Details of plagioclase grain size in a series of tabular basaltic intrusions (sills), together with the Holyoke Flood basalt lava flow and the prehistoric Makapuhi lava lake.

For each body, a suite of samples was collected to form a vertical traverse through the full height of the exposure to permit detailed microstructural analysis using optical microscopic examination of standard thin sections.

The plagioclase grain size was measured as the long axis length of each grain intersection in a thin section. The population of long axis lengths viewed in thin section can be analysed to constrain the true 3D shape of the grains only if the grains all have the same shape, which is unlikely for a sample of natural basalt. Accordingly, in the associated paper, only the arithmetic mean of this population is reported for each sample.