The continental slope is a steep, submerged geological feature that acts as a boundary between the shallow continental shelf and the deep abyssal plain of the ocean basin. It is the most dramatic downward change in elevation. The slope generally begins where the continental shelf ends, at a depth typically averaging around 135 meters, and extends down to depths of 3,000 to 4,000 meters. While the average global inclination is about 4 degrees, this gradient can vary significantly, ranging from 1 degree to as steep as 25 degrees or more in certain locations.
Formation Mechanisms at Passive Continental Margins
The formation of continental slopes along passive margins, which are not currently plate boundaries, is primarily a constructive process driven by the slow, massive accumulation of sediment. These margins, like the coastlines bordering the Atlantic Ocean, developed after continental rifting and subsequent thermal subsidence of the crust. The slope is built seaward through a process called progradation, where continuous deposition causes the continental shelf to grow outward and downward over geologic time. Massive quantities of terrigenous sediment are transported across the continental shelf and deposited at the shelf break.
The immense weight of this accumulating sand, silt, and clay causes the underlying crust to subside, creating accommodation space for even more material. This thick wedge of sediment then becomes structurally unstable due to the force of gravity acting on the steep face of the slope. Gravitational failure is a major mechanism in sculpting the passive slope’s angle, which typically sits around 3 to 6 degrees. The sheer volume and weight of the sediment lead to various forms of mass wasting, including large-scale slumping and rotational landslides, moving huge blocks of material downslope, building up the steep profile while transporting sediment toward the continental rise below.
Formation Mechanisms at Active Continental Margins
Continental slopes at active margins, such as those found along the Pacific Rim, are formed by powerful tectonic forces rather than just sedimentation. These margins are located directly at plate boundaries, usually where an oceanic plate is subducting beneath a continental plate. The formation process here is dominated by compression and deformation, resulting in a distinct and often much steeper structure. As the oceanic plate sinks into the mantle, it carries with it a layer of marine sediment that has accumulated on the seafloor. When this plate slides beneath the overriding continental plate, a significant portion of that sediment is scraped off and plastered against the continental margin, building a geological feature known as an accretionary prism or accretionary wedge.
Reshaping and Modification by Submarine Processes
Once formed, both passive and active continental slopes are continuously reshaped by powerful submarine processes, with erosion being the dominant modifying force. The most significant of these are turbidity currents, which are dense, fast-moving flows of sediment mixed with water that act like underwater landslides. These flows are triggered by events like earthquakes, overloading of sediment, or gravitational slumping on the upper slope. Turbidity currents travel down the slope at high velocities, carving away at the seafloor as they move. Over geologic time, this intense erosion incises massive features called submarine canyons into the slope face. These canyons often run perpendicular to the coastline, acting as conduits for transporting sediment from the continental shelf and slope to the deep-sea floor, while the slope is also constantly modified by smaller-scale mass movement events, including simple slumping, debris flows, and rock falls.