6 CONTINENTAL MARGINS

䊳 11.6 CONTINENTAL MARGINS

A continental margin is a place where continental crust meets oceanic crust. Two types of continental margins exist. A passive margin occurs where continental and oceanic crust are firmly joined together. Because it is not

a plate boundary, little tectonic activity occurs at a pas- sive margin. Continental margins on both sides of the Atlantic Ocean are passive margins. In contrast, an ac- tive continental margin occurs at a convergent plate boundary, where oceanic lithosphere sinks beneath the continent in a subduction zone. The west coast of South America is an active margin.

Figure 11–14

A scanning electron microscope photo of PASSIVE CONTINENTAL MARGINS pelagic foraminifera, tiny organisms that float near the surface of the seas. (Ocean Drilling Program, Texas A&M University)

Consider the passive margin of eastern North America. Recall from Chapter 2 that, about 200 million years ago, all of the Earth’s continents were joined into the super- continent called Pangea. Shortly thereafter, Pangea be- gan to rift apart into the continents as we know them

Pelagic sediment accumulates at a rate of about 2 to today. The Atlantic Ocean opened as the east coast of

10 millimeters per 1000 years. As mentioned earlier, its North America separated from Europe and Africa. thickness increases with distance from the ridge because

As Pangea broke up, the crust of North America’s the sea floor becomes older as it spreads away from the

east coast stretched and thinned near the fractures (Fig. ridge (Fig. 11–15). Close to the ridge there is virtually

11–16). Basaltic magma rose at the spreading center, no sediment. Close to shore, pelagic sediment gradually

forming new oceanic crust between the separating conti- merges with the much thicker layers of terrigenous sed-

nents. All tectonic activity then centered at the spreading iment, which can be 3 kilometers thick.

mid-Atlantic ridge, and no further tectonic activity oc- Parts of the ocean floor beyond the mid-oceanic

curred at the continental margins; hence the term passive ridge are flat, level, featureless submarine surfaces called

continental margin.

the abyssal plains (Fig. 11–7). They are the flattest sur- faces on Earth. Seismic profiling shows that the basaltic

The Continental Shelf

crust is rough and jagged throughout the ocean. On the On all continents, streams and rivers deposit sediment on coastal deltas, like the Mississippi River delta. Then ocean currents redistribute the sediment along the coast. The sediment forms a shallow, gently sloping submarine surface called a continental shelf on the edge of the con-

Mid-oceanic ridge

tinent (Fig. 11–17). As sediment accumulates on a con- tinental shelf, the edge of the continent sinks isostatically

Pelagic sediment

because of the added weight. This effect keeps the shelf slightly below sea level.

Basaltic oceanic

Over millions of years, thick layers of sediment ac-

crust

cumulated on the passive east coast of North America. The depth of the shelf increases gradually from the shore

Figure 11–15 Deep sea mud becomes thicker with increas- to about 200 meters at the outer shelf edge. The average ing distance from the mid-oceanic ridge.

inclination of the continental shelf is about 0.1º. A con-

190 CHAPTER 11 OCEAN BASINS

(a)

Pangea

Basalt flow

Rift valley

Continental crust

Crust uplifted and thinned

(b)

Rift valley

Fault blocks

Lithosphere

New oceanic

Abyssal plains

crust Mid-Atlantic

Continental shelf

ridge

(c) East coast of

West coast of North America

Continental slope

Continental rise

Rift valley

Atlantic Ocean

Africa

Figure 11–16

A passive continental margin developed on North America’s east coast as Pangea rifted apart and the Atlantic Ocean basin began to open. (a) A mantle plume forces central Pangea upward. (b) Faulting and erosion thin the uplifted part of Pangea, and the crust begins to rift apart. Rising basalt magma forms new oceanic crust in the rift zone. (c) Sediment eroded from the continent forms a broad continental shelf–slope–rise complex.

tinental shelf on a passive margin can be a large feature. the shelf or because ocean currents bypass that area. In The shelf off the coast of southeastern Canada is about

warm regions where sediment does not muddy the wa- 500 kilometers wide, and parts of the shelves of Siberia

ter, reef-building organisms thrive. As a result, thick beds and northwestern Europe are even wider.

of limestone accumulate in tropical and subtropical lati- In some places, a supply of sediment may be lack-

tudes where clastic sediment is lacking. Limestone ac- ing, either because no rivers bring sand, silt, or clay to

cumulations of this type may be hundreds of meters thick

Continental Margins 191

Continental Continental

Continental

crust sediment

Rock salt

Faults

Figure 11–17

A passive continental margin consists of a broad continental shelf, slope, and rise formed by accumulation of sediment eroded from the continent. Salt deposits, reef limestone, and basalt sills are common in the shelf sedimentary rocks.

and hundreds of kilometers across and are called car- In essence, then, the shelf–slope–rise complex is bonate platforms. The Florida Keys and the Bahamas

a smoothly sloping surface on the edge of a continent, are modern-day examples of carbonate platforms on con-

formed by accumulation of sediment eroded from the tinental shelves.

continent.

Some of the world’s richest petroleum reserves oc- cur on the continental shelves of the North Sea between