Contracting Earth Hypothesis

In the 19th century it was believed that, since its formation, the Earth had been cooling due to heat loss by thermal conduction. Computations by Lord Kelvin on the rate of cooling of an initially molten Earth provided the first, erroneous, estimate of the age of the Earth of 100 Ma. As a corollary, it was suggested that the accompanying contraction of the Earth on cooling might provide a mechanism for mountain building. It was estimated that the circumference of the Earth had decreased by 200-600 km since the Earth's accretion. The discovery of radioactivity at the end of the 19th century negated much of the early work as it provided a precise method of dating rocks and also demonstrated that the Earth possesses its own internal sources of heat.

The contraction hypothesis envisaged that the central region of the Earth underwent more rapid cooling and contraction than the outer part and was placed in a state of tangential tension. Above a horizon of no strain, the outer shell of the Earth was then subjected to tangential compression as it collapsed inwards upon the shrinking center (Fig. 12.1). The lithosphere is too thick to respond to this compression by buckling, but would yield by thrust faulting, producing mountain belts by the stacking of thrust slices.

A contracting Earth is no longer recognized as a possible mechanism for tectonic activity for two convincing reasons:

1 The Earth is not cooling sufficiently rapidly to be consistent with contraction, and modern

Region of tangential compression Level of zero strain Region of tangential tension

Figure 12.1 Contracting Earth model.

evaluations of cooling rates imply a total contraction of only a few tens of kilometers. Consequently, the contraction hypothesis cannot account for the many thousands of kilometers of crustal shortening which must have occurred in mountain belts throughout geologic time.

2 The hypothesis implies that the lithosphere is everywhere in compression, and cannot provide an explanation for phenomena that must have originated in tensional regimes, such as normal faults, ocean ridges, and rift valleys.

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