Lecture 10

CONTINENTAL MARGINS
Typical features and their characteristics
Contrasting types of continental margins
Submarine canyons on continental margins

Typical profile: Shelf, Shelf break, Slope, Rise

Continental shelf
Flooded continent and coastal plain
5-500 km wide; < 200 m deep

Shelf break
Edge of shelf
Abrupt change in slope

Continental slope
Continent/ocean boundary
10-100 km wide
Depth to base of slope varies
3,000 m in Atlantic
8,000 m in Pacific (marginal trenches)
Origin (?)
Faulting in initial rifting
Uplift at edge of shelf

Continental rise
Slope/ocean basin transition, gently sloping
Well developed in Atlantic; uncommon in Pacific
100-1,000 km wide
Origin: thick accumulation of sediment from land

TYPES OF MARGINS

"Passive" margins
Typical of Atlantic -- well-developed shelf, slope, rise
Origin (plate tectonics)
Rifting of continent to form new ocean
Margin is "trailing edge" of a continent-ocean plate
"Constructive" margin -- built seward by sediment deposition

"Active" margins
Typical of Pacific -- narrow shelf, narrow & deep slope,
rise uncommon
Origin (plate tectonics)
Convergent or transform boundary
Margin is "leading edge" of plate

SUBMARINE CANYONS
Steep-sided, V-shaped vallyes on shelf and slope
Origin -- erosion by turbidity currents, sediment-rich "slurries"
Initiated by earthquake or other "event"
Currents erode canyons as they flow downslope
Deposition of sediments on rise

Features of continental margins

A typical profile of a continental margin would show the following features
Shelf
Shelf Break
Slope
Rise

The continental shelf is the flooded extension of the adjacent continent and its coastal plain. The width of shelves varies from 5 to 500 km. Depth is less than 200 m.

The shelf break is the edge of the continental shelf. It marks an abrupt change in slope of the sea floor.

The continental slope marks the boundary between continents and ocean basins. The slope is relatively narrow, ranging from 10 to 100 km wide. Depth at the base of continental slopes varies from ocean to ocean. On Atlantic margins, the slope ends at about 3,000 m. Along most of the Pacific margins of North and South America, the slope descends abruptly into marginal trenches with depths up to 8,000 m. The origin of continental slopes is largely uncertain. Some slopes may have developed as faults during the intial rifting continents to form ocean basins. Others seem to be related to uplift at the edge of the continental shelf (e.g., volcanic activity, coral reef build-up).

The continental rise is the gently-sloping transition between the continental slope and deep-ocean floor (and abyssal plains). Continental rises are well developed in the Atlantic, Indian, and around Antarctica; but are uncommon in the Pacific. Where present, widths vary from 100 to 1,000 km. Continental rises result from the thick accumulation of sediment from land that is transported to the shelf and then down the slope.

Contrasting types of continental margins

As described above, the continental margins of the Atlantic and the Pacific differ in important ways. These differences are explained by plate tectonic processes.

The typical margin of the Atlantic (as well as the Indian Ocean and around Antarctica) is called a passive margin. This type of margin has well-developed shelf, slope, and rise. According to plate tectonics, passive margins developed during continental rifting and the formation of a new ocean basin. A passive margin is the "trailing edge" of a continent-ocean plate (e.g., the North American plate). Passive margins are also called "constructive" margins because they are built seaward by the deposition of sediment.

The margin of the Pacific side of the Americas is called an active margin. In this type, continental shelves are narrow, slopes are narrow and deep, and rises are often absent. According to plate tectonics, active margins are convergent or transform plate boundaries. Thus, an active continental margin is at the "leading edge" of one of the interacting plates. Active margins are sites of important geologic activity, including earthquakes, volcanism, and mountain-building (over a long time-scale).

Submarine canyons on continental margins

Submarine canyons are steep-sided, V-shaped valleys that begin on the shelf, extend down the slope, and end at the rise. They are common on both passive and active margins.

Submarine canyons are the result of erosion by rapidly flowing, sediment-rich currents called turbidity currents. Although no one has actually observed a turbidity current, we think they develop as follows: Sediment from land is transported to the edge of the continental shelf. Some "event," like an earthquake or violent storm, triggers the current -- a dense slurry of sediment a water. The turbidity current flows rapidly down the slope, eroding and transporting sediment as it goes. When it reaches the base of the slope, it slows down, spreads out, and deposits its sediment load in submarine "deltas" or fans on the continental rise and even well out into abyssal plains. We will consider ocean sediments deposited by turbidity currents later.
Study/Review Questions
10-1. Draw a profile of a typical continental margin. Label the important features. Indicate the general depth and width range of each feature.
10-2. Why is the base of the continental slope so much deeper on the Pacific side of South America than on the Atlantic side?
10-3. How do continental rises develop, that is, what is their origin? Why are rises uncommon around the margins of the Pacific? (Consider question 10-2)
10-4. What is meant by a "passive margin?" How do they develop according to plate tectonics? Would you think that passive margins become wider as they get older? Why?
10-5. What is meant by an "active margin?" How do they develop according to plate tectonics? [Look at the second part of 10-3 again. Note the connection of narrow margins and poorly developed rises with active margins.]
10-6. Describe the profile and distribution of submarine canyons on continental margins.
10-7. Describe how turbidity currents on margins can account for the origin of submarine canyons. Are turbidity currents important in transporting sediment to continental rises and adjacent abyssal plains?