Submarine Canyons: Meaning and Theories | Oceanography | Geography

In this article we will discuss about:- 1. Meaning of Submarine Canyons 2. Distribution of Submarine Canyons 3. Theories.

Meaning of Submarine Canyons:

Long, narrow and very deep valleys or trenches located on the continental shelves and slopes with vertical walls resembling the continental canyons are called submarine canyons because of their location under oceanic water.

On the basis of morphogenetic processes these are classified into:

(i) Glacially eroded canyons, and

(ii) Non-glacial canyons.

The non-glacial submarine canyons being more in number than the glacial canyons and widely spread in all the oceans have been studied in much detail. The non-glacial canyons, thus, will be described as submarine canyons in the following discussion. These, besides a few exceptions, are found transverse to the coasts and in front of the mouths of major rivers.

On an average, there is little difference in the transverse and longitudinal profiles of submarine and subaerial (continental) canyons. According to Sheppard the submarine canyons are similar to the youthful river valleys on the land but are decidedly deeper and a few of them have dendridtic pattern of tributaries of sec­ondary canyons.

The longitudinal course of submarine canyons is usually sinuous while that of the subaerial canyons is generally straight. The gradient of subma­rine canyons is steeper than the continental canyons. The submarine canyons are generally several kilome­tres wide at their heads and their average length is 16 km.

Though the gradient of longitudinal profiles of the canyons varies significantly but on an average it is 1.7 per cent. The canyons facing the river mouths are usually long (e.g., Congo Canyon) but have gentle gradient. The canyons located near the island are deep with steepest gradient (13.8 per cent). According to the studies of 102 submarine canyons by Sheppard and Beard average gradients of the upper, middle and lower segments of the canyons are 11.62 per cent, 6.63 per cent and 4.76 per cent respectively.

The depths of submarine canyons vary from 610m to 915m. At few places the depth has been noted up to 3,048m. The submarine canyons carry various types of ocean de­posits but the steep valley sides are devoid of unconsolidated materials. The floors of the canyons have coarser materials than the adjacent continental shelves. The deposits include sands, clays, silt, gravels and pebbles.

Distribution of Submarine Canyons:

The world distributional pattern of submarine canyons does not reveal any control of latitudes on their distributions and location. Francis Sheppard and Charles Beard have located 102 submarine canyons in the world on the basis of soundings of the continental shelves and slopes.

Generally, submarine canyons are more abun­dantly found along the straight coasts than highly indented and crenulated coastlines. They are found along the stable and unstable coasts alike. They are more commonly found off the east coast of the USA from Canada to Cape Hatteras; off the Californian and Mexican coasts; along the north Mediterranean, Phil­ippines, Jappan and Aleutian islands; off the coast of west Africa; off the east coast of India etc.

Atlantic Ocean:

Significant submarine canyons of the Atlantic Ocean are Hudson Cayon (facing the mouth of the Hudson river, 827m deep), Chesapeak Canyon, Mississippi Trough, Fosse de Cape Brenton Canyon (in the Bay of Biscay off the south-western coast of France), Nazare Canyon (off the western coast of Portugal, 4000m deep), Congo Canyon (near the mouth of the Congo river) etc.

Pacific Ocean:

Columbia Canyon; Monterey Canyon (which has several tributary canyons like Ascension canyon, Soquel canyon, Carnel canyon etc.); Mugu canyon, Scripps canyon and Dume canyon (all are off the Californian coast); Panama canyon (off Burica Peninsula) etc. are the important canyons on the western coast of North America while Piseu Chang canyon (off the coast of Korea), Philippine canyon (on the main coast of Luzon), Saganin canyon, Fizi canyon etc. are a few prominent canyons of the western Pacific Ocean.

Indian Ocean:

Canyons are found along the eastern coast of India (table 24.2), in front of the Indus river, along the north-eastern coast of Sri Lanka, along the eastern coast of Africa etc.

Theories of Submarine Canyons:

The following theories have been put forth to explain the origin of submarine canyons:

(1) Diastrophic Theory:

A few exponents (Andrade, Lawson, De la Roche Ponie, J.W. Gregory, Yanasaki, Jensen, Bourcart etc.) have related the ori­gin of submarine canyons to various types of earth movements and tectonic implications (faulting, fold­ing, warping, sinking of sea floor etc.). The tensional forces caused by earth movement due to endogenetic forces result in the formation of faults and graben on the continental shelves and slopes.

These fault-troughs and graben become submarine canyons. Similarly, warping and steep folding give birth to synclinal basins and synclinal troughs respectively which become sub­marine canyons. According to De Andrade submarine canyons are formed due to creation of a series of graben – like valleys during local coastal displacements.

Such tectonically originated submarine canyons have been reported by Lawson off the Californian coast, by De la Roche Ponie near the coast of Cyprus and Morocco, by J.W. Gregory (Hudson Canyon and St. Lawrence Trough), by Yanasaki (near Japan coast) etc. According to Jensen and Bourcart submarine canyons were formed during Quaternary period due to subsidence and drowning of river valleys along the continen­tal marginal flexure.

