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Hydrosphere

hydrosphere, discontinuous layer of water at or near Earth’s surface. It includes all liquid and frozen surface waters, groundwater held in soil and rock, and atmospheric water vapour.

Water is the most abundant substance at the surface of Earth. About 1.4 billion cubic kilometres (326 million cubic miles) of water in liquid and frozen form make up the oceans, lakes, streams, glaciers, and groundwaters found there. It is this enormous volume of water, in its various manifestations, that forms the discontinuous layer, enclosing much of the terrestrial surface, known as the hydrosphere.

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Central to any discussion of the hydrosphere is the concept of the water cycle (or hydrologic cycle). This cycle consists of a group of reservoirs containing water, the processes by which water is transferred from one reservoir to another (or transformed from one state to another), and the rates of transfer associated with such processes. These transfer paths penetrate the entire hydrosphere, extending upward to about 15 kilometres (9 miles) in Earth’s atmosphere and downward to depths on the order of five kilometres in its crust.

This article examines the processes of the water cycle and discusses the way in which the various reservoirs of the hydrosphere are related through the water cycle. It also describes the biogeochemical properties of the waters of Earth at some length and considers the distribution of global water resources and their utilization and pollution by human society. Details concerning the major water environments that make up the hydrosphere are provided in the articles ocean, lake, river, and ice. See also climate for specific information about the impact of climatic factors on the water cycle. The principal concerns and methods of hydrology and its various allied disciplines are summarized in Earth sciences.

Ocean waters and waters trapped in the pore spaces of sediments make up most of the present-day hydrosphere. The total mass of water in the oceans equals about 50 percent of the mass of sedimentary rocks now in existence and about 5 percent of the mass of Earth’s crust as a whole. Deep and shallow groundwaters constitute a small percentage of the total water locked in the pores of sedimentary rocks—on the order of 3 to 15 percent. The amount of water in the atmosphere at any one time is trivial, equivalent to 0.013 × 106 cubic kilometres of liquid water, or about 0.001 percent of the total at Earth’s surface. This water, however, plays an important role in the water cycle.

Hypsometric curve

hypsometric curve, also called Hypsographic Curve, cumulative height frequency curve for the Earth’s









surface or some part thereof. A hypsometric curve is essentially a graph that shows the proportion of land area that exists at various elevations by plotting relative area against relative height. In the hypsometric curve of the total Earth surface there exist two maxima of frequencies—at the 100-metre (109-yard) and the 4,700-metre (5,140-yard) elevations, which correlate with the mean level of the lowland continental areas and the deep-sea floor. This aspect of the Earth’s surface, revealed by hypsometric analysis, supports the theory of a crust consisting of simatic (peridotitic, specific gravity about 3.3) materials under the oceans and of sialic (granitic to gabbroic, specific gravity about 2.7) materials of the continents.

continental shelf

A continental shelf is the edge of a continent that lies under the ocean. Continents are the seven main divisions of land on Earth. A continental shelf extends from the coastline of a continent to a drop-off point called the shelf break. From the break, the shelf descends toward the deep ocean floor in what is called the continental slope.

Even though they are underwater, continental shelves are part of the continent. The actual boundary of a continent is not its coastline, but the edge of the continental shelf. The widths of the continental shelves vary. Along parts of the U.S. state of California, for example, the continental shelf extends less than a kilometer (.62 miles). But along the northern coast of Siberia, the shelf









extends about 1,290 kilometers (800 miles). The average width of a continental shelf is 65 kilometers (40 miles).

Most continental shelves are broad, gently sloping plains covered by relatively shallow water. Water depth over the continental shelves averages about 60 meters (200 feet). Sunlight penetrates the shallow waters, and many kinds of organisms flourish—from microscopic shrimp to giant seaweed called kelp. Ocean currents and runoff from rivers bring nutrients to organisms that live on continental shelves.

Plants and algae make continental shelves rich feeding grounds for sea creatures. The shelves make up less than 10 percent of the total area of the oceans. Yet all of the ocean’s plants and many types of algae live in the sunny waters.

In some places, deep canyons and channels cut through the continental shelves. Little light penetrates these submarine canyons, and they are sometimes the least-explored areas of continents. Often, submarine canyons are formed near the mouths of rivers. Strong river currents cut deeply into the soft material of the continental shelf, just like they erode rocks above ground. The Congo Canyon, extending from the mouth of the Congo River, is 800 kilometers (497 miles) long and 1,200 meters (3,900 feet) deep. The Congo Canyon is part of Africa.

