Formation of degree 1 structure for mobile-lid convection.

In this model, mantle convection always goes to degree-1 planform, provided that the upper mantle is ~300 times weaker than the top thermal boundary layer and ~30 times weaker than the lower mantle, independent of internal heating rate (30%-60% being tested), initial conditions (random perturbation, l=3, l=6, or l=100), and Ra number (the largest Ra tested that leads to ~6 cm/yr surface velocity). We believe that such a degree-1 planform is responsible for supercontinent formation, that is, all the continents are attracted to the downwelling hemisphere.