The continental margin
All continents (and large islands) are
surrounded by a continental margin, a
continuous band of mostly sedimentary
rock (figure 1). This makes up some 20%
of the ocean floor; the rest is the deep
ocean basins (abyssal plains).
Continental margins consist mainly
of a continental shelf and a continental
slope, although there is considerable
variety (see box next page).
The continental shelf is relatively flat,
dipping down very gently (less than 0.1°)
as it extends out from the shoreline to the
shelf break or shelf edge, which is where
the continental slope begins (figures 2
and 3). Its width varies considerably from
several kilometres (a few miles) to more
than 400 km (250 miles); the average is
80 km ( 50 miles). At least one shelf is
over 1,000 km (600 miles) wide.
wide shelves are found along the Arctic
Ocean, Bering Sea, and the Grand Banks,
Newfoundland (figure 1).
At the edge of the continental shelf, at
a consistent average depth of about 130
m (430 ft), the slope of the seafloor
suddenly increases from nearly flat to
about 4°, all the way down to depths of
1,500 to 3,500 m ( 4,900 to 11,500 ft).
This is the continental slope, and if all the
water were removed from the oceans, it
would be the most conspicuous geomorphological boundary on Earth (figure 4).
In a few places on some continental
margins the slope can be much steeper
than average—up to 90°.
Margin sedimentary rocks
The sedimentary rocks within the
continental margin can be extremely
thick—over 20 km ( 12 miles) deep in
places, especially in buried basins.
sedimentary units are generally sheetlike, thickening seaward with a slight
dip (figure 5). This profile reinforces
the evidence for a broad scale uplift of
the adjacent continental area while the
Rise Abyssal Plain
Deep - Sea
Figure 2. Schematic of an Atlantic type margin.
Figure 1. The continental margin shown in light blue (NOAA).
Figure 3. Schematic of a Pacific type margin.