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Features of the Sea floor Lesson

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Classifications of ocean floor: Continental Margins

Continental Margins

Passive margins

Are also called Atlantic-type margins. Recall how continents are rifted apart: Continental crust is stretched and rifted, then the spreading center creates oceanic crust between the two continental crusts. These margins ace the edges of diverging tectonic plates. Very little volcanic or earthquake activity are found in these areas.

Active margins

AKA as Pacific-type margins, and are located near the edges of converging plates (subduction zones). Active margins are the site of volcanic and earthquake activity.

Continental Margins

Submarine Canyons

Submarine canyons form in the same way terrestrial canyons due: from river erosion. When the river enters the sea, it keeps moving, producing a current of water moving towards the open ocean. This current also carves out a channel on the continental shelf that extends to the shelf break, when it begins to carve out a canyon. Turbidity currents are similar to snow avalanches as they are a tumbling flow of water and sediment that rushes down the canyon to the deep sea floor. Once on the sea floor the sediment stops and is deposited in deep sea (submarine) fans. The heavier sediment is deposited first, then the medium, then the finest, producing a sequence called graded bedding

Southern California

Southern California's margin is unique. We are classified as an active margin, but we don't have a trench/subduction zone. We are, however, on a plate boundary - the transform fault known as the San Andreas Fault Zone. Thus, our margin has features similar to passive margin, but with many extras. Our margin contains:

The image below shows a profile from our coastline at Long Beach out to the deep sea floor (the abyssal plane). The red line represents where the surface of a typical continental margin would be. Our margin is covered with basins, islands and ridges that formed due to the faults associated with the San Andreas Fault Zone.

Notice the orientation of the faults in the map above. The faults, mountains and islands south of Los Angeles (where UCR, UCI and UCSD are on the map) trend NW-SE. This trend is the same as nearly all of the mountains along the west coast of the US, Canada, Central and South America. Now look at the faults, mountains and islands where UCLA is located on the map. They trend in an E-W direction.

What caused this nearly 90° rotation? Geologists still aren't sure, but rotation began about 20 million years ago. This animation created by Tanya Atwater of UCSB Geology shows one possible explanation.

This satellite image also shows the mountains and islands associated with the Transverse Ranges.

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copyright Sonjia Leyva 2015