The continental slope refers to a relatively steeply sloping surface lying seaward of the continental shelf

The continental slope refers to a relatively steeply sloping surface lying seaward of the continental shelf. The continental slopes beyond the seaward margin of the continental shelf are features similar in their rate of slope to mountain ranges which are present on the continents.

The break at the top of the slope varies from 1 to 10 km above the deep ocean basin at its base. Even greater vertical relief is found in areas where the slope descends into submarine trenches.

Usually there is a gently sloping depositional surface at the base of the continental slope which is known as the continental rise. In fact, it is a deposit which is laid down at the base of the slope on the floor of the deep ocean basin.

Average slope of the continental slope is about 4°, but its steepness may vary from 1° to 25°. The slopes in the Pacific Ocean average more than 5°, while those in the Atlantic and Pacific Oceans are about 3°.

It should be borne in mind that since the slopes around the margin of the Pacific Ocean are associated with the coastal mountain ranges and submarine trenches, the continental slopes are steeper relative to other oceans of the world.

According to Shepard, the average slope of the continental slopes is 4° 17′, but on mountainous coasts their slope is about 3°30′. The most important point to remember is that the continental slopes extend from a depth of 3600 meters to 9100 meters.

On the basis of his observations of 500 profiles, Shepard concludes that the steepness of the slope depends on the nature of the coastlines. The continental slope associated with volcanic cones may measure up to 50°.

Moreover, wherever the submarine canyons traverse the slope, their margins are almost vertical and highly dissected. A comparison may be made between the submarine canyons and the Grand Canyon of Arizona in the United States.

The major characteristic of the continental slope is the fact that due to its steepness the accumulation of silt on it is only nominal. Another important factor for the thinner deposits is the distance from the continental margin due to which the supply of eroded material is dwindled.

Besides, the turbidity currents wash away the accumulated material on it. The availability of light and nutrients being deficient there is less abundance of marine flora and fauna on it. As we are aware, without adequate light plants are unable to carry on photosynthesis and the animals that feed on plants cannot survive.

The continental slopes cover 8.5% of the total area of all the oceans; however, the actual percentage in different oceans is dissimilar.

The most important characteristic of the relief of the continental slopes is the presence of very deep canyons and trenches the origin of which is controversial. Like the steep escarpment, there is a lot of structural control on the relief of the continental slope. There are certain continental slopes which are undoubtedly fault – scarps.

Besides, there are slopes which are only the reef talus. There exist such types of continental slopes in certain areas which have been formed by ridges behind which there are huge accumulation of debris derived from the continental shelves.

According to certain scientists, some of the continental slopes have been formed by the down-warping of such ancient lands which were formerly peneplanes. The continental slopes on the Atlantic coast of North America are actually the margins of the ancient rigid mass of Appalachia.

Shepard has laid great emphasis on the diastrophic origin of such slopes, even though he accepts the possibilities of the origin of the continental slopes by different processes.

Submarine Canyons:

The submarine canyons are usually the steep-sided erosional gorges cut into the sediment and rocks of the shelf and the continental slope. They are in fact the most striking of the negative or depressed features of the shelf and the continental slope.

They are similar to the canyons cut by rivers in the arid or semi-arid regions. Some of the submarine canyons are as large as or even larger than the Grand Canyon of Colorado. The submarine canyons are very often found opposite the mouths of major rivers such as the Congo or Hudson.

Because of their distribution, it is believed that the rivers are mainly responsible for the formation of these submarine canyons. However, there are such canyons that are not located opposite the river mouths, either present-day or ancient.

The submarine canyons, like the canyons formed by the river erosion, have tributaries and steep V-shaped walls that expose various types of rocks of different geological ages. The rocks exposed on the walls of the submarine canyons comprise many types ranging from soft shale’s to quartzite and granite.

One of the characteristic features of these canyons is that they are zig-zag. It was Lindencohl, who for the first time discovered the submarine canyons in 1885. He discovered a submarine valley on the coast of the Hudson Bay which extended for a distance of 160 km up to the continental slope.

Later on, in 1903, another American scientist, Spencer, discovered several such canyons. His discovery revealed that the Hudson Canyon extended up to a distance of 224 km from the continental slope.

The submarine canyons are characterized by a lot of variation in their shape, size and other characteristic features. For example, the canyons opposite the mouths of rivers like the Hudson, Congo and Indus extend from over the continental slopes to the mouths of these rivers.

