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Oceanography - Part 1
1.0 INTRODUCTION
The hydrosphere on earth covers almost 367 million sq km or roughly 70% of the earth’s area.
About 98% of all water is in the oceans, 2% is in streams, lakes, ground-water, and glaciers. Approximately 70.8% of the Earth is covered with water and 29.2 per cent is the land area. Thus, it is for a good reason that Earth has been called 'the water planet'. It has been estimated that if all the irregularities of the Earth's surface were smoothed out to form a perfect sphere, a global ocean would cover the Earth to a depth of 2.25 km! It is this great water mass that makes our Earth unique. Water permitted life to evolve and flourish and, therefore, every inhabitant on the Earth is directly or indirectly controlled by it.
Beneath the ocean waves lies the biggest and most unexplored landscape on the Earth. Here are coral reefs, enormous open plains, deep canyons, and the longest mountain range on the Earth - the Mid-Atlantic Ridge - which stretches almost from pole to pole.
2.0 Oceans and Coasts
Oceans: Water covers about 70.8 per cent of Earth's surface. There is only one world ocean, but the continents divide the ocean into five distinct parts: (i) Pacific Ocean, (ii) Atlantic Ocean, (iii) Indian Ocean, (iv) Arctic Ocean, and (v) Southern Ocean (Antarctic Ocean). The greater parts of the oceans, the ocean basins, are relatively young having evolved within the last 80 million years. One of the most recent plate collisions, between the Eurasian and African plates, created the present day arrangement of the continents and oceans.
Coasts: The total length of the Earth's coastlines is more than 5,00,000 km, that is the equivalent of 12 times around the globe. A high percentage of the world's population live in coastal zones. Of the ten most populated cities on the Earth, eight are situated on deltas, estuaries, or the coast.
3.0 Ocean Relief or Major Features of Oceanic Lithosphere
The oceanic crust differs strikingly from the continental crust with respect to rock types, structure, landforms, age and origin. The major features of the ocean floor are: (1) continental shelf (margin), (2) continental slope, (3) continental rise, (4) abyssal floor (deep ocean basins), (5) oceanic trenches, (6) oceanic ridges, and (7) seamounts.
3.1 Continental margin
The continental margin is the submerged shelf and slope forming the outer edge of a major landmass. The differences between oceanic and continental crust composition are fundamental in margin development. The transition between the two crustal types lies under the area of the continental slope with the shelf over continental crust and oceanic crust below the continental rise. The difference in densities and isostatic balance of the continental to oceanic crustal blocks is expressed dramatically by the presence of a continental slope, which is the great declivity from the shelf break downward to deep ocean depths. The continental shelf can extend upto 1500 kms in width (Siberian shelf in the Arctic ocean) but normally is upto 80 km wide. Depth is shallow, and can be upto 150 m only.
The continental shelf is the submerged part of the continent lying between the coastline and the shelf break, which is the change in inclination marking the boundary between continental shelf and slope. The average gradient of the shelf is only seven minutes of inclination (less than a degree), and off coastal plain shorelines, the shelf is a submerged continuation of the plain that has alternated between submergence and emergence during geologic time.
3.2 Continental slope
The slope that extends from the continental shelf down to the ocean deep is known as continental slope. In some areas, such as off eastern North America, the continental slope grades into the more gently sloping continental rise. At present continental shelves and continental slope constitute about 11.4 % of the Earth's solid surface out of which 6.4 per cent is the area of continental slope.
3.3 Continental rise
This is the gently sloping seafloor lying at the foot of the continental slope and leading gradually into the abyssal plain. They cover 3.8 per cent of the total area of the oceans.
3.4 Abyssal floor
The most widespread landform on earth, the abyssal floor is the large expanse of very smooth, flat ocean floor found at depths of 3000 to 5500 m. The abyssal plains cover from 80 to 85% of the floor of the Pacific Ocean.
