Expanding Earth vs. Plate Tectonics:
Timothy Casey B.Sc. (Hons.)
The Late Twentieth Century Refutation of the Expanding Earth Theory
Expanding earth theory, in both its mass accretion and mass conservation variants, calls plate tectonics into question. For the most part, the expanding earth theory relies heavily on the assumption that subduction does not occur. Verification of subduction by numerous cosmogenic isotope studies and common direct GPS measurements of subducting plate motion refuted the expanding earth model in the late Twentieth century.
The Plate Tectonics model used to explain observed convergence of pieces of crust, known as "tectonic plates", at subduction zones, and divergence of these tectonic plates at rift zones, gains independent support from a variety of sources including palaeogeography and geochemical studies of unstable isotopes such as Beryllium 10.
The concept of Plate Tectonics began with the idea of Continental Drift. Continental Drift was first proposed by Ortelius (1596) based on the fit of continental coastlines. Although loosely underpinned by the corresponding coastal geography, the idea lacked independent corroboration. The first empirical corroboration of Continental Drift was provided by Snider-Pellegrini (1858), a geographer who showed that the geographic distribution of fossil assemblages was continuous across continental boundaries as they first existed when the continents were together. Snider-Pellegrini (1858) lacked an empirical mechanism and when Wegener (1929) found that Continental Drift solved the apparent contradictions in palaeoclimatology between different regions, he was still unable to offer a verifiable mechanism. For this reason, the idea, although strongly underpinned by verifiable empirical evidence, remained largely incomplete. It is worth noting that many geologists ignored the evidence provided by Wegener on the grounds that he was a meteorologist with no formal training in geology - and therefore not an expert in the field. Ironically, during the same year, Arthur Holmes discovered that volcanic activity could not provide an adequate escape route for the amount of heat produced by the interior radioactivity of the earth. He thereby concluded that the heat in the interior of the earth must be redistributed by convection in the outer layers of the earth's interior. Holmes (1929), provided the first empirical notion for any Continental Drift mechanism. From here, the mechanism of mantle convection remained to be corroborated by more direct evidence. Arnold (1956) discovered that Beryllium 10 is cosmogenic. However, it was not until the 1980s that 10Be was used to track fluid motion in the upper mantle.
Objections to the idea of Plate Tectonics, usually based on the assumption that subduction is still an unsubstantiated process (Vine, 1987), have a history spanning several years now. The major weakness being a lack of any credible driver, this theory has sought support in the convolutions of the more complex theories such as fluid dynamics and quantum mechanics.
Personally, I'm intrigued by the implications in the emergence of theories such as expanding earth when used in a persistent campaign to question the validity of the Plate Tectonics model. What exactly is so wrong with Plate Tectonics, given that no-one appears to be offering a more comprehensive explanation of the evidence?
As an intellectual campaign, expanding earth theory lacks the usual cognitive extortion that gets packaged with pseudo-sciences. There is no impending catastrophe to hammer us into intellectual submission, and the one potentially pseudo-scientific characteristic is the juxtaposition of expansion as an unverifiable driver against convection, which without evidence of subduction can also be considered equally dubious. Such a brave assertion made to the degree of speculation of the plate tectonic idea in its early days, is perhaps a credit to Prof. Carey, whose job it was to sound the alarm in the event that consensus supplanted scepticism.
Expanding Planets in a Shrunken Nutshell
Mass accretion via condensation and collision are widely accepted processes by which mass is added to the planets by external deposition of extraterrestrial material in the case of planet earth. Thankfully, the rate of accretion is far too slow to account the rate of divergence measured at rift zones. Life would be interesting to say the least, if meteor showers were more common than rain storms!
However, the idea that mass was accumulating in the interior of the earth via some unexplained process dates back to when Yarkovsky (1888) suggested the accumulation and transformation of "aether" into chemical elements inside the earth; forcing the earth to expand. Although "aether" is soundly refuted, the nearest analogue in modern scientific models is the gravitational field. Yarkovsky's idea is perhaps the simplest and most natural depiction of a perpetual motion machine. Potential energy from matter in the pressure column is expended at the base of that column in the formation of new matter, which forces the column up and outward - effectively forcing the body to expand and increasing the potential energy of the system and forces acting at the core. The problem is that the expenditure of energy in this case dictates the acceleration of mass towards the base of the column instead of away. Yarkovsky's Aether expansion machine thus requires twice the energy that is theoretically available and an additional missing process by which the energy may be expended without depleting the reserve. Hilgenburg (1933, 1974) interestingly enough, also goes on to support the aether transmutation idea some time after it had been soundly discredited by empirical science.
