THE HYDROSPHERE
 

Approximately 70.8 percent of the terrestrial surface is covered by saltwater oceans, with a volume of about 1.4 billion cubic kilometres ,at an average temperature of about 4º C, not far above the freezing point of water.
The oceans contain about 97 percent of the planet’s water budget. The remaining 3 percent occurs as fresh waters, three-quarters of which are locked up in the form of ice at polar latitudes. Most of the remaining fresh water is groundwater held in soils and rocks; less than 1 percent of it occurs in lakes and rivers. In terms of percentage, atmospheric water vapour is negligible, but the transport of water evaporated from the oceans onto land surfaces is an integral part of the hydrological cycle that renews and sustains the ecosphere.

HYDROLOGICAL CYCLE

The hydrological cycle involves the transfer of water from the oceans through the atmosphere to the continents and back to the oceans over and beneath the land surface. The cycle involves processes such as precipitation, evaporation, interception, transpiration, infiltration, percolation, and runoff. These processes operate throughout the entire hydrosphere, which extends from about 15 kilometres into the atmosphere to roughly 5 kilometres into the crust.
About one-third of the solar energy that reaches the surface of the Earth is expended on evaporating ocean water. The resulting atmospheric moisture and humidity condense into clouds, rain, snow, and dew. Moisture is a crucial factor in determining weather. It is the driving force behind storms and is responsible for separating electrical charge, which is the cause of lightning. Moisture wets the land, replenishes subterranean aquifers, chemically weathers the rocks, erodes the landscape, nourishes life, and fills the rivers, which carry dissolved chemicals and sediments back into the oceans. For example, water plays a vital role in the carbon dioxide cycle. Calcium is weathered from continental rocks and is then returned to the oceans, where it combines to form calcium carbonates (such minerals constitute the shells of marine life). Eventually the carbonates are deposited on the seafloor and are lithified to form limestones. Some of these carbonate rocks are later dragged deep into the Earth’s interior and melted, resulting in a release of carbon dioxide (from volcanoes, for example) into the atmosphere. Cyclical processing of water, carbon dioxide, and oxygen through physical and biological systems on the Earth is fundamental to maintaining balance in the ecosphere.

Oceans

Earth is covered by one hydrosphere or one layer of connecting water. Even though the ocean is broken up into seven ocean parts, all the oceans are connected, one flowing into the other. It is due to this interconnection that the mean sea level is same across the world.

The Pacific Ocean, covering 64,186,000 square miles, is nearly as large as the Atlantic and Indian Oceans combined. It contains a little more than half of all the water in the world's oceans.

TYPES OF COASTS

Coastlines may be of two types – coastlines of submergence which have formed due to sinking of land or rise in sea levels; and coastlines of emergence which have formed due to uplift of the land or fall in sea levels.

Coastlines of Submergence

Fjord coasts- Fjords are submerged glacial U-shaped valleys marked by steep walls. Due to greater intensity of ice erosion, fjords are deeper inlands and become shallower as one approaches the sea. They are found in the temperate regions of the world and are very prominent in Scandinavia, Alaska and South America.
Ria coasts- In upland coastal regions where the mountain chains run at right angles to the sea, ria coasts were formed when the sea levels rose following end of ice age and sea waters submerged these valleys.
A ria coast differs from fjords in two aspects. First, they are not glaciated and second, there depth increases seawards.
Dalmatian coasts- Dalmatian coasts were formed when the mountain valleys which run parallel to the coastline are submerged following the rise in sea levels after end of ice age. These are longitudinal coastlines where former mountain systems rise above surface as series of islands.
Estuarine coasts- In submerged lowlands, the mouths of rivers are drowned in the sea so as to give rise to estuaries. These estuaries being de-silted by tidal action often serve as good ports.
It is to be noted that despite being deep sheltered coastlines, Ria, Fjords and Dalmatian coasts do not form good ports as the mountains in the background hinder communication with the mainland.

Coastlines of Emergence

Emergent upland coast- Faulting and movements along the coastline often thrust the coastal lands upwards thereby leading to formation of emergent upland coasts. The western coast of Indian peninsula is such emergent coast.
Uplifted lowland coast- such coasts are formed when the continental shelf gets uplifted and smooth gently sloping coastline is formed.