On the contrary, there are canyons which are found only on the continental slopes. As stated earlier, only some submarine canyons are zig-zag, while others form a dendritic pattern with their tributary canyons.

As regards their size, the submarine canyons range from small gorges to a giant-size canyon of the dimensions of the Grand Canyon cut by the Colorado River. There are yet other canyons which extend on the deep sea plain up to a depth of several thousands of meters.

It may be stated that the submarine canyons are usually found near the continental coasts, but sometimes they are also found along the coasts of islands situated in the oceans.

The walls of these canyons being very steep or almost vertical, there is complete absence of unconsolidated sediments on them. The walls are composed of sedimentary rocks, but there are certain exceptions too.

Monterey Canyon along the coast of California was carved out of granite rocks. The sediments deposited on the floor of these canyons consist of coarser materials than those on the continental shelves. The sediments may be as large as gravels and even pebbles.

Of all the submarine canyons found on the Indian coasts, those on the eastern coast deserve special mention. The following are the important submarine canyons formed on this coast; the submarine canyon opposite to the Ganga mouth, Pondicherry Canyon, Polar Canyon, Godavari” Canyon, Madras Canyon, and Puri Canyon.

From the structural point of view, there are variations in the longitudinal slopes of the submarine canyons. Though the average slope of these canyons is 1.7 percent, those associated with certain islands are characterized by having a slope of 13.8 percent.

Usually the depth of the bottom of these canyons is 334-500 fathoms, but in certain areas their depth exceeds 830 fathoms. Shepard has given the description of a submarine canyon off the coast of New England whose depth varies from 1166 to 1400 fathoms.

Canyons even deeper than 1667 fathoms have been discovered. The gradient or the rate of slope of the bottoms has been measured as 5 to 50 feet per mile. The walls of these canyons are reported to have rock exposures such as granite, limestone, sandy shale’s of the Cretaceous Period and clay of the Tertiary Period.

A large number of submarine canyons have been recently discovered on the continental shelves and continental slopes of the Pacific and Atlantic Oceans. Off the coast of New England at least 30 canyons have been discovered. Similarly, many canyons are known to exist in the Gulf of Mexico, and between Cape Cod and Cape Hatteras.

On the west coast of North America there are 40 submarine canyons discovered between South Mexico and Vancouver Island. Similar canyons are known to exist along the eastern, western and the southern coasts of the continent of Africa.

Portugal, France and the British Islands have also a few submarine canyons on their continental shelves and continental slopes. There is a long list of such canyons whose discussion is rendered impossible due to the limited scope of this book.

Theories of the Origin of Submarine Canyons:

Before discussing the salient features of various theories proposed to explain the origin of the submarine canyons, it would be advisable to bear in mind that the problem still remains unsolved. However, there are various hypotheses to solve this riddle; their brief discussion follows:

Broadly speaking, all the submarine canyons known until now may be placed in two groups; first, the canyons formed by glaciations, and second, the canyons formed by processes other than glaciations. The canyons formed as a result of the glacial activity resemble broad troughs which were produced by the process of glaciations.

This type of canyon is found on the continental shelves of Norway and the Arctic Ocean. On the other hand, the canyons belonging to the other group are found in the form of very deep and elongated depressions which were carved out of rocks by agents of erosion except the glaciers.

Such canyons have been discovered in almost all the seas and oceans of the world, and their numbers are very large. With the development of echo sounding the submarine canyons of the second group have been thoroughly investigated into and surveyed, so that a clear picture of their bottoms has been obtained.

In this group are found three distinct types: (i) a large number of canyons are simply drowned river valleys whose origin is indisputable and noncontraversal; (ii) some canyons are formed only on the continental shelves and are relatively shallower in depth; and (iii) the third type of canyons are very deep and can be traced right out in the deep-sea plains.

They extend from the outer edge of the continental shelf to some distance from the continental slope. Some of the important hypotheses proposed to explain the origin of the abovementioned submarine canyons are the following:

The Sub-aerial Erosion Hypothesis:

Because of their many similarities to the river-cut canyons formed on the land areas in arid and semi-arid regions, Shepard and many other investigators believe that the submarine canyons must have been the result of the sub-aerial erosion.

As a matter of fact, there is no geological theory that would explain the worldwide exposure of the shelf and the continental slope within relatively recent geological time.

However, to justify his hypothesis Shepard has suggested that during the Pleistocene Period huge amount of water was removed from the ocean and deposited on the continents as ice-caps.

