Ocean Forms - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 56

disappearing. The underlying basaltic crust here disappeared com-
pletely: all of it was pushed down into the mantle (see Plate 2).
A little to the south there was another ocean, the track of which
can be followed by another line of eroded mountains from southern
Britain through Spain and Germany into south-west Poland. This
ocean, the Rheic Ocean, essentially lay between Europe and Africa. It
was a forerunner of the mighty Tethys Ocean, the demise of which
gave birth to the Alps and the Himalayas, and of which the Mediter-
ranean Sea is the shrunken remnant.
Oceans of the past, therefore, have gone through cycles of opening
and closure, each cycle lasting a few hundred million years. This is
often called the Wilson Cycle, after John Tuzo Wilson, a Canadian
geophysicist and one of the great pioneers of the plate tectonics
hypothesis. It is also commonly referred to via its mirror image, the
supercontinent cycle; a term that reflects the innate terrestrial preju-
dice of human landlubber geologists. The continents here are simply
passengers, being carried almost literally on the backs of the ever-
moving ocean plates.
Today we can see oceans at all stages of their evolution. There are
incipient oceans, those initial fractures that are not yet filled by the
sea. The classic example here is the Great Rift Valley of east Africa: a
crustal slab tens of kilometres across and a few thousand kilometres
long, that has dropped by a kilometre or more along parallel vertical
fractures. Magma is already rising along these fractures, feeding
chains of volcanoes. Soon, geologically speaking, Africa will split
apart and a new ocean will be born. Linking with the Great Rift Valley,
and a little further advanced, there is the Red Sea. Here, ocean crust
has been forming for some 50 million years and has widened to some
300 kilometres across.
There are also the fully grown oceans, including those that are still
widening—like the Atlantic, where every year Europe moves some