 Basaltic shield volcanoes typically feature two main rift zones, situated with angles of 120° between in ideal situations.On shield volcanoes forming from level seafloor without neighboring vents, flank rifting occurs more evenly distributed around the vent.However, where the flanks of a volcano may be supported on one side by the presence of a pre-existing feature, or burdened with various planes of weakness, rift zone formation promulgates according to down-slope pull of gravity.