Advances in Conception about Earthquakes - Earthquake Engineering for Structural Design - page 110

4.1.5 A New Visionabout Plate Tectonics: Super-continentsCycle Theory
The most convincing evidence for the theory of plate tectonics comes from the
seafloor spreading. Millions of years ago, the super-continent Pangaea developed
major rifts along which the continents separated into their present configuration.
But what happened before Pangaea?
Studying the geology and geophysics of old mountain ranges, such as the
Appalachians and Urals, the sea-level history, the rock types and their magnetic
orientation, scientists have proposed that a Super-continents Cycle exists,
according to which super-continents form and break up at the intervals of tens of
millions of years. This theory is largely based on the Wilson's Cycle, which occurs
as part of the breakup and subsequent reassembly of the super-continents (Nichols
et al, 2003).
Wegener's theory was generalized in 1967 by Tuzo Wilson, who observed that
the evolution of the Earth is cyclical. This theory is born as an answer to the
question:
… did the Atlantic Ocean open, close, and reopen?
This theory is known as Wilson's Cycle Theory (Everything, 2002). In the frame of
this theory, Wegener's great Pangaea super-continent (formed about 300 million
years ago) was only the last actual step in these cycles. The theory is based on the
observation that cyclical opening and closing of the ocean basins occurs during the
history of the Earth's crust formation (Fig 4.7) (Geology 202, 2004).
At some late time, continental rifting begins again for the next cycle. Until
today, there may have been more than six occurrences of these cycles; each one
taking 300 to 500 million years (Bokor, 2007):
- First super-continent Ur -3.0 billion years;
- Assembly of Kenorland -2.5 billion years;
- Assembly of Columbia -1.8 billion years;
- Assembly of Rodinia -1.1 billion years;
- Breakup of Rodinia - 760 million years;
- Assembly of Pannotia -600 million years;
- Breakup of Pannotia -550 million years:
- Assembly of Pangaea -300 million years;
- Breakup of Pangaea -200 million years.
What will happen after Pangaea?
Figure 4.8 shows the presumed future world (about 250 million years from
now), when a new super-continent will be formed, by the disappearance of the
Atlantic Ocean, the collisions of North and South Americas with Africa and
Australia with Antarctica, and the closing of the Indian Ocean, the Pacific Ocean,
will take over the majority of the world's surface.
The main lesson of the Wilson's Cycle Theory, if it is accepted, refers to the fact
that today the continental configuration approach starts only after the breakup of
the Pangaea super-continent, ignoring previous cycles. For a complete scenario of
the configuration today of the existing faults, the plate movements for all the cycles
must also be considered (provided that it could be possible, knowing that the
present knowledge about the remote past is very limited).