  With the purpose of modelling the process of mountain building, we
investigate the evolution of the ridge produced by the convergent motion of a
system consisting of two layers of liquids that differ in density and viscosity
to simulate the crust and the upper mantle that form a lithospheric plate. We
assume that the motion is driven by basal traction. Assuming isostasy, we
derive a nonlinear differential equation for the evolution of the thickness of
the crust. We solve this equation numerically to obtain the profile of the
range. We find an approximate self-similar solution that describes reasonably
well the process and predicts simple scaling laws for the height and width of
the range as well as the shape of the transversal profile. We compare the
theoretical results with the profiles of real mountain belts and find and
excellent agreement.
