The expanding Earth hypothesis was first proposed in the 1920s and was subsequently adopted by several geologists as the mechanism behind the break-up of continents, the formation of continental rifts, and the presence of extensional features such as normal faults (Carey, 1976, 1988). Their proposal was that the continental lithosphere was originally continuous over the surface of an Earth of smaller radius and that, as the Earth expanded and its surface area increased, the continental lithosphere fragmented and dispersed, while mantle material welled up into the consequent gaps to form the oceans. Independent evidence for the expanding Earth hypothesis appeared to be provided by certain theoretical physicists, who suggested that
Figure 12.1 Contracting Earth model.
the universal gravitational constant was decreasing with time as the universe expanded and its constituent matter became more widely dispersed. Gravitational forces are responsible for binding the Earth into a spherical form, and since the gravitational constant directly controls the magnitude of the force of attraction between masses, its decrease would imply a progressive relaxation of the binding forces and an increase in the Earth's radius.
The most recent versions of the expanding Earth hypothesis correlate the period of rapid expansion with the break-up and fragmentation of Pangea in the past 200 Ma. These argue that continental reconstructions can be arranged more accurately on a globe of smaller radius, and propose that during this period the surface area of the Earth increased by a factor of 2.5 implying an increase in the radius from 63% of its present value and a mean radial expansion rate of about 12 mm a-1.
There are two methods available that can be used to test the expanding Earth hypothesis directly.
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