About 85 million years ago several oceanic plates formed the floor of the Pacific Basin.
The Pacific Plate was a small southern plate with other plates spreading away from it.
Over time the Pacific Plate drifted north and expanded in all directions by sea floor spreading.
New sea floor was continually added to its eastern edge
by spreading between the Pacific and Farallon Plates.
As plate movement continued the eastern side of the oceanic Farallon Plate slid
or subducted under the continental North American Plate.
It subducted faster than new sea floor was being created on its western side
causing the plate to gradually narrow.
As the Farallon Plates width decreased do to subduction
it broke into smaller plates some of which subducted entirely and disappeared.
Plates subduction occurred because the cool dense oceanic Farallon Plate sank into the
mantle beneath the more buoyant continental North American Plate.
As the plate descended it released water causing the hot mantle to melt.
The resulting buoyant magma rose and pooled beneath the surface.
From time to time some of the magma broke through to the surface and erupted
in explosive ash clouds and lava flows building volcanoes.
This process continued along the growing volcanic mountain chain in repeating cycles.
While the Farallon plate continued to subduct causing millions of years of volcanic activity along the coast.
About 20 million years ago the San Andreas fault system began to form when
and where the Pacific and North American plates first made contact with one another.
Along this boundary the Pacific Plate continued its motion to the
northwest toward the Aleutian Islands moving sideways along the rim of North America.
As it slid past it scraped off slivers of the North American plate and carried them with it.
Continental fragments along the borderland were captured by the Pacific Plate
in this process and moved away pulling open many of the basins in Southern California.
As the plate boundary lengthened southward, it first lay along the Pacific rim of Baja California.
About five million years ago a new section of
the San Andreas fault system broke inside the North American continent
along the line that is now the Gulf of California.
This broke Baja California away from the North American Plate and transferred it to the moving Pacific Plate.
The northern end of Baja California began to ram into Southern California
pushing up many of our local east-west trending mountains
and forming the big bend in the San Andreas fault system.
The Sierra Nevada Great Valley Block farther inland is currently in the early stages
of being captured by the Pacific Plate and carried northwestward.