RELIEF FEATURES OF OCEAN FLOOR

Like the continents, oceans floors too have relief features, which can be broadly divided into five zones: (i) Continental shelves (ii) Continental slopes and (iii) Deep Oceans Basins or Floors (iv) The Ocean Deeps (v) Submarine Canyons.

Continental Shelf
The portions of the land, which are submerged under seawater, constitute continental shelf. The continental shelf is shallow and its depth is not more than 200 meters. Its slope from the land to the sea is about 2 meters per km. The breadth of the continental shelf is not the same everywhere. The maximum breadth is almost equal to 1,100 km. found between Northern Norway and Novaya Zemlya along Berings Sea. The area of continental shelf is maximum in Asia, i.e. 9.38x106 sq. km. The second place is occupied by North America (6.74x106 square km.). The smallest shelf, i.e., 0.36x106 square km. is found in Antarctica. Areas of the shelf are composed of hard rocks. Mud, sand and glacial deposits are also found at many places. Glacial deposits are found on the continental shelf in New England and Canada. Many drumlins rise above sea surface and appear as islands.

Types of Continental Shelves
i. Glacier shelves
ii. Broad river shelves
iii. Coral shelves.

Continental Slope
At the outer edge of the continental shelf, the slope suddenly steepens. This is found to be 35 to 61 meters per km. One end of the slope connects it with continental shelf while the other one merges into the ocean floors. The area occupied by continental slope is 8.5% of the total ocean area. The continental slope consists of light continental rocks, which overlie the dense rocks of ocean floors. Hence, no deposit of the coast except some fine mud reaches the continental slope.

Continental Rise

Continental rise is the zone of transition between the continental slope and abyssal plains. The region represents the edge of continental blocks as it merges into the oceanic floor. It is marked by gentle slope of 0.5 to 1⁰ .

Deep Ocean Basins or Floors
About 2/3 of ocean surface is occupied by deep ocean basins or floors. They are situated on the base of the continental slope. The depth varies from 3,000 to 6,000 meters. Many long zigzag rides, plateaus, volcanic summits, etc., exist on the basin. Many mountain tops rise steeply from the ocean floors and emerge out of sea as islands. Such mountain tops are found in Indonesia and in Pacific Ocean. Many pelagic deposits and oozes are found on the basins.

The Ocean Deeps
A depression, long and narrow, existing on the deep ocean basins are called ocean deeps. The deeps are also called trenches or troughs. Sometimes their name suggests that the trenches have a steep slope. In fact, their slope is nowhere more than 7% Due to the nonexistence of sediments, the deeps are almost empty.

TIDES

Tides are defined as slight oscillations of sea level that occur approximately twice a day and attain exaggerated proportion in marginal seas, straits and estuaries. The major cause of the tides is the gravitational pull of the moon and, to a lesser extent, of the sun, as it is further from the earth.

Lunar Tides
As the moon travels in its orbit in the same direction as the earth’s rotation, a period of 24 hours 50 minutes elapses between two successive occasions when the moon is vertically above a point. The highest level the water reaches is called a high tide and the lowest level is called the low tide. High and low tide occur twice each during the period of 24 hrs. 50 minutes, giving an interval of about 12-1\2 hours between successive high (or low) tides.

Neap and Spring Tides
When the earth, the moon and the sun are in a straight line, the gravitational force is at its greatest because tide-producing forces of both sun and moon complement each other and they ‘pull’ together. This produces tides of unusually great ranges, called the spring tides. These occur about twice a month; at new moon, when the sun and the moon are in conjunction: and at full moon, when they are in opposition.
When the earth, the moon and the sun are not in a straight line, but are at right angles to the earth, the gravitational force is less, as the sun and the moon are not pulling together. This happens during phases of first and third quarter, i.e. at half moon, the sun’s tide producing force tends to balance the tide producing force of the moon, resulting in tides of usually small range known as neap tides. Amplitude or tidal range refers to the difference between high tide and low tide; it is high at spring tides and low at neap tides.

