- © The Mineralogical Society Of America
The vesicles (bubbles) in basaltic lava flows can be used to determine paleoelevation at the time of eruption. In the repertoire of paleoelevation proxies presently available to the research community, it represents one of very few direct proxies of elevation. The technique is based on the sizes of vesicles at the tops and bottoms of lava flows. We assume that bubbles do not know a priori that they will reside in one part of the flow or another when they are erupted from a volcanic vent. As such, the mass of gas is evenly distributed throughout the flow. The volume of the bubbles will therefore depend on pressure, which at the top of the flow is just atmospheric pressure, and at the bottom is atmospheric plus hydrostatic pressure from lava overburden. Since lava thickness can be measured in the field, and bubble size distributions (most notably the modal size) can be measured in the lab, a simple relation can be solved for atmospheric pressure, and using the standard atmosphere, elevation can be determined.
The key parameters that must be determined are lava flow thickness at the time of emplacement and cooling of the upper and lower parts of the flow, and the sizes of the vesicles. Each of these requires some analysis. Measured flow thickness in the field is not necessarily the same as that during solidification of the upper and lower several cm of the flow, as inflation and deflation of the flow could have occurred, thus changing the thickness from that which provided lava overburden to determine vesicles sizes in the lower part of the flow. Lava flows must be examined in cross-section in the field to ensure that the indications of inflation or deflation are absent. Either would alter the proper vesicularity profile as a function of stratigraphic position within the flow for a given flow thickness. Only flows displaying the correct vesicularity profile can be meaningfully sampled. Vesicle sizes can be measured in a number of ways, but the most accurate is Computed X-Ray Tomography, which provides 3D information regarding size, shape, distribution, and connectivity of vesicles in a sample. We have developed numerical routines for extracting quantitative size distributions from tomographic data.
The basalt paleoaltimeter can be applied to any tectonic region in which basalts are present. We tested the method by measuring the elevation of Mauna Loa, Hawaii, and then as a first application to a tectonic issue, placed constraints on the timing of uplift of the Colorado Plateau. While further testing of this tool for paleoelevation analysis would always be helpful, the technique is sufficiently refined for general use by the geologic community, provided appropriate samples are collected from lavas demonstrating simple emplacement history, and vesicle sizes are measured with sufficient quantitative accuracy to resolve elevation to within a few hundred meters. As for any proxy, the best results can be obtained with multiple sampling, and when possible, multiple independent proxies for the most reliable determination of paleoelevation history.