higher velocities can transport and even erode unconsolidated sediment.
Sea-level
The lowering of sea-level associated with the buildup of
Oligocene and Neogene ice sheets has caused erosion of
a very large amount of material off the continental shelves
and ultimately deposited on the continental rises.
Summary
The continental rise is the gently inclined slope between
the base of the continental slope and the deep ocean floor.
It overlies the ocean crust bordering the faulted and fractured continental margin. It is the ultimate site of accumulation of sediment shed from the continent into the deep
sea. A small amount of sediment reaches the continental
rise by settling through the water column. Much of the
detrital sediment is brought down by turbidity currents,
but a very large amount of the material is delivered
through failures of the sediments of the continental slope
descending as slumps or catastrophic slides. Deep sea bottom currents move and redistribute the sediments of the
continental rise. Sea-level fluctuations associated with
the buildup of ice sheets have caused a very large amount
of material from the continental shelves and slopes to be
deposited on the continental rises.
Bibliography
Bryn, P., Berg, K., Forsberg, C. F., Solheim, A., and Kvalstad, T. J.,
2006. Explaining the Storegga Slide. Marine and Petroleum
Geology, 22, 11–19.
Heezen, B. C., and Ewing, M., 1952. Turbidity currents and submarine slumps and the 1929 Grand Banks earthquake. American
Journal of Science, 250, 849–873.
Heezen, B., Tharp, M., and Ewing, M., 1959. The floors of the
oceans: I. The North Atlantic. Geological Society of America,
Special Paper, 65, 1–126.
Cross-references
Continental Slope
Ocean Margin Systems
Shelf
CONTINENTAL SLOPE
William W. Hay
Department of Geological Sciences, University of
Colorado at Boulder, Estes Park, CO, USA
Definition
The first use of the term “slope,” referring to the edge of
the continental block, appears to have been by Rollin
D. Salisbury in his book Physiography, published in
1907. “The continental platforms and the ocean basins
are topographic features of the first order. The contrast
between them is emphasized by the fact that there is
almost everywhere a rather steep slope from the one to
the other, À a steep descent from the continental platforms
to the ocean basins, or looked at from the other point of
view, a steep ascent from the ocean basins to the continental platforms” (pp. 5–6). Later in the book, he presented
the idea that the junction of the continent and the ocean
basin was bordered by a belt that represented a concaveupward part of the seafloor “100–300 miles wide” as indicated in Figure 681: Map of the world showing in black the
portions of the sea bottom which are concave upwards.
The term “continental slope” only appears in the book’s
index. Although it was obvious Salisbury considered the
top of the continental slope to be demarcated by the shelf
break, a lower boundary was defined by Heezen
et al. (1959) as the point at which the gradient becomes
less the 1:40 (about 1
25').
History
The discovery of the continental slope goes back to the
time of laying the first telegraph cables across the Atlantic
(see SHELF), but the detailed bathymetry of continental
margins did not become known until after World War II
when the United States and USSR began to make extensive measurements of the seafloor depth using SONAR
(Doel et al., 2006).
The geologic basis for the continental slope
It was recognized in the late nineteenth century that the
margins between the continents and ocean basins were a
fundamental geologic boundary, and in the twentieth century, it became evident that this was due to a fundamental
difference between the rocks forming the floors of the continental blocks and ocean basins. However, it was not until
the development of the theory of plate tectonics that the
nature of the slope became fully understood. There are
two kinds of continental slope, one associated with passive continental margins, the other with active margins.
On passive margins, the continental slope overlies the
region where the continental margin was thinned by
faulting during the process of continental breakup. Along
active margins, the continental slope marks the boundary
between a subducting oceanic plate and the continental
crust on the overriding plate. In some instances, the
subducting oceanic plate is physically eroding the continental crust.
What controls the configuration of the
continental slope
Crustal thinning
Along classic passive margins, it is the original width of
thinning of continental crust by faulting and intrusion
of ocean margin basalts, an area with a width typically of
100–300 km. Where the margin coincided with a transform fault, as on the E-W Guinea margin of West Africa,
the continental slope can be very narrow and abrupt.