Perigean and Apogean Tides
When the moon is nearest to the earth in its orbit (at perigee), its tide producing power is greater than average, resulting in perigean tides. These are 15-20 per cent greater than average. When the moon is farthest from the earth (in apogee), the tides are called apogean tides, which are about 15-20 per cent less than average. Coincidence of spring and perigean tides results in an abnormally great tidal range; while when neap and apogean tides coincide, the range is abnormally small.

River Tides
Tides are experienced in the lower parts of many of the great rivers. These are known as tidal rivers, where either the coastal area has recently subsided, or the ocean level rises, causing the lower part of the river to be drowned. River tides are distinguishable from ocean tides by one characteristic: the interval between high tide and the next low tide.

THE SEAS

With respect to their communication with the ocean the SEAS are classed into
(i) Marginal (fringing) seas,
(ii) Continental seas, or Mediterranean sea.

MARGINAL SEA
MARGINAL SEA is a semi-enclosed sea that borders a continent, and lies on a submerged portion of a continental mass, rather than within an ocean basin; for example: Sea of Okhotsk, Sea of Japan, Yellow Sea, Celebes Sea, Baltic Sea, North Sea, Red Sea, Persian Gulf.
The MARGINAL SEAS are characterized by a more or less free links with the oceans and, in a number of instances, are separated from it by a chain islands or by peninsulas. A relatively unobstructed communication of Marginal seas with the oceans is a factor determining their mutual similarity with respect to the ‘salinity’, ‘temperature’, and the abundance of ‘organic life’. They are swept by the ocean ‘tides’ and are subject to the influence of oceanic ‘currents’. On the other hand, some Marginal seas have close ties with land and are under sway of its effects. Among examples of this type of bodies are the Bering Sea, the sea of Okhotsk, the Japan Sea, the East China Sea, the South China Sea, the Caribbean Sea and other seas.

GULFS (BAYS) varying in their shape and size, indenting the land and broadly and freely communicating with the ocean, such as the Bay of Biscay, the Gulf of Guinea, the Bay of Bengal, the Gulf of Alaska, are also set apart. Some seas for instance the Arabian Sea, may also be referred to as Gulfs.

CONTINENTAL SEA
It is a partially enclosed sea, lying on or within a continent in the structural sense, linked with the open ocean through a narrow strait, for example Baltic Sea, Hudson Bay, Yellow Sea, Mediterranean Sea.
CONTINENTAL SEAS communicate with the ocean only through narrow and comparatively shallow straits. They include MEDITERRANEAN SEA with an average depth of 1498 metres and maximum depth of 4594 metres and a strait near the Gibraltar (about 320 metres deep); the BLACK SEA with a mean depth of 1271 metres and a link to the Mediterranean through the Bosporus strait (46 m deep); the sea of azov (8-13 metres deep).
A completely closed basin is the Caspian sea, which at the end of Pleistocene had lost all connection with the Azov – Black Sea basin and, consequently, with the ocean.

SEVEN SEAS
We have got only four oceans but SEVEN SEAS (especially in historical Tales). The proverbial seven seas are made up by dividing the first three oceans into north and south along the Equator and adding Arctic to them, thus: (i) North Pacific, (ii) South Pacific, (iii) North Atlantic, (iv) South Atlantic, (v) North Indian, (vi) South Indian and (vii) Arctic.

THE OCEAN DEPOSITS

There are four sources of ocean deposits. They are
(1) Terrigenous sediments: These deposits are obtained from erosion of rocks on land and are subdivided into the four subheadings:
i. Due to the work of rivers
ii. Due to the work of sea waves and currents.
iii. Due to the work of winds.
iv. Due to the work of glaciers.
The final destination of the sediment produced in these ways is mostly the seas and the oceans. They are also known as muds.

(2) Pelagic Sediments: These sediments are found on the floors of deep seas and oceans. Their origin is the marine vegetation and animal life. The sediments consist of shells and skeletons of marine vegetation and animals. They are also known as oozes.

(3) Sediment from Sub-Marine Volcanoes: The volcanoes submerged under the sea explode out volcanic material. This material is also very helpful in the formation of oceanic sediments. The ash of volcanic material ejected on the land also reaches the sea. They are also known as clays.

(4) Extra Terrestrial Sediment: Meteors and their ash falling on the earth also form a part of oceanic sediments.

