Influence of surface ice and water on geothermal flux

In suggesting that surface temperature changes experienced over typical glacial and interglacial periods, typically in the range of 20 to 110 ka, could penetrate far enough into the crust to interfere with the flow of geothermal energy I was in an earlier contribution (Croll, 2007a) clearly wrong. As Figure 1 shows, a cyclic thermal loading at the Earth’s surface with a periodicity of 110 ka will penetrate fully depths of just 1.75 to 3.5 km. These depths would be insufficient to trigger the forms of interaction with the flow of geothermal energy required to develop tectonic disturbances described in this earlier paper. However, over the periods of around 140 Ma, typifying the cyclic thermal loading seemingly responsible for the ice ages, no such problems exist. As the lower diagram of Figure 1 shows surface temperature variation having periodicity of 140 Ma are capable of full penetration to depths typical of both oceanic and continent and crust. This means that major changes to the heat transfer processes at the Earth’s surface, occurring over these lengths of geological time, will likely have significant effects upon the geothermal energy flux. And variations in the geothermal flux will in turn exert major controlling influences on the crust thickness. 

 

Figure 4 illustrates schematically how various surficial changes could play a role in determining the geothermal energy flux and consequently the crust thickness.

Ice sheets and crustal rise:
Figure 4(a)(1) shows a typical column of crust at or near a continental passive margin which is then overlain with a thick ice sheet Figure 4(b)(2). Note, for this illustration I have ignored the isostatic readjustment due to the weight of the ice, but this too will play a role as later discussion will attempt to show. The thermal gradient through the ice sheet will be determined by its thickness, and the average surface and base temperatures. Where permafrost exists, this too will over a very long period of time develop an equilibrium thermal gradient. This near surface thermal gradient will eventually exert a controlling influence over the level of geothermal heat flux. If for example the ice sheet, or combination of ice sheet and underlying permafrost, had a thickness of 2 km and an average temperature difference of -40^oC, over a period of time sufficient for thermal equilibrium to be reached through the thickness of the crust and ice sheet there will result a downward thermal gradient of +20^oC.km^-1.  Over an area of relatively thin crust, that might have perhaps been part of a continental passive margin with say a thickness of 10 km and a temperature difference of +1200^oC between top and bottom, the thermal gradient will be around 120^oC.km^-1. Note, for simplicity I have ignored any contributions to the geothermal gradient arising from the decay of radioactive elements within the crust – in any case these are thought to be relatively small in oceanic crust. This initial disparity in thermal gradient will

gradually be adjusted to ensure that the flux through the ice is matched by the flux through the crust. In other words depending upon the relative diffusivities of ice and crustal rock, the geothermal gradient in the crust will be lowered to match that controlled by the average ice thickness and temperature gradient. This is suggested in Figure 4(a)(3). The result of this adjustment over long geological periods (10’s Ma) will likely be some phase changes in the magma beneath the crust and a gradual thickening of the crust from below. Being lighter than the semi-liquid magma from which it has been formed, the now thickened crustal column will be forced upward through the effects of buoyancy, as suggested in Figure 4(a)(3). In other words there will be an isostatic readjustment for which the original free surface will have been elevated relative to its original location. At a given depth within the crust such a realignment of the geothermal gradient will be likely to produce very substantial changes in temperature, the consequences of which could be of major significance in many tectonic processes, some of which will be discussed later. A mechanism along these lines will for example be invoked later to explain how ocean crust of continental shelves could be elevated to become continental crust and continue to rise during the cyclic waxing and waning of ice sheets over the period of an ice age.

It might be objected that during an ice age there will not necessarily be continuous ice cover as seems to be implied in the above model. However, the long period component of the shorter period cyclic fluctuations of ice sheets will provide what could be regarded as a continuous average ice cover. That is, on average over the period of the ice age the effects will be as described. Equally, during any interglacial flooding of previously continental margin crust due to eustatic changes, it might be anticipated that the effects will be the opposite. Figure 4(b)(1) represents a column of crust that might perhaps have been exposed during an average sea level drop accompanying an ice age. During the warm age (sometimes referred to as the “hot house”), lasting for periods of circa 100 Ma between ice ages, this area may be flooded as depicted in Figure 4(b)(2). It might also be cyclically flooded during the glacial – inter-glacial fluctuations. With oceans seemingly providing more efficient crustal surface heat transfer mechanisms, there will initially be an incompatibility between the surface heat transfer from the crust and that associated with the geothermal gradient of the previous continental crust. Over time this increased heat flux will be reflected by an increased geothermal gradient and an associated decrease in the effective crustal thickness. In this case the reduced crustal thickness might be anticipated to have again arisen from the phase change at the lower crustal boundary to accommodate the increased geothermal flux and its associated geothermal gradient, as shown in Figure 4(b)(3). As a result of the increased overall density of the crustal column and its overlying water will there will be a lowering of the crust. This mechanism will be used to later to provide a possible explanation as to how continental crust could sink beneath the waves.            

What is being suggested is that during the very long period cycles associated with the ice ages (circa 140 Ma) and their intervening warm ages, major changes in geothermal gradients could give rise to a number of long term cyclic tectonic processes.  These will be explored more fully in later posts. 

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.