CONTINENTAL MARGIN
The CONTINENTAL MARGIN is the transition to the
deep ocean basin. The margin belongs mostly to the continent.
The shallowest portion of the margin and the portion nearest to the continent is the SHELF. This feature is simply an underwater extension of the adjacent above-water portion of the continent, the COASTAL PLAIN. That is, the shelf and coastal plain are the same feature, divided only by the position of the sea level. Because sea level can and has changed, the area of the shelf can also change. When sea level was lower, more of the shelf was exposed and became part of the Coastal Plain. Although they are the same feature, there are fundamental differences in the processes occurring in each part. The coastal plain experiences erosion, while the shelf is a site of deposition, as are all portions of the ocean bottom
The CONTINENTAL SLOPE,
as its name implies, is the sloping edge of the continent as it merges into the
deep ocean basin. The steepness of the continent slope differs from place to
place. Slopes along mountainous coasts are steeper than those offshore from
coastal plains. In either case, the slopes are not very steep, with the
gentlest ones being barely noticeable if you could drive up one of them in a
car. The steeper slopes would be comparable to a typical highway hill. The
apparent steepness of the slope on the cross section shown here is due to a
graphic distortion knows as VERTICAL EXAGGERATION. In order to illustrate
features of the earth that would be no greater than the thickness of the pencil
line used to draw them, the vertical dimension is expanded. In the diagram
shown here, the horizontal dimension is hundreds of miles but the vertical
dimension is only about 10,000 feet or about 2 miles. If the vertical dimension
was drawn to the same scale as the horizontal distance, the entire continental
margin would be about 1/50 inch high, and it would be entirely within the
thickness of a pencil line.
The continental rise is a wedge of sediment that has accumulated at the base of the slope due to the change in gradient from the steeper slope to the virtually flat abyssal plain. This build-up of sediment is similar to the debris that accumulates at the base of a cliff. The sediment is transported from the shelf and slope to the rise by slumping and by turbidity currents. Thus, the continental rise is a depositional feature built up by the accumulation of sediment transported from the shallower shelf and slope. The continental rise is named because of the “rise” of the bottom seen by fathometers as ships approach the continent.