Rocks on the floor of Gusev crater are basaltic in composition, as determined from measurements by the Mars Exploration Rover, Spirit. On the basis of compositional data, models of the basaltic lavas at the time of their emplacement suggest viscosities of 2.3 to 50 Pa ¿ s (dependent on the number of phenocrysts and vesicles that were present), which would be more fluid than terrestrial tholeiitic lavas and comparable to mare lavas on the Moon or Archean high-Mg basalts on Earth. Morphological data and crater counts derived from the High Resolution Stereo Camera on Mars Express and other orbiters suggest that the lavas flooded Gusev crater at about 3.65 b.y. and postdate older floor materials, such as putative sediments emplaced by Ma'adim Vallis.