In this paper we re-examine the relationship between seismically constrained variations in crustal structure along the southern East Pacific Rise (SEPR) and the segment-scale variations in axial depth, morphology, basalt geochemistry, and hydrothermal activity that have often been attributed to along-axis differences in the supply of magma to the mid-ocean ridge. Along >800 km of the fast spreading SEPR, good correlations exist between axial depth, ridge cross-sectional area, mantle Bouguer anomaly, and the MgO weight percent of basalts recovered from the rise axis. These correlations indicate along-axis changes in crustal thickness and temperature consistent with variations in magma supply on time scales of ~100,000 years. In contrast, we show that the depth and width of the midcrustal magma sill, the thickness of seismic layer 2A, and the intensity of hydrothermal venting are poorly correlated with regional variations in ridge depth and cross-sectional area. We suggest that the emplacement geometry (width of the intrusion zone and flow lengths), not magma supply, controls extrusive layer (seismic layer 2A) thickness. We hypothesize that magma lens properties and hydrothermal activity are closely linked to spreading events (dike intrusion, eruptions, faulting) which occur on much shorter timescales (~10--100 years) than the longer-term variations in magma supply reflected in along-axis changes in the shape and depth of the ridge axis. ¿ 1997 American Geophysical Union