by an oceanic trench, the edge of the continent may be
undergoing abrasion, and the slope may be very steep.
Detrital sediment supply
As the available accommodation space on the shelf is
reduced, detrital sediment may accumulate on the continental slope. Although initially unconsolidated, it may
be held in place by the cementing effect of gas hydrates,
especially where significant amounts of organic carbon
are present. However, sea-level fluctuations and currents
may destabilize slope sediments and landslide scars are
common.
Carbonate deposition
The production of carbonate occurs in shallow water, but
even minor sea-level fluctuations will expose the original
aragonitic sediment to fresh water and cause its conversion to well-cemented calcite or dolomite. As crustal subsidence occurs, carbonate banks may build upward very
rapidly, and the marginal slopes may be almost vertical.
Waves
Today, other than tsunamis, wave action has little effect on
slope deposits as they are below wave base of even long
swell. However, during glacial sea-level lowstands, when
the ocean surface was near the shelf breaks, waves
actively eroded the upper part of the continental slope.
Currents
Many continental slopes are sculpted by contour currents
flowing along them. These currents have velocities of up
to 40 cm/s and can easily erode and move sediment. They
generally flow in a direction opposite to the surface gyres.
Along the western margins of the ocean basins, the currents of the tropical-subtropical surface gyres (e.g., Gulf
Stream, Kuroshio) are narrow and deep and can sculpt
the upper parts of the continental slopes.
Sea-level
The large sea-level fluctuations associated with the development of high latitude ice sheets lowers the surface of the
ocean to the top of the continental slope. During
lowstands, rivers may cut into the edge of the continental
shelf and carve canyons in the upper part of the continental
slope. Their turbid waters can then flow down the slope
and their sediment load deposited on the lower slope and
continental rise.
Mass wasting
In many areas the continental slope is at the maximum
angle of repose for sediments, and very large mass wasting
events often triggered by earthquakes can occur. One of
the largest was the three Storegga Slides on the Norwegian
margin (Bondevik et al., 2005; Haflidason et al., 2005).
Their heads were at the edge of the continental shelf
(Storegga is Norwegian for “the Great Edge”). The landslides occurred along 290 km of the continental margin
with an estimated volume of 3,500 km
3 of debris. The last
of the slides occurred about 6,100 BCE and generated a
large tsunami, over 20 m high in Northern Scotland.
Continental slope profiles
While the continental slope forms the steeper part of the
transition from the shelf to the ocean basin, the profiles
differ greatly from place to place as shown in Figures 1
and 2.
Figure 1a shows the steep slope south of Vladivostok
into the Japan Sea which occupies a pull-apart basin
behind the Japanese archipelago. Figure 1b shows one of
the steepest slopes along the edge of a continent, near
Accra, Ghana, where the continent-ocean transition is
along a transform fault. Figures 1c and d show two very
tall, steep continental slopes on active margins associated
with oceanic trenches. Figure 1c shows that on the western
South American margin at 25
S, from the Andes into the
Peru-Chile Trench; note that a continental shelf is lacking.
Figure 1d shows the transition from the eastern Aleutians
across a shelf into the Aleutian trench. Figure 1e is an E-W
section from the Hebrides across the shelf with a slope into
Rockall Trough and then up again onto Rockall Bank, a
continental fragment. This is the area in which the term
“continental shelf” was first used. Figures 1f and g show
two continental margins with “borderlands.” Figure 1f
shows the complex bathymetry of the Southern California
Borderland with ridges and basins terminating oceanward
in a steep escarpment. Figure 1g is an E-W section off
northern Florida showing the transition from the shelf
down a slope onto the broad Blake Plateau and finally
down a steep slope into the western Atlantic Basin.
Figures 1h and i are profiles along the classic passive margin of the northeastern United States. Figure 1h is a
NW-SE transect from Cape Cod into the deep Atlantic
with a very steep upper slope and a classic concaveupward lower slope and rise. It is evident that there is no
distinctive morphological feature that separates the slope
from the rise. Figure 1i shows the broad shelf of the
New York Bight, the slope, the Hudson Submarine Fan,
and the transition onto the Hatteras Abyssal Plain. The
trace of the Hudson Canyon is indicated by a dashed line.
Figure 2a shows a typical Antarctic profile, with a
broad deep shelf generally sloping toward the ice-laden
continent, and a relatively gentle slope into the deep
ocean. Figure 2b is an W-E section from the Gulf of Mexico across south Florida to the Great Bahama Bank, all carbonate terrains. Off western Florida, the shelf is very
broad and ends at a steep escarpment into the depths of
the Gulf of Mexico basin. The shelf off Miami is very narrow, and the slope down into the Florida Strait is
interrupted by the Pourtalés Terrace. The slope up to
the Bahama Banks is almost vertical and swept clean of
sediment by the Florida Current/Gulf Stream.
Figure 2c shows the transition from continent to ocean
basin along an N-S section from the Ganges-Brahmaputra
Delta into the Bay of Bengal. This is the most gradual
CONTINENTAL SLOPE
125
undergoing abrasion, and the slope may be very steep.
