What Happens at Transform Boundaries?

The San Andreas Fault
An ariel view of the San Andreas Fault, a continental transform fault with right-lateral strike-slip movement. Chris Sattlberger / Cultura Exclusive / Getty Images

Transform boundaries are areas where the Earth's plates move past each other, rubbing along the edges. They are, however, much more complex than that.

There are three types of plate boundaries or zones, each of which features a different type of plate interaction. Transform boundaries are one example. The others are convergent boundaries (where plates collide) and divergent boundaries (where plates split apart).

Each of these three types of plate boundary has its own particular type of fault (or crack) along which motion occurs. Transforms are strike-slip faults. There is no vertical movement—only horizontal.

Convergent boundaries are thrust or reverse faults, and divergent boundaries are normal faults.

As the plates slide across from each other, they neither create land nor destroy it. Because of this, they are sometimes referred to as conservative boundaries or margins. Their relative movement can be described as either dextral (to the right) or sinistral (to the left).

Transform boundaries were first conceived of by Canadian geophysicist John Tuzo Wilson in 1965. Initially skeptical of plate tectonics, Tuzo Wilson was also the first to propose the theory of hotspot  volcanoes.

Seafloor Spreading

Most transform boundaries consist of short faults on the seafloor occurring near mid-ocean ridges. As the plates split apart, they do so at differing speeds, creating space—anywhere from a few to several hundred miles—between spreading margins. As the plates in this space continue to diverge, they do so in opposite directions. This lateral movement forms active transform boundaries.

Between the spreading segments, the sides of the transform boundary rub together; but as soon as the seafloor spreads beyond the overlap, the two sides stop rubbing and travel abreast. The result is a split in the crust, called a fracture zone, that extends across the seafloor far beyond the small transform that created it.

Transform boundaries connect to perpendicular divergent (and sometimes convergent) boundaries on both ends, giving the overall appearance of zig-zags or staircases. This configuration offsets energy from the whole process.

Continental Transform Boundaries

Continental transforms are more complex than their short oceanic counterparts. The forces affecting them include a degree of compression or extension across them, creating dynamics known as transpression and transtension. These extra forces are why coastal California, basically a transform tectonic regime, also has many mountainous welts and down-dropped valleys.

The San Andreas fault of California is a prime example of a continental transform boundary; others are the North Anatolian fault of northern Turkey, the Alpine fault crossing New Zealand, the Dead Sea rift in the Middle East, the Queen Charlotte Islands fault off western Canada, and the Magellanes-Fagnano fault system of South America.

Because of the thickness of the continental lithosphere and its variety of rocks, transform boundaries on continents are not simple cracks but wide zones of deformation. The San Andreas fault itself is just one thread in a 100-kilometer skein of faults making up the San Andreas fault zone. The dangerous Hayward fault also takes up a share of the total transform motion, and the Walker Lane belt, far inland beyond the Sierra Nevada, takes up a small amount too.

Transform Earthquakes

Although they neither create nor destroy land, transform boundaries and strike-slip faults can create deep, shallow earthquakes. These are common at mid-ocean ridges, but they do not normally produce deadly tsunamis because there is no vertical displacement of seafloor.

When these earthquakes occur on land, on the other hand, they can cause large amounts of damage. Notable strike-slip quakes include the 1906 San Francisco, 2010 Haiti, and 2012 Sumatra earthquakes. The 2012 Sumatran quake was particularly powerful; its 8.6 magnitude was the largest ever recorded for a strike-slip fault.