The Origin and the Mechanics of the Forces Responsible for Tectonic Plate Movements

Simplified version... By Robert Maurer

In 1912 Dr Alfred Wegener (Germany) published his Theory of Continental Drift based on fossil and meteorological similarities in the Americas, Africa and parts of Northern Europe. He postulated that all the Continents were joined together in one landmass, which he termed Pangea (Greek for one land).

Fig. 1. Figure 1

In 1937, Alexander du Toit (South Africa), published his book 'Our Wandering Continents'. In which he based his observations on the geological similarities of the coal deposits in Northern Europe and common glaciated markings in South America, Southern Africa and Australia. His detailed work also allowed him to demonstrate that the presently named Caledonian Mountain range, which starts in Cumbria in the United Kingdom, skirts through Ireland and continues as the Appellation Mountains in New England (USA) were joined together in Pangea. Du Toit also coined the terms Gondwanaland, Laurasia, Tethys Sea, and the acronym SAMFRAU syncline to describe a continuous fold line through South America, South Africa and Australia. Despite the overwhelming evidence, this work was not fully accepted, as du Toit, like Wegener before him did not offer a mechanism to explain how the continents were caused to wander.

Despite the scepticism, other evidence being collected by various field geologists, particularly with regard to the orientation of the magnetic lines in iron rich magmas showed that these anomalies could only really be accounted for if Pangea existed. In 1944 Holmes published the idea that heated convection currents in the mantle due to the heated core could account for orogenic activity.

The next major breakthrough came in the late 1950's when the experimental submersible Alvin mapped the mid Atlantic ridge and noted the creation of new oceanic crust at 'black Smokers' along the ridge [Fig.2.].

Fig.2.

Collected core samples from the Atlantic floor also showed mirror imaged reversed polarity paleo- magnetic lines either side of the Mid Atlantic ridge [Fig.3.], which varied in age from the present to the Jurassic era.

Fig.3.

Prof. A Hess (USA) checked these new discoveries against his own meticulous mapping of the Pacific Basin during his tenure in the US Navy in WW2. He correlated the Pacific Basin with it's deep trenches and truncated mountain ranges, the recycling of oceanic crust with mantle magma (andesite) that issued from continental volcanoes, with the creation of new oceanic crust at the mid Atlantic ridge [Fig.4.]. In 1962 Hess published his findings in which he concluded that the break-up of Pangea and subsequent Continental or Plate movement was due to the combined action of 'Ridge Push' forces at divergent boundaries [Fig.5.] with the 'Slab-Pull' forces at convergent boundaries. He attributed the forces involved to the action of heated circulation currents within the Earth's mantle. In this manner Hess was also able to balance the creation of new oceanic crust by the equivalent sub-duction of older oceanic crust.

Thus 30 years after he died Wegener was credited with being the father of the science of Plate Tectonics.

Fig.4.

 

Fig.5.

Despite the comparative simplicity of the Hess model, a number of major anomalies exist. In order to account for some of these anomalies, in particular the absence of an equivalent sub-duction activity on the African plate, various models for the convection current system have been put forward [Fig.6.].

Fig.6.

In 2001, the author, R.Maurer (Chartered Engineer, UK) while on a mineral collecting expedition in Bolivia, noted uplifted but undisturbed alluvial sedimentary beds at the top of the Andes near Potosi. Taken together with his finding of new species of fossilised cretaceous fish and stromatolite beds, these observations prompted him to look at the magnitude of the forces involved in lifting the vast Andean mountain range from approx. 3 km below sea level to approx. 8 km above sea- level.

The author (drawing on his engineering expertise) concluded that the forces needed to sustain the unrelenting and uni -directional movements of the continental plates in a conflicting omni directional heated convection current system (based on the Hess Model) was mechanically unsustainable.

The first clue came with the observation that the precession of the equinoxes (Milankovitch Cycles) [Fig.7.], the longer-term cyclical changes in the inclination and obliquity of the central axis of rotation of the Earth taken together with the observed magnetic polar wandering suggested that the Earth was behaving like an unbalanced rotating cylindrical body. A domestic spin dryer with an unbalanced wet load is a good everyday example of an unbalanced rotating system. Vibrations are induced in rotating bodies when the centre of mass is not co-incident with the centre of rotation. In these cases, counterbalance weights are fixed to balance the rotating system. Another simple everyday example is the positioning of balance weight on the wheels of motor vehicles.

Standard textbook calculations as well as experimental evidence on unbalanced rotating bodies show that the rim or outer surface will be in tension at the heavier side and in compression on the lighter side.

Fig.7.

The above observations were reinforced by noting that the geologically quiescent African Plate, which is splitting (Tension Zone) at the Rift Valley (Fig 9), is almost diametrically opposed to the geologically active Pacific Basin (Fig 4), with its deep trenches and sub-duction zones (Compression Zone). This is completely in keeping with standard textbook mathematics showing the build up of tension and compression either side of a rotating un-balanced spherical body. (The mathematical analysis is given in the main treatise).

