The San Andreas Fault was caused by the Pacific Plate and North American Plate starting to move sideways past each other along a transform boundary, rather than colliding head-on. It formed as the older subduction system changed, leaving a long strike-slip fault zone in California.

What happened geologically

  • Before the fault became what it is today, an oceanic plate called the Farallon Plate was being subducted beneath western North America.
  • As that plate system changed, the Pacific and North American plates came into contact and began sliding horizontally past one another.
  • That sideways motion created the San Andreas Fault, which is a right-lateral strike-slip fault.
  • The plates are still moving today, so the fault remains active and produces earthquakes when stress builds up and is released.

Simple picture

Think of it like two giant rough blocks of rock slowly grinding past each other. They do not move smoothly all the time, so strain builds up and then breaks loose in earthquakes.

Bottom line

So, the fault was not “caused” by a single event; it developed over millions of years from plate tectonics and the shift from subduction to sideways plate motion.