PALEOELEVATION RECORDED IN VESICULAR BASALTS

It is possible to measure the amount and timing of plateau uplift using size distributions of vesicles in basaltic lava flows. The rapidly cooled top and bottom of a flow preserved the erupted bubble population, so bubbles at the top are "frozen in" at atmospheric pressure, while those at the bottom have an additional hydrostatic pressure from the lava overburden. The modal size of the vesicle (bubble) population is larger at the top than at the bottom. This leads directly to paleoatmospheric pressure and thus elevation because the thickness of the flow can easily be measured in the field, and the vesicle sizes can be measured in the lab. Because this technique records paleoatmospheric pressure, it is not subject to uncertainties stemming from the use of climate-sensitive proxies, although like all measurements, has its own sources of potential error. To use the technique most effectively, it is essential to identify preserved flows in the field that show evidence of simple emplacement and solidification. This can be determined by the bulk vesicularity and size distribution as a function of stratigraphic position within the flow. By examining the stratigraphic variability of vesicularity, emplacement processes can thus be reconstructed. It is critical to be able to accurately measure the size distribution in collected samples from the tops and bottoms of flows because our method is based on the modal size of the vesicle population. Our analytical techniques involving High-Resolution X-ray Computed Tomography (HRXCT) allow us to analyze the large number of samples required for reliable interpretations. We have tested our technique on basalts emplaced at known elevations at the base, flanks, and summit of Mauna Loa, and applied them to the Colorado Plateau.