- The embryonic stage, involves uplift and continental crust extention, resulting in the the formation of rift valleys (e.g. the East African Rift System).
- The young stage involves the evolution of rift valleys into spreading regions with thin strips of ocean crust between the rifted continental sections. This forms a narrow, parallel-sided sea, like the Red Sea that is opening between NE Africa and Arabia.
- The mature stage is characterised by widening of basin and its continued development into a major ocean flanked by continental shelves and with the continual production of new, hot, oceanic crust along the ridge (e.g. Atlantic Ocean).
- Eventually, this expanding system becomes unstable and, away from the ridge, the oldest oceanic lithosphere sinks back into the asthenosphere, forming an oceanic trench subduction system, such as the situation in the western Pacific Ocean. Onset of subduction at the ocean boundary marks the subduction stage (e.g. the Pacific Ocean).
- Once subduction overtakes formation of new crust at the constructive boundary, the ocean begins to 'shrink'. Island arc's collide and create young mountain ranges around the periphery of the 'shrinking' ocean. This marks the terminal stage of the cycle (e.g. the Mediterranean).
- The end stage occurs when all the oceanic crust between the continental masses has subducted, and the continents converge along a collision zone characterised by an active fold mountain belt, such as the Himalayas. The plate boundary becomes inactive, but the boundary between the two plates remains as a zone of lithospheric weakness. Therefore it has the potential to the site of a new rift and so the cycle continues.
Wednesday, 8 February 2012
Wilson Cycle = Opening and Closing of Oceans
Rifting controls the opening and closing of oceans. The cycle is known as the Wilson Cycle and it illustrates the balance between new crust being created by volcanic eruptions and destruction of crust at subduction zones, to maintain Earth's fixed size. This helps to explain why there are subduction zones at the edges of most continents.