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 and vertically (e.g. mountain formation). The currently dominant theory of plate tectonics argues that a combination of seafloor spreading (Hess 1962), slab pull (Forsyth and Uyeda, 1975) and convection currents (Ampferer, 1906) in the planet's interior causes the Earth's crust to move (Meschede et al., 2022). The predictive power of the plate tectonics theory regarding plate motions appears convincing (see Fossen and Teysier (2025) for an overview) and was so decisive that other theories (incl. their derivations; see Meschede et al. (2022) for an overview) have already been rejected, incl. the theories of geological thermal cycles (Joly, 1925; Holmes, 1929, 1930), degassing (Barth, 1952), undation (van Bemmelen, 1933), oscillation (Haarmann, 1930), expansion (Carey, 1975), contraction (Dana, 1873), continental drift (Wegener, 1912, 1915) – the predecessor of the current plate tectonics theory – or the flowing hypothesis (Gutenberg, 1927). However, the precise mechanisms driving plate tectonics remain elusive due to a convergence of modelling complexity, observational constraints and theoretical uncertainty. The inability to directly measure deep Earth forces or accurately simulate key processes prevents definitive validation. Models are hampered by poorly constrained physical parameters, and geophysical imaging provides only indirect proxies for the dynamic forces at play. The same applies to the subduction hypothesis and the necessary recycling of the crust in the form of remelting. Without subduction, the plate tectonic model cannot work. However, it remains unclear how the process of subduction was supposed to have started in the first place, and this is a question that is still being debated. The main challenge is to identify a mechanism by which a strong lithosphere might break in the absence of plate tectonics-related weak zones and slab driving forces (Lu et al., 2021). It is not the purpose of this paper to discuss the weaknesses of the plate tectonic theory or other already rejected approaches in detail. It would simply go beyond the scope of this paper. However, we believe that the open questions and uncertainties about the driving forces of plate tectonics leave room for an alternative explanation. We therefore want to focus in this paper on a completely different, new explanatory approach. We will describe a mechanism that can be used to explain both horizontal and vertical movements as well as adjustment processes of a planet’s crust. In our view, our approach provides a coherent model that addresses causes and consequences, and we also claim that the predictions of our model are perfectly in line with the actual movements of the Earth's crust as measured by the GNSS Time Series provided by the Jet Propulsion Laboratory (Heflin et al., 2020). Among other things, our new theory can easily answer four key questions about the movement of the planetary crust. The first two are directly derived from the mentioned GNSS observations. First, why is there not a single mountain range on Earth where adjacent crustal fragments are moving towards each other? How can mountains still fold? Second, why are the only zones where crustal fragments move directly towards each other some transition zones between high plateaus of continents (and islands) and deep basins of oceans (and seas)? Third, why are crustal movements so intense on Earth in particular, while they are often not observable on other planets? Finally, why do earthquakes occur repeatedly even within large continents 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)? As novel as our approach may be, it does not require any new physical, chemical or other findings or assumptions about processes in the Earth’s interior that cannot be directly observed or measured. Only the GNSS dataset on the movement directions of the Earth's crust (Heflin et al., 2020), which helped us to identify the mechanism, can be described as new, as it was not available at the time the plate tectonics theory was first developed. The described mechanism is the result of analyzing this relatively new dataset and is based solely on fundamental scientific principles that are beyond dispute. 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. Everything else is just a matter of logical deduction. Unlike the explanatory approaches we are familiar with, however, the root cause here is not the horizontal movements of the Earth's crust, which subsequently might 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 (section 6). This subsequently necessitates ongoing adjustments to new isostatic equilibria in the form of (relative) vertical uplift and subsidence movements of the crust (see section 2). 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 (see section 3). 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 (see sections 4 and 5). 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, liquid 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 liquid water acts as a catalyst, further amplifying the resulting effects of regional differences in the rate of crustal thickening (see sections 7 and 8).