LAVA FLOW VESICULARITY ON EARTH AND ON MARS CONTROLLED BY EQUILIBRIUM DE-GASSING

Efforts to interpret the significance of vesicles in lava flows on Mars have resulted in insights into the process of vesiculation in terrestrial volcanoes and lava flows. Volcanic vents on Earth and Mars erupted under current environmental conditions are predicted to have significantly different landforms but the associated lavas are predicted to have similar vesicularity and morphology.

Based on a comparison of the vesicularity of the surface of a lava flows on Earth and on Mars, it is proposed that the vesicularity of lava flow surfaces is mainly a function of an initial lava vesicularity (volume of gas per unit volume of lava) that is established by degassing during eruption to a gas volume significantly less than that necessary for fragmentation (<70 percent). Vesicles are bubbles of magmatic volatiles (largely H2O, CO2, CO) exsolved on ascent and decompression from the melt source and by crystallization of anhydrous minerals isobarically during initial solidification and crystal growth. The bubbles are trapped within lava flows and pyroclasts after the enclosing silicate liquid cools and solidifies.

Differences in vesicularity in lava sections on Earth and Mars are expected to be relatively sensitive to differences in atmospheric pressure and manifested mainly as differences in the preserved gradients of vesicularity with depth. Also, samples of initial vesicularity preserved in the near surface of lava flows are characterized by size-distributions that differ from distributions in samples from lava flow interiors. The differences in vesicle population follow that expected from an initial diffusively grown population and one modified through coalescence. Tabular expanses of differing mechanical properties in lava field are one expected result.

Some characteristics of lava flows observed in MRO/HiRISE images and from MER surface observations, such as expanses of platy lavas and extremely vesicular basalt clasts on the surface, can result from widespread vesicle segregation influenced by equilibrium degassing in an environment similar to that on Mars today.