Some further remarks on the alternative earth tectonics hypothesis

The preceding few blogs have attempted to cover some of the apparent weaknesses of the Plate Tectonics (PT) explanation for how the Earth’s lithosphere has dynamically evolved throughout the eons of geological time. It has focussed upon the particular problems relating to the ways in which sediments are laid down and mountains are formed. There appears to be compelling evidence that mountains are generally the result of erosion of regionally uplifted crust, which is quite out of line with the mechanism envisaged by PT. Moreover, the evidence I have been able to gather suggests that these periods of regional uplift occur over quite short geological timescales (measured in 10’s Ma), whereas PT regards continents as having been impacted into one another at timescales measured over 100’s Ma. There also appears to be good grounds for supposing that the regional uplifts occur synchronously over very different regions of the Earth’s surface. For this to happen in the PT model would require some remarkable coincidences.  

Sedimentary evidence seems to provide a picture of crust rising and falling, sometimes above and sometimes below sea level, severely challenging the PT notion that continents have over many 100’s Ma had essentially unchanged plan forms, albeit they have broken away from their precursor super-continents, and been propelled into different spatial locations. Some of the important evidence I have been able to gather shows that the nature of sedimentary deposit also displays a cyclical form, with periods in which the forms of the sediments and their rates of deposition display globally synchronous characteristics. Moreover,  many cycles of sedimentary deposition, epeirogenic rise, continental erosion, epeirogenic subsidence, appear to have occurred over the time it is believed to have taken for the assembly, breakup and global journeys of the fragmented supercontinents envisaged in the PT hypothesis. PT seems to have very little useful to say as to why this might be the case.

Based upon my own reading of the current situation a new hypothesis has been presented that appears to provide an alternative explanation for what has and continues to help shape the Earth’s crust. It is based upon observations of how cyclical thermal processes are at work, albeit at greatly reduced spatial and temporal scales, in controlling both natural and man-made surface morphologies. There is growing evidence that long (10’s to 100’s ka) and very long (10’s to 100’s Ma) cyclic changes in thermal conditions on the Earth could be similarly playing an important role in global tectonics. Some of the earlier posts have argued that a great many features of the dynamic evolution of the lithosphere can be explained in terms of massive cyclic changes in thermal regime within the crust, including phase changes of lower lithospheric rock.

These earlier posts have suggested that the driving forces for these thermal changes are closely related to those responsible for glacial and interglacial episodes arising within the geologically recurring ice ages. Waxing and waning of thick ice sheets and the associated changes in sea levels are suggested to induce major changes in geothermal heat flux and its associated geothermal gradients. The consequential changes in lithospheric temperature are shown to provide models capable of explaining the cyclically developed massive horizontal tension and compression forces, and their associated kinematic and rheological consequences within the upper regions of the crust. But over longer geological periods these cyclic changes in surface ice and water and the thermal insulation they provide, have been argued to cause alternations in the thermal regimes and possibly associated phase changes at the lower lithosphere boundary, sufficient to induce substantial changes to the thickness of the Earth’s crust. Consequential isostatic and eustatic changes then help to explain the all too evident vertical motions of ocean and continental crust, including explanations of how ocean crust can be raised to form new continental crust and vice versa - issues which are left poorly accounted for by PT.

There appears to be some evidence in support of this new theory. The pulses of mountain building have been compared with paleoclimate temperature estimates (Post & Illis, 2009) and noted by Kalander et alia (2011) to coincide rather well with the geological evidence on the periods ice ages have occurred. Figure 11 reproduces Figure 2 but has had added the periods when major regional mountain forming episodes have been recorded. There does appear to be a remarkable coincidence between the ice age pulses and those of mountain building, including that of the Plieno-Pliestocene (Ollier, 2006b). And although the precise dating seem a little less clear, there does appear to be more than circumstantial evidence that the pulses in different forms and rates of sedimentation may also have strong links with the ice ages (Sloss, 1964; Wezel, 1992). Over shorter geological periods the seemingly synchronous and global development of river terraces (Bridgeland, 1988, 2010; Westaway, 2002, 2010) are strongly indicative of the work of the glacial – inter-glacial cycles being superimposed upon long term epeirogenic uplift.

All in all there does appear to be a compelling case that in contrast with the supposed monotonically evolving processes envisaged by PT, much of the dynamic of Earth lithosphere evolution has been one of cyclical processes. The preceding blogs have attempted to explain the close agreement between the phasing of the cycles of geological evolution and the forcing of climate caused by Earth’s interactions with the sun (whether directly or indirectly as controlled by variations in cosmic ray flux). They have also proposed a thermal-mechanical models for how this dynamic process might be working.     

 Much of the above post has been taken from the paper "On the Causes of Vertical Motions of Lithosphere", James G A Croll, Frontiers meeting, Geological Society of London, November, 2011.