3.5 Oceanic trench
The narrow, deep depression in the seafloor representing the line of subduction of an oceanic lithospheric plate beneath the margin of a continental lithospheric plate; often associated with an island arc. The ocean trenches are the lowest areas on Earth's surface. The Mariana Trench, in the Pacific Ocean, is the deepest part of the world's oceans -11022 m below sea level, and many other trenches are more than 8000m deep. Some of the important trenches of the oceans are:
North Pacific Ocean: Mariana Trench (11022 m), Ramapo Deep or the Japan Trench (10,554 m), Aleutian Trench (10,498 m), Philippines Trench (10,475 m)
South Pacific Ocean: Kermadec-Tonga Trench (the steepest trench of the world), New Hebrides Trench, Peru-Chile or Atacama Trench (7635 m)
North Atlantic: Puerto Rico Trench (8358 m), Romanche Trench (7631 m)
Indian Ocean: The Sunda Trench (Sumatra) and Java Trench (7454 m)
The oceanic ridge is perhaps the most striking and important feature on the ocean floor. They cover about 22.1 per cent of the surface of the oceans. Their total length is about 66 thousand kilometers. The oceanic ridge extends continuously from the Arctic basin, down the centre of the Atlantic Ocean, into the Indian Ocean, and across the South Pacific. The oceanic ridge is essentially a broad, fractured swell and generally more than 1400 km wide. Its higher peaks rise as much as 3000 m above the ocean floor. The Mid-Atlantic Ridge, having a length of about 14,450 km, extends from Iceland in the north to Bouvet Island in the south. The oceanic ridges are formed by lava which erupts beneath the sea and cools to form solid igneous rock. This process mirrors the creation of volcanoes from cooled lava. The ages of the sea floor rocks increase in parallel bands outward from central ocean ridges.
3.7 Island arcs
Island arcs are curving chains of volcanic islands and seamounts that are always found paralleling the concave edge of trenches. They are formed by the tectonic activity of subduction. Some of the important island arcs are the Aleution Islands, the Lesser Antilles, and the Mariana Islands.
3.8 Seamounts
It is a circular or elliptical projection form the seafloor, more than one kilometer in height, with a relatively steep slope of 20° to 25°.
Guyot is a flat topped, submerged, inactive volcano. They are tall enough to approach or penetrate the sea surface. Generally they are confined to the west-central Pacific.
3.9 Black smokers
The black smokers are vents in the ocean floor which disgorge hot, sulfur-rich water from deep in the Earth's crust. Despite the great depths, a variety of life-forms have adapted to the chemical-rich environment which surrounds black smokers.
4.0 MAJOR OCEAN RELIEFS
4.1 The Pacific Ocean
The Pacific Ocean is the largest and deepest of all water bodies. Together with its associated seas, it covers about one-third of the earth's surface and exceeds the total land area of the world in size. The average depth is generally around 7,300 metres. Its shape is roughly triangular with its apex in the north at the Bering Strait. Many marginal seas, bays and gulfs occur along its boundaries. Nearly 20,000 islands dot this vast ocean; those appearing in mid-ocean are coral and volcanic and the rest are continental islands.
North and Central Pacific: This part is characterised by maximum depth and a large number of deeps, trenches and island areas. Some well known trenches are Aleutian and Kuril, from 7,000 to 10,000 metres. There are also a large number of seamounts, guyots and parallel and arcuate island chains in the central part.
South-West Pacific: The average depth of this part is about 4,000 m, and this part is marked by a variety of islands, marginal seas, and continental shelf and submarine trenches. Mariana Trench lies in this portion and the Mindanao Trench is also very deep with a depth of more than 10,000 metres.
South-East Pacific: This part is conspicuous for the absence of marginal seas, and has submarine ridges and plateaus. The Tonga and Atacama are prominent trenches.
4.2 The Atlantic Ocean
The Atlantic is roughly half the size of the Pacific Ocean and resembles the letter 'S' in shape. It has prominent continental shelf with varying width - the largest width occurring off north-east America and north-west Europe. The Atlantic Ocean has numerous marginal seas occurring on the shelves, like the Hudson Bay, the Baltic Sea and the North Sea.
The most striking feature of the Atlantic Ocean is the presence of Mid-Atlantic Ridge which runs from north to the south paralleling the 'S' shape of the ocean itself, dividing the Atlantic into two deeper basins on the either side.
The ridge is about 14,000 km long and about 4,000 metres high. Several peaks of this ridge project out of the ocean surface to form islands of the mid-Atlantic. Examples include Pico Island of Azores, Cape Verde Island. Also, there are coral islands like Bermuda and volcanic islands like Ascension, Tristan da Cunha, St Helena and Gough.
