The time-dependent variability of the physical properties of upper oceanic crustal rocks have been characterized by compiling acoustic velocity, density, porosity, iron oxidation state, and H2O+ data from the upper 50 m of all Deep Sea Drilling Project and Ocean Drilling Program drill holes which recovered basalt samples from normal oceanic basement. This data compilation is interpreted in terms of the geological processes responsible for these changes as the seafloor ages. Drill site averages of both acoustic velocity and bulk density show a decrease with increasing age for crustal ages less than 100 Ma. Similarly, chemical data from the same drill cores indicate the presence of two distinct alteration processes. Oxidation of ferrous to ferric ions within the rock-forming minerals proceeds rapidly within the first 5 Ma after formation, slows in the subsequent 5- to 20-m.y. period, and then appears to stop. Hydration of the silicate minerals also appears to generally increase with age over the full 150-m.y. age range of the samples, although at a slower rate than oxidation. The observed distribution of the physical properties of the upper crustal rocks for the first 100 Ma can be explained by three processes: (1) a large initial variability of Vp and density, caused by the range of original porosity of the rocks, (2) an age-progressive alteration associated with oxidation that systematically reduces both Vp and density with increasing age for the first 5 m.y., and (3) an additional, nonoxidative alteration that increases the hydration of the silicate minerals, with a resulting major impact on the physical properties. Still unexplained, however, is the lack of an expected trend of decreasing porosity with increasing age and alteration and the large, nonsystematic scatter in the physical property data for samples older than 100 Ma.