New spacecraft data provide increasing evidence for a dynamic environment on present-day Mars. Exogenic processes such as impact cratering, mass wasting processes, and active dune migration have all been observed to modify the surface. No traces of current endogenic activity have been found yet, but some studies point to very localized volcanism in the last few millions of years. However, no systematic study of young volcanic surfaces had been performed so far. We present absolute model age determinations of plains volcanism on Mars as derived from impact crater size-frequency distributions. Extended areas in Tharsis, the largest volcano-tectonic region on Mars, have been resurfaced by lava flows in the last few tens of millions of years. We also present results on the rheologic properties of these lava flows, inferred from morphometric measurements. Yield strengths are in the range of 100–300 Pa, and viscosities reach values of 102 to 103 Pa s, indicating basaltic compositions. The results imply that Mars retained until recently, and probably still retains, enough internal heat to produce wide-spread plain-style volcanism, producing low-viscosity lava flows throughout large parts of Tharsis.