By and large the Atlantic Ocean lacks in troughs and trenches, which are more characteristic of the Pacific Ocean. North Cayman and Puerto Rico are the two troughs and Romanche and South Sandwich are the two trenches in the Atlantic Ocean.
4.3 The Indian Ocean
This ocean is smaller and less deep than the Atlantic Ocean. Since it is completely blocked in the north by the Asian landmass, it can be considered only half an ocean. It has few marginal seas. Linear deeps are almost absent. The only exception is Sunda Trench, which lies to the south of the island of Java.
There are a number of broad submarine ridges in this ocean, which include the Lakshadweep-Chagos Ridge, the St. Paul Ridge which widens into the Amsterdam St. Paul Plateau, the Socotra-Chagos Ridge, the Seychelles Ridge, the South Madagascar Ridge, the Prince Edward Crozet Ridge, the Andaman-Nicobar Ridge and the Carlsberg Ridge. These ridges divide the ocean bottom into many basins. Chief among these are the Central Basin, Arabian Basin, South Indian Basin, Mascarene Basin, West Australian and South Australian Basins.
Most of the islands in the Indian Ocean are continental islands and are present in the north and west. These include the Andaman and Nicobar, Sri Lanka, Madagascar and Zanzibar. The Lakshadweep and Maldives are coral islands and Mauritius and the Reunion Islands are of volcanic origin. The eastern section of the Indian Ocean is almost free from islands
This is a graph of the area of the Earth's surface above and given elevation or depth above or below the sea level. The highest mountain peak (Mt.Everest) is 8848 m, while the greatest ocean depth is 11,022 m. The average elevation of the exposed land is 840 m and the average depth of the oceans is -3790 m.
Temperature of the Oceans: Oceans absorb more than 80% of the solar radiation that reaches the Earth. Moreover, water has remarkable capacity for absorbing heat. The upper most 10 % of the oceans contain more "heat than the entire atmosphere.
The temperature of the oceans is not uniform. It differs from latitude to latitude and from surface to bottom. The major determinants of oceans temperature are:
Latitude: The surface temperature of the oceans declines from equator towards the pole as on equator the Sun's rays are vertical and slanting on the poles.
Prevailing Winds: Wind direction (trade and anti-trade winds) largely affects the surface distribution of temperature of the oceans.
Unequal Distribution of Land and Water: The Northern Hemisphere has more land area than that of the Southern Hemisphere. Consequently, the oceans of the Northern Hemisphere arc warmer than that of the Southern Oceans.
Evaporation Rate: The volume of water that evaporates each year from the surface of ocean is equivalent to 350,000 cubic kilometers. The evaporation rate is however, not uniform in different latitudes.
Density of Water: The density of water is mainly a function of its salinity and temperature. The density of water differs from latitude to latitude. In the areas of high salinity, there is relatively high temperature and vice-versa.
Ocean Currents: Surface temperature of the oceans are also controlled by warm and cold currents. The presence of a warm water current increases the temperature and the rate of evaporation. Consequently, the region records more rainfall, while the cold water current reduces the temperature of the moisture-laden wind. The coast along with a cold water current flows record more fog, but less precipitation.
Local Factors: Submarine ridges, local weather conditions like storms, cyclones, winds, fogs, cloudiness, rate of evaporation, lapse rate, condensation, and precipitation also affect the distribution of temperature of the oceans.
Horizontal Distribution of Temperature: In general, the temperature of the surface water in the lower latitudes is about 26°C which decreases towards poles. The oceans of the Northern Hemisphere record an average temperature of 19.4°C. In the Northern Hemisphere the average temperature reads 22°C at 20° latitude, 14°C at 40° latitude, and 0°C near the poles. In the Southern Hemisphere, the average temperature is only 16.1°C.
The maximum and minimum annual temperatures of ocean water in the Northern Hemisphere are in the months of August and February, respectively. The average annual range of temperature is about 12°C. The highest annual range of temperature is recorded in the North Atlantic Ocean. Moreover, the annual range of temperature is higher in the inland seas as compared to the open oceans.
