Recently the Orbiter Camera on the Mars Global Surveyor imaged on some lava flows. The sparse cratering and thence young age of these flows indicates that volcanic activity on Mars is long-lived and perhaps pulsating. These pulses beg for an explanation, not yet provided by one-dimensional convection models simulating the thermal and compositional evolution of the Martian interior. We have, for the first time, simulated the thermochemical evolution of Mars with a two-dimensional model for the entire planetary history. Our results show that variable thermal conductivity k(T, P) can maintain sublithospheric "hot spots" over time scales of several hundred Ma and is essential in the focussing of hot mantle upwellings at almost the same location. This scenario can explain the persistent pulsating and focussed volcanism in the Tharsis region.