain why volcanoes are generally absent at transform boundaries. [2 marks]

• Cue. No plate subducts and melts to form magma, and no gap opens for magma to rise, so there is no source of magma reaching the surface and therefore no volcanic activity, only earthquakes.