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Our Mattole Canyon 

Just west of Cape Mendocino there exists a submarine escarpment related to the boundary between the Pacific and Gorda plates. It descends into a canyon which exceeds the relief of the Grand Canyon (see profiles). As a consequence, the surface of the ocean off Cape Mendocino is not horizontal! Sailing north across the escarpment with a GPS receiver should reveal a five-meter drop in elevation, and I encourage someone to confirm this.

Satellites can determine sea-height variations to an incredible accuracy of three centimeters by bouncing very short radar pulses off the ocean surface. Reflections from a calm sea are sharp, whereas waves broaden the radar echo. Satellite radar can determine sea height, wave height and even wind speed.

The variations in sea surface that I wish to emphasize are not those caused by tides, winds and currents. Although the Gulf Stream raises sea level by a meter (via the Coriolis effect), and hurricanes can cause surges of several meters, gravity variations related to submarine topography have the dominant effect on sea height, amounting to over 100 meters. The elevated slab of dense rock on the south side of the Mendocino escarpment gravitationally pulls water away from the north, causing the sea-surface topography to simulate the submarine topography, albeit at a much subdued magnitude. Farther west, as one approaches the Gorda Rise, which is underlain by hot-swollen rocks, there is a reversal in the slope of the sea floor and sea surface. Continuing west, the sea-surface step can be detected as far away as Midway Island.

The first worldwide bathymetric maps were derived from satellite determinations of sea surface height. They revolutionized our understanding of the earth's plate tectonics and distribution of submarine volcanoes.

It is conceivable that an earthquake could collapse the Mendocino escarpment to yield a monster tsunami.

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About The Author

Don Garlick

Don Garlick is a geology professor retired from Humboldt State University. He invites any questions relating to North Coast science, and if he cannot answer it he will find an expert who can. E-mail [email protected].

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