Many of the tephra fall layers from Leg 165 sites have been altered to clay minerals or indurated by the process of cementation. It is not possible, therefore, to determine their overall grain-size characteristics. However, all the layers contain unaltered phenocrysts of feldspar, biotite, hornblende, or pyroxene. In this study we have used the size of the largest feldspar crystals as a measure of the grain-size characteristics of the layers and consequently, an indicator of the dispersal efficiency of the source eruption. The advantage of this approach is that it considers the settling behavior of grains with a uniform density and relatively simple shapes. In contrast, the more abundant, but often altered, glass shards have complex shapes that can significantly affect their settling rate in the atmosphere and make size comparisons between layers difficult (Wilson and Huang, 1979).
Tephra layers were selected randomly throughout the Miocene to Holocene sequences of Sites 998, 999, and 1000. Only samples from the base of the layers, where the grain size is typically largest, were used for the study. Grain mounts were prepared by treating the tephra samples with acetic acid, disaggregating them with an ultrasonic cleaner, and then sieving the sample to isolate the largest grain-size fraction. The 20 largest feldspar crystals in each slide were measured individually (longest and intermediate axes) using the National Institute of Health image analysis program. Crystal images were produced using a Zeiss Axioscopt petrographic microscope with a high-resolution charge coupled device video camera. Calibration of the video image allowed size measurements to be made with a precision of ±5 µm.