5.6 Implications of plate tectonics
5.6.1 The Wilson cycle
High-quality, palaeomagnetic data are now sufficiently abundant that it is possible to reconstruct the movement of the continents throughout the past 500-600 million years (i.e. the Phanerozoic) and, with increasing uncertainty, back to 750 Ma and possibly earlier. From these reconstructions it became apparent that the continental masses have been assembled previously into supercontinents that have broken apart, dispersed, and then later reassembled in a different configuration to form another supercontinent. This observation was noted by Wilson (Box 1) who proposed that an ocean basin has a lifespan with several stages: it begins with the initial opening, and then goes through a widening phase before starting to close and on to its ultimate destruction. This theory accounts for the cycle of continental break up and reassembly, and became known as the Wilson cycle in his honour. From the palaeomagnetic reconstructions, it appears that the cycle of supercontinent assembly - break-up and subsequent reassembly - takes about 500 million years to complete. This time period can be further explained by a simple calculation.
Imagine that a roughly circular supercontinent, 5000 km in radius, and located about the Equator, rifts in two along a north-south line. A new spreading centre between the rifted halves spreads at an average rate of about 3 cm y−1. How long would it take for the two halves to first meet again on the opposite side of the globe? (Assume that the circumference of the Earth is 40 000 km.)
The spreading rate is equivalent to an ocean width increase of 30 km Ma−1. For the continents to meet on the opposite side of the globe, the ocean will need to have opened to half the circumference of the Earth less the original width of the continent (i.e. 20 000 km - 5000 km = 15 000 km). The time taken to achieve this will be:
Clearly, this is an average estimate because spreading rates vary, and continental configurations are far more complex than the simple two-continent rifting model outlined in Question 10. But if it is correct, then, given that Pangaea formed about 300 Ma ago, the next supercontinent is due to begin to assemble in about 200 million years, perhaps once the Pacific Ocean has been closed by the subduction zones that surround it.
Various stages have been identified for the Wilson cycle, and all of these stages can be recognised in different parts of the Earth today.
The earliest stage, called the embryonic stage, involves uplift and crustal extension of continental areas with the formation of rift valleys (e.g. the East African Rift System).
The young stage involves the evolution of rift valleys into spreading centres with thin strips of ocean crust between the rifted continental segments. The result is a narrow, parallel-sided sea, for example the Red Sea that is opening between NE Africa and Arabia.
The mature stage is exemplified by widening of the growing 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 system (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 with a Wadati-Benioff zone demarking the descending plate and associated island arcs, such as the situation in the western Pacific Ocean, or Andean-type volcanism. The onset of subduction at the ocean boundary marks the subduction stage of the cycle (e.g. the Pacific Ocean).
Once subduction outpaces the formation of new crust at the constructive boundary, the ocean begins to contract. Island arc complexes, complete with their inventory of sedimentary and volcanic rocks, collide and create young mountain ranges around the periphery of the contracting ocean. These features mark the terminal stage of the cycle (e.g. the Mediterranean).
The end stage occurs once 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. Finally the plate boundary becomes inactive, but the site of the join, or suture, between the two continental masses is a zone of weakness in the lithosphere that has the potential to become the site of a new rift and so the cycle continues.