Until comparatively recently, the only crustal movements worth the name were thought to be either post glacial rebound, where surface ice melting induces crust rebound until a new equilibrium is established, or gravitational collapse, such as weathering and erosion. Both are gradual, isostatic processes.
The many extensional features such as rifts, horst/graben faulting, uplift, tilted blocks and basin formation were seen to be part of these isostatic processes, occurring over geologic time scales and with any lateral movement being a localized event.
However mounting geographical, geological, paleontological and botanical evidence was pointing to continental mobility on a global scale, and by the sixties the Isostatic Tectonics view was finally rejected in favour of a globally mobile surface with most opting for a surface plate model driven by mantle convection currents, …Plate Tectonics.
Others concluded that the abundant extensional features, the spreading ridge that surrounds Antarctica being one of many such examples, …that such features could only be accounted for by an increase in the surface area, and thus radial expansion, of the earth, …Expansion Tectonics.
‘Surface’ mobilists espoused Plate Tectonics while the ‘surface area’ mobilists advocated Expansion Tectonics. The ‘facts’ could be made to fit both explanations. But the absence of a plausible expansion mechanism was an insurmountable problem for Expansion Tectonics, and Plate Tectonics was adopted to replace the Isostatic model.
To recap, Plate Tectonics prevailed over Expansion Tectonics because Plate Tectonics requires less energy, no mass/volume variation, and, more importantly, it has a plausible mechanism to drive the process, i.e. convection. Earth expansion was a contender process in the sixties but is now considered a discredited theory. It has been considered, and, largely due to the absence of any causal mechanism, rejected by most, but not all, right-thinking geo-folk. This is despite the fact that much of the geological/geographical/botanical/fossil/seismic evidence for plate tectonics can be used to support the expansion model. The evidence is also consistent with a sub-crustal increase in mantle or core/mantle volume so that the increased surface area consists of primordial crust separated by recent ocean crust.
Both Plate and Expansion theories assume that the process occurs continuously over geological time spans. Whatever its driving mechanism, tectonic movement is considered to be a slow but inexorable process.
However recent work on the structure of the world that lies beneath our feet, specifically 3-D images of the crust and mantle generated by supercomputer processed, earthquake induced, seismic wave data, reveals that there is no obvious evidence for the simple convective system that one might expect from the tectonic model. From the link…
… One could a priori expect such [tomographic ] images to reflect a very simple convective system, where the ascending currents are found under mid-ocean ridges, and the descending currents are found at subduction zones, where the tectonic plates slide back into the mantle. In fact, seismic tomography … reveals a more complex situation that cannot yet be very clearly interpreted. …
– Physics of the Earth’s Interior. Barbara Romanowicz.
The images,From the maps it is evident that Mid-Ocean Ridge spreading centers do not persist below 600 kms. What emerges at deeper levels are ocean sized blobs located under the southerly region of the Pacific and African plates to create a more or less balanced global distribution of these low density/high temperature features.
This configuration does not support the simple convection model of a continuous column of ascending or descending material originating near the core-mantle boundary and extending to the base of the, …according to the Plate Tectonic model, very mobile and actively subducting crust.
Convection is an important part of the Plate Tectonics narrative in that it is the proposed mechanism that supplies the enormous amount of energy needed maintain the currents necessary to drive the plates across the surface of the earth. Without it, Plate Tectonics has difficulties with its energy budget as it assumes that surface plate mobility is powered by energy from below. Without convection, plate mobility is left without a plausible driving mechanism. It could not be maintained, let alone initiated. Something else would be required to explain the obvious separation of pieces of continental crust that both Plate Tectonics (and Expansion Tectonics) seek to explain.
The lack of evidence for simple convection in the tomographic data has prompted modifications to the model by way of the introduction of a stratified two-stage convection model. This is roughly analogous to a pot filled with water and oil, with separate convection cells in both oil and water layers and a conductive energy transfer process at the interface.of the liquid layers. The modifying effects of uncontained convection, regional lateral temperature variations and other tweaks are also being considered.
The picture that emerges, …a chemically stratified mantle with variable-depth chemical boundaries near 1,000 and 2,000 km and a lower mantle depleted in radioactive elements appears to satisfy available geochemical and geophysical constraints. … suggests a major revision of the Plate Tectonics energy budget and itself appears to be somewhat at odds with what one might expect were convection currents present. Convection is not generally associated with stratification.
The tomographic picture may offer more support to a model where the plates are relatively static with any relative movement due an increase in the overall surface area of the globe together with accompanying isostatic adjustment and curvature modification.
Separation by surface area expansion still doesn’t have a causative mechanism, but the lack of evidence for simple convection also casts doubt on the viability of the surface plate mobility claimed by Plate Tectonics.