LARGE-SCALE POROUS FEATURES IN THE CALDERA OF A SEAMOUNT

Seamounts have relatively high porosities in their upper crustal sections. At Axial Seamount, gravity surveys were conducted using a Bell Gravity meter within Alvin to determine a porosity of 23% for the uppermost 100 m of the summit and summit caldera. On the caldera floor, higher porosities are inferred for areas of active hydrothermal venting. Seafloor observations and models indicate this porosity is a result of large-scale volcanic and tectonic features such as lava tubes and cracks greater than 10 m across, lava drain backs, or regions of open pillow basalts in the near sub-surface. These high porosity zones can impact the sub-surface permeability, and models of hydrothermal upflow zones appear to correlate with observed localized gravity anomalies. The variety of hydrothermal alteration, hydrothermally active areas, and open porous features, all appears to be related to the high porosity that is inferred from geophysical measurements on this active seafloor volcano.