Vertical Distribution of Temperature: Both energy and sunlight decrease with depth in the oceans. Only about 45% of light energy striking the ocean surface reaches a depth of about one meter, and only 16% reaches 10 meter depth. On the basis of temperature the ocean depths may be divided into the following three zones:
Surface Zone or Mixed Zone (Photic or Europhic Zone): it is the upper layer of the ocean. In this layer, the temperature and salinity are relatively constant. It contains about 2% of the total ocean volume of water, and its depth is only about 100 meter.
Thermocline or (Pycnocline): It lies between 100 metres and 1000 metres. It contains about 18% of the total volume of oceanic water. There is a steep fall of temperature in this zone. The density of water increases with increasing depth.
The Deep Zone: This zone lies below 1000 meters in the mid-latitudes. The deep zone contains about 80% of the total volume of ocean water. Temperature near the bottom is always more than the freezing point - around one or two degree C.
6.0 Ocean Currents
Surface ocean currents are driven by the prevailing winds and by the spinning motion of the Earth which drives the currents into circulating whirlpools, or gyres. This is known as the Coriolis effect. Deep sea currents, over 100 m below the surface, are driven by differences in water temperature and salinity, which have an impact on the density of deep water and on its movement. The main causes of ocean currents are:
- Differential heating of the equatorial and polar oceans
- Trade and anti-trade winds
- Coriolis effect
- Variations in the salinity and temperature of the oceans
- Configuration of the coast line and ocean floor
In general, warm ocean currents bring warm water from the equatorial regions to polar regions and cold ocean currents bring cold water from the polar regions to the equatorial regions.
Sea-waves: It is a ridge of water between two depressions. A wave approach, they curl into an arc and break. The energy of surface waves is responsible for the erosion of the coast. Waves also initiate currents which run along the coast and which are the moving force in long-shore drift. The height of a wave is generally proportional to the square of wind velocity.
Tides: The periodic short-term rise and fall in the sea levels is known as tide. It is produced due to gravitational interaction of Earth, Moon and Sun. Moon exerts the strongest influence on tides because of its closeness to the Earth.
Spring Tides: On the full moon and the new moon, the tides occur are the highest and known as the Spring Tides, while in the 1st and 3rd quarters of a month, tides are lower than the usual, which is known as the Neap Tides. The highest tides in the world occur in the Bay of Funday. The highest tides in India are recorded at Okha, Gujarat.
It takes 24 hours 50 minutes for the rotating Earth to bring the same meridian vertically below the Moon everyday. Hence, tides occur at regular intervals of 12 hours and 25 minutes. Generally, tides occur twice a day.
Merits of Tides: Tides generally help in making some of the rivers navigable for ocean going ships. For example, London and Kolkata have become important ports owing to tidal nature of the mouths of Thames and Hooghly, respectively. Tides also wash out the sediments brought by rivers and thus, retard the formation of deltas and help in cleaning the coastal regions.
The tidal force may also be used as a source for the generation of electricity. The generation of tidal energy was started by France, followed by Japan. India produces tidal energy in the Gulf of Khambat and Kachchh.
Global winds cause the water surface to move and build up in the direction that the wind is blowing. The Coriolis effect deflects winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Due to the same effect major surface ocean currents are also deflected to the right in the Northern Hemisphere (in a clockwise spiral) and to the left in the Southern Hemisphere (in a counter-clockwise spiral). These major spirals of ocean-circling currents are called "gyres" and occur north and south of the equator. However, they do not occur at the equator since the Coriolis effect is absent there.
The five major ocean-wide gyres are the North Atlantic, South Atlantic, North Pacific, South Pacific, and Indian Ocean gyres. Each is flanked by a strong and narrow "western boundary current," and a weak and broad "eastern boundary current".
The North Atlantic subtropical gyre includes the Gulf Stream, the Labrador current, the East Greenland current and the Canary current. The Gulf stream becomes the North Atlantic Current at about 40 degrees N and 45 degrees W and flows easterly across the North Atlantic. The cold waters of the Labrador Current flow southeastward between Canada and Greenland while the East Greenland Current flows southwestward between Greenland and Iceland. Farther east, the North Atlantic Current splits into the Norwegian Current (which flows northeasterly between Iceland and Europe along the coast of Norway) and the Canary Current (which flows southward along the west coast of Spain, Portugal, and North Africa). The Canary Current merges with the North Atlantic Equatorial Current, thus completing the North Atlantic subtropical gyre.
