Brief descriptions of the vesicularity of each flow unit from Site 990A are given in Table 1. Aa flows, both in the Hawaiian drill cores and in Hole 990A cores, are characterized by having very low vesicularity (<5%) in their interior parts, which leads to high overall flow densities. This feature was noted by early observers in Hawaii (e.g., Jaggar, 1920; Macdonald, 1953) and has been documented to result from both loss of volatiles from the flow surface during lava transport (Lipman et al., 1985) and from collapse and compaction of vesicles as the lava increases in both thickness and viscosity. Thus, typically, the only low-density portion of aa flows is the highly brecciated upper flow crust. These characteristics of the flows can be seen directly in both P-wave velocity and density profiles through the Hole 990A core sequence (Fig. 2).

Pahoehoe flows, in contrast, are highly vesicular. Thin pahoehoe flows are typically vesicular throughout their entire thickness, whereas inflated pahoehoe sheet flows in general have well-defined thin lower and thick upper vesicular zones separated by a massive flow interior (Cashman and Kauahikaua, 1997). The upper vesicular zone represents flow crust generated during active flow emplacement and can be used to estimate flow emplacement duration (Hon et al., 1994).

A brief qualitative summary of groundmass textures preserved in Hole 990A flow units is given in Table 1. Quantitative analysis and SEM imaging have been performed for only selective flow units, results of which are included in Table 1, where appropriate.