Formation of a Continental Shelf

Over many millions of years, organic and inorganic materials formed continental shelves. Inorganic material built up as rivers carried sediment—bits of rock, soil, and gravel—to the edges of the continents and into the ocean. These sediments gradually accumulated in layers at the edges of continents. Organic material, such as the remains of plants and animals, also accumulated.

Many continental shelves were once dry land. Some 18,000 years ago, at the peak of the most recent ice age, much of the Earth’s water was frozen into huge masses of ice called glaciers. The sea level dropped, exposing continental









shelves. During this glacial period, scientists say that sea levels were perhaps 100 meters (330 feet) lower than they are today.

The continental shelves between North America and Asia were probably exposed during the Ice Age. Some scientists say that the shelves provided a “land bridge” between the two continents. People may have used this land bridge—now the Bering Strait—to migrate from Siberia to what is now Alaska, becoming the first human beings in North America.

Biologists have also found the remains of land-based plants and animals on shelves that are now underwater. For example, scientists have discovered 11,000-year-old mastodon teeth and spruce pollen off the coast of the northeastern United States. Scientific instruments can show that the mastodon and pollen lived during the time of the last ice age.

When the shelves were above water, glaciers moved over them and changed their surfaces. As huge alpine glaciers moved quickly downhill, they gouged

deep, narrow valleys. Now, the valleys are filled with seawater. These narrow, flooded valleys that descend into the continental shelf are known as fjords.

















Continental slope

continental slope, seaward border of the continental shelf. The world’s combined continental slope has a total length of approximately 300,000 km (200,000 miles) and descends at an average angle in excess of 4° from the shelf break at the edge of the continental shelf to the beginning of the ocean basins at depths of 100 to 3,200 metres (330 to 10,500 feet).

The gradient of the slope is lowest off stable coasts without major rivers and highest off coasts with young mountain ranges and narrow continental shelves. Most Pacific slopes are steeper than Atlantic slopes. Gradients are flattest in the Indian Ocean. About one-half of all continental slopes descend into deep-sea trenches or shallower depressions, and most of the remainder terminate in fans of marine sediment or in continental rises. The transition from continental crust to oceanic crust usually occurs below the continental slope.

About 8.5 percent of the ocean floor is covered by the continental slope-rise system. This system is an expression of the edge of the continental crustal block. Beyond the shelf-slope break, the continental crust thins quickly, and the rise lies partly on the continental crust and partly on the oceanic crust of the deep sea. Although the continental slope averages about 4°, it can approach vertical on carbonate margins, on faulted margins, or on leading-edge, tectonically active margins. Steep slopes usually have either a very poorly developed continental rise or none at all and are called escarpments.

Continental slopes are indented by numerous submarine canyons and mounds. The Blake Plateau off the southeastern United States and the continental borderland off southern California are examples of continental slopes separated from continental shelves by plateaus of intermediate depth. Slopes off mountainous coastlines and narrow shelves often have outcrops of rock.

The predominant sediments of continental slopes are muds; there are smaller amounts of sediments of sand or gravel. Over geologic time, the continental slopes are temporary depositional sites for sediments. During lowstands of sea level, rivers may dump their sedimentary burden directly on them. Sediments









build up until the mass becomes unstable and sloughs off to the lower slop and

the continental rise. During highstands of sea level, these processes slow down as the coastline retreats landward across the continental shelf, and more of the sediments delivered to the coast are trapped in estuaries and lagoons. Still the process continues, albeit slowly, as sediments are brought across the shelf break by winnowing of the shelf surface and by advection. Slopes are sometimes scoured by such major ocean currents as the Florida Current that work to erode their surfaces. Off active major deposition centres, such as the Mississippi delta, slope sequences may accumulate through progradation, while the active slope front is continuously shedding sediments downslope by gravity processes.

The Deep Sea Plains

The deep sea plains, also called abyssal plains, are a broad and nearly level area covering about two-thirds of the entire ocean floor. They extend from the base of the deep sea fans and the marginal trenches.

These plains lie at depths between 2000 and 3000 fathoms. They include flat surfaces with slopes of less than 1:1000. However, the deep sea plains are not as flat as was formerly thought.

The Albatross Expedition found from continuous echo-sounding that the bottom profile is rather rugged in detail. It was also discovered that despite BRANCHES IN – PUNE AND MUMBAI








the absence of steep slopes, the ocean floor is characterized by different types of topographical features.

The major characteristic of the abyssal plains is the complete absence of sediments brought by the Rivers. It certainly does not mean that the bottom is composed of naked rocks.

