Geology Reference
In-Depth Information
Continental margin
Continental margin
Continental shelf
Continental shelf
Continental slope
Sea level
Continental slope
0
Oceanic ridge
2
Continental rise
Oceanic trench
4
6
8
10
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
Distance (km)
◗
Figure 9.7
Features of Continental Margins A generalized profi le showing features of the
continental margins. The vertical dimensions of the features in this profi le are greatly exaggerated
because the vertical and horizontal scales differ.
terminates where the inclination of the seafloor increases
abruptly from 1 degree or less to several degrees. The outer
margin of the continental shelf, or simply the
shelf-slope
break
, is at an average depth of 135 m, so by oceanic stan-
dards, the continental shelves are covered by shallow water.
At times during the Pleistocene Epoch (1.8 million to
10,000 years ago), sea level was as much as 130 m lower than
it is now. As a result, the continental shelves were above sea
level where deposition in stream channels and fl oodplains
took place. In addition, in many parts of northern Europe
and North America, glaciers extended well out onto the con-
tinental shelves and deposited gravel, sand, and mud. Since
the Pleistocene ended, sea level has risen, submerging these
deposits, which are now being reworked by marine processes.
Evidence that these sediments were in fact deposited on land
includes remains of human settlements and fossils of land-
dwelling animals (see Chapter 23).
and Submarine Fans
In Chapter 6, we discussed the origin of graded bedding,
most of which results from
turbidity currents
, under-
water fl ows of sediment-water mixtures with densities greater
than that of sediment-free water. As a turbidity current fl ows
onto the relatively fl at seafl oor, it slows and begins depositing
sediment, the largest particles fi rst, followed by progressively
smaller particles, thus forming a layer with graded bedding
(see Figure 6.24). Deposition by turbidity currents yields a
series of overlapping
submarine fans
, which constitute a
large part of the continental rise (
Figure 9.8). Submarine
fans are distinctive features, but their outer margins are dif-
fi cult to discern because they grade into deposits of the deep-
ocean basin.
No one has ever observed a turbidity current in prog-
ress in the oceans, so for many years, some doubted their
existence; however, seafl oor samples from many areas show
a succession of layers with graded bedding and the remains
of shallow-water organisms that were displaced into deeper
water by turbidity currents (Figure 9.8).
Perhaps the most compelling evidence for turbidity
currents is the pattern of trans-Atlantic cable breaks that
took place in the North Atlantic near Newfoundland on
November 18, 1929. Initially, an earthquake was assumed to
have ruptured telephone and telegraph cables. However, the
breaks on the continental shelf near the epicenter occurred
when the earthquake struck, but cables farther seaward were
broken later and in succession (Figure 9.8b). The last cable
to break was 720 km from the source of the earthquake, and
it did not snap until 13 hours after the fi rst break. In 1949,
geologists realized that an earthquake-generated turbidity
current had moved downslope, breaking the cables in suc-
cession. The precise time at which each cable broke was
known, so calculating the velocity of the turbidity current
was simple. It moved at about 80 km/hr on the continen-
tal slope, but slowed to about 27 km/hr when it reached the
continental rise.
◗
The seaward margin of the continental shelf is marked by the
shelf-slope break
(at an average depth of 135 m), where the
more steeply inclined
continental slope
begins (Figure 9.7).
In most areas around the margins of the Atlantic, the conti-
nental slope merges with a more gently sloping
continental
rise
. This rise is absent around the margins of the Pacifi c,
where continental slopes descend directly into an oceanic
trench (Figure 9.7).
The shelf-slope break is an important feature in terms
of sediment transport and deposition. Landward of the
break—that is, on the shelf—sediments are affected by waves
and tidal currents, but these processes have no effect on sedi-
ments seaward of the break, where gravity is responsible for
their transport and deposition on the slope and rise. In fact,
much of the land-derived sediment crosses the shelves and is
eventually deposited on the continental slopes and rises, where
more than 70% of all sediments in the oceans are found. Much
of this sediment is transported through submarine canyons by
turbidity currents.
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