Cracks in the Ocean Floor

The concept that the ocean floor is spreading was studied for many years: new crust constantly forms at the bottom of the ocean. The ocean floor has deep trenches, plains, and mountain ranges. The mountain ranges are higher than those found on the continents but with different characteristics. The spines of these great mountain ranges, called mid-ocean ridges, exhibit incredible volcanic activity in rift zones. The rift zones are fissures(=splits) in relatively narrow regions of the crust, along which the crust splits and spreads. One hundred eighty million years ago, the paleocontinent Gondwana broke apart, forming a rift from which the Atlantic Ocean grew, and is still growing.

The Crust Under the Oceans

The constant generation of new ocean crust along rift zones powers a seemingly endless process that generates new lithosphere that is carried from the crest(=the top) of the ridges, as if on a conveyor belt. Because of this, scientists have calculated that in about 250 million years, the continents will again join and form a new Pangea as they are pushed by the continually expanding ocean floor. Ocean plates are in contact with land plates at the active boundaries of subduction zones or at passive continental boundaries (continental shelves and slopes). Undersea subduction zones, called ocean trenches, also occur between oceanic plates: these are the deepest places on the planet.