On the contrary, their surface is covered with various pelagic oozes. In certain areas volcanic peaks rise steeply from these plains, and sometimes reach the surface as isolated islands.

The pelagic oozes consist of skeletal remains of innumerable Dead Sea animals and the vegetables. In the deeper parts are found the deposits of red clay which appear to be the volcanic ash deposited there by the winds blowing over the oceans.









Another characteristic feature of these plains is that those erosional processes, which produce various types of landforms on the continents, are not present there. In fact, the volcanic and tectonic activities produce the bottom relief of these plains.

Moreover, the effect of weathering and sub-aerial erosion being nil on the deep ocean floor, all the topographical features are characterized by extreme sharpness. Landforms produced by submarine volcanic activities or by folding and faulting tend to conserve their original form.

Various research vessels with the latest observational techniques have successfully discovered various types of topographical features on the deep sea plains.

These submarine features include the oceanic ridges, trenches, seamounts and guyots etc. These landforms under the sea water are described under the respective oceans in which they occur.

Oceanic ridges are found in every ocean basin and appear to girdle the Earth. The ridges rise from depths near 5 km (3 miles) to an essentially uniform depth of about 2.6 km (1.6 miles) and are roughly symmetrical in cross section. They can be thousands of kilometres wide. In places, the crests of the ridges are offset across transform faults within fracture zones, and these faults can be followed down the flanks of the ridges. (Transform faults are those along which lateral movement occurs.) The flanks are marked by sets of mountains and hills that are elongate and parallel to the ridge trend.









New oceanic crust (and part of the Earth’s upper mantle, which, together with the crust, makes up the lithosphere) is formed at seafloor spreading centres at these crests of the oceanic ridges. Because of this, certain unique geologic features are found there. Fresh basaltic lavas are exposed on the seafloor at the ridge crests. These lavas are progressively buried by sediments as the seafloor spreads away from the site. The flow of heat out of the crust is many times greater at the crests than elsewhere in the world. Earthquakes are common along the crests and in the transform faults that join the offset ridge segments. Analysis of earthquakes occurring at the ridge crests indicates that the oceanic crust is under tension there. A high-amplitude magnetic anomaly is centred over the crests because fresh lavas at the crests are being magnetized in the direction of the present geomagnetic field.

The depths over the oceanic ridges are rather precisely correlated with the age of the ocean crust; specifically, it has been demonstrated that the ocean depth is proportional to the square root of crustal age. The theory explaining this relationship holds that the increase in depth with age is due to the thermal contraction of the oceanic crust and upper mantle as they are carried away from the seafloor spreading centre in an oceanic plate. Because such a tectonic plate is ultimately about 100 km (62 miles) thick, contraction of only a few percent predicts the entire relief of an oceanic ridge. It then follows that the width of a ridge can be defined as twice the distance from the crest to the point where the plate has cooled to a steady thermal state. Most of the cooling takes place within 70 or 80 million years, by which time the ocean depth is about 5 to 5.5 km (3.1 to 3.5 miles). Because this cooling is a function of age, slow-spreading ridges, such as the Mid-Atlantic Ridge, are narrower than faster-spreading ridges, such as the East Pacific Rise. Further, a correlation has been found between global spreading rates and the transgression and regression of ocean waters onto the continents. About 100 million years ago, during the early Cretaceous Period when global spreading rates were uniformly high, oceanic ridges occupied comparatively more of the ocean basins, causing the ocean waters to transgress (spill over) onto the continents, leaving marine sediments in areas now well away from coastlines.









Besides ridge width, other features appear to be a function of spreading rate. Global spreading rates range from 10 mm (0.4 inch) per year or less up to 160 mm (6.3 inches) per year. Oceanic ridges can be classified as slow (up to 50 mm [about 2 inches] per year, intermediate (up to 90 mm (about 3.5 inches) per year, and fast (up to 160 mm per year). Slow-spreading ridges are characterized by a rift valley at the crest. Such a valley is fault-controlled. It is typically 1.4 km (0.9 mile) deep and 20–40 km (about 12–25 miles) wide. Faster-spreading ridges lack rift valleys. At intermediate rates, the crest regions are broad highs with occasional fault-bounded valleys no deeper than 200 metres (about 660 feet). At fast rates, an axial high is present at the crest. The slow-spreading rifted ridges have rough faulted topography on their flanks, while the faster-spreading ridges have much smoother flanks.

