Page 398 - Global Tectonics

380 CHAPTER 12

12.1 INTRODUCTION

The mechanism behind the motion of plates is still
controversial. Older theories for the origin of the major
structural features of the Earth’s surface that relied
on the supposed contraction or expansion of the Earth
have now been discounted. The most likely mechanism
of heat transfer from depth appears to be convection.
The form of this convection and the manner in which
the thermal energy is utilized in driving the plates are
discussed in this chapter. Figure 12.1 Contracting Earth model.

12.2 CONTRACTING evaluations of cooling rates imply a total
contraction of only a few tens of kilometers.
EARTH HYPOTHESIS Consequently, the contraction hypothesis
cannot account for the many thousands of
kilometers of crustal shortening which must
have occurred in mountain belts throughout
In the 19th century it was believed that, since its forma-
geologic time.
tion, the Earth had been cooling due to heat loss by
thermal conduction. Computations by Lord Kelvin on 2 The hypothesis implies that the lithosphere is
the rate of cooling of an initially molten Earth provided everywhere in compression, and cannot provide
the first, erroneous, estimate of the age of the Earth of an explanation for phenomena that must have

100 Ma. As a corollary, it was suggested that the accom- originated in tensional regimes, such as normal
panying contraction of the Earth on cooling might faults, ocean ridges, and rift valleys.
provide a mechanism for mountain building. It was esti-
mated that the circumference of the Earth had decreased
by 200–600 km since the Earth’s accretion. The discov-
ery of radioactivity at the end of the 19th century 12.3 EXPANDING
negated much of the early work as it provided a precise
method of dating rocks and also demonstrated that the EARTH HYPOTHESIS
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 The expanding Earth hypothesis was fi rst proposed
in the 1920s and was subsequently adopted by several
placed in a state of tangential tension. Above a horizon
of no strain, the outer shell of the Earth was then sub- geologists as the mechanism behind the break-up of
continents, the formation of continental rifts, and the
jected to tangential compression as it collapsed inwards
upon the shrinking center (Fig. 12.1). The lithosphere presence of extensional features such as normal faults
(Carey, 1976, 1988). Their proposal was that the con-
is too thick to respond to this compression by buckling,
but would yield by thrust faulting, producing mountain tinental lithosphere was originally continuous over the
surface of an Earth of smaller radius and that, as the
belts by the stacking of thrust slices.
A contracting Earth is no longer recognized as a Earth expanded and its surface area increased, the con-
tinental lithosphere fragmented and dispersed, while
possible mechanism for tectonic activity for two con-
vincing reasons: mantle material welled up into the consequent gaps
to form the oceans. Independent evidence for the
1 The Earth is not cooling sufficiently rapidly to expanding Earth hypothesis appeared to be provided

be consistent with contraction, and modern by certain theoretical physicists, who suggested that