The South Atlantic Current is the southern branch of the subtropical gyre in the South Atlantic. This eastward flowing current is also known as the West Wind Drift. In the Southern hemisphere the coriolis effect is reversed. Hence the south Atlantic gyre circulates in a counter-clockwise direction.
Currents in the South Pacific and South Atlantic are narrowest and flow most rapidly along their western margins but are broad and slow along their eastern margins. The Brazil current is the South Atlantic Gyre's western boundary current and brings warm equatorial water to the pole. The Benguela current is the eastern boundary current, bringing cold Antarctic water towards the equator. The South Atlantic's northern boundary is the South Equatorial Current and it is bordered on the south by the Antarctic Circumpolar Current.
The North Pacific Gyre is an immense region of slowly spiralling, warm equatorial air that pulls in winds and converging sea currents. It is located between the equator and 50° N latitude, and comprising 20 million square kilometres making it the Earth's largest ecosystem. The four prevailing ocean currents which form this gyre are the North Pacific Current to the north, the California Current to the east, the North Equatorial Current to the south, and the Kuroshio Current to the west. This gyre is also the site of an unusually intense collection of man-made marine debris, known as the Great Pacific Garbage Patch.
The South Pacific gyre is bounded by the equator in the North, Australia in the west, South America in the east and the Antartic Circumpolar current in the south. Due to Earth's trade winds and Coriolis force the ocean currents in South Pacific Ocean circulate counter clockwise. The currents act to isolate the center of the gyre from nutrient upwelling and few nutrients are transported there by the wind (eolian processes) because there is relatively little land in the Southern Hemisphere to supply dust to the prevailing winds.
The Indian Ocean Gyre is a complex system of many currents extending from the eastern coast of Africa to the western coast of Australia. The northern part of the system circulates between the Horn of Africa and the Indonesian archipelago. It is sometimes called the Indian monsoon current. The Indian monsoon current takes its name from the wind-the monsoon-that drives it. It is one of the very few currents in an ocean gyre that change direction. In the summer, the current flows clockwise, as the monsoon blows in from the southwestern Indian Ocean. In the winter, the current flows counterclockwise, as the wind blows in from the Tibetan plateau in the northeast.
7.0 Salinity
Salinity is a measure of dissolved solids in seawater, usually expressed in grams per kilogram or parts per thousand by weight. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, or 599 mM). In waters of the oceans, 96.5% is water, and 3.5% dissolved salts. The most common substance in the sea water is sodium chloride or salt. So, one kilogram (roughly one litre by volume) of seawater has approximately 35 grams (1.2 oz) of dissolved salts (predominantly sodium (Na+) and chloride (Cl-) ions). Average density at the surface is 1.025 g/ml. Seawater is denser than both fresh water and pure water (density 1.0 g/ml @ 4 °C (39 °F)) because the dissolved salts add mass without contributing significantly to the volume.
Sources of Salinity: Much of the dissolved materials (salts) of the seas/oceans originated from land which was carried to the oceans by rain, running water, ground water, wind, sea-waves and glaciers. Some of the salts, however, have their origin in the deeper layers of the Earth. The volcanic lava adds to the salinity of the oceans. In addition to these, the dead and decomposed organic life adds to the salinity of the oceans.
7.1 Determinants of the salinity of oceans
The distribution of salinity of the oceans depends on the rate of evaporation, temperature, precipitation, cloudiness, influx of fresh water, winds, and ocean currents:
Evaporation: In general, higher the rate of evaporation, higher the salinity of oceans.
The highest rate of evaporation is recorded along the Tropic of Cancer, especially in the Red Sea and the Persian Gulf. Consequently, these water bodies have one of the highest salinity in the world.
Temperature: There is a direct relationship between temperature and salinity of the oceans. In general, the higher the temperature, the higher the salinity of the oceans. The Torrid Zone has higher salinity as compared to the Frigid Zone of the world.
