56 Origin and Morphology of Ocean Margins
filled. The fjords of Norway, Greenland, and western Canada are witnesses to this
powerful action of the ice.
Shelves fonned by large deltas off river mouths (Amazon, Mississippi, etc.) can be
very flat and monotonous, in striking contrast to both the rugged ice-carved shelf and
the irregular coral reef shelf. The rich supply of fine sediment associated with delta
environments allows redistribution and smoothing by waves and currents. Of course,
waves and currents can also build up dunes, barriers, beach benns, and sand waves,
depending on circumstances (see Chap. 7). Examples for these conditions of high
terrigenous sediment supply include the North Sea, the shelf off the Siberian rivers,
the shelf of the Yellow Sea. A prime example is the Senegal delta, where the shelf has
less than 10 cm relief over several miles!
In any study of present shelves, we are chiefly faced with the question of how
much of the observed morphology and sediment cover is inherited from the past and
how much may be ascribed to the activity of the sea today. The task is complicated
by the fact that "today" includes at least the last several hundred years. Within such
time spans the sea can produce effects whose causes may not be obvious, especially
if they include rare but powerful hurricanes and large earthquake-generated waves
(tsunamis).
Tsunamis are produced chiefly in the trenches rimming the Pacific; they travel
over thousands of miles in a few hours. They are very long waves, and so low in the
open sea that they are not noticed on board a ship. When such a wave reaches a shelf
area, however, it slows down and builds up, reaching enonnous heights (tens of
meters) where conditions are right. In this condition it can wreak havoc on the coasts
of exposed shelves and also influence shelf morphology through the powerful bottom
currents it produces.
2.8 The Shelf Break
The shelf break, where shelf joins upper slope, is a prominent morphological feature
of continental margins. In principle, the break marks the depth below which the
influence of sea level on erosion and deposition wanes rapidly (Chap. 5). The details
are by no means understood, however.
The shelf break is commonly represented by a distinct increase in the slope at
about 100 to 150 m. The global mean is near 130 m. In the Antarctic and Greenland
it is very deep, down to 400 m. Here the break may mark the depression by the ice
load and the maximum depth of carving by ice. However, it is equally deep off
southwest Africa, where this explanation does not work. Generally, the break in slope
is distinct (Fig. 2.9), but it can also be gentle in places.
The fact that the shelf break is commonly between 100 to 150 m deep, strongly
suggests that it marks the lowstand of glacial sea level. Apparently this lowstand was
reached repeatedly in the maximum glaciations during the late Quaternary, thus exerting considerable control on shelf evolution. In any discussion of the shelf and the
shelf break in a particular geographic area, the regional isostatic responses of the
shelf to loading and unloading with water due to the changing sea level also must be
filled. The fjords of Norway, Greenland, and western Canada are witnesses to this
powerful action of the ice.
Shelves fonned by large deltas off river mouths (Amazon, Mississippi, etc.) can be
very flat and monotonous, in striking contrast to both the rugged ice-carved shelf and
the irregular coral reef shelf. The rich supply of fine sediment associated with delta
environments allows redistribution and smoothing by waves and currents. Of course,
waves and currents can also build up dunes, barriers, beach benns, and sand waves,
depending on circumstances (see Chap. 7). Examples for these conditions of high
terrigenous sediment supply include the North Sea, the shelf off the Siberian rivers,
the shelf of the Yellow Sea. A prime example is the Senegal delta, where the shelf has
less than 10 cm relief over several miles!
In any study of present shelves, we are chiefly faced with the question of how
much of the observed morphology and sediment cover is inherited from the past and
how much may be ascribed to the activity of the sea today. The task is complicated
by the fact that "today" includes at least the last several hundred years. Within such
time spans the sea can produce effects whose causes may not be obvious, especially
if they include rare but powerful hurricanes and large earthquake-generated waves
(tsunamis).
Tsunamis are produced chiefly in the trenches rimming the Pacific; they travel
over thousands of miles in a few hours. They are very long waves, and so low in the
open sea that they are not noticed on board a ship. When such a wave reaches a shelf
area, however, it slows down and builds up, reaching enonnous heights (tens of
meters) where conditions are right. In this condition it can wreak havoc on the coasts
of exposed shelves and also influence shelf morphology through the powerful bottom
currents it produces.
2.8 The Shelf Break
The shelf break, where shelf joins upper slope, is a prominent morphological feature
of continental margins. In principle, the break marks the depth below which the
influence of sea level on erosion and deposition wanes rapidly (Chap. 5). The details
are by no means understood, however.
The shelf break is commonly represented by a distinct increase in the slope at
about 100 to 150 m. The global mean is near 130 m. In the Antarctic and Greenland
it is very deep, down to 400 m. Here the break may mark the depression by the ice
load and the maximum depth of carving by ice. However, it is equally deep off
southwest Africa, where this explanation does not work. Generally, the break in slope
is distinct (Fig. 2.9), but it can also be gentle in places.
The fact that the shelf break is commonly between 100 to 150 m deep, strongly
suggests that it marks the lowstand of glacial sea level. Apparently this lowstand was
reached repeatedly in the maximum glaciations during the late Quaternary, thus exerting considerable control on shelf evolution. In any discussion of the shelf and the
shelf break in a particular geographic area, the regional isostatic responses of the
shelf to loading and unloading with water due to the changing sea level also must be