CONTINENTAL MARGIN & THE SEA FLOOR

THE RELIEF OF THE OCEAN

If all water is removed from the ocean basins, the kind of surface revealed is not to be the quiet, subdued topography as was once thought, but a surface characterized by great diversity – towering mountain chains, deep canyons and flat plains. Infact, the scenery would be as varied as that on the continents.

FATHOMOMETER, a radar sounding and electric echo device is used to determine the depth of sea
Oceanographers studying the topography of the ocean basins have delineated three major units:
i. Continental Margins;
ii. The Ocean Basin Floor; and,
iii. Mid Ocean Ridges

I. Continental Margins
The zones that collectively make up the continental margin include the continental shelf, continental slope and continental rise.

1. Continental Shelf
The continental shelf is a gently sloping submerged surface extending from the shoreline towards the deep ocean basin. Since it is flooded by continental type crust, it is clearly a flooded extension of the continents. According to Goh Cheng Leong the shelf is in fact the seaward extension of the continent from the shoreline to the continental edge.
The depth of the continental shelf is conventionally taken as 100 fathoms (200 meters) but it is variable. Its width varies greatly. Off the coast of Ireland it stretches westward for a distance of 50 miles and off Siberia it reaches a maximum width of 800 miles. But it may be only a few miles, or it may be absent altogether. On the average, the continental shelf is about 80 km wide and 130 m deep at the seaward edge.
The width or extent of the shelf is determined by the adjoining landforms. In the case of plain coastal area, the shelf will be several miles in width. While if the coast is hilly and mountainous the continental shelf will be narrow.
The average inclination of the continental shelf is less than 1/10th of 1o, a drop of only about 2 m/km, (10 ft/mile), but seldom it is more than 2 or 3o (average), e.g. Ireland=5o, Cape Torinana=36o. The slope is so slight that it would appear to an observer to be a flat surface.

Origin of Continental Shelf
When compared with many parts of the deep ocean floor, the surface of the continental shelf is relatively feature less. This is not to say that the shelves are completely smooth. The most profound features are long valleys running from the coastline into deeper waters. Many of these valleys are seaward extensions of sea valleys on the adjacent landmass. Such valleys were excavated during the Pleistocene Epoch (Ice Age). During this time, great quantities of water were tied up in vast ice-sheets on the continents, causing sea level to drop by 90-120 m and exposing large portions of the continental central shelves. Due to this drop in sea level, rivers extended their courses, and land dwelling plants and animals inhabited the newly exposed portions of the continents. Today, these areas are again covered by the sea and inhabited by marine organisms. Dragging along the eastern coast of North America has produced the remains of numerous land dwellers, including mammoths, mastodons and horses. Bottom sampling has also revealed that fresh water peal bags existed, adding to the evidence that the continental shelves were once land areas.
Some smaller continental shelves may be formed by Wave Erosion or by the Deposition of land-derived or river-borne materials on the offshore terrace.
Shelves formed by erosion are found near Iceland and Faeroe Islands, Denmark. The breadth of erosional platform will depend upon:
i. the resistance of rocks;
ii. the strength of the waves and currents; and,
iii. the length of time during which the level of land and sea remains unchanged.
Glacial erosion has played a large part in the formation of shelves surrounding glaciated countries. These shelves exhibit the characteristic features – basins and troughs – resembling terrestrial features produced by ice. Such deposits have been modified by current or wave erosion. F. P. Shepard (Submarine Geologist) concludes;
“The most clear cut history of shelf development is found around the glaciated areas. The shelves in these places appear to have been greatly deepen specially on the inside by the movement of glaciers”.
The broken material due to the action of rain, rivers and waves of the sea also settles near the land. Off the mouth of Amazon it is said that the sea is sometimes discolored by mud at a distance of 300 miles. But still even the largest rivers must deposit most of their burden near the shore. The waves and currents may widen the settling area. The material derived from land accumulates as Sub-Marine Terrace upon the margins.
Beyond the edge of the continental terrace, the waves and currents cease to be effective. But as the terrace is built up, the water becomes shallower and the action of the transporting agents becomes more effective and they are able to carry the material farther than before. The terrace therefore grows gradually outwards by addition at its edge, in the same fashion as a railway embankment or the tip-heaps of a quarry.
Delta Growth is locally also an important factor in the formation of the shelf, e.g., Mississippi Delta. According to this view the continental shelf and slope are formed by deposits brought from the land. The deposition is smoothened and redistributed by waves and currents. The action of currents in general is said to cease at about 600ft. under the surface and as a result the shelf is usually found at this depth.
Shepard suggests that in some cases a combination of erosion and deposition may have produced wide shelves. Recent geophysical work on the origin of the shelf suggests that they are in part Gigantic Prism-Shaped Accumulation of sediments perhaps several thousand meters in thickness.
Significance
The continental shelves represent 7.5% of the total ocean area, which is equivalent to about 18% of the earth’s total land area. These areas have taken an increased economic and political significance.
They have been found to be sites of important mineral deposits, including large reservoirs of petroleum and gas (nearly 20%), as well as huge sand and gravel deposits.
The shallowness allows the sunlight to penetrate through the water, which encourages the growth of minute living organism. The plankton are a source of food to fishes. The waters of the continental shelf contain many important fishing grounds that are significant sources of food. For example the Grand Banks of New Foundland, the North Sea and the Sunda Shelf.
Their limited depth and gentle slope keep out cold undercurrents and increase the height of tides. This hinders shipping as ship can only enter and leave port on the tide. World’s greatest seaports are located on continental shelves, e.g., London, Singapore, Hong Kong.

