What is the slope of the line?
a. -1
b. 0
c. 1
d. 20

A, Sketch of a heterogeneous plate, modelled as a cylindrically symmetric region that is thicker owing to a combination of greater depletion and lack of extension. b, Rock uplift at the edge and at the centre of the 'plateau' over 50 Myr of conductive relaxation for different values of d (w = d/10 and h = 100 km) and Tb, the initial basal temperature before mid-Tertiary time (modelled as t =  0), assumed to be the time of the thermal perturbation due to removal of the Farallon plate. The smaller symbols correspond to Te = 20 km and the larger to Te = 30 km (ref. 19). The observed range in residual rock uplift of the Colorado Plateau (Fig. 1d) is indicated by the grey regions. c, Lateral migration of the 1,200 °C isotherm at 180-km depth as a function of d and h (results shown are for Tb = 1,300 °C). The slope of the lines represents the migration rate, which is a strong function of w, as illustrated by the lines with the smallest black symbols and fastest rate of encroachment: d = 800 km and w = d/100. All other results are shown for a larger value, w = d/10. Grey bars indicate rates observed in the migration of the onset of magmatism in Fig. 2b. d, Spatiotemporal pattern of rock uplift (every 5 Myr) for d = 800 km, w = d/100, Te = 30 km and Tb = 400 °C, showing higher values at the margins than in the interior and migration of the point of highest uplift slightly in from the margins of the plateau.