Forces of an astronomical nature such as solid tides play an active role in plate tectonics: this is what the results of a research resulting from the collaboration between the Sapienza University of Rome, ASI and INGV affirm.
The driver of tectonic plate dynamics is still relatively unclear. For decades it has been thought that the Moon and the Sun could contribute to the internal dynamics of the Earth, but, although there was much indirect evidence, their influence had never been convincingly demonstrated.
Not everyone knows that in addition to liquid tides there are also solid tides which continuously deform the earth's crust, displacing the soil both vertically and horizontally by several decimetres. With the study "Tidal modulation of plate motions” just posted on Earth Science Reviews, scientists from the Sapienza University of Rome, the Italian Space Agency (ASI) and the National Institute of Geophysics and Volcanology (INGV) have shed light on the importance of solid tides, (i.e. those deformations of the ground and of all the earth's crust that occur during the passage to the zenith of the two celestial bodies) proving the link between the displacements of the tectonic plates and the forces of an astronomical nature such as the tides, in particular in their horizontal component.
The periodic effects of tides occur at very different time intervals. Some have high frequency, that is, they occur on a semi-diurnal, diurnal, biweekly and monthly basis. Others, on the other hand, are low-frequency with longer cadences: semi-annual, annual, approximately 8,8 and 18,6 years, up to that of the precession of the equinoxes which has a period of 26.000 years. Those with periods of 8,8 and 18,6 years, on which the study focused, are due, respectively, to the precession of the perigee and of the ascending node of the Moon.
The high-frequency oscillations are mostly damped by the highly viscous Earth's outer shell, the lithosphere, which is about 100 km thick and whose motion relative to the underlying mantle has so far remained unexplained. Furthermore, the high-frequency oscillations are confused with climatic and seasonal factors due to oscillations of the atmospheric pressure and of the fluid cycles in the subsoil and in the ocean basins. Hence the idea of looking for low-frequency horizontal oscillations on the inter-continental baselines, because they are uniquely attributable to tidal stresses.
This was possible thanks to the global network of permanent GNSS stations (the acronym stands for "Global Navigation Satellite Systems", which includes both the American GPS and the European GALILEO system) through which it is possible to measure speed between plates even between stations thousands of miles away.
Thanks to important international services that have been operating for at least 30 years such as the International GNSS Service (IGS), to which ASI contributes significantly through its Space Geodesy Center in Matera, the stations have accumulated time series of their daily coordinates now long at least 20 years, necessary to carry out this type of analysis.
Thus Davide Zaccagnino, Francesco Vespe and Carlo Doglioni carried out the analysis of the variations over time of the speed of separation or approach between the plates.
Their studies revealed that the secular drift of the continents, i.e. of the lithospheric plates into which the planet's shell is divided, is modulated by a vibration that oscillates at the same low frequencies as the tides. An intra-plate baseline retesting was done to understand whether or not these oscillations persisted. Just the negligence found on intra-plate baselines confirmed that these astronomical forces play a decisive role in describing the drift motions of the continents which, therefore, slowly move towards 'west' thanks to the horizontal thrust of the solid tides with respect to the mantle below, along an undulating flow described by the so-called tectonic equator which makes an angle of about 30° with the geographic equator.
Astronomical forces such as solid Earth's tides play an active role on plate tectonics: this is the results of a research conducted by the Sapienza University of Rome, the Italian Space Agency (ASI) and the National Institute of Geophysics and Volcanology (INGV).
For decades it has been thought that the Moon and the Sun could contribute to the dynamics of the Earth. However, despite a lot of indirect evidence, their influence had never been convincingly demonstrated.
Few people are aware that tides are affecting not only the oceans, but also the solid Earth and its crust. With the study “Tidal modulation of plate motions”, just published in Earth Science Reviews, scientists from the Sapienza University of Rome, the Italian Space Agency (ASI) and the National Institute of Geophysics and Volcanology (INGV) have shed light on the importance of solid tides, those deformations that swing up and down but also horizontally the ground and the earth's crust of few tens of cm during the passage at the zenith of the two main celestial bodies affecting our planet, ie, the Moon and the Sun, proving the link between the displacement of tectonic plates and the horizontal component of the Earth's tides.
Periodic tidal effects occur at very different time intervals. Some have high frequency, that is, they occur on a semi-day, day, biweekly and monthly basis. Others, on the other hand, are low-frequency with longer cadences: semi-annual, annual, 8.8 and around 18.6 years, up to that of the precession of the equinoxes which has a period of about 26,000 years. Those with periods of 8.8 and 18.6 years, on which the study has focused, are due, respectively, to the precession of the perigee and the ascending node of the Moon.
The high-frequency oscillations are mostly buffered by the high viscosity of the Earth. Furthermore, high-frequency oscillations are often mixed with climatic and seasonal factors due to atmospheric pressure variations and fluid cycles in the subsoil and the ocean basins.
Hence the idea of looking at low-frequency horizontal oscillations because they are uniquely attributable to tidal stresses. This has been possible due to the global network of permanent GNSS stations (the acronym for Global Navigation Satellite Systems, which includes both the American GPS and the European GALILEO system), that allowed to perform speed measurements between sites at thousands of kilometers distance on different tectonic plates.
Thanks to the important international services operating for at least three decades such as the International GNSS Service (IGS), to which ASI significantly contributes through its Matera Space Geodesy Center, the stations have accumulated historical series of their daily coordinates of at least 20 years required to unravel this type of analysis. Davide Zaccagnino, Francesco Vespe and Carlo Doglioni carried out the analysis of the variations in time of the speed of lengthening or convergence among the plate pairs.
Their studies have shown that the continents drift, that is, the lithospheric plates into which the planet's shell is divided, is modulated by vibrations that oscillate at the same low-frequency of the horizontal component of the solid Earth's tides. A counter-test was also made for intra-plate baselines to understand whether these oscillations persisted or not.
Precisely the negligibility found on intra-plate baselines confirmed that these astronomical forces play a decisive role in describing the motion of the continents which slowly drift toward the 'west' thanks to the horizontal shift of the solid tides with respect to the underlying mantle, along the undulated flow of plates and so-called 'tectonic equator' that makes an angle of about 30° with the geographical equator.