2. Continental Slope
Making the seaward edge of the continental shelf is the continental slope, which leads into deep water and has a steep gradient compared to the continental shelf, i.e., about 1 in 20 (2o-5o), and extend upto 3,660 m. While the gradient varies from place to place, it has an average drop of about 70 meters/km (370 ft/mile). The continental slope represents the true edge of the continent.
Along some mountainous coasts, the continental slope descends abruptly into deep ocean trenches, which intervenes between the continent and ocean basin. In such cases, the shelf is very narrow or does not exist at all. The sides of this continental slope and the trench are essentially the same feature and grade into the adjacent mountains, which tower thousands of meters above sea level. This situation occurs along the West Coast of South America. Here the vertical distance from the high-peaks of the Andes Mountains to the floor of the deep Peru-Chile Trench, bordering the continent, exceeds 12,200 m (40,000 ft).
Perhaps the most significant features associated with continental slopes are deep, steep sided valleys known as Submarine Canyons. Originating on the outer continental shelf or on the continental slope, submarine canyons may reach water depths of 3 km. Although some appear to be seaward extension of valleys that were carved on the continental shelf during the Ice Age, others are not oriented in this manner. Further more, the canyons reach depths far below the maximum lowering of sea level, which indicates that they were created in some process that operates below the ocean surface. Most available information seems to favor the view that these deep scars on the continental slope are excavated largely by turbidity currents, which are dense slurries of sediment and water that flow downslope.

3. Continental Rise
In regions where trenches does exist, the steep continental slope merges into a more gradual incline known as continental rise. Here the gradient lessens to between 4 and 8 meters per kilometer (not more than 1o). It becomes very flat towards the deep sea floor and virtually merges with the Abyssal Plain. While the width of the continental slope averages 20 km. The continental rise may reach for hundreds of kilometers. This feature consists of a thick accumulation of sediments that moved downslope from the continental shelf to the deep ocean floor. More specifically, the sediments comprising the continental rise are delivered to the base of the continental slope by turbidity currents that follow submarine canyons. When these muddy currents emerge from the mouth of a canyon onto the relatively flat ocean floor, they deposit sediment that forms Deep Sea Fan. Deep sea fans have the same basic shape as alluvial fans, which form at the foot of steep mountain slopes on land. As fans from adjacent submarine canyons grow, they coalesce to produce the continuos apron of sediment at the base of the continental slope.

II. Ocean Basin Floor

Between the continental margin and oceanic ridge system, lies the ocean basin floor. The size of this region almost 30% of the earth’s surface is roughly comparable to the percentage of the surface that projects into the sea as land. Here we find ocean trenches, which are dramatically deep grooves in the ocean floor; remarkably flat regions, known as Abyssal Plains; and steep sided volcanic peaks, called Seamounts.

1. Deep Ocean Trenches
Deep ocean trenches are long, relatively narrow features that represent the deepest parts of the ocean. Several in the western Pacific approach or exceed depths of 10,000 meters and atleast a portion of one, the Challenger Deep in the Mariana Trench, southwest of the island of Guam is more than 11,000 m (39,960 ft.) below sea level.
Although deep ocean trenches represent only a very small portion of the ocean floor area, they are nevertheless very significant geological features. Trenches are the sites where moving crustal plates are destroyed as they plunge back into the mantle. In addition to the earthquakes created, as one plate descends beneath another, igneous activity also associated with trench regions. Trenches in the open ocean are paralleled by volcanic island areas, while volcanic mountains, such as the Andes, may be found paralleling trenches that are adjacent to the continents. Melting a descending plate produces the molten rock that leads to this igneous activity. Other examples of ocean deeps, which are around the shores of Pacific are; Mindanao Deep (35,000 ft.), Tonga Trench (31,000 ft.) and Japanese Trench (28,000 ft.); whereas the highest peak on land, Mt. Everest is only 8,848 meters (29,028 ft.).