LIFE IN THE OCEAN

The living organisms of the ocean can be divided into three groups, according to ‘where’ and ‘how’ they live in the ocean.
(i) The lowest order group is called PLANKTON. Plankton is made up of the ocean’s smallest-usually microscopic-plants and animals. These tiny plants and animals float freely with the movements of ocean water, are true ‘drifters’, and form the basis for the oceanic food chain.
(ii) The Second group is composed of the animals that swim in the water. This group is called NEKTONS and includes fish, sea mammals such as whales, and invertebrates such as the squid and octopus.
(iii) The third group is the plants and animals that live near or on the ocean floor. This group is called the BENTHOS. It includes coral and such burrowing or crawling animals as the barnacle, crab, Lobster, and oyster, and plants such as seaweed.

OCEAN CURRENTS

Fast-flowing current of seawater generated by the wind or by variations in water density and temperature between two areas. (Temperature differences across latitudes lead to uneven expansion of ocean waters thereby creating a gradient along which currents flow. For example the waters around equator are about 8 cm higher than that at middle latitudes.)

Ocean currents are partly responsible for transferring heat from the Equator to the poles and thereby evening out the global heat imbalance.
There are three basic types of ocean current: drift currents are broad and slow-moving; stream currents are narrow and swift-moving; and upwelling currents bring cold, nutrient-rich water from the ocean bottom.
Stream currents include the Gulf Stream and the Japan (or Kuroshio) Current. Upwelling currents, such as the Gulf of Guinea Current and the Peru (Humboldt) current, provide food for plankton, which in turn supports fish and sea birds. At approximate five-to-eight-year intervals, the Peru Current that runs from the Antarctic up the West coast of South America, turns warm, with heavy rain and rough seas, and has disastrous results for Peruvian wildlife and for the anchovy industry. The phenomenon is called El Niño (Spanish `the Child´) because it occurs towards Christmas.

Pacific Ocean Currents
Winds in the northern Pacific produce generally clockwise currents; a northward current from the equator flows past the Philippines and is joined by currents from the East Indies and China Sea at Taiwan to form the Kuroshio. The Kuroshio Current, off the shores of Japan, is the largest current. It can travel between 25 and 75 miles a day, 1 - 3 miles per hour, and extends some 3300 feet deep. The Gulf Stream is close to this current's speed. A cold current from the Bering Sea enters the Okhotsk and Japan seas, causing freezing in winter. In the South Pacific the trade winds cause anticlockwise equatorial currents which branch opposite southern Chile, to flow north as the cooling Peru Current (Humboldt Current), and south round Cape Horn.

Atlantic Ocean Currents
The trade winds, which determine the course of the ocean currents, have their origin in high-pressure areas in the centre of both the North and South Atlantic. They produce a warm equatorial current which divides and flows south as the Brazil current, and north through the Caribbean Sea and Gulf of Mexico, emerging as the Gulf Stream or North Atlantic Drift, which has an enormous warming influence on the climate of northwest Europe. A cold current flows south from the Arctic Ocean and, as the Labrador current, passes beneath the Gulf Stream off the Newfoundland Grand Banks. Between 40º and 80º west, and 25º and 30º north, there is an area of calm occupied by the Sargasso Sea, in which float extensive banks of weed.

Indian Ocean Currents
The Mozambique and Agulhas currents are warm and move southwards; in the east, the colder Western Australian current crosses the Indian Ocean moving northwards. North of the equator the currents vary with the monsoon. From November to March, there are northeast winds and the current flows from the North-east as North East Monsoon Drift. From May to September, southwest winds reverse the current to flow as South West Monsoon Drift. This is the only example of an annual reversal of direction by a large oceanic current.

Arctic Ocean Currents
The warm surface waters of the Atlantic flow up into the Arctic regions, passing between Greenland and Norway, where they are chilled by contact with the icy Arctic waters, and gradually sink to the bottom. Finally they return, along the east side of Greenland and down Davis Strait, as a cold current carrying with it the icebergs that are a danger to navigation in the Atlantic.

