Transform boundaries are found on every major ocean floor and on several continents, from California to New Zealand to Turkey. These are places where two tectonic plates slide horizontally past each other rather than colliding head-on or pulling apart. The most famous example is the San Andreas Fault in California, but dozens of transform boundaries exist across the globe, both on land and deep beneath the ocean.
The San Andreas Fault, California
The San Andreas Fault is the most widely recognized transform boundary on Earth. It marks the contact between the Pacific Plate, moving northwestward, and the North American Plate. The fault runs roughly 800 miles through California, from the Gulf of California in the south to Cape Mendocino in the north, where it merges with the Cascadia subduction zone offshore of Oregon and Washington.
The landscape along the San Andreas tells the story of millions of years of grinding lateral motion. The region around San Francisco Bay features long ridges separated by narrow valleys carved by erosion along individual fault lines. At Point Reyes National Seashore, Tomales Bay and Olema Valley sit directly on the main trace of the fault. During the 1906 San Francisco earthquake, a fence line was offset 16 feet in a single event. A 112-mile creeping section in central California moves slowly and continuously between the zones that ruptured in the great earthquakes of 1857 and 1906.
The Alpine Fault, New Zealand
New Zealand’s South Island is split by the Alpine Fault, a transform boundary where the Australian and Pacific plates grind past each other. In central South Island, this single fault accommodates over 70% of the total motion between the two plates. The Alpine Fault produces major earthquakes (magnitude 7 to 8+) on a remarkably regular cycle of roughly every 250 to 300 years. The last major rupture occurred in 1717, which means the fault is well within its expected window for the next large event. The Southern Alps themselves owe much of their dramatic height to the compression that accompanies the sideways motion along this boundary.
The North Anatolian Fault, Turkey
Northern Turkey sits on one of the most seismically dangerous transform boundaries in the world. The North Anatolian Fault runs across the width of the country, marking the boundary where the small Anatolian Plate slides westward past the Eurasian Plate. The driving force behind this motion is the Arabian Plate pushing northward into Eurasia from the south, essentially squeezing the Anatolian block sideways like a seed being pinched between two fingers.
Over the past century, major earthquakes along this fault have migrated in a broadly westerly direction, progressively rupturing new segments closer to Istanbul. This pattern has made the fault one of the most closely studied in the world.
The Dead Sea Transform, Middle East
The Dead Sea Transform runs from the Gulf of Aqaba northward through the Jordan Valley, the Dead Sea, and into Lebanon and Syria. It separates the Arabian Plate from the African Plate. Like the San Andreas, the two plates slide past each other horizontally. The Dead Sea itself sits in a pull-apart basin, a depression that forms where the fault bends and the two sides stretch slightly apart. The Jordan River Valley, the Sea of Galilee, and the remarkably low elevation of the Dead Sea (the lowest point on any continent) are all products of this transform boundary.
The Chaman Fault, Pakistan and Afghanistan
Less well known but highly active, the Chaman plate boundary in western Pakistan and eastern Afghanistan accommodates the sideways component of motion between the Indian and Eurasian plates at about 30 millimeters per year. The displacement is spread across a broad zone of faulting rather than a single clean line. The main Chaman Fault itself only takes up 12 to 18 millimeters per year of that total motion, with the rest distributed across parallel faults including the Ghazaband and Ornach Nal faults. This region has produced several magnitude 7+ earthquakes, including the devastating 1935 Quetta earthquake (magnitude 7.7) and the 2013 Balochistan earthquake (magnitude 7.7).
The Queen Charlotte Fault, Western Canada
Off the coast of British Columbia and southeastern Alaska, the Queen Charlotte Fault forms a transform boundary between the Pacific and North American plates. It is essentially the northward continuation of the same plate relationship that creates the San Andreas system farther south, though the geometry changes along the way. The Queen Charlotte Fault produced a magnitude 8.1 earthquake in 1949, one of the largest strike-slip earthquakes ever recorded in North America. The boundary here is slightly compressive, meaning the plates push together as well as slide past each other.
Oceanic Transform Faults
The majority of transform boundaries on Earth are actually on the ocean floor, not on land. Mid-ocean ridges, the long volcanic seams where new seafloor is created, are not continuous lines. They are broken into segments that are offset from each other, and the short faults connecting those offset segments are oceanic transform faults. There are hundreds of them worldwide.
Some well-documented examples include the Clipperton Transform Fault on the East Pacific Rise, where the seafloor spreads at about 103 millimeters per year, and the Marathon Transform Fault on the Mid-Atlantic Ridge, where spreading is much slower at roughly 25 millimeters per year. Others include the Vema, Chain, Kane, and Gofar transform faults. The speed of spreading directly shapes what these boundaries look like on the seafloor. At fast-spreading ridges like the East Pacific Rise, transform zones are relatively shallow and can even form slight ridges. At slow-spreading ridges like the Mid-Atlantic Ridge, they carve deep valleys with more than a kilometer of relief.
Fracture zones, the inactive extensions of these transform faults beyond the ridge segments, leave long scars across the ocean floor that can stretch for thousands of kilometers. They are consistently shallower than the active transform valleys by an average of about 650 meters, because the active fault zone is kept deep by the pulling forces of plate motion.
What the Landscape Looks Like
Transform boundaries leave distinctive marks on the surface. On land, you typically see long, straight valleys and ridges running parallel to the fault, with streams and rivers that make sharp bends where they cross the fault zone. These offset streams are one of the clearest signs of long-term horizontal displacement. Rocks on one side of the fault may have been carried tens to hundreds of miles from where they originally formed. Earthquakes along transform boundaries are shallow and occur in narrow zones because the rupture surfaces tend to be steep and vertical. The largest transform earthquakes reach about magnitude 8, which is powerful but generally smaller than the biggest subduction zone events.
In the ocean, transform valleys can be dramatic features, sometimes more than a kilometer deep at slow-spreading ridges. At faster ridges, magma from below partially fills the transform zone, creating shallower and sometimes ridge-like topography instead of a valley.