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.