TSUNAMI

The word tsunami is derived from the Japanese composition - 'tsu' means 'harbor' and 'name' means 'wave'. A tsunami is a series of water waves caused by the displacement of a large volume of a body of water, generally an ocean or a large lake. Earthquakes, volcanic eruptions and other underwater explosions (including detonations of underwater nuclear devices), landslides, glacier cavings, meteorite impacts and other disturbances above or below water all have the potential to generate a tsunami.
Tsunami waves do not resemble normal sea waves, because their wavelength is far longer. Rather than appearing as a breaking wave, a tsunami may instead initially resemble a rapidly rising tide, and for this reason they are often referred to as tidal waves. Tsunamis generally consist of a series of waves with periods ranging from minutes to hours, arriving in a so-called "wave train".
Although the impact of tsunamis is limited to coastal areas, their destructive power can be enormous and they can affect entire ocean basins; the 2004 Indian Ocean tsunami was among the deadliest natural disasters in human history with over 230,000 people killed in 14 countries bordering the Indian Ocean.

Classification
Based on the distance from the source and the travel time tsunamis are classified in to three types by UNESCO-IOC. They are:

•   Local Tsunami originates from a nearby source for which its destructive effects are confined to coasts within 100 km or less than 1 hour tsunami travel time from its source. These can often be the most dangerous because there is often little warning between the triggering event and the arrival of the tsunami.
• Regional Tsunami is capable of creating destruction in a particular geographic region, generally within 1,000 km or 1-3 hours tsunami travel time from its source. Regional tsunamis occasionally have very limited and localized effects outside the region.

Distant Tsunami (also called an ocean-wide, distant, tele or far-field tsunami) is a tsunami that originates from a far away source, which is generally more than 1,000 km away from the area of interest or more than 3 hours tsunami travel time from its source. A distant tsunami is capable of causing widespread destruction, not only in the immediate region of its generation, but across an entire ocean.

Generation Mechanisms
The principal generation mechanism (or cause) of a tsunami is the displacement of a substantial volume of water or perturbation of the sea. This displacement of water is usually attributed to earthquakes, landslides, volcanic eruptions, glacier calving or more rarely by meteorites and nuclear tests. The waves formed in this way are then sustained by gravity. Tides do not play any part in the generation of tsunamis.

Seismicity
Tsunami can be generated when the sea floor abruptly deforms and vertically displaces the overlying water. Tectonic earthquakes are a particular kind of earthquake that are associated with the Earth's crustal deformation; when these earthquakes occur beneath the sea, the water above the deformed area is displaced from its equilibrium position. More specifically, a tsunami can be generated when thrust faults associated with convergent or destructive plate boundaries move abruptly, resulting in water displacement, owing to the vertical component of movement involved.

Landslides
In the 1950s, it was discovered that larger tsunamis than had previously been believed to be caused by earthquakes could be caused by giant submarine landslides. These rapidly displace large water volumes, as energy transfers to the water at a rate faster than the water can absorb.

Meteotsunamis
Some meteorological conditions, such as deep depressions that cause tropical cyclones and sudden fast moving weather systems can generate a storm surge, called a meteotsunami, which can raise tides several metres above normal levels..

Destruction
Tsunamis cause damage by two mechanisms: the smashing force of a wall of water traveling at high speed, and the destructive power of a large volume of water draining off the land and carrying a large amount of debris and material with it.

TSUNAMI WARNING SYSTEMS

Tsunami warning systems exist in many places around the world. As scientists continuously monitor seismic activity (earthquakes), a series of buoys float off the coast and monitor changes in sea level. Unfortunately, since tsunamis are not very tall in height when they are out at sea, detection is not easy and there are many false alarms.
Appropriate actions to be taken by local officials may include the evacuation of low-lying coastal areas, and the repositioning of ships to deep waters when there is time to safely do so. Warnings may be updated, adjusted geographically, downgraded, or cancelled. To provide the earliest possible alert, initial warnings are normally based only on seismic information.

Ocean observing instruments used to detect tsunamis:

Tide Gauges- Tide gauges measure the height of the sea-surface and are primarily used for measuring tide levels.

