About 20 million years ago when the Pacific and North American plates first made contact with each other
the San Andreas fault system began to form at their common boundary.
The Pacific Plate was moving Northwest in relation to the North American Plate.
The block of crust that currently forms the Western transverse ranges and
the Channel Islands was submerged and lined up in a north-south direction along the coast of present-day San Diego.
Between about 20 million in 5 million years ago this block broke loose and pivoted along the edge of the
North American plate rotating clockwise about 60 degrees.
Beginning about five million years ago, Baja California broke away from the North American plate and moved
Northwest ward as part of the Pacific plate.
As it did so it compressed against the North American plate in Southern California.
This compression caused folding, faulting and uplift within the Western transverse ranges Channel Islands block.
During this time the block underwent as much as 30 degrees of additional clockwise rotation.
Similar deformation and rotation has continued in the block up to the present time.
The rotation of the western transverse ranges Channel Islands block
stretched thin and fractured the crust, drawing magma to fill the gaps.
In some places the magma erupted on the sea floor as volcanic lava that accumulated
to as much as 10,000 feet thick.
Marine basins that formed in subsided areas of the extended crusts collected sediments.
Rivers on the mainland that flowed into the sea deposited some of these sediments.
Submarine landslides were also deposited on the sea floor.
Over time diatoms and other marine organisms shells accumulated along with
the other sediments creating sedimentary layers of diatomaceous mudstone on the
seafloor top the underlying volcanic rock.