Models of Continent & Mantle Evolution

A simple computer study shows how the very land we stand on (brown), and the ocean basins (blue water drawn in) can be modelled with double diffusion.  A brown low-density fluid (different composition) representing the continents diffuses down into denser fluid (clear) and collects in clusters along the top because of convection cells (closed streamlines). These are buoyantly driven by temperature (red isotherms) that is hot at the bottom and cold at the top.  Clusters modify the convection cells so that each cold sinking region is split apart. The clusters become shaped like our continents with constant interior thickness and thickening near the edges. Edge surface elevation is consistent with mountains. The cold sinking (subducting) fluid at cluster flanks thickens the edges and makes each cluster and the dense fluid below it cold like continents and their cold basements. Between the clusters, warm denser fluid rises from the bottom and spreads apart at the top with the surface elevated in the center like mid-ocean ridges. If fluid is internally heated, or if the flow is slowly changing as in this figure, a warm plume slowly forms deep below each cluster and rises, splitting each cluster apart. They drift back and forth laterally and continue to form and re-form like the supercontinent cycle. The temperature and density (from different chemicals) diffuse at different rates in a liquid, all other physical properties are constant. The general area of study is called double diffusive convection that was discovered at Woods Hole in the 1950’s for ocean water, whose density changes with temperature and salinity. Although scores of applications for double diffusion are now known, the movement of continents and the formation of ocean basins is a new application. (Summary and image provided by Jack Whitehead)