Ocean Bottom Relief
OCEAN BOTTOM RELIEF Four Relief Zones- Continental Shelf Continental Slope Deeps & Plains Ridges & Trenches
A. Continental shelf extension of the continent and not really a part of the ocean basin. Depth & Slope: - average water depth of 100 or 180 m, and gently sloping (1° to 3°) towards the oceans. Width:- various from 60 km to more than 1,500 km The following factors are affected by the width. It is narrow where high mountains are very close and parallel to the coast. For example, the Pacific continental shelf along the western coast of South America is narrow (only 16 km wide) because of the presence of the Andes mountains. The shells are wide where the coast lands are wide plains. Though the continental shelves are generally wider in front of the river mouths.
Continental shelf Area: - 8.6 per cent of the total area of the ocean basin. Types:- 1. inner continental shelf 2. mid continental shelf 3. outer continental shelf. Importance:- provide ideal habitats for marine life including plants and animals . ideal fishing grounds. The coral reefs are considered the frontline natural buffers against storm, tidal surges and powerful tsunamis E.g.-The continental shelves of Maldives saved human lives from the fury of Sumatra tsunami of December, 26, 2004, as the human deaths were minimized to only 98. The shallow continental shelves support rich mangrove forests which provide ideal natural habitats for marine as well as land animals such as Bengal tigers in sunderban.
Origin of continental shelves 2. subsidence of the continental margin Result of marine erosion and fluvial deposition. 3. cliff erosion and submergence of wave cut platforms.
4. Tectonically formed continental shelves. 5. Continental shelves are formed due to glacial control and marine erosion. 6. Continental shelves are formed due to cliff erosion and submergence of wave cut platforms. 7. Continental shelves formed due to terrigenous fluvial deposit.
B. Continental slope The zone of steep slope extending from the continental shelf to the deep sea plains which varies from 5°to more than 60°. the transition between the shelf and deep ocean floor Depth & Slope: - The depth of water varies from 200 m to2, 000 m. Width:- about 20 km (12.4 mi) wide, consists of mud’s and silts, Area: - occupy only 8.5 per cent of the total area of the ocean basin. 12.4 % in the Atlantic, 7 % in the Pacific and 6.5 % in the Indian Ocean. Submarine canyons are significant features.
Origin:- According to the erosion theory slopes are formed due to erosion by marine processes mainly sea waves. According to tectonic theory faulting is held responsible for the origin of continental slopes. Some exponents believe that the continental slopes are formed due to bending and warping of continental shelves followed by sedimentation.
SUBMARINE CANYONS Long, narrow and very deep valleys located on the continental shelves and slopes with vertical walls. Several kilometers wide and their average length is 16 km. facing the river mouths are usually long (e.g. Congo Canyon) but have gentle gradient. Located near the island are deep with steepest gradient (13.8 per cent). depths vary from 610m to 915 m. For example, The Monterey Canyon of the coast of California Similar to Grand canyons.
C. DEEP OCEAN BASINS AND ASSOCIATED FEATURES It characterized by the following significant relief features of elevation hills, depressions, plains Abyssal plains Submarine/Abyssal hills Seamounts (guyots) and seatablemounts
1. ABYSSAL PLAINS found at depths between 3000 and 6000 meters. The average slope is about 0.5°. between the foot of continental rise and mid-ocean ridge. It cover 75.9 percent of the total area of the ocean , (80.3 percent -Pacific Ocean, 80.1 percent -Indian Ocean and 54.9 percent -Atlantic Ocean).
The solidified basalt crust Volcanic deposits. pelagic deposits of plants, marine animals and siliceous. The fine-grained sediments, mainly clay and silt. It is generally featureless but a few long, narrow and elongated ridges, guyots etc. Concentrations of metals, manganese, iron, nickel, cobalt, and copper. These nodules may provide a significant resource for future mining ventures. significant influence upon ocean carbon cycling, dissolution of calcium carbonate and atmospheric CO2 concentrations over timescales of 100–1000 years.
