DISCUSSION AND REPLY Analysis of Vesicular Basalts and Lava Emplacement Processes for Application as a Paleobarometer/Paleoaltimeter: A Discussion

Sahagian et al. (2002a, 2002b) have developed a methodology for determining paleoelevations of highlands using observed vesicularities of lava flows. Since vesicular lavas preserve a record of paleoatmospheric pressure during the time of emplacement, they can be used to directly infer paleoelevations. Their model could act as a powerful tool, particularly in view of the uncertainties induced by other methods. As their data suggest, elevations could be estimated within ∼400 m. However, their work also raises the issue of emplacement of pahoehoe lobes and flows. Sahagian et al. (2002a, 2002b) state that only flows with a simple emplacement history can be used to estimate paleoelevations from paleopressures. Critical to their model is the flow thickness at the time the bubbles at the top and bottom of the flow solidified. It is important that the upper and lower 10 cm or so of the flow solidified after the flow reached its final thickness. The question is whether the flow or flow lobe presently observable in the field was originally emplaced with the same thickness or whether it has undergone endogenous growth or deflation. To infer flow emplacement history, Sahagian et al. (2002a, 2002b) rely on a comparison of the observed vesicularity profile with “ideal” vesicularity (fig. 1 in Sahagian et al. 2002a), as discussed by Sahagian (1985) and Sahagian et al. (1989). According to them, multiple layers of small vesicles in a relatively thick flow are suggestive of inflation, while a vesicle profile similar to that in their figure 1 (Sahagian et al. 2002a) suggests a simple, noninflated mode of emplacement. They did not provide information about the vesicularity profiles of the flows used in their