Transform Boundary
A transform boundary is a place where two tectonic plates slide horizontally past each other, producing earthquakes but no volcanoes.
Definition
A transform boundary is where two plates slide past each other sideways, like two cars scraping side by side; crust is neither made nor destroyed there, but the grinding causes earthquakes. Plates move horizontally past one another along a strike-slip fault, so lithosphere is conserved. Most transform faults are short segments connecting offset pieces of mid-ocean ridge, but a few cut through continents, and they generate frequent, shallow earthquakes and little or no volcanism. Recognized by J. Tuzo Wilson in 1965, transform faults are plate boundaries along which relative motion is pure strike-slip, parallel to the fault trace. Between two offset ridge segments the sense of slip is opposite to the apparent offset, a prediction confirmed by earthquake focal mechanisms that helped establish plate tectonics, and beyond the ridge segments the trace continues as an inactive fracture zone. Continental transforms accumulate elastic strain that is released in large earthquakes, and bends and step-overs produce local compression (transpression) or extension (transtension).
Example
The San Andreas Fault in California is a transform boundary, and Los Angeles is sliding slowly northwest past San Francisco. The fault separates the Pacific Plate from the North American Plate, with relative motion of about $5\ \text{cm/yr}$, and the Alpine Fault in New Zealand and the North Anatolian Fault in Turkey are other continental transforms. The 1906 San Francisco earthquake (magnitude about $7.9$) produced up to $6$ meters of horizontal slip along $477\ \text{km}$ of the San Andreas. The Salton Sea and Death Valley occupy transtensional pull-apart basins, and the San Gabriel Mountains rise at a transpressional bend.
Key Insight
Plates at a transform boundary do not slide smoothly: they stick, build up stress for years, then jerk loose all at once, and that jerk is an earthquake. Transform faults explain why mid-ocean ridges look like a staircase on a map, since the ridge is offset in segments and transform faults link the steps. Elastic rebound, the model developed from the 1906 rupture, is the foundation of earthquake science. Rock on either side of a locked fault bends elastically for decades, then snaps back to its unstrained shape in seconds.