2. SUBMARINE/ ABYSSAL HILLS A variety of hills of volcanic origin project above the deep sea plains (abyssal plains), namely volcanic hills and islands, sea mounts, table mounts or guyots etc. When these hills appear above sea water surface (sea level), they are called volcanic islands or simply islands. hills are 1000 m high from the ocean
floor while their width ranges between 0.l km to 100 km. The volcanic hills of lower Height are called abyssal hills or sea knolls. The hills always below sea level are called sea mounts, while flat-topped volcanic hills are called table mounts or guyots. The submarine volcanic mountains with average height of 1,000 m from ocean floor, but they are always below ocean water. When numerous abyssal hills are found in cluster on ocean floors, the resultant morphological features are called abyssal hill provinces.
D. OCEAN RIDGES AND OCEAN TRENCHES. 1. Ocean Deeps and Trenches: - They are also the deepest parts of the ocean floor. Ocean deeps are grouped into two categories according to size:- Very deep but less extensive depressions are called deeps, while Long and narrow linear depressions are called trenches. These deeps and trenches are characterized by very steep slopes.
Major oceanic trenches Out of the explored and surveyed 57 deeps, the Pacific Ocean, the Atlantic Ocean and the Indian Ocean account for 32, 19, and 6 deeps respectively. Mariana Trench located to the west of Philippines in the North Pacific Ocean is the deepest (11.02 km deep) of all the ocean deeps. On an average, the ocean trenches are 3 to 5 km in depth from the surrounding surface of ocean floor. Major oceanic trenches Trench Ocean Depth Mariana Trench Pacific Ocean 11,033 m (36,198 ft) Tonga Trench 10,882 m (35,702 ft) Kuril–Kamchatka Trench 10,542 m (34,587 ft) Philippine Trench 10,540 m (34,580 ft) Kermadec Trench 10,047 m (32,963 ft) Izu-Bonin Trench 9,780 m (32,090 ft) Japan Trench 9,000 m (30,000 ft) Puerto Rico Trench Atlantic Ocean 8,800 m (28,900 ft) Peru-Chile Trench 8,065 m (26,460 ft)
2. Mid-Ocean Ridge underwater mountain system formed by plate tectonics. The uplifted seafloor results from convection currents which rise in the mantle as magma at a linear weakness in the oceanic crust, and emerge as lava, creating new crust upon cooling.
boundary between two tectonic plates longest mountain range in the world. 65,000 km (40,400 mi). occupy about one-third of the ocean floor. Not all are centered in the ocean basins. mid-Atlantic Ridge, and mid-Indian Ocean Ridge are of central & East Pacific Rise is certainly of non-central location. mid-Atlantic Ridge and East Pacific Rise are the most extensively explored and studied ridges. All of the ridges are of volcanic origin and consist of basaltic pillow lava. the width varies but on an average it is 1OOO km. The average height is about 2,500 m ridges are characterized by active volcanisms and seismic events. Description:- ridges are geologically active; the oceanic crust is made up of rocks much younger than the Earth Formation processes:-There are two processes, ridge-push and slab-pull.
2. Island arc archipelago composed of a chain of volcanoes which alignment is arc-shaped, situated parallel and close to a boundary between two converging tectonic plates. Island arcs are formed as one oceanic tectonic plate subducts another one. Classification debate about the usefulness of the distinction between island arcs and volcanic arcs. The term "volcanic island arc" is a sub-classification of "island arc." It is tectonically created arc-shaped mountain belts that are partly below sea level. a mountain belt is partly submerged in ocean. Many of these are composed of volcanoes, and can thus be further classified as volcanic island arcs.
Tectonic Formation The resulting volcano chain has the shape of an arc parallel to the convergent plate boundary and convex toward the subducting plate. On the subducting side of the island arc is a deep and narrow oceanic trench, which is the trace at the Earth’s surface of the boundary between the downgoing and overriding plates. Island arc Country Trench marginal sea Plate Subducting plate Aleutian Islands United States Aleutian Trench Bering Sea North American Plate Pacific Plate Japanese Archipelago Japan Japan Trench、Nankai Trough Sea of Japan North American Plate, Eurasian Plate Pacific Plate, Philippine Sea Plate Andaman and Nicobar Islands India Northern Java Trench Andaman Sea Eurasian Plate Indo-Australian Plate Mariana Islands Mariana Trench Philippine Sea Plate Tonga islands Tonga Tonga Trench Australian Plate
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