Fig. 5

Free-slip versus free surface steady-state stagnant lid. Small-scale convection experiments l i d C2 with a free-slip surface (left hand side column) and l i d C5 with a free surface (right hand side column) both employing a composite yield stress with a yield stress gradient of Δ σ _y =0.0054 and a dimensional, constant yield stress of σ _y,const=8.7 MPa. a Surface topography of the free-slip top boundary model that is calculated from the normal stress occurring at the vertically fixed, top domain boundary and f the actual topography that occurs in the model with the free top boundary. b, g Effective viscosity and the 700 K contour (black line), c, h second invariant of the strain rate, d, i second invariant of stress and e, j horizontal velocity are shown for both models, where green colours indicate flow in clockwise direction. If applied, the sticky-air layer is removed visually and replaced with a hatched grey area. Very high values are clipped in regions where they exceed a certain maximum value (black areas). White arrows and grey streamlines indicate the flow direction and pattern