seas and oceans
The seas and the oceans are stretches
of water salted, unlike wide and soft rivers such as the lakes, the rivers
and the rivers.
The seas and the oceans cover 71 %
with the surface of the Earth and
play an essential role in the climatic balance of the environment.
In the Northern hemisphere, the seas and
the oceans represent 61 % of surface, against 81 % in the Southern hemisphere
(also called "marine" hemisphere).
SEAS
The seas are smaller than the oceans.
There are 3 types of seas: closed seas, inland seas and seas of the
Mediterranean type.
The closed seas are the Caspian Sea and the sea of Aral
(in Asia) and the Dead Sea (in the Middle East). These seas, which do not
communicate with any other sea nor any ocean, are fed only by rivers. It is in
fact of immense salted lakes; salt is there besides in quantity more
significant than in the other seas and oceans.
The inland seas open on other seas. It is the case of the
Black Sea or North Sea (which gives on the Baltic) (which communicates with the
Mediterranean by the Bosphorus in Turkey). These seas are low depth.
Lastly, the seas of the Mediterranean type are the largest seas and communicate with
oceans, as the Mediterranean (which gives on the Atlantic Ocean), the sea of
the Philippines (Pacific Ocean), the Caribbean Sea (Atlantic Ocean) or the sea
of Arabia (Indian Ocean).
OCEANS
The oceans have for principal
characteristics to be of a surface quite higher than that of the seas and to be
delimited by several continents.
The Pacific Ocean is largest of all the oceans (165 million
km² approximately, is 300 times the size of France) and deepest (more than 11
000 m to the pit of Mariannes). It is delimited in the east by the American
continent and the west by Asia and Oceania.
Second by its surface (more than 80
million km²), the Atlantic Ocean is delimited in the east by Europe and Africa
and in the west by the American continent (North America, Central America and
South America).
Lastly, the Indian Ocean (more than
70 million km²) is almost entirely located in the Southern hemisphere. It is
bordered in the east by Oceania and Indonesia, in north by India and the west
by Africa.
To these three oceans are added the Arctic Ocean (also called sea Arctic Glaciale because of
its dimensions, "small" for an ocean) and the ocean Glacial the Antarctic (or Antarctic Ocean).
The principal points of passage between
these oceans are:
–the
Bering Strait: located between Alaska (in the United States) and Russia, it
connects the Pacific Ocean to the Arctic Ocean, in the Northern hemisphere;
–the
Magellan Strait, Cape Horn and the Drake passage: located at the
southern point and Chile, they connect the Pacific Ocean to the Atlantic Ocean,
in the Southern hemisphere;
–the
strait of Bass: located at the south of Australia, it connects the Indian
Ocean to the Pacific Ocean, in the Southern hemisphere;
–the
strait of Torres: located between New Guinea-News-Guinea and Australia, it
connects the Indian Ocean to the Pacific Ocean, in the Southern hemisphere;
–the
course of the Needles: located at the southern point of South Africa, it
marks the limit between the Indian Ocean and the Atlantic Ocean, in the
Southern hemisphere.
MARINE WATER
The water of the seas and the oceans is
naturally salted. The average content salt (35 grams per liter) varies
according to the fresh water arrival: the more there is fresh water, the less
there is salt, and conversely.
The temperature of the water of the seas and the oceans
varies according to the depth of water, the latitude (position compared to the equator) and of the sun contribution , but also of the
importance of the marine currents. The sea waters of the Caribbean, for example,
can reach a temperature of 30 °C, while in the Arctic Oceans and the Antarctic,
the surface water is below 0 °C all the year.
The tides are a phenomenon which appears by the
rise (flow) of water on the coasts
then their descent (the backward flow).
This movement is the consequence of the
attraction of the Moon and the Sun on the Earth. The tides function according
to a
periodic rate/rhythm (one or two
tides per day). The amplitude of the tides is more or less significant (the
coefficient goes from 20 to 120) and depends on the seas (of 10 cm at sea the
Mediterranean with nearly 20 m in the Atlantic Ocean).
The waves are undulatory movements on the surface of
marine water, due to the winds. The waves are littoral when they arrive to the
coast. If not, they are oceanic waves: one speaks then about swell,
left bearing of surface water on the open sea. More the spacing between two
waves and their rate of travel are significant, more the swell is strong. The
highest waves can measure up to 30 m (in the Pacific Ocean).
The marine currents are constant or periodic movements marine
water: they are due to the winds or are produced by the descent and the rise of
water (indeed, the difference of salinity, temperature and density of water
causes this type of movements, in-depth). The importance of the currents is capital
for the climatic balance of certain areas of the Earth.
The Gulf Stream, for example, is a current heat: it is
born on the coasts from the Gulf of Mexico and goes up the North Atlantic at
broad coasts of the North-American continent, under the blast pressure of
south-west. Then, it deviates towards the east and joined Western Europe
(France in particular) to which it brings soft and wet winters; without this
marine current, this part of Europe would have the same climate as Canada.
LIFE AND RESOURCES OF THE SEAS AND THE
OCEANS
The marine ecosystem particularly rich and is varied. Indeed,
water of the seas and the oceans abounds in life, at the same time vegetable
(the vegetable plankton and algae) and animal (the animal plankton, of many
invertebrates, the fish, marine mammals). They have considerable,
but also energy resources living (the energy of the waves and tides for
example, used to produce electricity) and
mineral (oil and the gas).
For a long time, the man largely exploits these resources: they are for example the
activities of fishing (become industrial
to the
xx E century), of
intensive trade or exploitation of oil reservoirs (at sea of North in particular), etc.
However, this exploitation is not
without dangers: many fish species are threatened, are
in process of extinction or disappeared from the seas and the oceans, and the marine
water pollution became at
certain places of planet a true plague.
TO GO FURTHER
? geography
? Earth
? water on the Earth
? rivers and rivers
? the man and water
? the water pollution
? life in the seas and the oceans: the plankton –sea water fish –marine mammals –marine invertebrates –marine birds