Thus, during the Ice Age the fall in the ocean level was assumed to have been about 33 to 38 fathoms as a result of which the continental shelf and the slope were exposed to the agents of sub-aerial erosion.

The rivers then cut their channels on these newly emerged lands. However, with the subsequent increase of temperature the ice-caps would melt and return water to the ocean, which in turn would rise in level.

Subsequently the river-cut valleys were submerged under ocean water. Thus, the sub-aerially eroded river valleys became the present-day submarine canyons.

Even though the arguments in favour of this hypothesis appear to be the most convincing and straightforward, there are certain very serious objections to it.

The main objection against this hypothesis is that the lowering of the sea level as proposed by Shepard could not have been possible as it seems very difficult to envisage enough ice to allow this fall in the sea level.

In other words, this hypothesis implies the change in the sea level of many thousands of feet, where­as the usual estimates of these fluctuations hardly exceed 50 fathoms. Therefore the sub-aerial erosion hypothesis fails to explain the formation of canyons of more than 850 fathoms depth.

Shepard, in 1952, has himself disapproved this idea, and proposed a much more realistic theory that the canyons were formed by a number of composite processes.

It is true that the sub-aerial erosion alone may not account for the formation of all the submarine canyons. However, it seems quite likely that some of them must have originated in this way.

There are many submarine canyons off Corsica coast which appear to be the drowned river vallays, and which are very similar in form to them. According to Gaskell, it is just possible that canyons were formed by the erosive action of the rivers and later on have been down-faulted to attain their present position.

The geophysical evidence is in favour of this viewpoint. Even the presence of hard rock on the walls of the submarine canyons can be explained in this way.

Diastrophic Theory:

According to the diastrophic theory, there are many such canyons which have originated due to the tectonic movements. As we know, the zone of contact between the ocean bottom and the continental shelf is rather weak and the convectional currents from beneath the continents and from under the oceanic crust would cause a subsidence wherever they meet.

This would ultimately lead to faulting and folding. Thus, between the faulted blocks rift valleys and grabens are formed. In the opinion of Andred, the submarine canyons form as a result of the joining together of a series of grabenlike depressions or valleys.

On the continents such grabenlike valleys are destroyed by the agents of erosion, while those submerged under the sea water are safer because of the absence of sub-aerial erosion.

The submarine canyons off the coasts of California, Cyprus, and Morocco etc. appear to be the result of tectonic movements of the earth’s crust. The Hudson as well as the St. Lawrence Canyons is considered to have been formed in the same way.

Similarly the deep-cut submarine valleys as found in the continuation of continental valleys of the Congo and Indus are considered to be of the tectonic origin.

Despite the divergence of opinion regarding the tectonic origin of the submarine canyons, all the scientists and investigators agree that a few submarine valleys are undoubtedly of tectonic origin.

Even the critics of the diastrophic hypothesis are of the view that there are certain continental shelves where there is the possibility of their submarine canyons being of diastrophic origin, but this hypothesis cannot be applicable to the formation of all the submarine valleys.

There is every possibility that the canyons of the Mediterranean Sea and the Pacific Ocean are of the tectonic origin, because in their coastal regions there are evidences of the tectonic movements in the Tertiary and Quaternary Periods.

But there is no evidence of any tectonic movements along the coasts of the Atlantic Ocean in the recent geologic period where the submarine canyons have originated by cutting the Pliocene rocks. It is just possible that these canyons must have originated after the formation of the Tertiary rocks.

Besides, all the structural land forms which were produced as a result of faulting near the coasts affected by the tectonic movements are invariably parallel to them.

On the contrary, almost all the submarine canyons are found at right angles to the sea coasts. Moreover, the dendritic pattern of the tributaries of these canyons is not possible in the valleys formed as a result of active diastrophism.

Even the critics of the diastrophic theory do agree on this point that the submarine canyons formed by other agencies may be deepened by the tectonic forces. The protagonists of this hypothesis of the origin of submarine canyons include such eminent scientists as Andred, Lawson, Gregory, Bourcart and Jensen.

Submarine Mudflow and Landslides Hypothesis:

In connection with his investigations of the ancient submerged river canyons, Shepard assumed that some of the submarine canyons were formed during the Palaezoic Era. Later on, they were filled up by terrigenous and pelagic deposits, which at a later period sided under the continental shelf due to landslides.

The earthquakes also contributed in loosening the deposited materials which were transferred by the landslides with the result that the filled up canyons reopened.

Canyons formed before the advent of the Pleistocene Period were partially filled with debris derived from land and because of the fall in the sea-level water continued to flow through these valleys into the ocean.