Along active margins, where the continent is bordered
124
CONTINENTAL SLOPE
Sea-level
The lowering of sea-level associated with the buildup of
Oligocene and Neogene ice sheets has caused erosion of
a very large amount of material off the continental shelves
and ultimately deposited on the continental rises.
Summary
The continental rise is the gently inclined slope between
the base of the continental slope and the deep ocean floor.
It overlies the ocean crust bordering the faulted and fractured continental margin. It is the ultimate site of accumulation of sediment shed from the continent into the deep
sea. A small amount of sediment reaches the continental
rise by settling through the water column. Much of the
detrital sediment is brought down by turbidity currents,
but a very large amount of the material is delivered
through failures of the sediments of the continental slope
descending as slumps or catastrophic slides. Deep sea bottom currents move and redistribute the sediments of the
continental rise. Sea-level fluctuations associated with
the buildup of ice sheets have caused a very large amount
of material from the continental shelves and slopes to be
deposited on the continental rises.
Bibliography
Bryn, P., Berg, K., Forsberg, C. F., Solheim, A., and Kvalstad, T. J.,
2006. Explaining the Storegga Slide. Marine and Petroleum
Geology, 22, 11–19.
Heezen, B. C., and Ewing, M., 1952. Turbidity currents and submarine slumps and the 1929 Grand Banks earthquake. American
Journal of Science, 250, 849–873.
Heezen, B., Tharp, M., and Ewing, M., 1959. The floors of the
oceans: I. The North Atlantic. Geological Society of America,
Special Paper, 65, 1–126.
Cross-references
Continental Slope
Ocean Margin Systems
Shelf
CONTINENTAL SLOPE
William W. Hay
Department of Geological Sciences, University of
Colorado at Boulder, Estes Park, CO, USA
Definition
The first use of the term “slope,” referring to the edge of
the continental block, appears to have been by Rollin
D. Salisbury in his book Physiography, published in
1907. “The continental platforms and the ocean basins
are topographic features of the first order. The contrast
between them is emphasized by the fact that there is
almost everywhere a rather steep slope from the one to
the other, À a steep descent from the continental platforms
to the ocean basins, or looked at from the other point of
view, a steep ascent from the ocean basins to the continental platforms” (pp. 5–6). Later in the book, he presented
the idea that the junction of the continent and the ocean
basin was bordered by a belt that represented a concaveupward part of the seafloor “100–300 miles wide” as indicated in Figure 681: Map of the world showing in black the
portions of the sea bottom which are concave upwards.
The term “continental slope” only appears in the book’s
index. Although it was obvious Salisbury considered the
top of the continental slope to be demarcated by the shelf
break, a lower boundary was defined by Heezen
et al. (1959) as the point at which the gradient becomes
less the 1:40 (about 1
25').
History
The discovery of the continental slope goes back to the
time of laying the first telegraph cables across the Atlantic
(see SHELF), but the detailed bathymetry of continental
margins did not become known until after World War II
when the United States and USSR began to make extensive measurements of the seafloor depth using SONAR
(Doel et al., 2006).
The geologic basis for the continental slope
It was recognized in the late nineteenth century that the
margins between the continents and ocean basins were a
fundamental geologic boundary, and in the twentieth century, it became evident that this was due to a fundamental
difference between the rocks forming the floors of the continental blocks and ocean basins. However, it was not until
the development of the theory of plate tectonics that the
nature of the slope became fully understood. There are
two kinds of continental slope, one associated with passive continental margins, the other with active margins.
On passive margins, the continental slope overlies the
region where the continental margin was thinned by
faulting during the process of continental breakup. Along
active margins, the continental slope marks the boundary
between a subducting oceanic plate and the continental
crust on the overriding plate. In some instances, the
subducting oceanic plate is physically eroding the continental crust.
What controls the configuration of the
continental slope
Crustal thinning
Along classic passive margins, it is the original width of
thinning of continental crust by faulting and intrusion
of ocean margin basalts, an area with a width typically of
100–300 km. Where the margin coincided with a transform fault, as on the E-W Guinea margin of West Africa,
the continental slope can be very narrow and abrupt.
Along active margins, where the continent is bordered
124
CONTINENTAL SLOPE