Fig. 4. (above)

Pacific Basin in Compression

Subduction Zones & Tension

Fig.9. (right)

African Plates in Tension splitting at Rift Valley

From this analysis it is reasonable to conclude that the African plate side of the Earth is heavier than the Pacific side of the Earth. A first order simple equations invoking the principle of isostatic equilibrium put the Centre of Mass of the Earth between 1 and 2 km off centre on the African side (Fig 10). Although 1 Km offset is used for the mathematical demonstration of the unbalanced state of the planet Earth, it is probable that the offset is a combination of the shift in the core plus differences in the composition of the mantle.

Fig.10.

Further mathematical modelling (see main treatise) takes into account the situation in which the crust is allowed to slide relative to the mantle (Fig11). This yielded formulae that allowed for the estimation of the circumferential stress acting on the Continental and Oceanic crusts in the direction of the Pacific Basin as well as allow for the estimation from first principles of the coefficient of friction (ì) between the mantle and the sliding crust (Fig. 12). The values obtained for ì closely corresponded with the independently obtained values determined under laboratory conditions. Thus the main assumptions can be demonstrated to be within the sensible bounds.

Fig.11.

Although the Core is only offset by 1km in an equatorial radius of 6400 km (c 0.015%), the generated unbalanced centripetal forces at the surface of the earth are substantial and more than sufficient to move continental plates. The calculated circumferential force of approx. 11.0 psig (75.8 Kpa) acting against the side of a continental plate such as the South American plate is more than sufficient to push it over the Nazca plate. In a similar manner this force will push the Indian sub-continent into the Asian continent to create the giant Himalayan mountain range.

In order to get an idea of the magnitude of this force, consider the pressure of 11 psig applied to the rear area (approx. 2600 ins2 (1.7 m2)) of a family motor vehicle to push it along a flat surface. The force available is approx. 12 tonnes. A three tonne hoist will pull the vehicle up a 1:3 slope on a tow away truck. Also consider that a developed pressure of 3.5 psig over the wingspan of a large passenger aeroplane will support its 300 tonne weight in flight.

Fig.12.

The calculations allowed for the estimation of the coefficient of friction (μ) between the sliding crust and the mantle from first principles [Fig.12.]. The values obtained closely corresponded with the independently obtained values determined under laboratory conditions. Having established that the unbalanced centrifugal forces (that have to now been almost totally disregarded) are sufficiently large to be considered as the primary driving force for tectonic activity, it is now possible to re-interpret the presently accepted Hess based 'slab-pull' force diagrams for tectonic plate movement, which are based in what is now obviously an omni-directional convection current system.

As the 'ridge-push' force has now been largely discounted, it is self evident that tectonic activity is a function of the rate at which the heavier density oceanic crust enters into the lighter lighter density mantle. The proposed 'force –push' model (Fig13) postulates that the sustained inertial based force moves the sliding crust to the lighter side of the planet, (which is already in compression) and in so doing, forces the continental crust over the oceanic crust.

Fig.13.

The unrelenting uni-directional movement of the South American Plate over the Nazca plate that is presently creating the Andean mountain range is a case in point. Fig13 shows the system in pictorial form. In a similar manner, the Indian sub- continent is being forced into Asia where the mighty Himalayas are being pushed up. Under these conditions, the omni directional convection currents (Fig14) play a passive rather than an active role in tectonic plate movements.

The new interpretation takes into account some of the major anomalies associated with the convection current theory. In particular, the extrusion of magma and the creation of new oceanic crust will take place where the oceanic crust is split by the resultant tensile forces on the 'heavier side' of the rotating planet. The magma that extrudes will be from the hottest regions of the mantle in proximity to the split ridges. Complex convection currents will arise under these conditions.

Fig.14.

By having the heavier African plate as the 'anchor', the break- up will be on both sides of what was Pangea. Under these conditions, sub-duction will not occur either side of the present African Plate [Fig.14.]. This major anomaly associated with the convection current theory is now taken into account.

Furthermore the consideration of the unbalanced centrifugal force model, which describes opposing forces of compression and tension, will create and area between them where the hoop stress reaches its maximum value [Fig.15.]. If this stress is high enough, surface cracking will occur and magma will extrude. In a similar manner, older and now almost extinct ridges will have been created when the principal axis of rotation and thus the equatorial belt were in a different plane to what they are at present. Geological evidence shows this change in obliquity of the axis of rotation.

Fig.15.

It is therefore postulated that the original creation of Pangea (pre-Permian) followed by it's subsequent break-up at the start of the Permian era is an inevitable consequence of the unbalanced centripetal forces, which stem from the mass imbalance of the rotating planet. As the movement of the centre of mass is small, but the ramifications are significant, tectonic plate movement will continue to be dynamic and with it the slow but inevitable regeneration of the Earth's crust.

The figures have been taken from various journals and books. As the pictures have been seen in many publications, the origin or copyright is not obvious. Please advise if your copyright is being used and we will apply for permission and acknowledge the origin

By Robert Maurer, 31A The Avenue, Cowley, Uxbridge, Middlesex. UB8 3AD, United Kingdom

This work is the copyright of Robert Maurer. MSc. CEng and must not be copied or used without the written permission of the Author. The manuscript [not this simplified version] was submitted to the British Geological Association on the 14 July 2003 for possible publication.

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