Ocean basins cover two-thirds of our planet’s surface. They are made of oceanic lithosphere, which is continuously being created from upwelling asthenospheric mantle at mid-oceanic spreading ridges, and destroyed at subduction zones where it sinks back into the mantle. This behaviour happens sufficiently rapidly and over large enough lengthscales that the entire ocean floor gets replaced every ~200 million years.
Oceanic lithosphere facilitates a lot of the communication between Earth’s surface and its deep interior. All across the seafloor, heat escapes to the surface that is either left over from the planet’s formation or has been produced by radioactive decay. Every year, approximately 20 km3 of volcanic melts are produced at spreading ridges, exposing minerals to surface weathering and releasing gasses into the oceans and atmosphere. At subduction zones, sediments and water bound up in minerals form part of the return chemical flux into the mantle. Given its fundamental role in modulating thermal and biochemical cycles, it comes as no surprise that geologists and geophysicists have dedicated a lot of energy to studying oceanic lithosphere over the last half a century.
In an invited review paper in Physics of the Earth and Planetary Interiors (PDF), we have synthesised the disparate and diverse observations that have been widely used to constrain the structure and dynamics of the oceanic lithosphere-asthenosphere system. We re-examine models of lithospheric cooling that have been put forward to explain its thermal evolution, and compare and contrast observed behaviours between each of the major ocean basins. Despite considerable recent success reconciling laboratory estimates of the thermal properties of mantle rocks with observed basement depths, heat flow measurements, basalt geochemistry, gravity and seismic studies, there remain several significant controversies and open questions within this research field. What process is responsible for resupplying heat beneath older portions of the oceanic plate? Why do cooling models that incorporate the thermodynamics of mineral phase changes not yield good fits to observational datasets? And can we build a new generation of models that are capable of accurately capturing the dynamics of mantle convection and flow in the thermal boundary layer? We hope that this review paper will provide a helpful summary of the progress that has been made so far in understanding the oceanic lithosphere-asthenosphere system, and a useful starting point to discuss some of the outstanding challenges that remain.