Contraction and expansion tectonics

A long-running debate concerns the change in size of the solid Earth - has it stayed the same, shrunk, or swollen? A steady-state view, in which the Earth has had constant dimensions, is associated with the ruling plate-tectonic theory and with fixed continents and oceans model (e.g. Meyerhoff and Meyerhoff 1972). Proposals of an expanding or contracting globe are controversial but not without foundation. Earth contraction, once regarded as a good explanation of tectonic episodes, is no longer given credence, but Earth expansion still has several supporters.

A contracting Earth

René Descartes, Gottfried Wilhelm von Leibnitz, and Isaac Newton raised the possibility that the Earth started out as an incandescent ball, and subsequently cooled and shrank. Jean Baptiste Armand Louis Léonce Élie de Beaumont (1831) favoured the view that the Earth's mountain chains resulted from episodic contraction, each bout of contraction leading to shrinkage in the area of the crust and the production of mountains. James Dwight Dana (1846) thought that the Earth had contracted because of its cooling or its separation from the Moon or both. However, by the end of the nineteenth century, a better understanding of the nature of orogenic belts, isostatic movements, and the tensional nature of many epeirogenic movements led to the abandonment of the contraction hypothesis by most geologists, although a contracting Earth was central to Eduard Suess's theory of global tectonics (Suess 1885-1909, 1904-24).

During the twentieth century, a few astronomers and geologists attempted to resurrect the notion of a contracting Earth, but their efforts proved futile. George Martin Lees (1953) attributed fold and thrust mountain structures to adiabatic compression of the crust produced by a contraction of the Earth's surface due to shrinkage of the interior. The most recent proponent of the contraction hypothesis is the astronomer Raymond Arthur Lyttleton (1982). Evidence for a contracting Earth is scanty, but a reading of some palaeo-magnetic data suggested that the Earth has contracted slightly over the last 400 million years (McElhinny et al. 1978, 217).

An expanding Earth

Credit for being the first to suggest that the Earth might be expanding should be given to William Lowthian Green (1857, 1875, 1887). Subsequently, isolated voices have spoken out in favour of Earth expansion and it still stands as a reasonable hypothesis.

Evidence for a smaller Earth

What is the evidence for Earth expansion? The improved fit of the Triassic continents on a globe with a reduced radius is perhaps one of the strongest pieces of empirical evidence in favour of expansion (Carey 1958, 1976; Owen 1976, 1981; Shields 1979; Scalera 2003). Reassemble Pangaea on a globe of modern dimensions, and the fit between continents is good at the centre of the reassembly but becomes increasingly bad as one moves away from it; reassemble the supercontinent on a globe with a smaller radius, and the fit is much more precise (Figure 2.8). In addition, a smaller globe covered almost entirely by a supercontinent would explain a lack of extensive oceanic crust during the Proterozoic (Glickson 1980). Several curious distributions of fossil groups also bolster the case for a smaller Earth in the past (e.g. Scalera 2003). Particularly suggestive are the disjunct sister taxa (and matching geological outlines) that span the Pacific west and east margins (Figure 2.9). Dennis McCarthy (2003) believes these remarkable biological correspondences, which

Figure 2.8 The expanding Earth, showing the growth from around 220 million years ago (radius 3,000 km), through the present (radius 6,370 km), to 250 million years in the future (radius 9,000 km). Source: Adapted from Scalera (2003).

Earth Outlines

Figure 2.9 Biogeographical sister areas and matching geological outlines of the Pacific. The curve of the current Mariana Trench reflects the curve of Mesozoic south-east Asia; likewise, the curve of north-west South America corresponds to the actual Mesozoic outline of that region. Source: Reprinted by permission from Blackwell Publishing: D. McCarthy (2003) The trans-Pacific zipper effect: disjunct sister taxa and matching geological outlines that link the Pacific margins. Journal of Biogeography 30, 1545-61.

Figure 2.9 Biogeographical sister areas and matching geological outlines of the Pacific. The curve of the current Mariana Trench reflects the curve of Mesozoic south-east Asia; likewise, the curve of north-west South America corresponds to the actual Mesozoic outline of that region. Source: Reprinted by permission from Blackwell Publishing: D. McCarthy (2003) The trans-Pacific zipper effect: disjunct sister taxa and matching geological outlines that link the Pacific margins. Journal of Biogeography 30, 1545-61.

form a zipper-like system of sister areas running up both sides of the Pacific, strongly indicate a vicariance origin - the opening of a closed Pacific Ocean in the Upper Triassic-Lower Jurassic associated with an expanding Earth. Other researchers, studying various extant and fossil species, support a vicariance event, rather than long-distance dispersal, to explain the disjunctions (e.g. Shields 1998).