The DART System- In 1995 the National Oceanic and Atmospheric Administration (NOAA) began developing the Deep-ocean Assessment and Reporting of Tsunamis (DART) system. An array of stations is currently deployed in the Pacific Ocean. These stations give detailed information about tsunamis while they are still far off shore. Each station consists of a sea-bed bottom pressure recorder which detects the passage of a tsunami.
Satellites- Satellite altimeters measure the height of the ocean surface directly by the use of electro-magnetic pulses. These are sent down to the ocean surface from the satellite and the height of the ocean surface can be determined by knowing the speed of the pulse, the location of the satellite and measuring the time that the pulse takes to return to the satellite.

ITEWS- Indian Tsunami Early Warning System. The system implemented in phases became full-fledged 24X7 operational early warning systems at the Indian National Centre for Ocean Information Services (INCOIS), Hyderabad, under the Earth System Sciences Organization (ESSO), Govt. of India in October 2007.
The Indian Tsunami Early Warning System has the responsibility to provide tsunami advisories to Indian Mainland and the Island regions. Acting as one of the Regional Tsunami Advisory service Providers (RTSPs) for the Indian Ocean Region, ITEWS also provide tsunami advisories to the Indian Ocean rim countries along with Australia & Indonesia

The Tsunami Early Warning System comprises a real-time network of seismic stations, Bottom Pressure Recorders (BPR), tide gauges and 24 X 7 operational warning centre to detect tsunamigenic earthquakes, to monitor tsunamis and to provide timely advisories following the Standard Operating Procedure (SOP), to vulnerable community by means of latest communication methods with back-end support of a pre-run model scenario database and Decision Support System (DSS).
The Warning Centre is capable of issuing Tsunami bulletins in less than 10 minutes after any major earthquake in the Indian Ocean thus leaving us with a response/lead time of about 10-20 minutes for near source regions in the Andaman & Nicobar and a few hours in the case of mainland.

LAKES

In simplest terms lakes can be defined as the hollows on the earth surface in which water accumulates. They vary a great deal in their shapes, sizes, depth and the mode of formation.
Some lakes being consistently fed by fresh water streams or rivers and also having outlet streams are fresh water lakes. On the other hand lakes which form a part of Endorheic or Inland drainage basin are saline lakes. Also, lakes are only a temporary feature of earth’s crust as they will eventually be eliminated by the twin process of draining and silting up.

Types of lakes

Lakes can be of several types depending on their modes of formation.
There are tectonic lakes and rift valley lakes formed by tectonic movements beneath the earth’s crust that give rise to depressions on earth’s surface. The rift valley lakes like Tanganyika, Malawi etc. and tectonic depressions like Caspian sea, Titicaca lake are few examples.
Glaciation also produces lakes. The most common ones are cirque or corrie lakes and the lakes formed in the drumlin depressions. Apart from these there are lakes formed by valleys being dammed by moraines carried by glaciers. Such lakes formed by glacial actions are very common in North and North West Europe.
Lakes can also be formed by volcanic activity. Crater or caldera lakes like Lake Toba in Sumatra are very common at the mouths of large volcanic cones. Similarly at times a stream of lava may solidify and block the flow of a river forming Lava Blocked Lakes like the Sea of Galilee.
Erosion of earth’s surface may also produce hollows which if occupied by water can form lakes. Depressions formed in the limestone regions by the solution of calcium carbonate by water are very common. Such depressions form lakes called poljes. Similarly wind deflated hollows in the arid regions may get transformed into salt lakes or oasis.
Lakes can also be formed by the deposition action of rivers, winds or ocean waves. Ox-bow lakes are very common among the mature stages of a river. Similarly wave action may produce bars and spits that may cut off a bay to produce lagoons.
Apart from these there are lakes made by animals like the beaver lakes in USA and a series of man-made lakes made by human beings.

Role of lakes in human life

Lakes have immense significance for human beings as they satisfy a series of their needs.
• Lakes provide a wide range of foods and minerals.
• Lakes are a crucial source for regulating river flows. They absorb excess waters during heavy rains to prevent downstream flooding. Also lakes absorb excessive sediment load from rivers thereby enhancing their flow capacity.
• Large lakes like the Great Lakes have a moderating effect on the climate of the region.
• Lakes are used as a reservoir of water which then can be used for a variety of purposes like power generation, irrigation etc.
• Lakes, especially those connected to the large rivers are an effective means of transport and have acted as a main agent of economic development in several regions. The Great Lakes are the best example of this.

MAN MADE LAKES