Detrital sediment supply
As the available accommodation space on the shelf is
reduced, detrital sediment may accumulate on the continental slope. Although initially unconsolidated, it may
be held in place by the cementing effect of gas hydrates,
especially where significant amounts of organic carbon
are present. However, sea-level fluctuations and currents
may destabilize slope sediments and landslide scars are
common.
Carbonate deposition
The production of carbonate occurs in shallow water, but
even minor sea-level fluctuations will expose the original
aragonitic sediment to fresh water and cause its conversion to well-cemented calcite or dolomite. As crustal subsidence occurs, carbonate banks may build upward very
rapidly, and the marginal slopes may be almost vertical.
Waves
Today, other than tsunamis, wave action has little effect on
slope deposits as they are below wave base of even long
swell. However, during glacial sea-level lowstands, when
the ocean surface was near the shelf breaks, waves
actively eroded the upper part of the continental slope.
Currents
Many continental slopes are sculpted by contour currents
flowing along them. These currents have velocities of up
to 40 cm/s and can easily erode and move sediment. They
generally flow in a direction opposite to the surface gyres.
Along the western margins of the ocean basins, the currents of the tropical-subtropical surface gyres (e.g., Gulf
Stream, Kuroshio) are narrow and deep and can sculpt
the upper parts of the continental slopes.
Sea-level
The large sea-level fluctuations associated with the development of high latitude ice sheets lowers the surface of the
ocean to the top of the continental slope. During
lowstands, rivers may cut into the edge of the continental
shelf and carve canyons in the upper part of the continental
slope. Their turbid waters can then flow down the slope
and their sediment load deposited on the lower slope and
continental rise.
Mass wasting
In many areas the continental slope is at the maximum
angle of repose for sediments, and very large mass wasting
events often triggered by earthquakes can occur. One of
the largest was the three Storegga Slides on the Norwegian
margin (Bondevik et al., 2005; Haflidason et al., 2005).
Their heads were at the edge of the continental shelf
(Storegga is Norwegian for “the Great Edge”). The landslides occurred along 290 km of the continental margin
with an estimated volume of 3,500 km
3 of debris. The last
of the slides occurred about 6,100 BCE and generated a
large tsunami, over 20 m high in Northern Scotland.
Continental slope profiles
While the continental slope forms the steeper part of the
transition from the shelf to the ocean basin, the profiles
differ greatly from place to place as shown in Figures 1
and 2.
Figure 1a shows the steep slope south of Vladivostok
into the Japan Sea which occupies a pull-apart basin
behind the Japanese archipelago. Figure 1b shows one of
the steepest slopes along the edge of a continent, near
Accra, Ghana, where the continent-ocean transition is
along a transform fault. Figures 1c and d show two very
tall, steep continental slopes on active margins associated
with oceanic trenches. Figure 1c shows that on the western
South American margin at 25
S, from the Andes into the
Peru-Chile Trench; note that a continental shelf is lacking.
Figure 1d shows the transition from the eastern Aleutians
across a shelf into the Aleutian trench. Figure 1e is an E-W
section from the Hebrides across the shelf with a slope into
Rockall Trough and then up again onto Rockall Bank, a
continental fragment. This is the area in which the term
“continental shelf” was first used. Figures 1f and g show
two continental margins with “borderlands.” Figure 1f
shows the complex bathymetry of the Southern California
Borderland with ridges and basins terminating oceanward
in a steep escarpment. Figure 1g is an E-W section off
northern Florida showing the transition from the shelf
down a slope onto the broad Blake Plateau and finally
down a steep slope into the western Atlantic Basin.
Figures 1h and i are profiles along the classic passive margin of the northeastern United States. Figure 1h is a
NW-SE transect from Cape Cod into the deep Atlantic
with a very steep upper slope and a classic concaveupward lower slope and rise. It is evident that there is no
distinctive morphological feature that separates the slope
from the rise. Figure 1i shows the broad shelf of the
New York Bight, the slope, the Hudson Submarine Fan,
and the transition onto the Hatteras Abyssal Plain. The
trace of the Hudson Canyon is indicated by a dashed line.
Figure 2a shows a typical Antarctic profile, with a
broad deep shelf generally sloping toward the ice-laden
continent, and a relatively gentle slope into the deep
ocean. Figure 2b is an W-E section from the Gulf of Mexico across south Florida to the Great Bahama Bank, all carbonate terrains. Off western Florida, the shelf is very
broad and ends at a steep escarpment into the depths of
the Gulf of Mexico basin. The shelf off Miami is very narrow, and the slope down into the Florida Strait is
interrupted by the Pourtalés Terrace. The slope up to
the Bahama Banks is almost vertical and swept clean of
sediment by the Florida Current/Gulf Stream.
Figure 2c shows the transition from continent to ocean
basin along an N-S section from the Ganges-Brahmaputra
Delta into the Bay of Bengal. This is the most gradual
CONTINENTAL SLOPE
125