The detailed structure and morphology of the TAG active hydrothermal mound (Mid-Atlantic Ridge) are investigated using near-bottom, high-resolution bathymetry data and digital photographic imagery. The mound comprises two distinct circular platforms with one superposed asymmetrically on the other. Areas of hydrothermal discharge and a surface depression are identified that have not been previously described indicating that the hydrogeology of the system is modified on very short time scales (years). We propose a model for the growth of the TAG active mound in which changes in the near-surface permeability of the mound caused by both fissuring and mineral precipitation result in the development of a structure with a surface morphology consistent with that observed. ¿ American Geophysical Union 1996