Distribution of major ridges and spreading centres

Oceanic spreading centres are found in all the ocean basins. In the Arctic Ocean a slow-rate spreading centre is located near the eastern side in the Eurasian basin. It can be followed south, offset by transform faults, to Iceland. Iceland has been created by a hot spot located directly below an oceanic spreading centre. The ridge leading south from Iceland is named the Reykjanes Ridge, and, although it spreads at 20 mm (0.8 inch) per year or less, it lacks a rift valley. This is thought to be the result of the influence of the hot spot.

Atlantic Ocean

The Mid-Atlantic Ridge extends from south of Iceland to the extreme South Atlantic Ocean near 60° S latitude. It bisects the Atlantic Ocean basin, which led to the earlier designation of mid-ocean ridge for features of this type. The Mid-Atlantic Ridge became known in a rudimentary fashion during the 19th century. In 1855 Matthew Fontaine Maury of the U.S. Navy prepared a chart of the Atlantic in which he identified it as a shallow “middle ground.” During the 1950s the American oceanographers Bruce Heezen and Maurice Ewing proposed that it was a continuous mountain range.









In the North Atlantic the ridge spreads slowly and displays a rift valley and mountainous flanks. In the South Atlantic spreading rates are between slow and intermediate, and rift valleys are generally absent, as they occur only near transform faults.

Submarine canyon

submarine canyon, any of a class of narrow, steep-sided valleys that originate on the continental shelf and cut into continental slopes and continental rises of the oceans. They are rare on continental margins that have extremely steep continental slopes or escarpments. Submarine canyons are so called because they resemble canyons made by rivers on land.

Unlike deep-sea trenches, which are found in areas where one tectonic plate slides beneath another, undersea canyons are found along the slopes of most continental margins. They also occur along the slopes of the Hawaiian Islands and possibly certain other ocean islands. The majority of these V-shaped depressions have steep, rocky walls thousands of metres high. Those of the Grand Bahama Canyon, which are thought to be the highest, rise nearly 5 km (3 miles) from the canyon floor. The walls of the Grand Canyon of the Colorado River, by comparison, measure about 1.6 km (1 mile) high. Most submarine canyons extend only about 48 km (30 miles) or less in length, but a few are more than 320 km (200 miles) long. They usually are many kilometres in width; for instance, the Grand Bahama Canyon measures 37 km (23 miles) at its widest point.









A relatively large number of submarine canyons are located directly offshore of river canyons of adjacent land areas and may have once been connected to extensions of the latter. In most cases, however, the characteristics of the submarine variety and those of the nearby land canyons are quite distinct. The submarine canyons, for example, tend to have steeper side slopes, much higher gradients, and considerably narrower floors. Moreover, the drainage pattern of submarine canyons differs from that of their terrestrial counterparts. The submarine canyons have a substantial number of tributaries at their heads but generally do not have as many tributaries in their lower courses as do the land canyons. Even so, many submarine canyons end in a series of distributary channels through the continental rise, and these channels often emerge in the deep sea on abyssal plains that are found adjacent to the continents, especially in the Atlantic basin.

Submarine canyons act as conduits to bring sand-sized sediments from the continental margins to the deep sea. During lowstands of sea level, rivers empty directly into the heads of many Atlantic canyons. Sand and mud are carried down these systems, many times bypassing the slope-rise system to be carried directly out onto the abyssal plains of the ocean floor. Core samples taken in abyssal plains in the Atlantic Ocean off North Carolina show distinctive shelf molluscan fragments in gravity-flow deposits that have traveled hundreds of kilometres and that can be traced back through the Hatteras submarine canyon system. During highstands of sea level, the submarine canyons off the east coast of North America are many tens of kilometres from the coastline, and the downslope movement through them is slowed down dramatically or perhaps even ceases. On the west coast of the continent, where active tectonism results in a narrow margin backed by high mountains, a situation exists today that is roughly analogous to the one that prevails on a passive margin during lowstands of sea level. Submarine canyons originate close to the coast, intersecting the longshore current system and siphoning off sand to basin floors in the Borderland—that is, the continental margin of southern California and northern Baja California.









For years the origin of submarine canyons has been the subject of much debate among investigators. Various ideas have been proposed, but prevailing theory favours subaerial erosion as the starting point for a good number of undersea canyons. Such erosion is thought to have begun with the lowering of sea level during the glaciations of the Pleistocene Epoch (about 2,600,000 to 11,700 years ago). It is perceived, however, that subaerial erosion alone could scarcely have excavated deep canyons that extend down to the seafloor. Evidence seems to suggest that the principal agents responsible for the formation of submarine canyons are marine processes, most notably the erosion and transportation of sediments by turbidity currents activated by the slumping of unconsolidated rock material near the heads of the canyons.