A less secure piece of evidence adduced in favour of Earth expansion is the apparent secular emergence of continents (decrease of sea level) during the Phanerozoic (Egyed 1956a, 1956b). The argument is that a progressive decline in the proportion of continents sub merged beneath oceans, both individually and collectively, demands that the Earth's radius should have increased, on average, at a rate of 0.5 mm/yr. However, a secular decrease in sea level could have arisen from the 20 per cent reduction in heat production over the last 500 million years (Armstrong 1969). Heat production in the solid Earth ultimately causes sea-level rises, and a 20 per cent tail off in heat production would lead to sea-level falling by about 80 m, which would account for most of the progressive Phanerozoic sea-level decline. The area covered by sea in North America suggests a roughly constant relationship between the elevation of continents and sea level since the Cambrian period, but the data are not sufficient to document secular emergence or submergence of the continent (Wise 1973). A reduced radius would call for a sea-level 1-1.6 km above the present level in Triassic and Jurassic times, whereas sea-level was actually lower then (Hallam 1984; Weijermars 1986; but see below).

Problems with a smaller Earth

Several 'frequently raised objections' to Earth expansion concern the character of the pre-Jurassic Earth, the source of the oceans and atmosphere, mountain building, subduction, palaeomagnetic data, and the source of the additional mass.

What would be the nature of a smaller pre-Jurassic Earth? According to some Earth-expanders, during the Archaean, the Earth's radius was about 1,700 km, and had expanded by just 60 km by the late Mesoproterozoic, the dominance of tensional tectonics during the Archaean and Proterozoic however suggesting that some degree of expansion might have occurred by crustal dilation associated with faulting and rifting (Maxlow 2003). On such an Earth, deep ocean basins would not exist before the Early Jurassic. Rather, the continental lithosphere would form the supercontinent Pangaea, which would cover the entire surface of the reduced-radius globe. Oceanic areas would exist as shallow, intracontinental or epicontinental seas, within which sediment deposition in deeper 'geosynclinal' basins would mask evidence of sea-floor spreading. A problem with such an arrangement is the fate of the hydrosphere. If the present oceans were decanted onto a pre-Jurassic small Earth with an unbroken continental crust, a 6.3-km deep ocean would flood the entire planet. However, some Earth expanders argue that during expansion, material produced by mantle devolatilization and accreted chiefly at the growing mid-ocean ridges and in rift zones has added to the atmosphere, hydrosphere, oceanic lithosphere, and underlying mantle at an accelerating rate. In others words, the expansion process partly created the ocean waters.

Another problem on a smaller, continent-covered planet is the building of mountains. The presence of mountains seems to rule out Earth expansion, because radial expansion would be the main tectonic force on an expanding planet and compressional forces to build mountains would be absent. However, the Earth has not expanded uniformly - the southern continents have separated more than the northern continents, and there is much more new oceanic lithosphere in the southern hemisphere. This asymmetrical expansion creates radial and tangential forces. A new construal of mountain building sets the radial expansion force at the hub of the process. An interesting, if highly controversial, spin-off from the expanding Earth hypothesis, is the explanation of mountain building offered by Cliff Ollier and Colin Pain (2000). In his characteristically iconoclastic style, Ollier writes that 'Most explanations of the origin of mountains in current textbooks are naive, simplistic and wrong' (Ollier 2003b, 129). In collaboration with Colin Pain, he promulgates this decidedly contentious view with gusto (Ollier and Pain 2000). The following crucial points summarize their view of mountain building:

1 Mountains are topographical, rather than geological, features. They are regions of high land, either plateaux or plateaux eroded by rivers or glaciers.

2 Plateaux form when low-lying erosional plains suffer vertical uplift. The uplift that created plateaux occurred in the last few million years (the 'Neotectonic Period').

3 Rock structures on which plains, plateaux, and mountains sit may have no causal link with the plains, plateaux, and mountains themselves. Geologists traditionally feel that explaining the structures found in mountains explains the origin of the mountains themselves. Ollier and Pain reject this view because there is no single structure under mountains.

4 Some rock structures, notably monoclines and vertical faults, may be associated with uplift.

5 Fold mountains, in the sense of mountains built by some force that produces mountains and folds rocks at the same time, do not exist. Geologists claim that the compres-sional forces responsible for folding rocks also produced the mountains in which the folds lie. Ollier and Pain are adamant that mountains have nothing to do with the folding of rocks or with the compression of the Earth's crust.

6 A plateau may spread laterally after uplift, which produces thrust faults and post-uplift folds.

7 Isostatic response following the deep incision of plateaux may lead to the production of new structures, including anticlines along major valleys and even major mountain ranges.

