Earthquakes can be bad. Earthquakes blocking off a key water source are likely worse. Earthquakes blocking water that eventually trigger volcanic eruptions would be… ridiculous? It may sound like a script to a summer blockbuster from the 1990s, but this story actually took place in the Miocene Epoch, around seven million years ago, leaving the Mediterranean Sea dry enough to walk across. The ground, and water levels, have obviously moved again since that time, but layers of earth all around the Mediterranean still tell the story the Messinian salinity crisis, when the sea was walkable and apparently lined by unusually active volcanoes.
Sediment layers under the sea
In cliffs along the Mediterranean, you can see layers of salt and gypsum crystals that were deposited thanks to evaporating seawater. It’s not surprising to see those sediment patterns in cliffs now exposed to the Sun, giving evidence to higher water levels long ago. The plot thickened in 1961, when researchers drilling under the Mediterranean Sea found the same patterns under water. For evaporation to have laid down salt and gypsum there, the only explanation was that what’s now the bottom of the sea was temporarily exposed to the Sun, although more than sunshine would have been required to dry up 965,300 square miles of water.
The loss of water was thanks to some major earthquakes near the strait of Gibraltar. As the European and African tectonic plates ground into each other, land was pushed up, blocking up the flow of water between the Atlantic ocean and the Mediterranean Sea. Without a regular supply of new water, the Mediterranean Sea was more susceptible to evaporation, drying up up enough to loose a half-mile of depth in places where water remained. The Sea started to look more like lakes, and which a new study suggests is what started to make some local mountains look more like volcanoes.
Water goes down, magma comes up
As much as we associate magma with being hot water with being cool, the big issue here was actually weight. Water is heavy enough to simply weigh down the ground, demonstrated recently by the 0.8 inches Houston, Texas was measured to have dropped under the weight of Hurricane Harvey’s rainfall. The dried Mediterranean greatly reduced the pressure on the ground below it, which allowed magma to more easily rise to the Earth’s surface. As a result of the magma’s freed movement, the rate of volcanic eruptions in the area more than doubled, with at least 13 eruptions taking place around 5.9 to 5.4 million years ago. To put it in a nutshell, blocking up the water effectively unblocked the magma.
Since the Mediterranean doesn’t currently look like Mordor, it’s easy to surmise that these changes were all reversed at some point. Another series of earthquakes reopened the strait of Gibraltar bit by bit, allowing the Atlantic to come rushing back in. Sediment layers suggest that these floods occurred more than once, although it’s unknown if that would have made for another disaster movie, or just welcome relief from the salty land and extra eruptions.
My kids asked: What about animals that got stuck out there when the water was gone and came back?
These transitions were probably difficult for most organisms. Even as water slowly dried up, the increased salinity would have giving many marine species a run for their money in whatever water remained. At least a few animals probably wandered out when the water was low, only to be isolated on newly defined islands when sea levels returned to normal. Once trapped on an island, it’s likely that some species underwent island dwarfism or gigantism thanks to the environmental pressures of living in a very different space than their ancestors did.
Source: Disappearance of Mediterranean Sea Seven Million Years Ago Triggered Widespread Volcanic Activity by David Bressan, Forbes