BASIC FACTS ABOUT CORAL REEFS

Coral reefs are some of the most diverse ecosystems in the world, housing tens of thousands of marine species. About one-third of all marine fish species live part of their lives on coral reefs.

Reefs in the Florida Keys, for example, hold at least 45 species of stony coral, 37 species of octocoral, five species of sea turtles, 500 species of fish, about 1,700 species of mollusks and hundreds of species of sponges. In addition to their incredible value as wildlife habitat, coral reefs protect coastlines from storms and provide billions of dollars of food and jobs every year to people around the world.

Behavior & Diet

Corals are ancient animals related to jellyfish and anemones. An individual coral is known as a polyp, a very small and simple organism consisting mostly of a stomach topped by a tentacle-bearing mouth. The polyps extend their









tentacles at night to sting and ingest tiny organisms called plankton and other small creatures.

Thousands of identical polyps live together and form a coral colony. Each polyp excretes a calcium carbonate exoskeleton beneath it and, over long periods of time, the skeletons of many coral colonies add up to build the structure of a coral reef. Many other species – fish, invertebrates, algae and microorganisms – make their homes on and around this reef. Reefs only occur in shallow areas that are reachable by sunlight because of the relationship between coral and algae. Various types of microscopic algae, known as Symbiodinium, live inside of the coral, providing them with food and helping them to grow faster. In many ways, reef-building corals are animals that act like plants – they stay in one place and get some of their energy from the sun.

Range

Coral reefs are found all around the world in tropical and subtropical oceans. They are usually found in shallow areas at a depth of less than 150 feet.

However, some coral reefs extend even deeper, up to about 450 feet deep. Despite how important coral reefs are to life in the ocean, all of them in the world add up to less than one percent of the sea floor – an area about the size of France.









Gulf

a deep inlet of the sea almost surrounded by land,

Bay

A bay is a body of water connected to an ocean or lake, formed by an indentation of the shoreline. A large bay may be called a gulf, a sea, a sound, or a bight. A cove is a smaller circular or oval coastal inlet with a narrow entrance; some coves may be referred to as bays. A fjord is a particularly steep bay shaped by glacial activity.

Bays can exist as the estuary of a river, as the estuary of the Parramatta River in Australia. Bays may be nested in each other; for example, James Bay is an

arm of Hudson Bay. Some large bays, such as the Bay of Bengal and the Hudson Bay, have varied marine geology.









The land surrounding a bay can often block waves and reduce the strength of winds. Thus bays were significant in the history of human settlement because they could provide a safe place for fishing. Later they were important in the development of sea trade as the safe anchorage they provide encouraged their selection as ports.

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, 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.









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.

Bank, Shoal and Reef:

These marine features are formed as a result of erosional, depositional and biological activity. Also, these are produced upon features of diastrophic origin. Therefore, they are located on upper parts of elevations.

A bank is a flat topped elevation located in the continental margins. The depth of water here is shallow but enough for navigational purposes. The Dogger Bank in the North Sea and Grand Bank in the north-western Atlantic off Newfoundland are famous examples. The banks are sites of some of the most productive fisheries of the world.

A shoal is a detached elevation with shallow depths, since they project out of water with moderate heights, they are dangerous for navigation.A reef is a predominantly organic deposit made by living or dead organisms that forms a mound or rocky elevation like a ridge. Coral reefs are a characteristic feature of the Pacific Ocean where they are associated with seamounts and guyots. The largest reef in the world is found off the Queensland coast of Australia (Fig. 3.3). Since the reefs may extend above the surface, they are generality dangerous for navigation.

Significance of Study of Oceanic Morphology:

The study of the morphology of the oceans is important because the relief controls the nature, character and the motion of sea water. The oceanic movement in the form of currents, in turn, causes many variations, which are important to the character of marine fauna and flora. The bottom relief of oceans also influences navigation, fishing and other important activities of man. BRANCHES IN – PUNE AND MUMBAI








Ocean Bottom Relief with the Floors of the Three Majors’ Oceans of the World

Ocean bottoms are not plain as believed earlier; they reveal many complex and varied features which rival the relief features on land.

Four major divisions can easily be identified on the ocean floor—(i) the continental shelf, (ii) the continental slope, (iii) the continental rise, (iv) the abyssal plain. Besides these, there are many associated features—ridges, hills, seamounts, guyots, trenches, canyons, sleeps, fracture zones, island arcs, atolls, coral reefs, submerged volcanoes and sea-scarps.

