Some past explanations of crustal dynamics

There is convincing evidence that the earth’s crust has undergone periodic changes with timescales, both very long measured in 100’s of ma, and shorter measured in10’s or 100’s ka,. Over the very long term continents would on a periodic basis seem to sink to become ocean floors and ocean floors rise to again become continents. How many such cycles have occurred during the circa 4.5 ba of the Earth’s existence and when exactly a significant crust of the form we know it today actually formed to make such movements possible, seem to be largely unresolved. However, it appears more than conceivable that the number of such very long-term cycles could be many. It also seems clear that the PT model, dominated as it is by the tangential motions of the crust, is incapable of explaining the occurrence of these very long-term vertical tectonic cycles.

Within these long-term geological cycles there seem to be other shorter timescale processes at work. These shorter period processes could be responsible for the alternations between compressive and tensile actions occurring within the crust. Before going on to outline a model that could provide an explanation for this dynamical system, involving as it appears to do both horizontal and vertical motions, accompanied by both tensile and compressive actions, it may be useful to consider some of the previously postulated explanations, other than PT, that have been advanced for the development of the earth’s crust as we know it today.

One major differentiation of Earth models is between those that take the line that what we see today has been the result of a gradual evolution in which the processes at work in the past should be evident from those that are at work today. This so called “uniformist” model contrasts with those that see the evolution in terms of more discrete and often cataclysmic changes.

Cataclysmic models: 

Among the latter were those that tried to explain the Earth as we find it today in terms of a Biblical flood. This idea, prevalent in the 18^th and early 19^th C, incorporated the growing recognition that the match between the coastlines of the Americas and Africa/Europe was due to a rifting apart of the Atlantic following the flood referred to in the Bible. Others have suggested that the spin-off of the Moon left a great hole in what is now the Pacific Ocean with the great void so created being filled by the splitting apart of Americas and Europe/Africa to form the Atlantic Ocean (this view is associated with George Darwin nephew of Charles). Various other ideas have included the colliding or near colliding bodies, taking different forms but including the idea that the gravity field generated by the Earth’s capture of the Moon developed at an early stage the forces needed to drag the continents towards the equator, creating enormous mountain building forces (Taylor 1908). A variant was the idea that the close approach of Venus (Baker, 1911) created the gravity field needed to drag the Moon from the Earth, with other orbital interactions being elaborated by Velikovsky (1950).

Uniformist models:

In the former, more traditional, uniformist, view the models have included the shrinkage (contraction) model relying upon the idea that the shrinkage of the Earth’s interior against the crust created the compression forces needed to build mountains. This idea appears to have been first put forward by Newton (1681) using the analogy of the wrinkling of the skin of an ageing apple (must have come a few days after his gravity observations from the falling apple!). Jeffreys (1976) too has argued that since its inception the Earth as a whole has contracted while cooling. Clearly these models fail to account for the clear evidence that in certain places and at some times, the crust has been and continues to be torn apart by tensile actions. To accommodate the very clear evidence of tensile stretching action, and at the other extreme, the expansion model advocates that the tearing apart of the oceans has been the result of a massive increase in the diameter. Carey (1958) argued that the diameter of the Earth could have been increased by as much as 100%. How these expansions occurred has been explained in a number of ways. Others account for the increased diameter by suggesting changes in phase or molecular composition of the Earth’s matter, or on a more modest level through a gradual decline in the strength of the gravity force (Dicke, 1962). Each of these uniformist models fail to account for the massive compressions needed to either explain upward folding and mountain building or the downward folding to form ocean trenches. None of the models are able to account for clear spatial and temporal evidence of periodic cycles of tension and compression being involved.

 

An attempt to reconcile the clear evidence of periods of tension and other periods of compression, the mixed shrinkage and expansion model, recognises that during the earliest period of the Earth’s formation the largely gaseous materials gradually changed phase to become liquid and some eventually solid. This gradual compaction of the molecules would have been accompanied by a massive decrease in the diameter of the Earth. When later these dense liquids and solids were broken-down into less compact molecular forms the volume was once again increased. This latter period would cover the formation of the Earth’s crust, during which the breakdown in molecular forms would have started to produce the water that now forms such an important ingredient in the dynamics of the Earth. This view (see for example MacDonald, 1959) is attractive but is more concerned with the period prior to the dynamic crust of present interest. It would suggest however, that underlying any shorter periodicities there may continue to be a gradual expansion occurring as the average thickness of the crust and the associated volumes of free water and other low density molecules increase. Along similar lines the so called antimobilists believe that the Earth’s crust has been shaped by cycles of heating and cooling, causing expansion and contraction of the land masses. They took the opposite view to the mobilists who supported Wegener’s notions of continental plates in motion. The concept of a pulsating earth has also been advocated by Wezel (1992) and Dickins (2000).

 The truth, if and when found, will undoubtedly find that most of these models contain elements required to explain what has occurred. It seems evident that certain phenomena are associated with sudden and cataclysmic changes. It seems equally clear that other phenomena have been the result of gradually emerging processes. It is also very clear that whether one adopts a steady state or a transient model, the evolution of the Earth’s crust has been and remains a highly dynamical process. There is strong evidence that at a given location the crust has at times experienced tensile action and at others compression. This is incompatible with either the uniformist view or many of the prevailing notions of the nature of the Earth’s dynamical system. There is also unquestionable evidence that the various regions of the Earth’s crust have experienced, on a periodic basis, major changes in vertical elevation, which is also at odds with most of the past models and certainly at variance with PT.

 

What therefore might be an alternative model that could explain all of the essential processes known to have taken place and which continue to take place in the shaping of the earth’s crust?

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.