During the Triassic Period, some 200 million years ago (MYA), the Tethys Sea opened across Pangea, rifting the northern Continents (Laurasia) from the southern Continents (Gondwana). Over the following Jurassic and Cretaceous Periods, the Atlantic Ocean, Indian Ocean and lesser seaways opened, splitting apart the components of these large land masses and leading to the continental geography that we find today.
During this process, the African Plate drifted northward and rotated counterclockwise, gradually compressing the Tethys Sea and cutting off its eastern connection with the Indian Ocean. In concert, a broad, relatively shallow arm of the Tethys, known as the Paratethys Sea, spread across central Europe and eastern Asia, initially forming a connection between the North Sea and the Mediterranean (the direct remnant of the closing Tethys). Beginning about 40 MYA, compression and subduction along the African-Eurasian margin forced up the Alps and other ranges across the northern edge of the Mediterranean, isolating the Paratethys as a vast inland sea.
Tectonic activity throughout the late Tertiary, followed by periodic glaciation during the Pleistocene, continued to mold the Paratethys Sea as drainage patterns were altered and ocean levels rose and fell. Today, the Aral, Caspian and Black Seas represent remnants of the Paratethys; the first two, entirely cut off from the ocean, have become "saline lakes" while the Black Sea, connected to the Mediterranean via the Bosporus Strait, has settled into a unique, layered hydrology. Cool freshwater, flowing in via the Danube and other Eurasian rivers, overlies warm (but more dense) saltwater that flows up from the Mediterranean; the volume of the latter is balanced by the outflow of freshwater toward the ocean, making the Bosporus Strait a dual flow conduit.