2. Abyssal Plains
Abyssal plains are incredibly flat features, infact, these regions are likely the most level places on the earth. The abyssal plains found off the coast of Argentina, for example, have less than 3 meters (10 feet) of relief over a distance exceeding 1300 km (300 miles). The average depth of an abyssal plain is about 3,000-6,000 m and it covers nearly 40% of the ocean. The monotonous topography of abyssal plains will occasionally be interrupted by the protruding summit of a buried volcanic structure.
By employing seismic profiler instruments whose signals penetrate far below the ocean floor, the researchers have shown that abyssal plains consist of thick accumulations of sediments that were deposited atop the low, rough portions of the ocean floor. The nature of the sediment indicates that these plains consist of sediments transported far out to sea by turbidity currents. The turbidity deposits are inter-bedded with sediments composed of minute clay sized particles that continuously settle onto the ocean floor.
Abyssal plains are found as part of the sea floor in all of the oceans. However, they are more wide spread where there are no deep ocean trenches adjacent to the continents. Since the Atlantic Ocean has fewer traps to act as traps for the sediments carried down the continental slope, it has more extensive abyssal plains than the Pacific.

3. Seamounts
Dotting the ocean floor are isolated volcanic peaks called seamount that may rise hundreds of meters above the surrounding topography. These steep-sided conical peaks are found on the floors of all the oceans, but the greater number and density have been identified in the Pacific.
Many of these undersea volcanoes begin to rise near oceanic ridges, divergent plate boundaries where the plates of the lithosphere move apart, they continue to grow as they ride along on the moving plain. If the volcano rises fast enough, it emerges as an island. Examples in the Atlantic include the Azores, Ascension, Tristan de Cunha, and St. Helena. During the time when they existed as islands, some of these volcanoes are eroded to near sea level by running water and wave action. Over a span of millions of years, the islands gradually sink as the moving plate slowly carries them from the oceanic ridge area. These submerged, flat topped seamounts are called Guyots. In other instances guyots may be remnants of eroded volcanic islands that were formed away from the ridge crest, possibly by hot spot activity. Here subsidence occurs after the volcanic activity ceases and the sea floor cools and contracts.

III. Mid Ocean Ridges

The ocean floor’s continuity is interrupted by islands rising above the sea level. These are of various types according to their origin:
Volcanic: made of basaltic rocks, e.g., Hawaii, Pacific Coral Islands
Continental Fragments or Sundered Islands: Detached from parent island masses, e.g., Madagascar, New Foundland.
Orogenic Islands or Island Areas, e.g.; islands along the Asiatic corner of the Pacific Ocean.
Mid ocean ridges are found in all major oceans and represent more than 20% of the earth’s surface. They are certainly the most prominent topographic features in the oceans for they form an almost continuous mountain range, which extends for about 65,000 km. In a manner similar to seam on a baseball.
Although ocean ridges stand high above the adjacent deep ocean basins, they are much different from the mountains found on the continents. Rather than thick sequences of folded and faulted sedimentary rocks, oceanic ridges consist of layer upon layer of basaltic rock that have been faulted and lifted.
The term “Ridge” may also be misleading since these features are not narrow, but have widths from 500 to 5,000 km and, in places, may occupy as much as ½ of the total ocean floor area. Ridge crests are marked by deep clefts or rifts (cracks), and are flanked by ridges and lines of peaks that extend outward for hundreds of kilometers. Axes of the ridges are marked by frequent earthquakes and characterized by a much higher heat flow through the crust. The rifts at the center of the ridges are the sites where the new magma wells-up from the Asthenosphere below, continually creating new oceanic crusts. The rifts, therefore, represent divergent plate boundaries where sea floor spreading is taking place.
The primary reason for the elevated position of a ridge system is the fact that newly created oceanic crust is hot, and therefore occupies more volume than cooler rocks of the deep ocean basin. As the young lithosphere travels away from the spreading center, it gradually cools and contracts. This thermal contraction accounts in part for the greater ocean depths that exist away from the ridge. Almost 100 million years must pass before cooling and contraction cease completely. By this time, rock that was once a part of a majestic mountain system is located in the deep ocean basin where it is mantled by thick accumulations of sediments.
Echo soundings have revealed scores of underground mountains. These mountains in Pacific are mostly of a volcanic origin. Many have flat tops, which were eroded by waves at the surface before sinking to their present depths.
Down the middle of the Atlantic runs a formidable mountain range, the “Mid Atlantic Ridge”, sometimes identified as the lost continent of Atlantic.