The San Andreas Fault is a transform plate boundary, where two tectonic plates slide horizontally past one another.

Plate boundary type

  • Geologists classify the San Andreas Fault as a transform plate boundary, a type of boundary defined by lateral, side‑by‑side motion rather than plates colliding or pulling apart.
  • This differs from convergent boundaries (where plates crash together) and divergent boundaries (where plates move apart to create new crust).

Which plates are involved

  • The fault marks the boundary between the Pacific Plate on the west and the North American Plate on the east.
  • Along this boundary, the Pacific Plate moves generally northwest relative to the North American Plate, producing significant horizontal displacement over millions of years.

How the fault behaves

  • The San Andreas is also described as a right‑lateral strike‑slip fault, meaning if you stand on one side and look across, the opposite side appears to move to the right during motion.
  • Stress builds up as the plates lock in places and is released in earthquakes, which is why this transform boundary is closely associated with California’s major seismic risk.

Quick comparison of boundary types

[5][1][3] [9] [2][3]
Boundary type Plate motion Example
Transform Plates slide past each other horizontally. San Andreas Fault (Pacific–North American plates).
Convergent Plates move toward each other and may subduct or crumple. Subduction zones of the Pacific Northwest.
Divergent Plates move apart, creating new crust. Mid‑ocean ridges (general example).
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