The powerful water currents continued to remove the accumulated debris from those canyons, so that the pre-existing canyons were made deeper and deeper.

However, after the investigation of sediments of the last phase of the Tertiary Period on the walls of the submarine canyons off the coasts of New England and California, it became obvious that the origin of these submarine valleys is not as old as it was supposed to be according to this theory.

Due to mudflow the walls of these canyons are very steep, and very fine particles of sediments get accumulated on the valley-floors. Shepard and Emery conclude that in the Tertiary Period there was an uplift of the continental margins, so that the erosive power of the rivers flowing into the ocean increased.

Now, the sediments brought down by the rivers were deposited on the bottom of the sea. Later on, due to subsidence of the land the river valleys were submerged under the sea-water and the clay deposits filled up the canyons.

The sea waves by their erosive action carried most of the deposited materials seaward. That is why such drowned valleys have their mouths open, whereas their landward portion is filled with sediments.

Since landslides are not so common, they along with the mudflow are capable of only deepening the canyons, but their origin cannot be attributed to them.

Submarine Density Currents:

Submarine density currents are produced due to the difference in temperature. In fact, the density currents generated at the bottom of the sea are not capable of forming the submarine canyons; they are rather more effective in lakes.

In the ocean water the stratification of water of different densities does not allow the vertical motion of any kind. Under such conditions the creation of density currents in the oceans and seas appears to be a remote possibility.

However, according to some, when the cold and dense surface water reaches the ocean floor, it creates density currents, which in turn contributes in the creation of a submarine canyon. Again some scientists are of the view that density currents are produced because of the combined effect of temperature and salinity.

In Florel’s opinion, density difference alone could create submarine density currents. He applied this hypothesis to the formation of sub-lacustrine trenches in the Lake Constance and Lake Geneva.

The trenches of the Rhine and those of the Rhone Rivers are formed because of the fact that along the axis of the current there is no deposition of sediments, while on the valley side’s deposition goes on creating lateral dykes.

However, later on he rejected his own idea that the erosion of the trenches by density currents was the result of such currents. His modified view was that erosion in front of the river mouths by density currents forms the upper part of a trench or depression. Some distance away from the river mouths, erosion is replaced by deposition.

Florel’s earlier contention that the Congo Canyon was the creation of density currents was strongly opposed by other investigators, who argued that other canyons are not associated with the rivers.

Daly was one of the protagonists of this theory. He also considered the density currents as one of the important factors leading to the formation of the submarine canyons.

According to him, when the sea-level registered a fall in the Ice Age, the muddy waters of the rivers flowing over the slope of the oceans cut deep valleys there, which later on drowned in the sea – water and became the submarine canyons.

To conclude, the density currents are of little significance in the formation of such canyons.

Turbidity Currents Theory:

The Turbidity Currents Theory was proposed as a result of the investigations of the sediments comprising very fine particles of sediments accumulated on the floor of submarine canyons.

Daly for the first time presented his view that the turbidity currents were mainly responsible for the formation of the submarine canyons. Daly’s arguments found full support when the trans-Atlantic cables laid on the bottom of the ocean broke down due to Grand Banks earthquake that occurred on November 18, 1929.

It was made clear that the velocity of the turbidity currents could be about 55 knots and that these currents are able to form these canyons. There is little doubt that the turbidity currents even with lesser velocity are also capable of producing the canyons by their great erosive capacity.

Most of the scientists are of the view that in the absence of these currents the submarine canyons would have been filled with sediments.

Regarding the origin of turbidity currents, Daly argues that during the Pleistocene Period when the sea-level was lowered, the exposed continental shelves were eroded considerably by the on-shore storms, and the resultant attack of waves.

Huge amount of wave-cut materials got mixed with sea water. At the same time the rivers also carried with them large amount of silts and poured them down in the ocean water, so that the coastal waters were charged with huge amount of silts and sediments.

The result was that the coastal waters being denser started flowing towards the sea. During the process additional help was received from the sea waves generated by on-shore high- velocity winds which proved to be a contributory factor in raising the sea-level near the shore.

The water-level being higher at the coasts, submarine counter-currents started flowing towards the sea. This is how the turbidity currents (currents with large amount of sediments or mud) erode the continental shelves and continental slopes, and form deep channels known as submarine canyons.

The critics of this theory hold the view that the velocity of the turbidity currents is not so high that they are able to erode the harder rocks. Besides, the steepness of the walls and the depth of these canyons are certainly against the validity of this theory.

