The Strait of Gibraltar was once a rocky barrier between the Atlantic and the Mediterranean (Image: Corbis)
Advertisement
Standing at Punta de Tarifa, the southernmost point of mainland Europe, the mountains of Morocco are clearly visible across the Strait of Gibraltar. This busy stretch of water, just 14 kilometres across at its narrowest, is the gateway between the Atlantic Ocean and the Mediterranean Sea, and the closest thing to a border between Europe and Africa.
Visiting it 5.4 million years ago, the picture is very different. The mighty Atlantic is there, but the Med is nowhere to be seen. In its place is a vast basin, glittering with salt crystals and dappled with lakes of hypersaline water. This land, connecting what would become Europe and Africa, is 2.7 kilometres below sea level at its lowest point. It鈥檚 quite a spectacle: Earth鈥檚 lowest land today is the Dead Sea basin, a mere 430 metres below sea level.
You have arrived at the height of the Messinian Salinity Crisis, when tectonic movements have closed the Strait of Gibraltar, cutting off the Mediterranean. In the hot and dry climate, it took perhaps 1000 years for the sea to evaporate almost completely. Its remains can still be found today, under the sea floor and along its shores in the form of thick deposits of salt and gypsum.
The basin didn鈥檛 stay desiccated for long. As time rolled on, the climate grew cooler and wetter, and rivers flowing into the basin turned it into a type of wetland called a lago mare, or 鈥渓ake sea鈥�. But to the west, a cataclysm was brewing.
If you want to see it, hop back in your time machine and set the dial for 5.33 million years ago. A combination of tectonic subsidence, erosion and sea-level rise is just about to let the sea back in.
The Zanclean flood 鈥� named after the geological age in which it happened 鈥� probably started slowly, gradually filling about 10 per cent of the basin over thousands of years. But we arrive in time for the ending 鈥� a deluge of biblical proportions, according to Daniel Garcia-Castellanos of the Institute of Earth Sciences Jaume Almera in Barcelona, Spain (). For some reason the rate of inflow suddenly soared, filling the basin completely in the space of a few months, raising the Mediterranean by about 10 metres a day. Every second a billion cubic metres of water roars past, 5000 times more than the Amazon today. The sight is awe-inspiring.
In all, 3 million cubic kilometres of Atlantic water floods in, gouging a channel 250 metres deep and 200 kilometres long that can still be seen on the bed of the Strait of Gibraltar.
鈥淚n all, 3 million cubic kilometres of Atlantic water floods into the basin鈥�
And it could happen again. Not enough water comes from rivers flowing into the sea to compensate for evaporation: it needs the Atlantic to keep it topped up. If tectonic forces were to seal off the strait, the Med would eventually dry up once more.
This article appeared in print under the headline 鈥淭he Zanclean flood鈥�
