When people think about the Mediterranean Sea they usually visualize tranquil seaside resorts. That’s what you’ll find on its coasts these days for sure, but it has an interesting and violent geological history. The slide show illustrates key points in its evolution, starting at six million years ago (6 Mya). FYI, that first slide is not entirely accurate … that’s the Mediterranean as it looked in 1994. 6 Mya the Mediterranean was actually part of the Tethys Sea, a large inland body of water that encompassed both the Black Sea and Caspian Sea.
Eventually, the movement of several tectonic plates (African, Eurasian, Indian) eliminated two critical connections: It sealed off the Mediterranean from the Atlantic Ocean, and separated it from other parts of the east Tethys Sea. Over the next 700,000 years water in the Mediterranean Sea evaporated, ultimately leaving behind brackish lakes 3–5 km below sea level. Salt deposits tens of meters thick covered much of the original basin. This event is known as the Messinian Salinity Crisis.
There were still several rivers feeding the desiccated basin, but they couldn’t keep up with the rate of evaporation in that warm climate. So this is how it remained until (best estimate) 5.33 Mya. That was when the Strait opened up again. The exact cause is uncertain: Some argue it was due to plate tectonics lowering the land bridge separating the Atlantic from the Mediterranean, some say it was rising sea levels, others speculate it was a residual stream from the Atlantic that eventually eroded a large enough channel to let the ocean in. However it happened, this event is known as the Zanclean flood.
It would have been a spectacular and (geologically) rapid event. Estimates are that the incoming water could have dropped through an initial waterfall over a kilometer tall, with inflow rates of 100 million cubic meters per second and a speed of 40 m/s (90 mph). That’s 1000X the outflow of the Amazon River! Mediterranean sea levels rose by 10 meters per day for the next several months to two years, at which point 90% of the Mediterranean basin had been refilled.
To this day, the Mediterranean remains saltier than the Atlantic due to evaporation. The high evaporation rate maintains a net inflow of water from the Atlantic. The Suez Canal, which connects the Mediterranean to the Red Sea, has a cyclic inflow and outflow that depends primarily on tides. And its total flow is negligible compared to the Strait of Gibraltar.
The Strait is expected to close again as the African plate continues moving north. But this will happen over geological rather than human timescales. It is estimated that, were the Strait closed today, most water in the Mediterranean basin would evaporate within a thousand years — possibly faster depending how global warming progresses and rainfall patterns change.
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