This great variety of relief is largely due to interaction of tectonic, volcanic, erosional and depositional processes. At greater depths, the tectonic and volcanic phenomena are more significant processes. Given below is a survey of typical relief features of the ocean floor.

A Hypsographic Curve Indicates Distnctive Features of the Ocean Bottom Relief

Continental Shelf:

This is a gentle seaward sloping surface extending from the coasts toward s the open sea. In all, about 7.5% of the total area of the oceans is covered by the continental shelves. The shelf is formed by the drowning of a part of a continent with a relative rise in sea level or marine deposition beneath the water.

The average width of the continental shelf is about 70 km and mean slope is less than one degree, but the width shows great variety from location to location. For instance, it is almost absent in the eastern Pacific, especially off South America and is upto 120 km wide along the eastern coast of USA. The seaward edge of the shelf is usually 150-200 metres deep.









The continental shelves are mostly covered by sediments of terrestrial origin. There are various types of shelves—glaciated shelf, coral reef shelf, shelf of a large river, shelf with dendritic valleys and the shelf along young mountain ranges.

The shelves are of great use to man:

(i) Marine food comes almost entirely from them;

(ii) They provide the richest fishing grounds;

(iii) They are potential sites for economic minerals; already, about 20% of the world production of petroleum and gas comes from shelves;

(iv) They are large stores of sand and gravel.

Continental Slope:

As the continental shelf nears its seaward edge, the gradient becomes steeper—two to five degrees. This is the site of the continental slope which descends to a depth of 3,500 metres and joins the shelf to the deep ocean floor. The site of the slope also indicates the end of the continental block. The slopes may be furrowed by canyons and trenches.

Continental Rise:

The continental slope gradually loses its steepness with depth. When the slope reaches a level of between 0.5° and 1°, it is referred to as the continental rise. With increasing depth the rise becomes virtually flat and merges with the abyssal plain.

Abyssal Plains:

Beyond the continental rise, at depths from 3,000 m to 6,000 m, lie the deep sea plains, called abyssal plains or abyssal floors. Covering nearly 40% of the ocean floor, the abyssal plains are present in all major oceans and several seas of the world. They are uniquely flat with a gradient of less than 10,000. The









large supply of terrigenous and shallow water sediments buries the irregular topography to form a generally flat relief.

Submarine Ridges:

Submarine ridges are mountain ranges, a few hundred kilometres wide and hundreds and often thousands of kilometres in length on the floors of oceans. Running for a total length of 75,000 km, these ridges form the largest mountain systems on earth.

These ridges are either broad, like a plateau, gently sloping or in the form of steep-sided narrow mountains. These oceanic ridge systems are of tectonic origin and provide evidence in support of the theory of Plate Tectonics.

Abyssal Hills:

These are elevated features of volcanic origin. A submarine mountain or peak rising more than 1,000 metres above the ocean floor is known as a seamount. The flat topped mountains are known as guyots

Seamounts and guyots are very common in the Pacific Ocean where they are estimated to number around 10,000.

Submarine Trenches or Deeps:

These are the deepest parts of the oceans with their bottoms far below the average level of the ocean floors. A trench is a long, narrow and steep-sided depression on the ocean bottom, which is usually 5,500 metres in depth. The trenches lie along the fringes of the deep-sea plain and run parallel to the bordering fold mountains or the island chains.

They are believed to have resulted from down faulting or down folding of the earth’s crust and are, therefore, of tectonic origin. The trenches are very common in the Pacific Ocean and form an almost continuous ring along the western and eastern margins of the Pacific. The Mariana Trench off the Guam Islands in the Pacific Ocean is the deepest trench with a depth of more than 11 kilometres.









Submarine Canyons:

These are steep valleys, forming deep gorges on the ocean floor. They are mainly restricted to the continental shelf, slope and rise.

Broadly, there are three types of submarine canyons:

1. Small gorges which begin at the edge of the continental shelf and extend down the slope to very great depths, e.g., Oceanographer Canyons near New England.

2. Those which begin at the mouth of a river and extend over the shelf, such as the Zaire, the Mississippi and the Indus canyons.

3. Those which have a dendritic appearance and are deeply cut into the edge of the shelf and the slope, like the canyons off the coast of southern California. The Hudson Canyon is the best known canyon in the world. The largest canyons in the world occur in the Bering Sea off Alaska. They are the Bering, Pribilof and Zhemchung canyons.

