Geological Work of Oceans PDF

Oceans GEOLOGICAL WORK OF OCEANS Oceans cover about 71% of the surface of the Earth. The greatest ocean depth of 11,035 m occurs in the Mariana Trench. Oceans have an average depth of 3,800 m. Oceans have a present volume of about 1.35 billion cubic kilometers, bu...

Oceans GEOLOGICAL WORK OF OCEANS Oceans cover about 71% of the surface of the Earth. The greatest ocean depth of 11,035 m occurs in the Mariana Trench. Oceans have an average depth of 3,800 m. Oceans have a present volume of about 1.35 billion cubic kilometers, but the volume fluctuates with the growth and melting of glacial ice. Geological work of oceans, like other agents, Comprises the processes of erosion, transportation and deposition, which depends on large number of factors such as: Relief of the floor Chemical composition of sea water Temperature, Pressure and density of sea water Gas regime of sea water Movement of sea water Work of sea organisms etc. Relief of the floor It has been established that the floor of the oceans exhibits an uneven topography with prominent elevation and depressions. On the basis of available bathymetric maps, the ocean is divided into definite regions as indicated below: Littoral zone Continental shelf Continental slope Continental rise Ocean floor Littoral zone It is the area between the high tide and low tide level. The sediments found this zone are very coarse grained. 1 of 14  Oceans Continental shelf The ocean floor gradually slopes down wards away from the shore. It is called Continental shelf. It varies in width from a few kilometers to several hundred kilometers. The continental shelves cover about 7.6 per cent of the total area of the oceans and 18 per cent of the land. About 20% of the world production of oil and gas comes from them. Continental Slope From the edge of the continental shelf, the sea floor commonly descends to the ocean basin, with an average gradient of 3.50 to 7.50-and is known as continental slope. Its depth ranges between 200-2500 meters and covers about 15%of the total area of the ocean. It has an average width of 16 to 32 Kilometers. Continental rise It extends from the bottom of the continental slope to the floor of the ocean basins. The rise has a slope of 10to 60. Its width varies from a few kilometers to a few hundred kilometers. 2 of 14  Oceans Ocean floor It begins at a depth of 2000 meters and goes down to 6000 meters. It covers 76 per cent of the total area of the oceans. Ocean floor contains a number of distinctive topographic units such as Mid-oceanic ridges Island arch Deep-oceanic trenches Abyssal plains & Hills Seamounts & Guyots Mid-ocean Canyons Mid-oceanic ridges A mid-oceanic ridge is an underwater mountain range. They form an almost continuous mountain chain that is about 75,000 kilometers (45,000 miles) long and 3 kilometers (nearly 2 miles) high. At the centre, a “V” shaped ocean floor is seen. It is known as rift valley. Island arch These are submarine ridges of ocean floor. Deep trenches occur along the concave side of the island arch. They are usually produced by volcanic activity. 3 of 14  Oceans Deep-oceanic trenches These are long, relatively narrow features that represent the deepest part of the ocean. Trenches usually have a depth of more than 10000m. The deepest trenches in the world mariyana trench has a depth of 11033m. Abyssal plains & Hills These are extremely flat features of the deep ocean floor. It consist of thick amount of sediments. Small hills having a height of few tens of meters to 100 meters above the ocean basin floor are known as Abyssal hills. Seamounts & Guyots They are isolated steep sided peaks rising the above from the floor with a height of 1000m or more. Most of the sea mounts are volcanic cones. Flat topped sea mounts are known as Guyots. 4 of 14  Oceans Submarine Canyons These are narrow V shaped submarine valley with high steep walls & irregular flow. They are valley of considerable depth. Chemical composition of sea water The oceanic water contains a large number of dissolved salts and have almost a uniform composition. The average salinity of sea water is 35 parts per thousand. i.e. one liter of sea water contains 35 grams of various dissolved salt. Sea water of normal salinity contains mostly chlorides which aggregate above 88% followed by sulphates more than 10% and small amounts of carbonates and other compounds. Sodium chloride constitute the bulk of the dissolved salts in sea water, followed by Magnesium- chloride, Magnesium sulphate, Calcium sulphates, Potassium-sulphates. Apart from these salts, there are also elements like iodine, fluorine, zinc, lead, phosphorous etc.in sea water. Salinity determines features like compressibility, thermal expansion, temperature, density, absorption of insolation,evaporation, humidity etc. It also affects the movements of the ocean waters. 5 of 14  Oceans Movement of sea water The sea is a mobile mass of saline water and the movements of sea water are of great geological importance as they determine the intensity of destruction caused by the oceans and seas on the shore and the floor and also the distribution and differentiation of the sedimentary materials that enter the seas and oceans. There are three main types of Ocean movements. Waves Currents Tides Waves Waves are generated mainly by the wind blowing over the surface of the ocean and sea water. The friction of wind moving over the water surface causes the water particles to move along circular or near circular orbits in a vertical plane parallel to the direction of wind. The ocean waves are oscillatory waves (or transverse waves) as they cause an oscillatory wave motion. The waves consist of alternating crests and troughs. Wave length is horizontal distance from crest to crest or trough to trough. Wave height is the vertical distance between trough and crest. Wave period is the time taken by two consecutive crests to pass any reference point. Wave Velocity is the ratio between the wave length and wave period. 