Qu’elle est la différence entre océan et mer?

Texte de Prof.Maurice Hakizimana posté sur son page facebook

1.C’est avant tout une histoire de conventions

Il faut savoir que les océans et les mers sont définis par des conventions décidées par l’homme, et non réellement sur des critères objectifs. Par exemple, le nom “océan” n’est devenu officiel qu’à la fin du XIXe siècle, après que Magellan ait navigué sur l’océan Pacifique et le nomma ainsi par temps calme.À noter que le mot “mer” vient du latin, mare et maris. Le terme “océan”, lui, a pour origine le mot mot grec Ôkeanos, une divinité marine qui symbolisait l’eau qui entoure le «disque» de la Terre.

Donc, selon les conventions, un océan est désigné par une grande étendue d’eau salée bordée par des continents.

On compte 5 océans sur la planète :

Pacifique,

Atlantique,

Indien,

Arctique,

Austral.

2.C’est aussi une question de taille.

L’océan est plus grand !

Pour vous donner un ordre d’idée, le plus petit océan, l’Arctique, fait 14,09 millions de km². Alors que la mer la plus grande, la mer d’Arabie, s’étend sur 3,6 millions de km².

3.Les mers sont bordières ou annexes

Parmi les mers, les océanographes en distinguent deux types différents:

D’abord, les mers bordières, disposées le long du contour des océans et fermées par des péninsules, des îles ou des hauts-fonds.

Les mers annexes, elles, communiquent avec l’océan uniquement par un détroit et sont entourées par des terres émergées.

C’est le cas de la Méditerranée et le détroit de Gibraltar.

Il existe trois mers qui sont totalement fermées :

la mer Caspienne, la mer d’Aral et la mer Morte.

Sans oublier que certaines font carrément partie des océans, comme la mer Jaune dans l’océan Pacifique.

4.Des mers sont plus salées que les océans

Autre critère à prendre en compte, les mers sont beaucoup plus salées que l’océan à cause d’une plus forte évaporation.

À titre d’exemple, la salinité de l’océan tourne en moyenne autour de 35 grammes/litre. En mer Rouge, cela monte à 41 g/l. C’est 275 g/l pour la mer Morte !

Exception qui confirme la règle, pour la mer Baltique, c’est entre 5 et 10 g/l, qui est alimentée par de nombreux fleuves.

5.Profondeur et diversité biologique

Est prise en compte également la profondeur. Par exemple, la mer de Norvège peut atteindre les 4 000 mètres. Encore mieux, la mer de Corail, située entre l’Australie et la Nouvelle Calédonie a son point maximum à 9 140 m. À côté, la Manche ou la mer du Nord font office de flaque d’eau avec leurs 200 mètres de profondeur…

Quant à l’océan, près de la moitié de ses eaux dépasse 3 000 m de profondeur. La fosse des Mariannes, avec ses 11 020 m de profondeur, est le point le plus profond.

Les océans sont plus grands mais c’est dans les mers que l’on trouve la plus grande diversité biologique. La mer Méditerranée abrite ainsi entre 4 et 18 % des espèces marines de la planète alors qu’elle représente moins de 1 % de la surface de l’océan mondial. Petite mais costaude !

source: https://business.facebook.com/Hakizimana-Maurice-2220484434682831/?xts[0]=68..

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