However, it cannot be denied that there are certain canyons which were definitely formed by the erosive power of the turbidity currents.

Submarine Spring Sapping:

Johnson, after a thorough review of the literature regarding the character and origin of submarine canyons, developed the hypothesis that “solution and erosion resulting from the outflow of underground water might contribute to the formation of the canyons”.

According to him, submarine springs produce notches on the continental slopes which gradually grow in size, and ultimately become submarine canyons.

During the Tertiary and Cretaceous Periods, and the Paleozoic Era the outcrops of porous sedimentary rocks were found on the land, and at certain places they extended up to the continental slopes in the oceans.

The rain water had percolated into the layers of porous rocks. Because of the steep incline of the layers of such rocks towards the oceans and seas, their outcrops were present on the continental slopes.

It was on the margins of these outcrops that springs of fresh water were produced in the sea. Under favourable conditions several such springs are produced on the continental slopes.

As the water gushing out of springs first produces a notch or opening on the ground and thereafter the notch develops into a large depression, and finally the depression changes into a very deep canyon, in the same way the submarine water flowing out of the submarine springs slowly and gradually carves out a submarine canyon.

However, a very long period of time is required for the formation of a canyon in this manner. It is to be borne in mind that the spring water being the mixture of different chemicals is capable of dissolving the rocks.

Besides, because of the force of its flow the spring water easily erodes the rocks also. Although this method of formation may apply in some areas, it may not account for all the submarine canyons.

Tsunamis or Earthquake Waves:

One group of scientists believes that stronger water motions from the long surface waves originating from strong seaquakes, which are called tsunamis in the Pacific Ocean, are capable of forming the submarine canyons.

This Japanese term (tsunamis) has been used for progressive long waves caused by submarine volcanic activity and eruptions and which spread in all directions from the epicenter. In the immediate vicinity of the epicenter waves of large amplitudes have been observed.

These high – amplitude waves break in shallow water and may reach heights of up to 35 meters. However, this phenomenon is most frequent only in the Pacific Ocean because of the great frequency of submarine earthquakes at the marginal zones of this ocean.

The Japanese coasts are very often hit by tsunamis. Thus, if at all, the tsunamis- formed canyons must be confined to the Pacific Ocean only.

According to Bucher, a strong opponent of the tsunamis theory, the velocity of sea waves in the submarine canyons is so low that they cannot form such topographic forms on the ocean bottom.

Even if in certain areas tsunamis is powerful enough to produce high-velocity waves on the ocean bottom which can form depression there, this theory cannot be applicable to wider areas. The worldwide distribution of the submarine canyons cannot be explained by the Pacific tsunamis.

The above discussion of various hypotheses proposed to explain the origin of the submarine canyons makes it amply clear that no single hypothesis is comprehensive enough to account for the formation of all the canyons of the world.

The Deep Sea Fans and the Continental Rise:

The deep sea fans and the continental rise are basically the topographic features associated with the continental margins. The deep sea fans are the depositional features associated with the formation of the submarine canyons.

These fans are formed at the foot of the submarine canyons. These fan-shaped deposits are the product of turbidity currents which comprise flows of sediment-laden water that move down the canyons from time to time.

The debris brought down by the turbidity currents are deposited at the base of the continental slope. The deposition occurs because of the slowing down of the flow of sediment-laden water as it reaches the bottom. These deposits are graded according to the grain size of the deposited sediments. The name ‘turbidities’ is given to these graded deposits.

These fans often have deep channels carved out by the flow as it moves across the fans. Thus, a pattern of distributaries develops. These distributaries are flanked by levees that are deposited by the turbid water overflowing the relatively shallow distributary channels.

The continental rise is a feature commonly found at the base of the continental slope because of the merging together of the deep sea fans. It is defined as a gently sloping surface that leads gradually into the deep-sea plains of the deep ocean basin.

It links the deep ocean floor to the continental slope. Its gradient varies from 1:100 to 1:800. The width of the conti­nental rise shows great variations. In certain areas the continental rise extends to several hundred kilometers in the abyssal plain.

Off North Africa near Dakar, the width of the rise exceeds 640 km. How-ever, it is nonexistent off the Bay of Biscay, and is only 112 km wide off England.

Because of numerous trenches located at the base of the continental slopes in the Pacific Ocean, only a few continental rises are found in it. But the continental rise forms a characteristic feature of the bottom topography of the Atlantic and Indian Oceans.