Some 100 years after Yarkovsky, Professor Carey puts forward an expanding earth idea based on an unknown mass accretion process he speculates is connected with the expansion of the universe (Carey, 1988). According to Einstein's most famous equation, an energy mechanism sufficient to produce matter on a scale that would explain the amount of tectonic rifting on this planet, requires an energy source more powerful than the sun.
The short answer to mass accretion earth expansion after Yarkovsky, Hilgenburg, & Carey, is that an energy source of the magnitude that could produce the material expansion they propose without vaporising the planet in the process, such as a cold star, has never been observed - unlike the alternative mechanisms said to drive plate tectonics, such as convection.
A year after Yarkovsy's publication, the idea of Continental Drift - usually attributed to Wegener - was proposed by Montovani (1889) using earth expansion as a possible mechanism. This was not the first time Continental Drift had been proposed. Continental Drift was proposed by Ortelius (1596), corroborated by Snider-Pellegrini (1858) who suggested the rather fanciful and unsubstantiated mechanism of continents driven apart by a massive volcanic eruption on the sixth day of creation, and later corroborated by Wegener (1912) who declined to argue a mechanism. Montovani (1889) presents us with the first attempt to explain the mechanism of Continental Drift without invoking any supernatural meddling. With Continental Drift the product of Yarkovsy's Earth Expansion, it still remained to find a more plausible mechanism for Earth Expansion than Yarkovsy's aethereal mass accretion idea, especially given the refutation of the aether hypothesis by the Michelson-Morely experiment conducted the previous year (Michelson & Morley, 1887).
Expansion due to phase transitions in the earth's core is put forward by Halm (1935), Egyad (1956), and Owen (1983). However, phase transition as it applies to material volume under the temperature and pressure conditions applicable to the earth's core is purely speculative and to date, phase transition mechanisms capable of driving the dramatic expansion inferred from plate divergence in isolation from plate convergence, cannot be reproduced in the laboratory, and otherwise have never been observed.
A somewhat more complex model for mass conserved earth expansion could involve a chemical breakdown from denser component sets to expanded reaction products, somewhat similar to an explosion. However, a quick audit of the energy account is very revealing. A reaction of this nature would once again, have to produce sufficient quantities of energy to effect the expansion proposed and drive plate motions as they are observed. This requires the spontaneous storage of tremendous amounts of energy in what is an evidently less stable chemical assemblage early in the planet's formation. Once again, the mystery source of all this energy is not independently observed, and the supposed alteration of mantle chemistry to the degree commensurate with chemical expansion is not observed in either extruded xenolith chemistry over geological history, or alterations to magma chemistry for a given setting over geological history.
Also of note, the inverse square law as it applies to all fields including gravity dictates that gravity at the surface of a planet of equal mass to earth, but half the radius would be four times that of earth's gravity. The implicit decay of gravity at the earth's surface is not supported by anatomical changes as they are observed in faunal progression.
Cyclones and anti-cyclone spin is the product of Coriolus effect as applied the difference in tropospheric temperature between equator and poles. Coriolus motion is an offshoot of gravitational phenomena, such as settling of denser fluids when initially situated above lighter fluids; in conservation of angular momentum as it applies to portions of mass as they approach the moment. This is about as mind-boggling as rotational dynamics get.
Coriolus produces strictly rotary motion in the relative horizontal, but vertical motion remains the child of gravity. In terms of non rotary motion in the horizontal such as the propagation of oceanic crust from divergent to convergent plate boundaries, this has nothing to do with rotary motion and is strictly related to the sinking of dense material and the upwelling of light material in response to gravity.
Differential rotation is the product of differences in angular velocity between portions of a single rotating fluid body. As a result, the empirical evidence for differential rotation is shear between portions of differing angular velocity. Differential rotation is observed in the sun, Jupiter, Saturn, and accretion disks. Not all galaxies and protostars show evidence of differential rotation indicating that this phenomenon is not an automatic feature of rotary fluid systems.
However, differential rotation cannot be a persistent product of viscous rotary bodies without a significant kinetic energy source to counteract the effect friction has of unifying the distribution of angular momentum in accordance with the second law of thermodynamics.
Longer Divergent Boundaries then Convergent Boundaries
The greater length of mid oceanic ridges than known subduction zones has raised the objection that if subduction is actually occurring, there should be mid oceanic deformation that reflects the apparent bottleneck through which extruded material from the longer divergent margins, must now pass at the shorter convergent margins.
