The effect of cracks and mineralogy on the seismic velocities of subaerially erupted basalts from the Southeast Greenland margin were modeled using the pore aspect ratio theory of Kuster and Toksöz (1974). The influence that cracks, or voids, have on the seismic velocities was separated from the effects of the apparent grain moduli. The principal results of this research can be summarized as follows:

1. Crack modeling of the seismic velocities of slightly altered samples from the interiors of lava flows from Hole 990A showed similar aspect ratio spectra among the samples. Forward calculations using an average, normalized aspect ratio spectrum (Table 3) fit the seismic velocity data for each sample to within an RMS misfit of <0.050 km s^-1 (<2%).
2. The percentage of total porosity with aspect ratios 0.01 (thin cracks) is lower in the more altered samples than in the less altered ones, even though the bulk porosity is higher in the more altered samples. This suggests that low aspect ratio pores (cracks) are the first portion of the pore space to be filled by alteration products.
3. The addition of low grain density alteration products (e.g., clays) into a basalt lowers the grain density and apparent grain bulk and shear moduli of the rock and, thus, lowers its P- and S-wave velocities.