6 of 14  Oceans Currents In the currents, there is an actual movement of the water over great distances, which may be caused by various factors, such as-the differences in temperature, salinity, action of steady and periodic winds etc. Tides Tides are periodic movements of the ocean waters due to the gravitational attraction of the sun and moon on the earth. Twice a day, about every 12 hours 26 minutes, the sea level rises and it also falls twice a day. When the sea rises to its highest level, it is known as 'high tide‘. When it falls to the lowest level, it is called 'low tide'. Spring tide & neap tide MARINE EROSION Seas and oceans do immunes erosion of the rocks constituting the coast and the bottom through the action of moving water. Erosion by the sea is accomplished mainly by the following processes: (a) hydraulic action (b) abrasion (c) attrition (d) corrosion Features Formed By Marine Erosion Sea cliff Wave cut platform Sea caves Sea arch Sea stack Sea cliff & Wave cut platform The hard bed rocks with steep slopes found along the coast are known as Sea cliff. A relatively flat bench like surface is left behind the wave cut cliff is called Wave cut platform. 7 of 14  Oceans Sea cliff Wave cut platform Sea caves, Sea arch, Sea stock The cliff may retreat by undercutting and resulting mass-wasting processes. In areas where differential erosion takes place, the undercutting may initially produces sea caves. If sea caves from opposite sides of a rocky headland meet, then a sea arch may form. Eventual weakening of the sea arch may result in its collapse to form a sea stack. 8 of 14  Oceans TRANSPORTATION The products of erosion by sea water and those which are dumped into the sea by the rivers are carried to various depths by the movement of the sea water. When the current is strong and the waves reach the bottom of the sea all the clastic material get stirred up. These are transported by rolling at low velocities and may pass in to a suspended state in high velocity. DEPOSITION Marine basins are the main depositional sites where most of the material obtained by the wear and tear of the rock masses. The waves and currents have a general tendency to move sediment seaward from the shore and to deposit coarser particles near the shore and finer sediments more in the seaward direction. Depending on the distance of the deposits from the shore, the depth of a basin, the movement of the water, the organic activities, etc, marine deposits are of various character. Accordingly they are classified into four main groups. 1. Littoral deposits 2. Neritic or Shallow-water deposits 3. Bathyal or Deep-sea deposits 4. Abyssal or Very-deep water deposits. Littoral deposits These are the shore-zone deposits. The littoral zone includes the area between the extreme levels of the high-tides and low-tides. Depositional shore features includes Beaches Bars Spits Beach barriers Barrier island Tombolos. 9 of 14  Oceans Beaches A beach is "the wave-washed sediments along a coast extending throughout the surf zone". Beaches are composed of unconsolidated materials ranging in size from pebbles to sand and mud. Bars, Lagoons, beach barrier or barrier island. Bars are ridge-like deposits which is parallel to the coast and are composed of sand, pebble and shell accumulations. Some times they are hundreds of kilometers long, 20-30 km wide and as high as 15-30 meters. Bars often confine a portion of the sea, separating it form the main body of water. This separated part of the sea is called a lagoon. When the bar lies far offshore and is completely detached from the main land, it is known as an off shore bar. It is also termed as beach barrier or barrier island. Bars Lagoons Barrier Island 10 of 14  Oceans Spit These are often considered as the most common types of bar. These are ridge-shaped deposits of sand and gravel which projects out from the land mass in to the sea. Thus its one end is attached to the land and the other end terminates amidst the sea. Sometimes spits may reach a considerable length. Tombolo It is a spit like ridge of sand or gravel that connect an island to the main land or another island. 11 of 14  Oceans Neritic or Shallow-water deposits These are the shallow water deposits formed on the continental shelf and at similar depths in the flanking regions of oceanic islands. Coral reefs are of important neritic deposits. On the basis of their forms and relation to the land three main types of reefs are recognised: (i) fringing reefs (ii) barrier reefs (iii) atolls Fringing Reefs These are narrow belts of reef which grow around island or along the coast. The reef is closely attached to the coast and the island. At low tide they are seen to be in continuity with the shore. Its seaward edge is higher than the landward portions. It is separated from the coast by a narrow, shallow strip of water. Barrier Reefs These are located at a considerable distance from the coast or the island and are separated from the mainland by a quite broad and relatively deep stretch of sea. This lagoon may be tens of kilometers wide. Small channels cut across & the barrier connect the lagoon with the open sea. The great Barrier Reef of Australia is the best example of this type. It extends for more than 2000 kms with an average width of 150 kms. 12 of 14  Oceans Atolls These are circular coral Reefs with a shallow central lagoon but no central island. A large number of channels cutting across the atoll reef connect the lagoon with the open sea. Atolls are more common in the Pacific than in any other ocean. Bathyal or Deep-sea Deposits 13 of 14  Oceans These deposits are formed in the continental slope zone which is characterised by low mobility of the sea water. Abyssal Deposits Beyond the foot of the continental slope, there is very little deposits of terrigenous material, and the abyssal plain is covered for most of the part by pelagic deposits which are formed to a great extent of the shells and skeletons of animals and plants which when alive float on the surface of the water. Important Abyssal deposits are: Polygenic sediments Volcanogenic sediments Globigerina Ooze Radiolarin Ooze Diatomaceous Ooze 14 of 14