Volcanoes serve an important role in the geological and atmospheric evolution of planetary bodies by providing new material to the crust and adding volcanic gases to the atmosphere. On Earth, H2O emitted by volcanoes is negligible compared to the total amount of water in the atmosphere, but on Mars, the amount of water in the atmosphere is about 10,000 times smaller, meaning that larger eruptions can have a much larger effect. Modeling of Cerberus plains volcanism has suggested possible lava effusion rates up to 106 m3/s and assuming a water content of 1.8% this would translate to a water vapor output rate of 1.8 × 107 kg/s, or a doubling of the atmospheric water content in less than one day of eruption. This amount of water would quickly overwhelm the atmosphere’s holding capacity and cause ice to accumulate on the surface. Creation of sulfuric acid from the combination of H2O and SO2 would also have a profound cooling effect on the atmosphere, which would increase the likelihood of ice accumulation. We will determine the effect of large Amazonian volcanic eruptions on the martian climate by: (1) developing improved constraints on the distribution, thicknesses, and ages of lava units in the Cerberus plains region and estimate eruption fluxes; (2) simulating the effect of ash on the condensation of water and the accumulation of ice; (3) simulating the effect of fissure eruptions on the Amazonian climate. The key to completing these objectives will be modeling of volcanic effects on the martian atmosphere using the Laboratoire de Météorologie Dynamique Global Climate Model (LMD-GCM). This model has the ability to advect dust particles, ash particles, and other condensation nuclei as separate tracers, each defined by a mass-mixing ratio and number density. Particles can be transformed into condensation nuclei, advected inside ice crystals, and sedimented from the atmosphere. The LMD-GCM also has the capability to model the radiative effect of sulfur gases and aerosols on the climate. By combining these two capabilities, we propose to simultaneously model the effect of a sulfur and water-rich volcanic eruption on the climate and the resulting distribution of ice on the surface. The Elysium Volcanic Province provides a test-bed for the exploration of the behavior of erupted water vapor and its effect on the Amazonian ice budget. Input constraints of eruption fluxes will be primarily developed at the University of Arizona in Years 1 and 2, and the GCM modeling will be primarily completed at JPL in Years 2 and 3.