Zanclean

(adj.) describing the ancient flood said to have formed the Mediterranean Sea

Imagine what resorts like the Majorca, Monte Carlo and the Amalfi Coast would be like without the Mediterranean Sea. Or, perhaps more accurately, without an ancient event in the geological prehistory of the Earth known as the Zanclean flood.

Zancle was the Greek word for the Sicilian port of Messina, and it’s from that name that this great flood, believed to have occurred a little over 5 million years ago, takes its name.

Before then, much of what is now the Mediterranean Sea would have been a vast, empty salt basin—but before that, it would have been an enormous expanse of open sea marking the western edge of what was called the Tethys Ocean. So how did we get from a sea, to a salt pan, to a sea again? Well, this particular story plays out over around 200 million years.

Head back in time to the age of the dinosaurs, and the layout of the Earth would have looked vastly different. The original single supercontinent of Pangea had begun to break apart thanks to the shifting tectonic plates beneath the Earth’s surface, to form two neighbouring landmasses known as Laurasia, to the north, and Gondwanaland, to the south. And surrounding them was an enormous single expanse of water, called the Tethys Ocean (after a sea goddess in Greek mythology).

The angle at which these two great landmasses met essentially formed a tight C-shape, and the bowl of that C was filled with Tethyan seawater to form an enormous gulf. As time (and we mean, a lot of time) went by, the tectonics of the Earth continued to shift so that the eastern edge of Gondwanaland drifted slowly northwards, to collide with the southern edge of Laurasia. As that happened, much of the vast gulf of water that had once separated these regions became closed off, forming the Mediterranean Sea.

The Mediterranean wasn’t entirely sealed off, however. While its eastern edge was closed (by the same continental forces that formed the Caucasus and the Himalayas), its western edge remained open to the Tethys via a series of sea channels or so-called ‘corridors’. Sometime during the Miocene epoch, however, all that changed.

Around 6 million years ago, that the last two channels connecting the Mediterranean Sea to the wider oceans—known as the Guadalhorce and Rifian corridors—were closed off. Quite what caused or forced these corridors to close remains unclear, but the result for the Mediterranean was catastrophic. In the scorching heat of the sun, the remaining water evaporated at a much faster rate than the rivers of Europe alone could replace it. Sea levels fell by many hundreds of metres, and although some water likely remained in the very depths of the basin, much of the Mediterranean region as it is today would have been transformed into a vast arid salt pan.

Geological samples from deep within the Mediterranean seabed prove this was the case: the salt-rich composition of a rocky mineral known as Messinian evaporite, obtained from the coast of Messina, shows that the entire Mediterranean region was once completely dried up. And so it would have remained, were it not for the Zanclean flood.

Around 5.33 million years ago, a new opening to the Atlantic Ocean was formed. It’s unclear whether this was a gradual or a catastrophic development—and it’s equally unclear quite what started it. One theory claims that a stream flowing close to what is now Gibraltar, in southern Spain, could have steadily eroded and weakened the landbridge that sealed the Mediterranean from the Atlantic. Once this bridge had been breached, the ocean waters tumbling over it would have hastened its destruction, and quickly reclaimed the basin behind it.

Whether or not this account is indeed the case, over time the Mediterranean was refilled by the incoming floodwaters, forming the sea as it has remained to this day.