Active and passive margins. Continental margins are defined as active or passive according to the presence or absence, respectively, of plate tectonic activity. Earthquakes and volcanoes are associated with active continental margins, which are marked by a landward continental shelf, a much steeper continental slope that ends at an active ocean trench, and an irregular ocean bottom that may contain volcanic hills (Figure 1).
Figure 1
An Active Continental Margin
A passive continental margin has a landward, shallow continental shelf, a deeper continental slope, a continental rise, and a flat abyssal plain (Figure 2).
Figure 2
A Passive Continental Margin
Continental shelves. A continental shelf is a shallow, almost flat platform that extends seaward from the edge of the continent. The nearshore sediment is mostly sand that grades outward toward finegrained mud at the deeper edge. Continental shelves range in width from a few kilometers to over 1,000 kilometers; depths increase from a few meters to about 200 meters. Being well within the 200‐meter deviation for sea level variations during glacial epochs, sedimentation on continental shelves frequently shows marine transgressions and regressions. The continental shelf is underlain by sialic (high in silicon and aluminum) crust, which is part of the continental mass. Continental shelves cover about 8 percent of the ocean floor.
Continental slopes. The continental slope extends from the seaward edge of the continental shelf into the deep ocean (15,000 feet) at an average angle of 4 to 5 degrees. It is thought that the sediments of the continental slope cover the transition zone between continental and oceanic crust, a zone that may be structurally complex and contain block faults and thrust faults.
Submarine canyons. Submarine canyons are erosion features that cut continental shelves and slopes. The heads of some of these V‐shaped canyons may have been carved by river erosion when the sea level was lower during Pleistocene glaciation. These canyons can be over 3 kilometers deep. Sprawling abyssal fans (deep‐sea fans) are often found at the mouths of submarine canyons (Figure 3). Abyssal fans resemble alluvial fans in shape. The canyons are sometimes eroded by the constant movement of sand by longshore currents, which also erodes the underlying bedrock. Sand movement that is quite rapid and steep is termed a sand fall. Regular bottom currents streamline and shape abyssal fans.
Figure 3
Submarine Canyons
Turbidity currents. Turbidity currents are large volumes of dense, sediment‐laden water that result when sand and mud on the continental slope are dislodged by landslides or earthquakes and become suspended in the water. The turbidity currents are denser than water and behave as a separate flow that rolls down the slope at speeds up to 60 kilometers per hour. The resulting sedimentary deposits are called turbidites. Although large‐scale turbidity currents have never been witnessed, they are likely the culprits that periodically break telephone cables on the ocean floor. Both graded beds and shallow‐water fossils found in abyssal plain sediments are also indirect evidence that large‐scale turbidity currents exist.