58 Origin and Morphology of Ocean Margins
Fig. 2.10. Physiographic diagram of the ocean margin off California. Note the narrow shelves
bounded by sea cliffs toward the land (uplift!). The Continental Borderland in the south is a
submerged basin-and-range province. The continental slope essentially consists of enormous coalescing fans which transgress over the abyssal hills province. [Sketch based on physiographic diagram
of H. W. Menard, 1964.]
The various physiographic diagrams commonly depicting the rapid descent from
shelf edge to deep sea are somewhat misleading in that the slopes are in reality quite
gentle: between I ° and 6° (see Table 2.1). A 1 ° slope, of course, would appear as a
plain if one were standing on it.
From the variety of types of slopes, it is clear that a number of different forces are
at work in shaping them. We have referred to some of the endogenic forces when
comparing Atlantic-type and Pacific-type margins. Changes in the thickness of continental crust, trough remelting and assimilation into the mantle (subcrustal erosion),
have been proposed as one mechanism influencing the evolution of margins. Such
processes may be active, for example, in the Southern California Borderland.
Basically, most slopes are the surfaces of thick accumulations of sediment washed
off the continents and mixed with marine materials of biologic derivation. The deep
sea receives a rather miserly share of the continent's products: the bulk accumulates
on the margins. The high rate of accumulation on many slopes leads to a precarious
balance in places, especially off deltas. When there is not enough time to de-water
and solidify the sediment, immense landslides can result from rather small disturbances, even in very gentle slopes. The slides tend to move on surfaces defined by
clayey layers with high water content. Water pressure may be unusually high in such
Fig. 2.10. Physiographic diagram of the ocean margin off California. Note the narrow shelves
bounded by sea cliffs toward the land (uplift!). The Continental Borderland in the south is a
submerged basin-and-range province. The continental slope essentially consists of enormous coalescing fans which transgress over the abyssal hills province. [Sketch based on physiographic diagram
of H. W. Menard, 1964.]
The various physiographic diagrams commonly depicting the rapid descent from
shelf edge to deep sea are somewhat misleading in that the slopes are in reality quite
gentle: between I ° and 6° (see Table 2.1). A 1 ° slope, of course, would appear as a
plain if one were standing on it.
From the variety of types of slopes, it is clear that a number of different forces are
at work in shaping them. We have referred to some of the endogenic forces when
comparing Atlantic-type and Pacific-type margins. Changes in the thickness of continental crust, trough remelting and assimilation into the mantle (subcrustal erosion),
have been proposed as one mechanism influencing the evolution of margins. Such
processes may be active, for example, in the Southern California Borderland.
Basically, most slopes are the surfaces of thick accumulations of sediment washed
off the continents and mixed with marine materials of biologic derivation. The deep
sea receives a rather miserly share of the continent's products: the bulk accumulates
on the margins. The high rate of accumulation on many slopes leads to a precarious
balance in places, especially off deltas. When there is not enough time to de-water
and solidify the sediment, immense landslides can result from rather small disturbances, even in very gentle slopes. The slides tend to move on surfaces defined by
clayey layers with high water content. Water pressure may be unusually high in such