Thermal History of the Earth: On the Importance of Surface Processes and the Size of Tectonic Plates

Abstract

The Earth's interior has been cooling over the past 3 billion years. Chemical analyses of ancient rocks show that the mantle was then about 250 degrees hotter than today, corresponding to a fairly low amount of heat released until now. However, when the mantle was hotter in the past, it was less viscous, and the dragging forces on tectonic plates were weak: plate motion was probably much faster. Heat from the Earth's interior is released mainly at oceanic ridges, where new seafloor is created. The faster the plates move, the more heat is released. How so little heat has been released with significantly faster plates in the past? Several authors proposed complex phenomena playing on plate tectonics to "slow down" plates in the past. Here, we propose a new model including surface processes controlling plates creation and disappearance, leading to evolving plate sizes. We show that the geometry of plates is a key factor: for a hotter Earth in the past, plates were larger, so that ridges were less numerous and a moderate amount of heat was released, even if plates were faster. This new paradigm (larger plates in the past) is compatible with observations in geological data.

Experimental Research on Electrical Conductivity of the Olivine-Ilmenite System at High Temperatures and High Pressures

“…The conductivities of the olivine aggregates with various ilmenite contents can be applied to invert the magnetotelluric (MT) profiles, and further research electrical structures of the Earth's upper mantle. Combining with the electrical conductivities of olivine-ilmenite systems and the thermal models of the Earth's continental upper mantle and oceanic upper mantle (Grigné and Combes, 2020), the electrical conductivity-depth profiles of the olivine aggregates with various ilmenite contents in the continental and oceanic upper mantles were constructed in detail (Figures 12A−C). As shown in Figure 12, the conductivities of dry olivine aggregates with a certain content of ilmenites at the same depth beneath the continental plate and oceanic plate were different due to the various thermodynamic conditions.…”

“…FIGURE 12 |The thermal models for the Earth's continental upper mantle and oceanic upper mantle (A)(Grigné and Combes, 2020), which were used to construct electrical conductivity-depth profiles for olivine aggregates with various ilmenite contents in the Earth's oceanic upper mantle (B) and continental upper mantle (C). Electrical conductivities of olivine aggregates were calculated based on the thermodynamic parameters of the (Continued )…”