WEBVTT
Kind: captions
Language: en

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About 85 million years ago several oceanic plates formed the floor of the Pacific Basin.

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The Pacific Plate was a small southern plate with other plates spreading away from it.

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Over time the Pacific Plate drifted north and expanded in all directions by sea floor spreading.

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New sea floor was continually added to its eastern edge

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by spreading between the Pacific and
Farallon Plates.

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As plate movement continued the eastern side of the oceanic Farallon Plate slid

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or subducted under the continental North American Plate.

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It subducted faster than new sea floor
was being created on its western side

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causing the plate to gradually narrow.

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As the Farallon Plates width decreased do to subduction

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it broke into smaller plates some of
which subducted entirely and disappeared.

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Plates subduction occurred because the cool dense oceanic Farallon Plate sank into the

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mantle beneath the more buoyant
continental North American Plate.

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As the plate descended it released water
causing the hot mantle to melt.

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The resulting buoyant magma rose and pooled beneath the surface.

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From time to time some of the magma broke through to the surface and erupted

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in explosive ash clouds and lava flows building volcanoes.

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This process continued along the growing volcanic mountain chain in repeating cycles.

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While the Farallon plate continued to subduct causing millions of years of volcanic activity along the coast.

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About 20 million years ago the San
Andreas fault system began to form when

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and where the Pacific and North American plates first made contact with one another.

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Along this boundary the Pacific
Plate continued its motion to the

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northwest toward the Aleutian Islands
moving sideways along the rim of North America.

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As it slid past it scraped off slivers of the North American plate and carried them with it.

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Continental fragments along the borderland were captured by the Pacific Plate

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in this process and moved away pulling
open many of the basins in Southern California.

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As the plate boundary lengthened southward, it first lay along the Pacific rim of Baja California.

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About five million years ago a new section of

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the San Andreas fault system broke
inside the North American continent

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along the line that is now the Gulf of
California.

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This broke Baja California away from the North American Plate and transferred it to the moving Pacific Plate.

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The northern end of Baja California
began to ram into Southern California

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pushing up many of our local east-west
trending mountains

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and forming the big bend in the San Andreas fault system.

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The Sierra Nevada Great Valley Block farther inland is currently in the early stages

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of being captured by the Pacific Plate and carried northwestward.