Why should one half of our world be losing its internal heat like a runny kettle, and the other retain it like a snug thermos? That is the mystery geophysicists have been trying to solve with a new set of geodynamic models, and the solution, it seems, lies in the interaction between seafloor geology, continental insulation, and tectonic motion over hundreds of millions of years.
A University of Oslo study has found that the Pacific hemisphere cooled about 50 Kelvin more than the African hemisphere in the last 400 million years. The researchers utilized a computational reconstruction going almost twice as far back in time as previously done, integrating datasets of seafloor age, positions of continents, and lithospheric properties into a half-degree global grid. This enabled them to compute the thermal budget of every grid cell and monitor the way heat loss changed between the two hemispheres.
The difference ultimately comes down to the physics of heat loss through various kinds of lithosphere. Oceanic lithosphere, which dominates the Pacific hemisphere, is thin and topped by massive amounts of cold seawater. This arrangement encourages conductive and convective cooling to be rapid, particularly along mid-ocean ridges where hydrothermal circulation can drain heat much more effectively than conduction alone. Oceanic heat flow models, ranging from the basic Half-Space Cooling and GDH1 plate models to more recent fractal-density models, demonstrate young seafloor loses heat at rates much higher than continental crust, which is a thermal blanket.
The African hemisphere, on the other hand, has a greater proportion of continental lithosphere. Thick continental plates are buoyant and possess low thermal conductivity, and thus they can retain mantle heat for thousands of millions of years. This “continental insulation” is a key process within the supercontinent cycle wherein accreted landmasses change the patterns of mantle convection, elevate geotherms of continents, and even cause mantle plumes. In past arrangements like Pangaea, this insulation potentially retained the mantle of the African hemisphere warmer for longer, thereby reducing its cooling rate.
The new modelling also crosses paths with our knowledge of plate tectonics and mantle convection. Mantle convection is a large heat engine, where upwellings at ridges and downwellings at subduction zones power plate movements. The “consistently higher plate velocities” of the Pacific hemisphere in the past 400 million years, noted by the study, indicate that it started that period with a warmer mantle. More hot mantle material is less viscous, facilitating more rapid motion of plates, but also loses heat more rapidly through the thin oceanic lid. This is compatible with grain-damage models of plate boundary formation, which assert that hotter mantle can still support mobile-lid convection, albeit in a more sluggish, drip-like subduction mode.
Heat loss from the seafloor is non-uniform, and hydrothermal systems at spreading centers have a disproportionate influence. Black smoker hydrothermal vents and ridge-flank circulation can abstract so much heat that conductive cooling models overestimate observed heat flow unless these advective mechanisms are accounted for. The CHABLIS model and its variants include basal heat flux from the asthenosphere to more closely fit measurements, particularly in older lithosphere. In the Pacific, with its large ridge systems and fast spreading rates, these mechanisms have been very effective at bleeding off internal heat.
The hemispheric cooling asymmetry has also profound implications for deep time. Supercontinent assembly and break-up repositions subduction zones, which can concentrate mantle return flow and produce large igneous provinces (LIPs). Some of the largest oceanic LIPs, including the Ontong Java Plateau, were emplaced in the Pacific basin following the break-up of Pangaea and may have been facilitated by the same intense mantle circulation that hastened heat loss. These magmatic processes not only recraft ocean basins but potentially affect climate and biosphere by releasing enormous volcanic gas volumes.
Finally, the study’s longer 400-million-year window emphasizes that terrestrial thermal evolution is not a homogeneous, global process. It is conditioned by the changing mosaic of plates, differences in thermal properties between continents and oceans, and dynamic regimes of mantle convection underneath. The Pacific hemisphere’s sudden cooling is a testament to how planetary heat loss is inscribed in the fabric of plate tectonics, from the grain scale in shear zones to the scale of whole hemispheres.
