Example of the "memory effect" present in the stagnant-lid carbon cycle. The blue line shows the age of the crustal layers upon reaching the decarbonation depth z_decarb, and therefore the time that has passed since the weathering rate was "recorded" into the crust. The shaded region marks where the crust is not yet old enough to reach the decarbonation depth. At 2.3 Gyr, the surface begins to heat up and thus younger and younger crust decarbonates as the decarbonation depth decreases. This example correspond to the Earth-like model shown in Fig. 1a–c.