Why the Mediterranean Almost Dried Up
Oceans look permanent. The Mediterranean is not. Roughly six million years ago its only connection to the Atlantic, through what is now southern Spain, shut. For something like 600,000 years the sea sat in a hot, dry basin with almost no way to refill, and it very nearly disappeared.
A sea cut off
The Mediterranean loses far more water to evaporation than rivers and rain replace. Today the Atlantic keeps it topped up through the Strait of Gibraltar. Close that gap and the basin drains at roughly half a metre a year — a geological blink. Geologists call the episode the Messinian salinity crisis, after the period in which it happened.
The salt left behind
When a restricted sea evaporates, dissolved minerals precipitate in order: carbonate first, then gypsum, then thick beds of halite — rock salt. Drill cores under the Mediterranean floor find exactly this sequence, in places more than two kilometres thick. That volume of salt could only form from an enormous body of water evaporating repeatedly as the barrier opened and closed.
The flood that refilled it
About 5.33 million years ago the Atlantic broke through for good. The Zanclean flood carved a channel across the Gibraltar area and refilled most of the basin in a geologically brief span — estimates run from a few years to a few thousand, which for a strait this size is still violent. Rivers that had cut deep canyons into the empty basin, including the Nile, were drowned and partly filled.
Why it matters now
The episode is a natural experiment in what happens when a sea is isolated: extreme salinity, regional climate drying, and mass extinction of marine species. It also reminds us that coastlines and gateways are temporary features — the geography we navigate is a snapshot.
Knowledge check
Two questions, no score
What is the main physical evidence that the Mediterranean dried out?
Why does the Mediterranean need constant inflow from the Atlantic?
Now prove it
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