Precipitation: The amount of precipitation is inversely related to salinity. In general, the higher, the precipitation, the lower, the proportion of salinity. The oceans of the equatorial region, recording the heavy rainfall throughout the year, have relatively low salinity.
Ocean Currents: Ocean currents also affect the spatial distribution of salinity in the ocean bodies of the world. The equatorial warm water currents drive away salts from the eastern coastal areas and accumulate them along the western coastal areas of the oceans. The Gulf Stream and the North Atlantic Drift increase the salinity in the North Sea and the North east Atlantic Ocean.
Influx of Fresh (River) Water: Comparatively low salinity is found along the mouth of the Amazon, Congo, and Ganga deltas mainly due to the influx of fresh water.
Horizontal Distribution of Salinity: The salinity of the oceans decreases on both sides of the tropics. For example, the average salinity along the equator is about 35%o, while along the Tropic of Cancer, it is over 36%o.
On the basis of salinity, the oceans and seas of the world may be divided into the following categories:
Seas having salinity above normal: The salinity of the Red Sea is about 39 to 41%, Persian Gulf 38% and the Mediterranean Sea 37 to 39%.
Seas with normal salinity: The Caribbean Sea, Gulf of Mexico, Gulf of California, and Yellow Sea have a salinity between 35%o and 36%.
Seas having salinity below normal: The Arctic Ocean, Antarctic Ocean, Bering Sea, Japan Sea, Baltic Sea, etc. have low salinity (21%).
Vertical distribution of salinity: So far as the vertical distribution of salinity is concerned, no definite trend is found and, therefore, no generalisation can be made. In general, with increasing depth of sea water, both the trends of increase and decrease of salinity are found. The vertical distribution of salinity is plotted on T-S Diagram, advocated by Helland-Hensen. The various patterns of salinity in the Halocline.
The characteristics of vertical distribution of salinity may be summarised as follows:
- The salinity of the oceans increases with increasing depth.
- Salinity is low at the surface water in the equatorial water.
- In the middle latitudes, the salinity increases with increasing depth.
The sediments, derived from various sources, deposited at the oceans floors are known as ocean deposits. The main sources of ocean deposits are:
Terrigenous: Gravel, sand, silt, clay, mud, etc. Sediments derived from the land and transported to the ocean by running water and wind, etc.
Volcanic Deposits: Ash, pumice, cinder, etc.
Biotic Deposits: Neretic deposits, pelagic deposits (ooze), globigerenia ooze, radiolarian ooze, red clay, etc. These deposits consist material mainly of marine organic origin, i.e. shells, skeletons of marine animals and plants. The most common of the biogenous sediments are calcareous ooze and siliceous ooze.
Cosmogenous Deposits: Dust, tektites and particles.
9.0 Coral Reefs
Coral reef is a linear mass of calcium carbonate (aragonite and calcite) assembled from coral organism, algae, mollusks, worms, etc. Coral may contribute less than half of the reef material. In most reefs, the predominant organisms are stony corals that secrete calcium carbonate (limestone). The coral reefs are also referred as the 'Rainforests of the oceans'because of their tremendous biodiversity.
The coral reefs may be broadly classified in the following categories: (i) Fringing Reef: Reef that is directly attached to a shore or borders it with an intervening shallow channel or lagoon.
Barrier Reef: Reef separated from a mainland or island shore by a deep lagoon. The Great Barrier Reef lying to the east of Queensland is the largest living structure on the Earth. It extends for more than 2000 km.
Atoll Reef: A more or less circular or continuous barrier reef extending all the way around a lagoon without a central island. Distribution of Coral Reefs in the World: Most of the coral reefs are found between 30° N and 30° S, where the average temperature of the sea surface is about 21 °C.
According to one estimate coral reefs cover about 2,84,300 square kilometers, with the Indo-Pacific region (including the Red Sea, Indian Ocean, South East Asia and the Pacific) accounting for 92% of the total coral formations. The Caribbean coral reefs account for about 7.5% of the total coral formations of the world.
Coral reefs are almost absent from along the west coast of Americas and the west coast of Africa. Corals are also restricted from the coastline of South Asia from Pakistan to Bangladesh. They are also not found in the delta region of the Ganga-Brahmaputra river and the Amazon river due to the addition of vast quantities of fresh water.
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