Plate tectonics

_{The Southern Alps rise dramatically beside the Alpine Fault on New Zealand's West Coast. About 500 kilometres (300 mi) long; northwest at top.}

Transform boundary

In plate tectonics, a transform boundary (also known as transform fault boundary, transform plate boundary, transform plate margin, strike-slip boundary, sliding boundary, or conservative plate boundary) is said to occur when tectonic plates slide and grind against each other along a transform fault. The relative motion of such plates is horizontal in either sinistral or dextral direction.

Most transform boundaries are found on the ocean floor, where they often offset active spreading ridges to form a zigzag plate boundary. However, the most famous transform boundaries are found on land. Many transform boundaries are locked in tension before suddenly releasing, and causing earthquakes.

Because of friction, the plates cannot simply glide past each other. Rather, stress builds up in both plates and when it reaches a level that exceeds the strain threshold of rocks on either side of the fault the accumulated potential energy is released as strain. Strain is both accumulative and/or instantaneous depending on the rheology of the rock; the ductile lower crust and mantle accumulates deformation gradually via shearing whereas the brittle upper crust reacts by fracture, or instantaneous stress release to cause motion along the fault. The ductile surface of the fault can also release instantaneously when the strain rate is too great. The energy released by instantaneous strain release is the cause of earthquakes, a common phenomenon along transform boundaries.

A good example of this type of plate boundary is the San Andreas Fault which is found in the western coast of North America and is one part of a highly complex system of faults in this area. At this location, the Pacific and North American plates move relative to each other such that the Pacific plate is moving northwest with respect to North America. Other examples of transform faults include the Alpine Fault in New Zealand and the North Anatolian Fault in Turkey. Transform faults are also found offsetting the crests of mid-ocean ridges (for example, the Mendocino Fracture Zone offshore northern California).