8 Major drainage patterns exist on the same time-scale as global tectonics and they commonly pre-date the formation of rift valleys, mountain ranges, and continental margins.

9 Theories of mountain building need to account for (1) a period of tectonic quiet that allowed the erosion of a planation surface, and (2) the usually young and rapid uplift that produced a plateau.

10 Plate subduction, if it occurs at all, is a continuous and long-lived process that fails to explain tectonic quiet, the erosion of planation surfaces, and the young age and rapid uplift of most mountains.

These 10 points prompt Ollier (2003b, 157) to conclude that some deep-seated force is needed 'to produce vertical uplift in the past few million years, and it must be a discontinuous force that did not operate at all over a previous period long enough to create wide erosion surfaces'. He argues that such a force is difficult to conjure on steady-state Earth, whereas with a expanding Earth, all that is required is for some parts to expand more rapidly than other, and that expansion varies through time.

Interestingly, Earth expansion would obviate the need for large-scale subduction. Take the example of subduction around the Pacific margins. By reassembling the circum-Pacific continents on a smaller Earth, the necessity for a huge pre-Mesozoic ocean - Panthalassa (and Tethys) - disappears, and the subduction of between 5,000 and 15,000 km of Pacific oceanic lithosphere becomes unnecessary. Instead, the north Pacific Ocean region is interpretable as a region of Mesozoic asymmetric spreading followed by Cenozoic symmetric spreading. Indeed, a planetary radial expansion rate of 21 mm/yr suffices to account for all ocean floor growth since at least the Early Jurassic, without the need to invoke subduction of oceanic lithosphere. Equally interesting are the implications of Earth expansion for palaeomagnetic results, which hinge crucially on an Earth of essentially constant radius. If the Earth has expanded, then the premises underpinning palaeomagnetic studies would require a reassessment, since inferred pole positions, apparent polar wander paths, and displaced terranes would be invalid.

The biggest unknown in the Earth expansion hypothesis is the source of additional mass to build a larger planet. Suggestions are plentiful (and in some cases fanciful). Richard Owen (1857) believed that the Earth had changed convulsively, the last convulsion involving an expansion from a tetrahedron to a sphere associated with a large displacement of continents and the ejection of the Moon. Bernhard Lindemann (1927) attributed the fragmentation and dispersal of Pangaea to an expansion of the Earth's interior associated with radioactive heating (see Moschelles 1929). Michael Bogolepow (1930) also saw radioactive heating as the primary cause of Earth expansion. Ott Christoph Hilgenberg, in his book Vom wachsenden Erdball (1933), maintained that the volume of the Earth and its mass are increasing, the extra mass coming from the transformation of aether! He was still sticking to his views 40 years later (Hilgenberg 1969, 1973). Other suggestions include variations in the effective sizes of atoms (Halm 1935); the expansion of the Earth along with all other things in the universe (Keindl 1940); and the Earth's possessing a core of dense hot plasma which is excited by a flux of cosmic particles modulated by the Sun, Moon, and planets (Shneiderov 1943, 1944, 1961). In 1954, working wholly independently of all other hypotheses of Earth expansion, Robert Tunstall Walker and Woodville Joseph Walker (1954), two economic geologists, came round to the view that the Earth was increasing in volume owing to the expansion of mass at its centre. Hugh Gwyn Owen (1976, 1981), an eloquent advocate for the expanding Earth hypothesis, believed that for any planet or satellite to expand, not only must the universal gravitational constant reduce, but also the material in the core must be in a plasma state and the core itself must be larger than a certain size. This would explain why there is no evidence of expansion on the Moon, Mars, or Mercury. A more recent study concluded that the Earth's radius has increased by 17 per cent owing to upper mantle formation resulting from gravitational differentiation of matter within the barysphere and phase transitions (Kozlenko and Shen 1993). A level-headed and recent review of possible expansion mechanisms by a physicist has this to say:

We have reviewed the implications for geophysics of the Hubble expansion, the cos-mological constant, vacuum energy, phase changes, a variation in the strength of gravity, continuous creation, exotic particles and higher dimensions. Of these, probably the last holds out most hope of a driving mechanism for Earth expansion. However, while there is some evidence in favour of the latter, we should recall that the surfaces of the Moon, Mars and Mercury show little to suggest either expansion or contraction. This should not necessarily be seen negatively: it would be naive to think that there is no connection between planetary physics and cosmology, and data from the Earth especially can be used to constrain modern theories of gravity.

(Wesson 2003, 416)

In short, Earth expansion remains a sound, if highly contentious, hypothesis. Further work will decide its fate.

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