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

Simply put, 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. 

Transform boundaries are one of three different ways the plates interact with each other, known as plate boundaries or zones. And while they move differently than convergent (plates colliding) or divergent (plates splitting apart) boundaries, they are almost always connected to one or the other.


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. Tuzo Wilson, initially skeptical of plate tectonics, was also the first to propose the theory of hotspot  volcanoes. 

Facilitating 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 (see the "String Cheese and Moving Rifts" section of the Divergent Plate Boundaries article for a deeper look). As the plates in this space continue to diverge, they now do so in opposite directions.

This lateral movement forms active transform boundaries.

Between the spreading segments, the sides of the transform are rubbing 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 a staircase. 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 called transpression and transtension respectively. These extra forces are why coastal California, basically a transform tectonic regime, also has many mountainous welts and downdropped valleys. Movements across the fault are up to 10 percent as much as the pure transform motion.

The San Andreas fault of California is a prime example of this; 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 southernmost South America.

Because of the thickness of the continental lithosphere and its variety of rocks, transforms on continents are not simple cracks but wide zones of deformation. The San Andreas fault, itself, is just one thread in a 100-kilometer-wide skein of faults making up the San Andreas fault zone. The dangerous Hayward fault takes up a share of the total transform motion, for instance, 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. 

Edited by Brooks Mitchell