Explosive Volcanism on Hecates Tholus, Mars' Investigation of Eruption Conditions

Evidence is presented supporting the identification of a discrete, well-preserved air fall deposit generated by explosive volcanism on Mars. The deposit is located immediately to the west of the summit caldera of Hecates Tholus (32øN, 209øW), the northernmost of the three volcanic constructs in the Elysium Planitia region. The absence of superposed impact craters larger than the resolution limit of the Viking images (40 m per pixel) indicates a very young age for this eruption, which evidently postdates even the most recent collapse episode of the Olympus Mons caldera (--•300 m.y.B.P.; Neukum and Hiller (1981)). From the distribution of craters smaller than 2 km in diameter, it appears that the air fall material mantles an area about 50 x 75 km in extent to an estimated thickness of about 100 m. By adapting numerical models for terrestrial volcanic eruptions to the Martian environment, these dimensions imply an eruption cloud height of about 70 km. In order to attain such an altitude, the mass eruption rate must have been --• 107 kg/s and the volatile content either --• 1 wt % for H20 or --•2 wt % for CO2. Although it is not possible to distinguish between silicic and basaltic volcanism on Hecates, were the volatile species CO2, the depth of most recent storage prior to eruption of the magma would have to be 50-100 km (due to the solubility of CO2 in the melt). If H20 were the driving volatile, a minimum depth of storage would be 0.2-4 km, thereby permitting the possibility of ground water assimilation by the magma.