Indian Ocean, body of salt water covering approximately one-fifth of the total ocean area of the world. It is the smallest, youngest, and physically most complex of the world’s three major oceans. It stretches for more than 6,200 miles (10,000 km) between the southern tips of Africa and Australia and, without its marginal seas, has an area of about 28,360,000 square miles (73,440,000 square km). The Indian Ocean’s average depth is 12,990 feet (3,960 metres), and its deepest point, in the Sunda Deep of the Java Trench off the southern coast of Java, is 24,442 feet (7,450 metres).

The Indian Ocean is bounded by Iran, Pakistan, India, and Bangladesh to the north; the Malay Peninsula, the Sunda Islands of Indonesia, and Australia to the east; Antarctica to the south; and Africa and the Arabian Peninsula to the west. In the southwest it joins the Atlantic Ocean south of the southern tip of Africa, and to the east and southeast its waters mingle with those of the Pacific.









The question of defining the oceanic limits of the Indian Ocean is complicated and remains unsettled. The clearest border and the one most generally agreed upon is that with the Atlantic Ocean, which runs from Cape Agulhas, at the southern tip of Africa, due south along the 20° E meridian to the shores of Antarctica. The border with the Pacific Ocean to the southeast is usually drawn from South East Cape on the island of Tasmania south along the 147° E meridian to Antarctica. Bass Strait, between Tasmania and Australia, is considered by some to be part of the Indian Ocean and by others to be part of the Pacific. The northeastern border is the most difficult to define. The one most generally agreed upon runs northwest from Cape Londonderry in Australia across the Timor Sea, along the southern shores of the Lesser Sunda Islands and the island of Java, and then across the Sunda Strait to the shores of Sumatra. Between the island of Sumatra and the Malay Peninsula the boundary is usually drawn across the Singapore Strait.

There is no universal agreement on the southern limit of the Indian Ocean. In general (and for the purposes of this article), it is defined as extending southward to the coast of Antarctica. However, many—notably in Australia—consider the portion closest to Antarctica (along with the corresponding southern extensions of the Atlantic and Pacific) to be part of the Southern (or Antarctic) Ocean. Australians often call the entire expanse south of that continent’s south coast the Southern Ocean.

The Indian Ocean has the fewest marginal seas of the major oceans. To the north are the inland Red Sea and Persian Gulf. The Arabian Sea is to the northwest, and the Andaman Sea to the northeast. The large gulfs of Aden and Oman are to the northwest, the Bay of Bengal is to the northeast, and the Great Australian Bight is off the southern coast of Australia.

The Indian Ocean differs from the Atlantic and Pacific Oceans in several other respects. In the Northern Hemisphere it is landlocked and does not extend to Arctic waters or have a temperate-to-cold zone. It has fewer islands and narrower continental shelves. It is the only ocean with an asymmetric and, in the north, semiannually reversing surface circulation. It has no separate source of bottom water (i.e., the Indian Ocean’s bottom water originates









outside its boundaries) and has two sources of highly saline water (the Persian Gulf and the Red Sea). Below the surface layers, especially in the north, the ocean’s water is extremely low in oxygen.

The origin and evolution of the Indian Ocean is the most complicated of the three major oceans. Its formation is a consequence of the breakup, about 150 million years ago, of the southern supercontinent Gondwana (or Gondwanaland); by the movement to the northeast of the Indian subcontinent (beginning about 125 million years ago), which began colliding with Eurasia about 50 million years ago; and by the western movement of Africa and separation of Australia from Antarctica some 53 million years ago. By 36 million years ago, the Indian Ocean had taken on its present configuration. Although it first opened some 125 million years ago, almost all the Indian Ocean basin is less than 80 million years old.

Temperature of Ocean Water

This is a simple temperature-depth ocean water profile. You can see temperature decreases with increasing depth. The thermocline are layers of water where the temperature changes rapidly with depth. This temperature-depth profile is what you might expect to find in low to middle latitudes.

Click on image for full size

Windows to the Universe original image

If you want to know about the temperature of the ocean, you have to learn about the parts of the the ocean first. The top part of the ocean is called the surface layer. Then there is a boundary layer called the thermocline. The thermocline separates the surface layers and the deep water of the ocean. The deep ocean is the third part of the ocean.

The Sun hits the surface layer of the ocean, heating the water up. Wind and waves mix this layer up from top to bottom, so the heat gets mixed downward too. The temperature of the surface waters varies mainly with latitude. The polar seas (high latitude) can be as cold as -2 degrees Celsius (28.4 degrees Fahrenheit) while the Persian Gulf (low latitude) can be as warm as 36 degrees BRANCHES IN – PUNE AND MUMBAI








Celsius (96.8 degrees Fahrenheit). Ocean water, with an average salinity of 35 psu, freezes at -1.94 degrees Celsius (28.5 degrees Fahrenheit). That means at high latitudes sea ice can form. The average temperature of the ocean surface waters is about 17 degrees Celsius (62.6 degrees Fahrenheit).

