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Sidescan of ocean floor off Vancouver Island
A marine side scan imaging system produced a sonar image of the sea floor, which is comparable to an aerial photograph of a land surface. This side scan was taken just at the position where the Juan de Fuca Plate is plunging beneath North America.
This causes sediments of the ocean basin to be scraped off the plate and thus attached to the continent.

The straight, blank strip crossing the middle of the image is the northwest-directed track which the survey ship is following. The scanner makes a 10-km wide image of the sea floor (see the scale), but leaves out the blank area directly below the ship.

The light grey image area with no features, seen below the blank strip, shows the flat ocean bottom. Above the strip, the darker grey areas depict the first rumpling and thrusting of the sediments covering the ocean floor. As the heavier Juan de Fuca Plate slides under the lighter North American continent, the latter acts like a bulldozer pushing the sediments resting on the ocean plate into hills and ridges which are hundreds of meters high.

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