Where does the water of the Mediterranean Sea come from?

Alcyon, the boat which undertakes an expedition "from the Mediterranean Sea to the Spitzberg", just went through the Strait of Gibraltar. This is a good opportunity to wonder where the water of the Mediterranean comes from.

Did you know that, each year, the Mediterranean Sea "looses" nearly one meter of water through evaporation? Where comes the water which balances this loss from ?

The Mediterranean Sea "looses" its water

The climate in the Mediterranean area is very warm and pleasant, and has very nice hot and dry summers. These are the best conditions for wonderful summer holidays at the beach, but also for a high rate of water evaporation. The Mediterranean Sea looses in this way nearly one meter of its water height every year!

This evaporating water leaves all its salts in the remaining water. This is why you feel a lot lighter when you swim in the Mediterranean than when you swim in the ocean: the water of the Mediterranean contains more salt than the water of the Atlantic (36 to 38 grams per litre in the Mediterranean against 34.9 in the Atlantic).

The origins of the Mediterranean waters

Well the Mediterranean is not emptying itself, so water must come from somewhere to "fill the gap". The origins of this water are:

  • a small quantity coming from the rivers and the rain (1200 km3 per year);
  • a very small quantity coming from the Black Sea (200 km3 per year);
  • and most of it comes from the Atlantic Ocean, through the Strait of Gibraltar (35 000 km3 per year).

It has been calculated by scientists that the Mediterranean waters need 100 years to renew themselves totally (through evaporation and water coming in through the Strait of Gibraltar).

The Strait of Gibraltar is not very wide (around 14 km at the narrowest point) and quite shallow (300 m deep). Therefore, there is a very strong current going through the Strait, coming from the Atlantic into the Mediterranean Sea. This was observed by the crew of the Alcyon boat, when they began to go through the Straight, willing to go out of the Mediterranean:

31st of March 2005
At last some good wind! Now let's go to Gibraltar... that we were going to have with port side during 5 hours, going forward with a lot of difficulties against the current! Finally, at the last point, going at a too slow pace amidst the cargos, we gave in and turned on the Perkins (the motor of the boat)...

When the Mediterranean totally dried up

7 million years ago, the system that we just described was already working in a very similar way, and the general shape of the Mediterranean was nearly the same one as today. The Strait of Gibraltar didn't exist yet, but several small straits replaced it.

The first Hominids in Africa appeared more or less 6 million years ago. At that time, the ancestors of many animals like Felids, Canids or Rodents were also already roaming on the earth's surface.

It is around that time that the water coming from the Ocean into the Sea was totally "cut off" or nearly so. This resulted from different factors, amongst which the movement of tectonic plates (for a definition, see another article on our website: "What if mountains came out of the Mediterranean Sea?")

Little by little, the Mediterranean totally dried up, just as if we left a full saucepan of salt water to evaporate on the fire: it would leave only a thin crust of salt at the bottom.
This is exactly what happened, and important layers of salt piled up at the bottom of the Mediterranean. From time to time, some water would succeed to come through and would fill in, for a while, the bottom of the basin before evaporating. The Mediterranean had turned into a vast salt desert.

This situation lasted for nearly 700'000 years. This is why, in some places under the bottom of the sea, you can find layers of salt which are more than 1000 meters thick.

A gigantic water fall

Finally, around 5.3 million years ago, the Strait of Gibraltar opened because of more tectonic plates' movements. Water from the ocean arrived into the Mediterranean basin, creating a gigantic waterfall, with a flow rate a thousand times bigger than the flow rate of the Niagara falls!

And this is how, only a few centuries later (which is a really short time on the geological scale), the Mediterranean Sea was normally filled again.

More information

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