9. Momentum Considerations

The introduction of the concept of momentum in continental plate movements is an essential element in the full understanding the subduction cycle. Momentum is defined as being the Mass of the moving object multiplied by its velocity

The basic text equation describes Momentum as Mass x Acceleration (a)

Acceleration is defined as the change in velocity.

Using the break-up of Pangea an example, the initial velocity of the South American plate whilst attached to the African Plate must be considered as zero. The movement of the South American Plate at the end of the Rodina era tectonic movements was halted by the Pangean accumulation of major plates

Thus, the change in velocity of the South American Plate will be from

Zero Velocity = Vo to Vp the Plate Velocity (b)

Taking the Mass of a moving plate =M

The Momentum will be M multiplied by Vp = MpV (c)

Ultimately the momentum associated with the movement of the large Continental Crust albeit with an exceedingly small velocity (12 mm/year) will override the less massive and possible opposite slower moving Oceanic crust. Thus, it is the momentum of a moving continental plate over the oceanic lithosphere that will start and maintain the process of subduction. This will continue until the moving plate is stopped at the convergent margin of a separate continental plate. At this junction subduction will grind to a halt and orogenic processes will be brought into play.

9.1 The Momentum of the moving Continental Plate drives the Subduction Process

With reference to Fig 16, the following sequence is offered as a viable explanation of the subduction process: Pangea will be used as the example:

  1. The breakup of the Supercontinent will result in continental plates being shed both east and west of the heavy African where the Centre of Mass (COM) is situated (sections 4&5)

  1. Subduction starts the moment the breakaway continental plate starts pushing against the oceanic lithosphere.

  1. During this compression stage the accretion process and trench formation will be started.

  1. As the continental plate continues to move forward, its weight will overcome the upward buoyancy forces and cause the oceanic lithosphere to start to deform as it is forced downwards towards the mantle. The forced downwards movement will be noted by the creation of stress related faults in lithosphere.

  2. The Slab Pull cycle will start when the deformed oceanic lithosphere is bent downwards towards the mantle. The resistive viscous drag forces will be overcome by the combined forces associated with the weight of the overriding continental plate and the continuously enlarging slab

  1. As the weight of the descending slab increases, the subduction rate of movement may increase relative to the faster moving opposite direction Continental plate. This in turn may alter the magnitude of tensile stress patterns in the horizontal sections of the oceanic lithosphere and may also make itself manifest by the thickening and shortening of the Continental crust

  1. It is unlikely that the tension induced in the basaltic oceanic crust will result in the creation of a major mid ocean ridge (East Pacific Rise) where the lithosphere is cracked down to the mantle. Minor ridges may also open.

  1. Slab finally breaks off or melts in the Asthenosphere/Mantle

  1. At this junction, the SLAB-PULL Force=0 as the slab falls away

  1. Continental mass continues its unrelentless movement driven by the circumferential stress forces without loss of momentum

  2. Subduction starts again as the Continental plate starts pushing down on the slab free

9.2 Topography and orogenic activity related to slab detachment

The four major basins making up Alaska appear to suggest the four cycles of slab formation followed by slab breakaway and the re-start of subduction.

With reference to the subduction process as described in (fig 16) it is feasible to explain the South Alaskan sequence (fig 17) as being due to the continuous movement of the overriding Continental plate. This movement was responsible for the successive separate orogenic cycles resulting from oceanic lithosphere bending followed by slab detachment. This cyclical activity led to the formation of the St. Elias Range which in turn led to the formation of the Alexander and Yukon Terranes and the present-day Accretionary Wedge as the Continental Plate moved south.

Fig 17

9.3 Summary. Subduction is a consequence of Tectonic Plate movements

Present wisdom regarding the role of subduction with respect to tectonic plate movements needs to be re-examined as the absence of a slab and thus the absence of a slab-pull force does not impede the movement of the continental plate. It therefore follows that subduction cannot be considered as the major driving force of plate tectonics. As such, subduction of the oceanic plate must be considered as the consequence of continental plate movements rather than as the driving force of plate tectonics. Using the above hypothesis in which subduction and with or without Slab pull is not the driving force for tectonic processes it is not unrealistic to relate the tectonic forces to the unbalanced rotation of the Earth.

9.4 Tectonic Plate Movement sequence diagram

Fig 17 reinterprets the original Hess model of subduction by circulatory convection currents with one that shows tectonic plate movements as being a function of the centripetal and differential tensile forces associated with the constant rotational velocity of the Earth. Thus ‘Slab Pull’ on the oceanic crust is replaced by ‘Tensile Stress Pull’ on the continental crust as being the major force for moving continents together or apart. In this respect the variable omnidirectional convection current driving force is replaced by a permanent constant force related to the rotational speed of the Earth. Furthermore, ‘Seafloor Spreading’ as described by Hess is replaced by ‘Upper Mantle Stretching’ and magma intrusion onto the sea floor is considered as an inevitable consequence of the propagation of the rifting of the mantle. As such, magma intrusion has no contribution to the forces moving tectonic plates. This interpretation does not invalidate research work at the convergent and divergent margins as the mineralogical and geological outcomes will be the same. As the stress calculations are based on the Earth’s constant rotational velocity, the forces available for all tectonic processes are not subject to conjecture regarding both the source and direction of the omnidirectional convection currents. This approach allows for the mathematical analysis that is applicable to fixed rotating bodies to be modified and utilised for the study of tectonic, orogenic and metamorphic processes.

Sequence of Tectonic Movements due to the circumferential tensile forces which are a function of the unbalanced rotating Earth

Fig 18. Copyright R. Maurer, www.tectonic-forces.org, 2020