The ocean floor gets deeper as the age of the underlying tectonic plate increases. This phenomenon results from the cooling and thickening of the plate as it moves away from the mid-ocean ridge where it formed. However, rather than deepening forever, after the plate reaches ages of 70–100 million years, the ocean floor stops subsiding. This 'seafloor flattening' signal suggests that some process must be resupplying heat to the base of old oceanic plates, preventing them from cooling further, but what exactly this process is remains highly debated.

Here, we use computer models and multiple geophysical datasets to test which process best accounts for observed flattening. We find that, while deep mantle flow and interactions between the plate and mantle plumes can explain localised ocean depth anomalies, only small-scale convection—whereby cold rock at the plate's base becomes unstable, drips off, and is replaced by warm upwelling rock—can reproduce the global scale of the signal. We also find that the onset of small-scale convection appears to be delayed if mantle temperatures were high when the plate formed, suggesting deeper and more extensive mantle melting may freeze a thicker stabilising layer of dry, sticky, and light residual rock into the cooling plate.