THE MECHANISM BEHIND PLATE MOTION

The mechanism behind plate motion is still controversial, other theories for the origin of the major structural features of the earth's surface that relied upon the supposed contraction of expansion of the earth have been discounted.
The most likely mechanism of heat transfer from depth appears to be thermal convection.

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Contracting Earth Hypothesis

• It was believed in the 19th century that the earth had been cooling due to heat loss by thermal conduction.

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• Computations on the rate of cooling on large iron spheres gave rise to the first erroneous estimates of the age of the earth (20-80million years ago).

• It was estimated that the circumference of the earth had decreased by 200-600km since accretion of the earth.

• The discovery of radioactivity of the end of the 19th century negated much of the earth's work by providing precise method of dating rocks and demonstrated that the earth processes its own internal heat sources.

• The contraction hypothesis envisaged that the central region of the earth underwent most rapid cooling and contraction and was placed in a state of tangential tension.

• The lithosphere is too rigid to respond to tangential compression by buckling as experienced by the outer shelf of the earth above the horizon of no strain, it will yield by trust faulting, producing mountain ranges by the stacking of thrust slices.

• A contracting earth is no longer recognised as a possible mechanism for tectonic activity for two reasons which are.

The earth is not cooling sufficiently/rapidly to be consistent with contraction and modern evaluations of cooling rates imply a total contraction of only few tens of kilometers. As a result, the contraction hypothesis cannot account for the many thousands of kilometers of crustal shortening that must have occurred in mountain belts throughout geologic time.

*The hypothesis implies that the lithosphere is everywhere in compression, and cannot explain the multitude of phenomena that must have originated in tensional regimes example, normal faults, ocean ridges, rift valleys etc.

Expanding Earth Hypothesis

• It was first proposed in the 1920's , subsequently adopted by several geologists as the mechanism for the break up of continents, the formation of continental rifts and presence of extensional features example, normal fault.

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• The continental lithosphere was proposed to originally be continuous over the earth of smaller radius and that as the earth expanded with increased surface area, 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 was provided by theoretical physicists that suggested that 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 indicates progressive relaxation of the binding forces and an increase in the earth's radius.

• The most recent version of expanding earth hypothesis correlate the period of rapid expansion with the break up and fragmentation of pangaea since 200-300million years ago.

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