A Dry Sea
Did you know that six million years ago the entire Mediterranean Sea nearly dried up?
“Big Stuff” like that is always hard to imagine, even for an earth scientist. I’ve seen dry rivers, dry lakes, and maybe a dry reservoir, but how can a whole sea 4000 meters deep and a couple 1000 km long vanish?
Take a look at a map and you will see that the Mediterranean Sea's only connection with the Atlantic Ocean is a narrow bottleneck called Gibraltar Strait. The Strait today is about 500 m deep and 15 km across. Gibraltar houses not only a famous “Rock”, but also the boundary between the African and European tectonic plates. Currently, relative plate motions there are very, very small, but some geologists think that six million years ago, they were sufficiently rapid as to squeeze the Strait completely shut.
Without access to Atlantic waters, rivers like the Nile, flowing into the Mediterranean could not keep pace with evaporation. The Sea dropped by 1000s of meters in elevation, perhaps drying out completely. How do we know this? As the Sea shrunk, the residual water became more and more saline. Eventually, the salts fell to the seabed forming expansive deposits. These salty layers have been discovered in drill cores, and their origin interpreted as a “Salinity Crisis” --- a fancy term for “drying out”.
You'd rightly suppose that when the Strait breached, one super colossal waterfall occupied the gap. I calculate that the waterfall flowed at the rate of 70,000 Niagara Falls. Even so, it took a year to refill the basin.
Geological “Big Stuff” is hard to imagine, but it happens nonetheless.
Steven N. Ward Santa Cruz
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John Rundle is a Distinguished Professor of Physics and Geology at UC Davis and the Executive Director of the APEC Collaboration for Earthquake Simulations. He chaired the Board of Advisors for the Southern California Earthquake Center from 1994 to 1996. Read John's blog.