VARIATIONS IN VESICULATION AND LAVA FLOW EMPLACEMENT, EARTH AND MARS

Terrestrial field study of the record of volatiles in lava flows is applicable to the general process of vesiculation and lava flow emplacement on planetary surfaces. Vice versa, efforts to understand the influence of planetary environments on lava flows foster efforts to assess the role of volatiles in terrestrial lava flows. Substantial differences in influences on emplacement of lava flows are predicted for plains lavas and lava flows on the flanks of large volcanoes on Mars.

Vesicle dimensions within lava flows are a function of ini-tial lava flow volatile retention, pressure at the crust-melt interface, and rate of solidification (time available for bubble coalescence). The gas law controls the vesicularity (ratio of the volume of vesicles to volume of lava) at a given level within a lava flow. Be-cause the pressure that satisfies the ideal gas law has both atmospheric and hydrostatic terms, differences in vesicularity are expected to be relatively sensitive to differences in atmospheric pressure in the upper lava section.

The atmospheric pressure of Mars is two orders of magnitude less than that on Earth. Terrestrial lava volatile abundances (~0.05 wt.%) in Martian lavas would exceed that required for surface fragmentation (violent frothing). Surface observations of vesicularity of surface rocks on Mars are limited, but current data suggest that surface vesicularities are not much greater than that on Earth. Dynamic equilibrium at the vent, the level at which volatiles are less than that necessary to drive pyroclastic eruption, is likely to be an important control in yielding this similarity. While volatiles in terrestrial lava flows may be several hundred ppm, equivalent values for Mars, all else being assumed equal, are several ppm. The result is similar initial surface vesicularities, but a substantially thinner vesicular zone and thicker vesicle-free interior for Martian lavas under current atmospheric conditions. Significant variations in the upper vesicle zone thickness can affect thermal and mechanical properties of lava flows, potentially accounting for many differences in lava flow characteristics unrelated to melt viscosity and temperature.