لطفا منتظر باشید ...
Water-Seas
More than %70 of the Earth is covered with
water.The seas and oceans account for %97 of
this water the oceans cover 361 million
square kilometers .
The extent of the seas and oceans used to
be different.Before the first geological era,
all the lands on the Earth were connected
together . The Urasia was on the northern
hemesphere , and on the southern hemesphere
there was the Gondwana.During some geological
changes in the history of the planet Earth,
the first breaking of the land took place
in the first geological era and as a result
the Thetis sea was formed;then gradually due
to different geological events and also the
important process of horizontal movement of
the crust of the Earth the continents got
separated from each other the separation
theory of continents and hence , various
oceans were formed.
The present oceans are :
1 - The Pacific ocean with the surface area
of 177 million square kilometers and maximum
depth of 11000 m.
2- The Atlantic ocean with
the surface area of 106 million square
kilometers and maximum depth of 8380 m.
3 - The Indian ocean with the surface area
of 77 million square kilometers and maximum
depth of 7450 m.
4 - The Arctic ocean with
the surface area of 14 million square
kilometers and maximum depth of 5450 m.
5 - The Antarctic ocean which lies on
the southern hemesphere.
The main difference between the oceans and
seas lies in their relative hugeness.Oceans
are vast expanses of water which separate
continents from each other,and their surface
area can reach up to tens or hundreds of
millions of square kilometers,while seas,
which are generally located on the edges of
oceans never exceed more than few million
square kilometers.There are some cuts in
a varieties of forms at the edges of oceans
near where they meet the land which are,
in some places,more or less deep;in fact,
these cuts form seas and gulfs.
Seas are divided into two distinct groups
according to their morphology :
1- the seas which are somehow connected with
the oceans and are generally separated from
them by a group of islands or peninsulas .
The free connection between oceans and these
seas is the main reason for the very close
relationship which exists between the morine
life and the water temperature and saltiness
in these two environments. Examples of these
are the Bering and the Barents and the Japan
seas.2- the seas which have fully and deeply
penetrated the land and formed semi-closed
basins of water.These basins are separated
from the oceans by under sea ridges.
Connection between these seas and oceans is
very difficult.The only way of connection is
through the semi-deep and narrow straits,
like the Black Sea and the Mediterranean Sea.
Each sea is divided into four zones according
to its depth :
1- the coastal or tidal zone littoral zone.
This is a zone located between the flow
and ebb of the sea.
2- the shallow zone
neritic zone starts from the lower limit
of the littoral zone and extends upto the
depth of 200 m.This zone is also called the
continental shelf. B3- the average depth zone
of the sea bathgal zone which begins from
the depth of 200 m ,and extends downward for
2000 to 3000 m.
4- the deep zone abyssal zone consists of
some parts with the depth of more than 3000 m
the lowest part of the Earth or the deepest
part of the oceans of the world is near Japan
and is called Marien graben with the depth of
more than 11 km .
The bottom surface of the seas and oceans are
not even or flat , but are similar to lands
with heights and depressions such as valleys,
rifts , ocean ridges , mountains subduction
areas . Old volcanic cones or guyots and
corals.
The surface of the seas are also constantly
on the move and changing. Most of the waves
seen on the surface of the seas are caused
by winds. Of course single, dangerous waves
are sometimes resulted from earthquakes and
explosions happening under the seas. Studies
show that the top surfaces of the oceans are
also changing in the same way as level of the
underground water . On a world scale these
changes are called static changes . These
phenomena are affected by such factors as
newly formed oceanic water basin which are
the result of the separation of continents
and the melting of icebergs.
The degree of saltiness of ocean waters
varies in different parts it varies according
to time and location. Generally the degree of
saltiness increases from the equator to the
Cancer Orbit and the Capricorn Orbit , and
decreases as it reaches the poles.It is worth
mentioning that the degree of saltiness
largely depends on the evaporation process.
There are different gases dissolved in the
waters of the oceans the most important one
is oxygen.The main role of this gas is for
the respiration of marine life.
