<P> An unusual type of flow feature occurs in the Cerberus plains south of Elysium and in Amazonis . These flows have a broken platey texture, consisting of dark, kilometer - scale slabs embedded in a light - toned matrix . They have been attributed to rafted slabs of solidified lava floating on a still - molten subsurface . Others have claimed the broken slabs represent pack ice that froze over a sea that pooled in the area after massive releases of groundwater from the Cerberus Fossae area . </P> <P> The second type of volcanic plains (ridged plains) are characterized by abundant wrinkle ridges . Volcanic flow features are rare or absent . The ridged plains are believed to be regions of extensive flood basalts, by analogy with the lunar maria . Ridged plains make up about 30% of the Martian surface and are most prominent in Lunae, Hesperia, and Malea Plana, as well as throughout much of the northern lowlands . Ridged plains are all Hesperian in age and represent a style of volcanism globally predominant during that time period . The Hesperian Period is named after the ridged plains in Hesperia Planum . </P> <P> Scientists have never recorded an active volcano eruption on the surface of Mars; moreover, searches for thermal signatures and surface changes within the last decade have not yielded any positive evidence for active volcanism . </P> <P> However, the European Space Agency's Mars Express orbiter photographed lava flows interpreted in 2004 to have occurred within the past two million years, suggesting a relatively recent geologic activity . An updated study in 2011 estimated that the youngest lava flows occurred in the last few tens of millions of years . The authors consider this age makes it possible that Mars is not yet volcanically extinct . </P>

What is the principal reason for the lack of geological activity on mars today