Within the time span of geologic history, geologic forces
and climate change must have combined to create events difficult for us
to imagine. This is the story of one such time....the day the Mediterranean
Today the Mediterranean Sea is a large inland body of
water, bounded to the south by Africa, to the north by Europe,and to the
east by Asia Minor. The only outlet to the oceans ofthe world is through
the western portal adjacent to Gibraltar.
The Mediterranean Sea is but a remnant of a great seaway,
the Tethys, that formed a continuous passageway between the Indo-Pacific
and the Atlantic. This seaway separated Africa from Eurasia throughout
the Mesozoic (Age of Dinosaurs) and continued as a barrier until some 18
to 14 million years ago when the eastern connection with the Indian Ocean
closed forming the Mediterranean. Since the eastern closure of the Tethys,
the Mediterranean has continued to narrow.
Closure of the eastern Tethys was the result ofthe northward migration of Africa and the collision of Arabia and Iran with Asia Minor (the later producing the Tarus and Zagros Mts.). Today the African plate continues a squeeze of the Mediterranean with subduction beneath Italy and the Greek Islands. Eventually North Africa seems destined to collide with Europe in the final elimination of the Mediterranean.
The compressional events described here resulted in mountain
building from the Alps and WSW through Asia Minor. For a long time (until
about 5-6 million years ago), these mountains divided two seas. The first,
being the Tethys and later Mediterranean; and the second being the Paratethys,
extending from central Europe along a line connecting the Black Sea, Caspian
Sea, and Aral Sea.
The Mediterranean is in a region where evaporation greatly exceeds precipitation. Today water enters the Mediterranean from the Atlantic replacing water lost through evaporation. Ifit were not for this "renewal", the Mediterranean would dry up into a deep and dry basin! All that is required for this to become a reality is either a lower of sea level below the Gibraltar sill depth or uplift of the sill above sea level.
In 1961, seismic reflection profiling by a ship in the Mediterranean discovered pillar-shaped structures of the Mediterranean seafloor that appeared to be salt domes. Structures like these could only have formed by evaporation of the Mediterranean. In 1970, the first successful deep drilling of the seafloor confirmed this evidence. This drilling and subsequent drilling suggests the following:
During the late Miocene (~6 million years ago) compressional tectonic forces and lower sea level combined to bring the Gibraltar sill above sea level. As the last Atlantic waters dripped over into the Mediterranean, this body of water began to become a hypersaline lake as evaporation exceeded precipitation. Over the next 1000 years, this lake became lower and progressively saltier. Finally deep basins thousands of feet deep became little more than salt flats surrounded by the exposed marginal walls of the European and African continental slopes. Surrounding rivers flowed outover the exposed continental shelves and then cascaded in enormous braided waterfalls into the deep basin. The Mediterranean had become a deep hole, dwarfing the Grand Canyon. The surrounding climate of the region became cooler and drier as an important moisture source disappeared.
A single drying up of the Mediterranean would only form salts or evaporites 100 meters thick, however, thicknesses of 1000 to 2000 meters floor parts of the basins. This tells usthat over 2 million years, the Mediterranean was not continually dry. Periodically, waters would cascade of the sill, partially filling the basin only to evaporite again. Such occurrences may have occurred as many as 40 times.
Surrounding rivers, having had their base levels reduced to the floor of the Mediterranean began rapidly cutting their river valleys. Rivers such as the Rhone in France, Po in Italy, Nile in Egypt, cut deep canyons far upstream. In Egypt this now buried canyon was discovered when drilling to find asolid foundation for the Aswan Dam.
Streams and rivers from the Paratethys, cut rapidly down where they entered the empty Mediterranean Basin. The Paratethys began emptying into the basin, spelling the death of the ParaTethys.Today only the remnants remain: the Black Sea, Caspian Sea, andAral Sea.
Finally, approximately 4.5 million years ago, arise of sea level and a relaxation of tectonic forces caused Atlantic waters to begin spilling over the Gibraltar sill. What a sight it must have been as waters cascaded over the Gibraltar waterfall!