WEBVTT
Kind: captions
Language: en

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About 20 million years ago when the Pacific and North American plates first made contact with each other

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the San Andreas fault system began to form at their common boundary.

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The Pacific Plate was moving Northwest in relation to the North American Plate.

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The block of crust that currently forms the Western transverse ranges and

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the Channel Islands was submerged and lined up in a north-south direction along the coast of present-day San Diego.

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Between about 20 million in 5 million years ago this block broke loose and pivoted along the edge of the

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North American plate rotating clockwise about 60 degrees.

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Beginning about five million years ago, Baja California broke away from the North American plate and moved

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Northwest ward as part of the Pacific plate.

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As it did so it compressed against the North American plate in Southern California.

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This compression caused folding, faulting and uplift within the Western transverse ranges Channel Islands block.

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During this time the block underwent as much as 30 degrees of additional clockwise rotation.

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Similar deformation and rotation has continued in the block up to the present time.

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The rotation of the western transverse ranges Channel Islands block

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stretched thin and fractured the crust, drawing magma to fill the gaps.

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In some places the magma erupted on the sea floor as volcanic lava that accumulated

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to as much as 10,000 feet thick.

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Marine basins that formed in subsided areas of the extended crusts collected sediments.

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Rivers on the mainland that flowed into the sea deposited some of these sediments.

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Submarine landslides were also deposited on the sea floor.

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Over time diatoms and other marine organisms shells accumulated along with

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the other sediments creating sedimentary
layers of diatomaceous mudstone on the

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seafloor top the underlying volcanic rock.