Earth’s geological structure and processes - Fundamentals of the Physical Environment - page 271

A supercontinental cycle - the Wilson cycle - is at work. Continents converge and
coalesce during one phase of Earth history and subsequently rift apart through relentless
plate motion. Supercontinents surrounded by a single global ocean become fragmented
continents separating several smaller oceans (Figure 10.2). Earth is small enough for
rifted fragments to reassemble eventually elsewhere. Moving on average at 10 1 cm yr −1
today - fingernail growth rates! - the cycle may seem imperceptibly slow but can be
completed within 500 Ma. This is short enough to have occurred eight to ten times during
Earth's history, especially as greater heat flow may have driven the cycle faster during
Archaean time.
Earth's present crust is half-way through such a cycle, which commenced with the
rifting of the supercontinent Pangaea c . 200 Ma ago, in the Mesozoic era of the
Phanerozoic aeon. Its global Panthalassic Ocean has been replaced by the new,
equatorially centred basins of the Pacific, Atlantic and Indian Oceans, whilst its Tethys
Sea arm was closed as Africa converged with Eurasia. Modern oceans are partially
enclosed by Pangaea's fragmentation into North and South America, Antarctica,
Australia and India and the emergence of South East Asia-Pacific Island arcs. Our
modern polar, landlocked Arctic micro-ocean contrasts with the south polar Antarctic
continent surrounded by the southern ocean. We need to think of the global map as
mobile and dynamic, rather than fixed in a position which we take for granted. Major
topographical features which profoundly influence modern global ocean and atmospheric
circulation such the Panama isthmus, linking North and South America, and the Tibetan
plateau are less than 3 Ma old. Closer inspection of Earth's crust reveals the global
morphotectonic landforms of current plate dynamics, clear evidence of past rifts and
collisions and the potential sites of future ocean basins and mountain ranges. Plate
tectonics provide the framework for understanding the geological evolution of the crust.
Its related supercontinental cycle and rock cycle drive the formation, degradation and
recycling of rock material and create distinctive landform assemblages. Most
geographical references in the text refer to the modern location and identity of crustal
fragments, which acquired their form and global position only recently. The age of events
in their geological history is indicated where appropriate.