scienceMultiple Countries

3 Kilometers of Salt Under the Med

When the Mediterranean dried up 5.96 million years ago, it left a salt layer 3 km thick. The refilling through Gibraltar may have been the largest waterfall ever.

Paubahi, CC BY-SA 3.0, via Wikimedia Commons

The Great Drying

Between 5.96 and 5.33 million years ago, the Mediterranean Sea experienced a large geological event known as the Messinian Salinity Crisis. Tectonic plate movements between Africa and Eurasia gradually closed the ancient seaways that connected the Atlantic Ocean to the Mediterranean. Cut off from its primary source of water, the sea began to evaporate much faster than rivers could replenish it.

This isolation led to a dramatic drop in sea level, with the basin floor sinking between 1.5 and 2.5 kilometers below the global ocean level. As the water vanished, the remaining sea became a series of hypersaline lakes. The extreme concentration of dissolved minerals caused them to precipitate out of the brine, blanketing the seabed. This process laid down a colossal layer of evaporite minerals, primarily gypsum, anhydrite, and halite (common salt). In some areas, these salt deposits reach a thickness of over 3 kilometers, with a total estimated volume exceeding 1 million cubic kilometers. This event also carved the landscape, as rivers like the Nile and Rhône cut deep canyons into the newly exposed continental margins, with the Nile's canyon reaching 2,500 meters below sea level near modern-day Cairo.

The Zanclean Deluge

The crisis ended abruptly 5.33 million years ago with one of the most catastrophic floods in Earth's history. The natural dam at the Strait of Gibraltar breached, allowing the Atlantic Ocean to pour back into the desiccated basin. This event, known as the Zanclean Flood, was of massive proportions.

The flow rate is estimated to have reached 100 million cubic meters per second, a volume about 1,000 times greater than the modern Amazon River. Water cascaded down a drop of more than a kilometer, creating what might have been the largest waterfall ever to exist on the planet. At the flood's peak, the sea level inside the basin may have risen by more than 10 meters per day. The entire Mediterranean basin, which had taken hundreds of thousands of years to dry up, was likely refilled in a period as short as a few months to two years. The force of the water carved a massive channel through the Strait of Gibraltar, the remnants of which are still visible in modern seafloor data. This sudden influx of water also triggered earthquakes as the immense weight pressed down on the Earth's crust.

💡Fun Facts

The immense weight of the salt has caused it to flow under pressure over millions of years, creating large underground salt domes called diapirs. These structures are effective at trapping oil and natural gas.
The crisis caused a major extinction of marine life. When the basin refilled, only 11% of the original endemic species survived, and biodiversity took over 1.7 million years to recover.
Evidence for the sea level drop was first discovered when geologists found that rivers like the Rhône had carved canyons hundreds of meters below the current sea level, far inland from the modern coast.
The removal of over 1 million cubic kilometers of salt from the world's oceans—about 5% of their total salt content—would have slightly lowered global ocean salinity and potentially affected the climate.

References

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Practical Information

Opening Hours: The geological evidence is a permanent feature of the Mediterranean basin, accessible through scientific data at any time.
Admission: Free
Accessibility: The primary evidence lies thousands of meters below the sea surface or is buried under subsequent sediment layers. However, exposed Messinian evaporite formations can be visited on land in locations like the Realmonte salt mine in Sicily, Italy, and in the Sorbas Basin in Spain.
Website: Visit website →

The Messinian Salinity Crisis EvaporitesMediterranean Sea, Multiple Countries

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