90 % of the total volume of ocean is found below the thermocline in the deep ocean. The deep ocean is not well mixed. The deep ocean is made up of horizontal layers of equal density. Much of this deep ocean water is between 0-3 degrees Celsius (32-37.5 degrees Fahrenheit)! It's really, really cold down there!

There is a neat program that is measuring the temperature and salinity of ocean surface waters around the world. The Argo program deploys floats that measure salinity and temperature throughout the surface layer of the ocean. Over 3,000 free-drifting floats have been deployed all over the ocean and each float is programmed to sink 2,000 meters down, drifting at that depth for about 10 days. The float then makes its way to the surface measuring temperature and salinity the whole time. Data is transmitted to a satellite once the float reaches the surface, so that scientists and the public have access to the state of the ocean within hours of the data collection. At a greater depth in the water, measurements are often made with a CTD instrument (CTD = conductivity, temperature, depth), where the instrument is placed in the ocean water from a ship or a platform. These instruments are used by the Bermuda Institute of Ocean Sciences (BIOS), where they have been tracking ocean measurements like temperature, salinity and oxygen concentrations for over 55 years.

The Argos and BIOS program have both published results that confirm that the ocean is warming. Surface water temperatures obviously change from season to season and year to year, but the whole ocean has warmed about 0.1 degree Fahrenheit (0.055 degree Celsius) in the past 30-50 years. This may not seem like much of a temperature change, but it is significant. Think about a pot of water heating on a stove. A small pot of water will heat quickly, while a large pot of water at the same heat setting will heat very slowly. This is due to a difference in heat capacity. The ocean has an enormous heat capacity because









of its large size. So it is like an enormous pot of water, and it takes a great amount of heat to warm the ocean. The fact that the ocean has warmed significantly in 30 to 50 years is remarkable and concerning.

Ocean Deposits

Ocean deposits are the unconsolidated sediments (Inorganic or Organic ) derived from various sources deposited on the oceanic floor.

What are the sources of these ocean deposits ?

These ocean deposits are derived from three major sources:

1- Terrigenous Sources

2- Volcanic Deposits

3- Pelagic Deposits

Labrador Current

The Labrador Current is a cold current in the North Atlantic Ocean which flows from the Arctic Ocean south along the coast of Labrador and passes around Newfoundland, continuing south along the east coast of Nova Scotia. It is a continuation of the West Greenland Current and the Baffin Island Current.It meets the warm Gulf Stream at the Grand Banks southeast of Newfoundland. The combination of these two currents produces heavy fogs and also created one of the richest fishing grounds in the world.In spring and early summer, this current transports icebergs from the glaciers of Greenland southwards into the trans-Atlantic shipping lanes. The waters of the Labrador Current have a cooling









effect on the Canadian Atlantic provinces and USA upper Northeast c oa s t f r om Ma i ne s ou th t o Massachusetts.

Peru Current

Peru Current, also called Humboldt Current, cold-water current of the southeast Pacific Ocean, with a width of about 900 km (550 mi). Relatively slow and shallow, it

transports only 350,000,000–700,000,000 cu ft (10,000,000–20,000,000 cu m) of water per second. It is an eastern boundary current similar to the California Current of the North Pacific. The West Wind Drift flows east toward South America south of latitude 40° S, and while most of it continues through the Drake Passage around the southern tip of South America to the Atlantic, a shallow stream turns north to parallel the continent as far as latitude 4° S, where it turns west to join the Pacific South Equatorial Current.As it is a cold current, except at times of the phenomenon known as El Niño, the Peru Current brings fog to the nearby coast but also helps to keep the coast one of the most intensely arid areas in the world. The cold flow is intensified by upwelling of deep water caused by the combined effects of the drag of surface winds of the Southeast Trades and the Earth’s rotation. Upwelling brings abundant nutrients close to the surface, and the beneficial effects of sunlight, which allow for rich plankton growth, make the waters off Peru, Chile, and Ecuador one of the world’s greatest fishing grounds for anchovies and the larger fish (e.g., tuna) that feed upon them. Another economic benefit is the guano, used for fertilizer, deposited by the flocks of birds that feed on the anchovies.

The current’s alternative name is taken from that of the German scientist Alexander von Humboldt, who in 1802 took measurements that showed the coldness of the flow in relation to the air above it and the sea around it.







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