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New spacecraft data provide increasing evidence for \na dynamic environment on present�day Mars. Exogenic \nprocesses such as impact cratering, mass wasting processes, \nand active dune migration have all been observed to modify \nthe surface. No traces of current endogenic activity have been \nfound yet, but some studies point to very localized volcanism \nin the last few millions of years. However, no systematic \nstudy of young volcanic surfaces had been performed so far. \nWe present absolute model age determinations of plains \nvolcanism on Mars as derived from impact crater sizefrequency \ndistributions. Extended areas in Tharsis, the largest \nvolcano�tectonic region on Mars, have been resurfaced by \nlava flows in the last few tens of millions of years. We also \npresent results on the rheologic properties of these lava flows, \ninferred from morphometric measurements. Yield strengths \nare in the range of 100�300 Pa, and viscosities reach values \nof 10^2 to 10^3 Pa s, indicating basaltic compositions. The \nresults imply that Mars retained until recently, and probably \nstill retains, enough internal heat to produce wide�spread \nplain�style volcanism, producing low�viscosity lava flows \nthroughout large parts of Tharsis