Believe it or not, when you have a deep rock layer (such as salt) that is less dense than the rock above it, the rock below can actually begin to form very-slow-moving “bubbles” that push up toward the surface of the upper layer, creating pressure and ultimately leading to a mound or hump in the top layer. In geology, this bump is called a “diapir” (sounds like “die up here”), and in the Mediterranean region the salts that form these diapirs come from a geological event known as the Messinian Salinity Crisis.

 

During a period of time between five and six million years ago, the African continental plate actually pushed the European plate up so far that the mouth of the Mediterranean Sea closed up and the Straits of Gibraltar that bring water in from the Atlantic Ocean were dry. Because the rivers that flow into the Mediterranean don’t bring enough water into the basin to replace what is lost from evaporation, it was only a matter of time before the water levels left inside dropped so far that the concentration of salts in the sea basin got high enough that they began to precipitate, or fall out of solution, and become salt crystals again - primarily gypsum salt, which is the first to precipitate from sea water. Whether the Mediterranean Sea completely dried up or its level simply dropped so far that salt crystals formed, what is known for certain is that it happened many times over the 600-700 thousand years following the Straits of Gibraltar closing up (perhaps as many as 40 drying-out periods!). The resulting deposits (4 x 10¹⁸ kg or 8.8 quintillion pounds) of salt left over from this event have been covered by sediment 7-10 km (c. 4-6 miles) thick from the outflow of rivers and streams emptying into the Mediterranean basin. This much salt under that much sediment makes for an unstable situation, in terms of the density of the two layers of material, since the salt is less dense than the overlying sediments.

 

So let’s think back to our cake in the oven: as it heats up, the bubbles of carbon dioxide inside expand with heat, until a large bubble forms in the middle of the cake, rises up into the cake above it, and eventually bursts, releasing the gas inside and causing the rest of the cake to fall into a large hole in the center. This is like what can happen if the rock that sits on top of a large quantity of salt gets disturbed somehow (e.g. as a result of seismic shifting) and the resulting looseness of the sediment allows the salt below to bubble up. Once the bubble reaches the surface of the rock and meets sea water, however, it dissolves back into solution, and the rock that it so recently pushed up falls back into the resulting hole, leaving behind a large pockmark feature. It is these pockmark features which we have found in the seafloor off the coast of Israel that have the geologists on board so excited. On last year’s mission, scientists noticed that the sidescan sonar surveys they got back from the region showed a number of large pockmark features in the seafloor, and we’ve now been able to go back and take a much closer look at more of them, collecting sediment samples using push cores and sediment scoops. The sediments we bring back will be sent to the geology labs at the Leon Charney School of Marine Sciences at Haifa University and the data will be shared with the larger science community, in order to help further our understanding of the geological activity in this and other regions of the world’s oceans.

 

 

When it comes to the science we do here aboard Nautilus, the things that gather the biggest crowds - the fish, the sharks, the shipwrecks - are compelling, of course, but if you take the time to look more deeply (oh, say, beneath 7000-plus meters of sediment), you can find equally fascinating material wherever you go. Even if it's just gypsum salt...