Age dependent porosity of young upper oceanic crust: Insights from seafloor gravity studies of recent volcanic eruptions

An October 1997 ALVIN program collected a series of seafloor gravity stations over the February, 1996, volcanic eruption site on the northern Gorda Ridge (46°41′N, 126°47′W). These measurements yielded estimates of the density and porosity of mid‐ocean ridge crust in the first few decades following formation. The estimated crustal density for this pillow eruption of 2270 ± 260 kg/m3 is very low, but in good agreement with the estimated densities of other recent seafloor volcanic flows. Rock matrix densities, measured from hand samples retrieved from the study area, give a bulk porosity for this flow of 34% ± 16%. This data, when included with similar estimates of upper crustal porosity, argue for a rapid evolution of upper crustal porosities from very high values (≥30%) for recent eruptions to much lower values (∼10–15%) where the seafloor is older than 0.5 million years. These variations of density and porosity are of a magnitude consistent with previously observed changes in seismic compressional wave velocity and permeability of ocean crust.