The driving mechanism of plate tectonics remains elusive. The contribution of the Moon and the Sun to the Earth's inner dynamics has been taken into account for decades but, despite a large amount of indirect evidence, their influence has never been convincingly proved.

Not everyone knows that not only liquid tides but also solid tides constantly deform the Earth's crust, swinging the land both horizontally and vertically by several centimetres. Through the study "Tidal modulation of plate motions" recently published on Earth Science Reviews Journal, the scientists of Sapienza University of Rome, the Italian Space Agency (ASI) and the Italian National Institute of Geophysics and Volcanology (INVG) have shed new light on the importance of solid tides (that is the deformation of land and the Earth's crust which takes place during the zenith passage of the two celestial bodies) proving the link between plate tectonics and astronomical forces such as tides, especially in terms of their horizontal component.
 

Periodic tidal effects take place at very different time intervals. Some are high frequency, with semidiurnal, diurnal, biweekly and monthly tidal cycles.
Others are low frequency and occur over more extended periods: semiannual, annual, 8.8 and 18.6 years approximately, up to that which occurs at the precession of the equinoxes and has a period of about 26,000 years. The study focused on those with periods of 8.8 and 18.6 years taking place, respectively, at the perigee precession and the moon ascending node.

High-frequency oscillations are mostly softened by the high viscosity of the Earth's outermost shell, the lithosphere, about 100 km thick. To date, the movement of the underlying mantle is still unexplained. Moreover, the high-frequency oscillations might be mistaken for climatic and seasonal factors caused by atmospheric pressure oscillation and fluid cycles in the subsoil and ocean basins. Hence the idea of studying the low-frequency horizontal oscillations since they are uniquely attributable to tidal stresses. The study was possible thanks to the permanent GNSS global network ("Global Navigation Satellite Systems", which includes both the American GPS and the European GALILEO system), that allowed speed measurements of tectonic plates even between stations situated thousands of kilometres from one another.

Thanks to important international services with at least 30 years of experience, such as the International GNSS Service (IGS), to which ASI makes a significant contribution through its Matera Space Geodesy Centre, the stations have collected historical series of their daily coordinates covering periods of at least 20 years, essential for carrying out this kind of analysis. These data were key to the work carried out by Davide Zaccagnino, Francesco Vespe and Carlo Doglioni in their analysis of the variations in time of the speed of lengthening or convergence among the plate pairs. Their studies have pointed out how the drift of the continents, that is, the lithospheric plates into which the planet's shell is divided, is modulated by vibrations that oscillate at the same low frequency as the horizontal component of the solid Earth's tides. Intra-plate baselines were then counterchecked to understand whether these oscillations persisted or not.

The same negligibility found on intra-plate baselines confirmed that these astronomical forces play a crucial role in the continental drift. Continents slowly move westward due to the horizontal displacement of solid tides as against the underlying mantle, along an undulated flow of plates traced by the so-called 'tectonic equator' which creates an angle of about 30° with the geographical equator.

References:

Tidal modulation of plate motions - Davide Zaccagnino, Francesco Vespe, Carlo Doglioni - Earth Science Reviews (2020) DOI https://doi.org/10.1016/j.earscirev.2020.103179

Further Information

Carlo Doglioni
Department of Earth Sciences
carlo.doglioni@uniroma1.it