This diastrophic theory of the origin of subma­rine canyons is criticised mainly on three counts:

(i) Majority of submarine canyons are found transverse to the coast whereas faulting generally occurs parallel to the coasts,

(ii) Many of the submarine canyons have dendritic pattern of their tributaries which cannot be explained through faulting, and

(iii) Not all the continental shelves and slopes show evidences of faulting.

This theory may explain the formation of canyons along the Pacific coasts (western coasts of North and South Americas and eastern coasts of Asia) and Mediterra­nean Sea where Tertiary and Quaternary earth move­ments were most active but the canyons along the western (eastern coasts of North and South Americas) and eastern (off the western coasts of Europe and Africa) of the Atlantic Ocean may not be explained in the absence of such movements. The canyons on the eastern coast of North America cut across the lithology of Tertiary and Quatenary periods.

(2) Subaerial Erosion Theory:

Several exponents (e.g., J.D. Dana, F.P. Sheppard, Hull etc.) on the basis of resemblance of submarine canyons to the continen­tal canyons in shape and deposition have related the formation of the former to the entrenching of river valleys by running water and subsequent drowning of these valleys due to subsidence and submergence of continental margins.

According to them the rivers eroded their valleys very deep forming deep gorges during the period of emergence when land rose higher well above the sea-level and the channel gradient was steepened. Later on the continental margins were ei­ther subsided due to earth movements or the sea-level rose (due to deglaciation) and thus these deep and long valleys were drowned and submarine canyons were formed.

The drowned valleys in Java Sea, Philippine Canyon, Monterey Canyons etc., have been cited as typical examples of submarine canyons formed due to sub-aerial erosion because their longitudinal profiles show upward concavity like continental canyons and there is significant terrigenous deposits in them.

W.M. Davis while contradicting the above theory argued that the formation of submarine canyons through subaerial erosion required vertical oscillation of land say upheaval of the continental margins upto thou­sands of feet above sea-level and subsequent equivalent regional subsidence to submerge the entrenched river valleys.

This would require long geological period as the aforesaid tectonic mechanism is not possible within short geological time. Secondly, if the submarine can­yons are the result of sub-aerial erosion during emer­gence and subsequent drowning during submergence, these canyons must have continued over the land also but these are found far away from the river mouths.

Emery and Sheppard while reacting to the first objec­tion of W.M. Davis maintained that the lowering of sea-level upto 1000 m during Pleistocene glaciation provided ideal continental platforms for the entrench­ing of valleys by the rivers and subsequent rise of the sea-level due to deglaciation submerged the deeply entrenched valleys to form submarine canyons. If this explanation is accepted, the submarine canyons be­yond the depth of 2000m remain unexplained.

(3) Submarine Density Current Theory:

Holimann (1883), Adolf Von Salis (1884) and Florel have related the formation of submarine canyons to the submarine density currents. These density currents are originated due to difference in density caused by temperature and salinity variations. Such density currents erode the continental shelves and form trenches while stagnant water on either side of the trenches allows sedimenta­tion and dyke formation (levees).

The density currents are originated mainly in front of the river mouths because of differences (in terms of temperature and salinity) in the water brought by the rivers and sea water. It may be pointed out that density currents are confined to enclosed seas, reservoirs and lakes only and these are seldom originated over shallow conti­nental shelves and thus density currents may not be taken as causative factors of the formation of subma­rine canyons.

(4) Turbidity Current Theory:

Turbidity currents having fine materials in suspension have been held responsible by several exponents (W.M. Davis, W.E. Rither, Tangier Smith, P.D. Trask, Lawson, Daly, Buchanan etc.) for the origin of submarine canyons in one way or the other. Strong onshore winds pile up water near the sea-shore with the result undercurrents are generated which flow towards the sea.

These un­dercurrents bring fine materials in suspension and so they are called turbidity currents. The higher density of these currents due to suspended sediments with them forces them to flow seaward under the surface water. The turbidity currents erode the continental shelves and form submarine valleys and canyons.

According to Daly there is increased rate of erosion of coastal land through marine waves due to fall in sea-level during glacial period, with the result turbidity of sea water is increased due to which density of sea water is also increased, consequently seaward turbidity currents are originated. These currents while moving over the con­tinental shelves and slopes erode them in linear manner and form submarine cayons and valleys.

Many critics (Zeppelin, Heim, Bucher etc.) have doubted the efficiency of turbidity currents to form submarine canyons. According to them the velocity of these currents is not such that they can powerfully erode the hard rocks of continental shelves to form canyons. Bucher is of the opinion that currents gener­ated through earthquakes and volcanic eruptions are more rapid and powerful and hence are more capable of eroding the continental shelves to form canyons.

Following Kunen it may be forwarded that submarine canyons in different localities having vary­ing lithologies and structures should be explained separately. The canyons developed in stable areas of compact and tenaceous lithologies are formed due to drowning of subaerial valleys, while those carved in unconsolidated lithogloies might have been formed through landslides, turbidity currents etc.