A quantitative analysis method is developed to objectively identify significant bathymetric changes between multibeam sonar surveys of the same seafloor area. The technique involves mathematical gridding of bathymetric surveys from different years, co-registration of grids based on prominent features covered in both surveys, calculation of slope-weighted depth difference grids, and determination of the boundaries of statistically significant differences. The resulting difference grids are used in interpreting geological processes and calculating the volume of new material added to the seafloor. The method is applied to data from the southern Juan de Fuca Ridge, where a seafloor volcanic eruption of 0.05 km3 is inferred to have occurred between 1983 and 1987. Camera-tow data and submersible observations provide ground truth which confirms the validity of the quantitative analysis method, and indicates empirically that the detection limit of the method allows new seafloor landforms measuring 1¿105 m2 area (or about 200--300 m in diameter) and 5--15 m in thickness to be detected through repeated Sea Beam bathymetric surveys. ¿ American Geophysical Union 1992