Approximately 70.8 percent of the terrestrial surface is covered by saltwater oceans, with a volume of about 1.4 billion cubic kilometers.

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.

The average depth of sea is 3790 m, compared with average elevation of land i.e. 875 m.

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 evaporation, transpiration, precipitation, infiltration, percolation, and runoff. These processes operate throughout the entire hydrosphere, which extends from about 15 kilometers into the atmosphere to roughly 5 kilometers 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[1]. 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.

Maritime Zones

United Nation Convention on Law of Seas (UNCLOS)

UNCLOS replaces the older 'freedom of the seas' concept, dating from the 17th century: national rights were limited to a specified belt of water extending from a nation's coastlines .

The issue of varying claims of territorial waters was raised in the UN in 1967 and in 1973 the Third United Nations Conference on the Law of the Sea was convened in New York. In an attempt to reduce the possibility of groups of nation-states dominating the negotiations, the conference used a consensus process rather than majority vote. With more than 160 nations participating, the conference lasted until 1982. The resulting convention came into force in 1994 . The convention introduced a number of provisions. The most significant issues covered were setting limits, navigation, archipelagic status and transit regimes, exclusive economic zones (EEZs), continental shelf jurisdiction, deep seabed mining, the exploitation regime, protection of the marine environment, scientific research, and settlement of disputes.

Internal waters

Covers all water and waterways on the landward side of the baseline . The coastal state is free to set laws, regulate use, and use any resource. Foreign vessels have no right of passage within internal waters.

Territorial waters

Out to 12 nautical miles (22 kilometres; 14 miles) from the baseline, the coastal state is free to set laws, regulate use, and use any resource. Vessels were given the right of innocent passage through any territorial waters .Fishing, polluting, weapons practice, and spying are not "innocent", and submarines and other underwater vehicles are required to navigate on the surface and to show their flag. Nations can also temporarily suspend innocent passage in specific areas of their territorial seas, if doing so is essential for the protection of its security.

Contiguous zone

Beyond the 12-nautical-mile (22 km) limit, there is a further 12 nautical miles (22 km) from the territorial sea baseline limit, the contiguous zone, in which a state can continue to enforce laws in four specific areas: customs, taxation, immigration and pollution, if the infringement started within the state's territory or territorial waters, or if this infringement is about to occur within the state's territory or territorial waters . This makes the contiguous zone a hot pursuit area.

Exclusive economic zones (EEZs)

These extend from the edge of the territorial sea out to 200 nautical miles (370 kilometres; 230 miles) from the baseline. Within this area, the coastal nation has sole exploitation rights over all natural resources. In casual use, the term may include the territorial sea and even the continental shelf. The EEZs were introduced to halt the increasingly heated clashes over fishing rights, although oil was also becoming important. The success of an offshore oil platform in the Gulf of Mexico in 1947 was soon repeated elsewhere in the world, and by 1970 it was technically feasible to operate in waters 4,000 metres deep. Foreign nations have the freedom of navigation and overflight, subject to the regulation of the coastal states. Foreign states may also lay submarine pipes and cables.

High Sea :

The Oceans beyond the limit of continental shelf . All countries have equal rights of navigation , aviation , flying , mining , research and exploration .

Aside from its provisions defining ocean boundaries the convention establishes general obligation for safeguarding the marine environment and protecting the freedom of scientific research on the high seas and also creates a legal regime for controlling mineral resources exploration in deep sea bed areas through an International Sea Bed Authority  , ISA , and “the common heritage of mankind” principle .

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.

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.

Mid-Oceanic Ridges or Submarine Ridges

A mid-oceanic ridge is composed of two chains of mountains separated by a large depression. [Divergent Boundary].The mountain ranges can have peaks as high as 2,500 m and some even reach above the ocean’s surface. 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. Iceland is a part of the mid-Atlantic Ridge

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.

nents) because they blow from the land towards the oceans whereas these trade winds raise the tempera­ture in the western margins of the oceans or the eastern coastal areas of the continents because of their onshore position.

Similarly, the eastern margins of the oceans in the middle latitudes (western coasts of Europe and North America) have relatively higher temperature than the western margins of the oceans because of the onshore position of the westerlies.

4. Ocean currents: Surface temperatures of the oceans are controlled by warm and cold currents. Warm currents raise the temperature of the affected areas whereas cool currents lower down the temperature. For example, the Gulf Stream raises the temperature near the eastern coasts of N. America and the western coasts of Europe.

Kuro Shivo drives warm water away from the eastern coast of Asia and raises the temperature near Alaska. Labrador cool current lowers down the tem­perature near north-east coast of N. America. Similarly, the temperature of the eastern coast of Siberia becomes low due to Kurile cool current.

5. Other Minor factors:

(i) Submarine ridges spewing hot magma,

(ii) Local weather conditions like storms, cyclones, hurricanes, fog, cloudiness, evaporation and conden­sation, and

(iii) Location and shape of the sea - Longitudinally more extensive seas in the low latitudes have higher temperature than the latitudinally more extensive seas as the Mediterranean Sea records higher temperature than the Gulf of California.

The enclosed seas in the low latitudes record relatively higher temperature than the open seas whereas the enclosed seas have lower temperature than the open seas in the high latitudes (Baltic Sea records 0°C (32°F) and open seas have 4.4°C or 40°F).