An Alternative Explanation for Recent Volcanism on Mars

Although Mars is significantly smaller than the Earth and therefore should have cooled more quickly, new mapping of the Martian surface shows that the planet was volcanically active even within the last few millions years. Evidence for this are some young lava flows in the Tharsis and Elysium regions. Most often this recent volcanic activity is explained by mantle plumes which transport hot material from the deep mantle and produce melt by pressure released melting. However, on Mars the development of plumes and even more their continued sustainment is difficult to reconcile with what we know from Martian interior dynamics as the temperature difference at the core-mantle boundary necessary to create a plume decreases rapidly during the first few hundred million years of the evolution. For today's conditions the temperature jump across the lower thermal boundary layer is between 20 and 90 K depending on the depth dependence of mantle parameters like viscosity and thermal conductivity. The excess temperature of a potential plume compared to its surroundings at the base of the stagnant lid is even less by a few tens of K due to the adiabatic temperature decrease of rising material and the heat consumption at the exothermic mantle phase transitions. An alternative explanation for recent volcanism is the influence of a low conducting crust with variable thickness on the temperature distribution in the upper mantle. In regions of thicker crust than the average the underlying mantle is efficiently insulated resulting in increased temperatures and the generation of partial melt. For Mars, a thick crust is in particular suggested underneath the large volcanic regions of Tharsis and Elysium. To test this hypothesis we have combined thermal evolution models with 2-D heat conduction calculations to examine the occurrence of a probable partial melt zone due to lateral variations in crustal thickness and topography. It is shown that the inefficient heat transport results in a local partial melt zone at the base of the stagnant lid. This partial melt may rise in form of diapirs through the lower and ductile part of the stagnant lid. Thus, recent volcanism may simply be due to the existence of the low conducting crust and not to the influence of the convecting mantle by a hot upwelling plume