Mafic volcanism is and has been the dominant type of volcanic activity on Earth and Mars and both environments have featured episodes of mega-scale basaltic flood lava eruptions throughout their geological history (e.g., Head and Coffin, 1997). On Earth typical flood lava events have volumes of 100-1000 km3 and may last for months to decades. Recurring eruptions have constructed Large Igneous Provinces with volumes of 105-106 km3 over periods as short as 0.5-1 Myr. Estimates on the volatile mass released by a terrestrial flood lava eruption is staggering or ~10.000 megaton (Mt) SO2 and ~1900 Mt HF+HCl per 1000 km3 of magma erupted. The amount of H2O and CO2 released is likely of the same magnitude as the SO2 emissions. More than 70% of this volatile mass was liberated at the vents and lofted by the eruption columns to stratospheric altitudes where they produced aerosol plumes that most likely had global dispersal. Subsequently, these events may have produced atmospheric perturbations of the magnitude predicted for a severe "volcanic" winter that lasted for decades (Thordarson and Self, 1996). Also, judging from the total volume of terrestrial LIPs, the volcanism that produced them is likely to have released 109-1010 Mt of volatiles (i.e. SO2, H2O, CO2, and HF+HCl) into the atmosphere during times of peak activity. Flood lava eruptions and episodes of similar and larger magnitude are known from the geologic past on Mars (e.g., Keszthelyi et al., 2000; Head and Coffin, 1997). Consequently, the large-scale degassing associated with these events and episodes may not only have contributed significantly to the development of the atmosphere on both planets, but may also have resulted in major shifts of the planetary climate.

Head, J. W. and Coffin M., 1997. Large Igneous Provinces: A planetary perspective. In: J.J. Mahoney and M.F. Coffin (Eds), Large Igneous Provinces: Continental, Oceanic, and Planetary Flood Volcanism. AGU Geophysical Monograph. AGU, Washington D.C.: 411-438. Keszthelyi, L., McEwen, A.S. and Thordarson, T., 2000. Terrestrial analogs and thermal models for Martian flood lavas. J. Geophys. Res., 105: 15027- 15049. Thordarson, T. and Self, S., 1996. Sulfur, Chlorine and fluorine degassing and atmospheric loading by the Roza eruption, Columbia River Basalt group, Washington, USA. J. Volc. Geotherm.Res., 74: 49-73.