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Internal Tides and the Continental Slope

Curious waves coursing beneath the surface of the sea may shape the margins of the world's landmasses

David Cacchione, Lincoln Pratson

For many of us, a drive to the ocean is, at the very least, a yearly pilgrimage. Imagine for a moment what it would be like if, during one of these trips, you arrived at the beach to discover that the water was gone. After getting over your initial disappointment, you would probably decide that exploring what had been hidden territory would be just as much fun as playing in the surf. So you'd drive on cautiously, wondering what was in store.

At first it's easy going: You are traversing the continental shelf—a nearly flat plateau made of sediments eroded from the land. To the left and right, you might spy relict river channels, which were formed during former ice ages when the buildup of glaciers lowered sea level by more than 100 meters, exposing the continental shelf to the open air (truly). Here and there on this largely flat plain, you could happen on fields of ripples or maybe even dunes, which were created by the motion of waves and current in the now-missing ocean.

Driving farther, you eventually arrive at the edge of the continental shelf. Depending on where you are in the world, this change could come just 5 kilometers out from shore (as it does, for example, off central Chile) or more than 100 kilometers (the situation for much of the East Coast). Here, where the ocean used to be more than 100 meters deep, you stop your car and get out to take in the magnificent view. Before you is the continental slope, which descends some 3 kilometers vertically as it ramps downward toward the abyssal ocean floor. This incline represents the seaward face of the vast pile of sediments eroded from the interior of the continent and deposited along its watery edge by rivers, waves, currents and, in some regions, glaciers.

As with the continental shelf, the surface of the continental slope varies from place to place. There are spots where the shelf drops off precipitously and where the descent to the base of the slope is quite rugged. This is generally the case where submarine canyons—some of which are larger than the Grand Canyon—cut into the continental slope. But between these chasms, the ground falls away much more gradually: If you were to climb back in your car and drive down such a slope, it would be like descending a steep highway pass out of the Rockies or the Appalachians.

What is perplexing (and thus intriguing) for marine geologists is the question of why the continental slope is not any steeper. When only gravity acts on them, sediments piled underwater can easily maintain stable slopes of 15 degrees or more. Yet some 80 percent of the continental slopes around the world dip downward at less than 8 degrees, and their average incline is only about 3 degrees. Acting alone, the constant influx of sediment eroded from the land would tend over time to steepen the grade, so, clearly, one or more natural forces are ensuring that these slopes remain low. We believe that a primary factor—one not recognized before—is something called the internal tides: submerged waves largely hidden from view that pulse through the body of the ocean with the same twice-daily period as the normal lunar tides.

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