This, however, is a classic example of confusing length and displacement. The total length of divergent boundaries is exaggerated by the contribution from transform faults along their length, that give the mid oceanic ridges their apparent curvature. However, the spreading ridges as exclusively extrusive features are only as long as the sections from which extrusion occurs. This excludes the many transform faults that account for vectors in divergent boundary length that are parallel to plate motion. Thus tallying up the extrusion width may only account for the vector perpendicular to motion. The same goes for convergent boundaries and when convergent and divergent boundary vectors perpendicular to motion are compared, the sum is roughly equal.
Expanding earth theorists have successfully expanded the gross width of divergent margins by adding in the combined length of associated transform faults, which length contributes nothing to extrusion width. Summed widths along vectors perpendicular to motion (as opposed to lengths following any direction) of rifting and subduction zones are quite obviously equivalent and so do not predict any sort of the mid oceanic bottleneck deformation.
The Evidence for Subduction: Verified, not Assumed
Expanding Earth theory rests heavily on the denial of subduction as a real observed process. Dated sources such as Vine (1987), are often cited as some sort of admission that subduction is the central assumption of Plate Tectonics, when this is not the case. Subduction is verified by plate motion observed at and near convergent plate boundaries and places such as oceanic trenches where seismic equipment is placed and tracked. The interest in these regions arises because of the high degree of seismic activity that the specific structure of Wadati-Benioff shear zones confirm as caused by subduction. The ultimate verification of subduction and its confluent mantle flow is the presence of strictly cosmogenic isotopes in andesitic lavas erupted directly above where subducted plate can be seen on its way down into the mantle in seismic charts.
Transport of Cosmogenic Isotopes
How do we determine that oceanic crust is actually subducted into the mantle? The answer comes in the form of cosmogenic isotopes such as 10Beryllium (10Be) that are produced exclusively by the interaction of cosmic radiation with atoms in the earth's atmosphere and on the earth's surface. With a half-life of only 1.5 million years, 10Be's presence in the lava erupted in continental and island arc settings is only possible if it is carried down beneath the eruption site by subduction, and subsequently mobilised by partial melting to be incorporated into the source magma. Tera et. al. (1984), Morris (1991), Morris & Zheng (1993), You et. Al. (1994), are a sample of studies that observe the presence of 10Be in magmas produced in island arc and continental margin settings; ultimately providing the geochemical evidence of subduction.
Plate Motion Measured at Oceanic Trenches
Plate motion is directly measured by a number of methods including the use of GPS equipment, and significantly such measurements are also made in the vicinity of subduction zones. Holt (1995), Regelous et. al. (1997), Hochstein (1995), Stevens et. al. (2002), Bock et. al. (2003), and Thiebot & Gutscher (2006) are just a small sample of authors using direct measurements of subduction rates to relate factors such as partial melting, fractionation, and seismic activity to the observed speed of subduction.
It doesn't matter if the intelligent looking man in the suit who says "subduction doesn't occur", is a professor (Carey, 1988), the fact that the definitive plate motions of subduction are measured proves that subduction does occur.
The vast majority of expanding earth theorists believe that subduction does not occur, and the fact that subduction has been verified by cosmogenic isotope geochemistry and by direct GPS measurement of subduction rates may well lend limited support to the hypothesis that most of the people are wrong most of the time! Both direct measurement of motion at convergent boundaries, and cosmogenic isotope studies confirm, when taken in light of geothermal gradients measured during drilling (Eg. Morris, 2001), that convection is the best explanation of the process that drives plate tectonics. Whether slab pull or thermal gradient are the dominant sources of kinetic energy in plate tectonics, mantle flow confluent with plate motion is confirmed by scientific observation.
The most tenable position for expanding earth theorists is taken by Owen (1983), who instead of denying readily observable processes such as subduction, incorporates it as a partial process of an expansion model in which plate tectonics plays a vital role. Owen (1983), as the title suggests, still lacks a viable and observed process to drive planetary expansion. However, history of scientific controversy shows that most often, when evidence loosely supports more than one idea, the answer lies in the admixture of ideas. For example, gradual evolution rates vs. sudden bursts of speciation reflect a mode of punctuated equilibrium where the rate of evolution changes smoothly in response to the ratio of radiation to extinction. Like Wegener (1929), Owen (1983) is a compelling and elegant idea that lacks the independent evidence of a driving process, which is necessary to transform the idea into a hypothesis. As with Wegener's Continental Drift, time will tell, but for now, Plate Tectonics is the best planetary tectonic theory available to science.
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