Geology
The Mediterranean Basin is a depression in the Earth's crust caused by the African Plate slipping under the Eurasian Plate. Typically in geologic history the depression is filled with sea water under various geologic names such as Tethys Sea. In the last period of the Miocene Epoch, the Messinian (7-5 mya), the Messinian salinity crisis, a near drying of the Mediterranean, was caused by the sea level dropping below the sill at the Strait of Gibraltar and the equilibrium between evaporation and replenishment shifting in favor of evaporation. At that time the Po Valley and the Adriatic depression were a single canyon system thousands of feet deep. On the southwest the Apennine Mountains bordered a land mass termed Tyrrhenis geologically. Their orogeny was just being completed in the Miocene. On the north the Alpine Orogeny had already created the Alps.
At the end of the Messinian the ocean broke through the sill and the Mediterranean refilled. The Adriatic transgressed into all of northern Italy. In the subsequent Pliocene sedimentary outwash primarily from the Apennines filled the valley and the central Adriatic generally to a depth of 1,000 m (3,300 ft) to 2,000 m (6,600 ft) but from 2,000 m (6,600 ft) to 3,000 m (9,800 ft) off the current mouth of the Po, with pockets as deep as 6,000 m (20,000 ft). At the start of the Pleistocene the valley was full. Cycles of transgression and regression are detectable in the valley and the Adriatic as far as its centre and in the southern Adriatic.
From the Pleistocene alternation of maritime and alluvial sediments occur as far west as Piacenza. The exact sequences at various locations have been studied extensively. Apparently the sea advanced and receded over the valley in conformance to an equilibrium between sedimentation and glacial advance or recession at 100,000-year intervals and 100 m (330 ft) to 120 m (390 ft) fluctuation of sea level. An advance began after the Last Glacial Maximum around 20,000 years ago, which brought the Adriatic to a high point at about 5500 years ago.
Since then the Po delta had been prograding. The rate of coastal zone progradation between 1000 BC and 1200 AD was 4 m/yr. Human factors, however, brought about a change in the equilibrium in the mid-20th century with the result that the entire coastline of the northern Adriatic is now degrading. Venice, which was originally built on islands off the coast, is most at risk due to subsidence, but the effect is realized in the Po delta as well. The causes are first a decrease in the sedimentation rate due to the locking of sediment behind hydroelectric dams and the deliberate excavation of sand from rivers for industrial purposes. Second, agricultural use of the river is heavy; during peak consumption the flow in places nearly dries up, causing local contention. As a result of decreased flow salt water is intruding into the aquifers and coastal ground water. Eutrophication in standing waters and streams of low flow is on the increase. The valley is subsiding due to the removal of ground water.
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