Why the Earth's crust moves: thermodynamics, Isostasy and the enabling role of liquid water

It is undisputed that the Earth's surface moves both horizontally ("plate tectonics") and vertically (e.g. mountain formation). However, the forces and mechanisms responsible for this movement remain controversial. In this thesis paper, we describe a mechanism that explains these horizontal and vertical movements and provides answers to the following questions, among others but by no means exclusively:

• Why is there not a single mountain range on Earth where - adjacent crustal fragments are moving towards each other?[1] How can mountains still form?
• Conversely, why are the only zones where crustal fragments move directly towards each other transition zones between high plateaus of continents (and islands) and deep basins of oceans (and seas)?
• Why are crustal movements so intense on Earth in particular, while they are often not observable on other planets?
• Why do earthquakes occur repeatedly even within large continental plates far away from known fault zones, whereas this is rarely the case in the interior of ocean floor fragments (with the exception of their edges)?

While other approaches fail to convincingly answer all of these questions, our approach does not even require any new physical, chemical or other findings. The mechanism described is based solely on a combination of fundamental, already known and undisputed scientific principles. We refer to the laws of thermodynamics, Archimedes' principle of buoyancy in liquids, and material properties such as thermal conductivity, density, and breaking and tensile strength.

Unlike the explanatory approaches we are familiar with, however, the starting point here is not the horizontal movements of the Earth's crust, which subsequently cause vertical movements (such as the folding of the crust during mountain formation). Based on the laws of thermodynamics, we will describe how differences in the average thermal conductivity of the regionally predominant rock types lead to different rates of crustal thickening due to the continuous cooling of the planet. This subsequently necessitates ongoing adjustments to new isostatic equilibria in the form of (relative) vertical uplift and subsidence movements of the crust.

Each upward and downward movement not only deforms the rigid crust, but also, due to the spherical shape of the planet, is accompanied by an (layer-by-layer) enlargement of the surface of the crust. Due to its severely limited elasticity and deformability, the existing crust cannot simply adapt to the new conditions like a balloon, for example, but must inevitably tear, break, or move horizontally in order to resolve the horizontal tension caused by the vertical movements. The cavities caused by fractures and cracks subsequently lead to changes in the average density of the crust (per base area) and cause further disruptions in the isostatic equilibrium, the consequences of which have a similar effect.

On planet Earth, this mechanism is particularly strong compared to other celestial bodies, as there are significant amounts of liquid water on the surface. Due to its significantly lower thermal conductivity compared to rocks, water acts as an additional strongly heat-insulating layer and delays the cooling of the planet (and thus also the further growth of the crust's thickness), especially in areas where it collects in large quantities. However, since these deep basins of today's oceans and seas have only formed due to the low thermal conductivity of the rock types prevailing there, the water further exacerbates the global differences in the crust’s thickness growth. The basins of the oceans are becoming deeper, while the high plateaus of the continents are rising ever higher. The water acts as a catalyst, further amplifying the resulting effects of regional differences in the rate of crustal thickening.