We investigate the magnetic signature of volcanic and nonvolcanic seafloor areas along the Southwest Indian Ridge between 61 degrees E and 67 degrees E and analyze their relationship with crustal thickness variations and past to present ridge segmentation. This part of the Southwest Indian Ridge is an end-member for the global ridge system in terms of low melt supply, thin crust, and ultraslow spreading rates. It is characterized by large expanses of seafloor that show no evidence for a volcanic upper crustal layer. We find that variations of intrinsic magnetization and thickness of the basaltic extrusive layer, where it is present, dominate the present-day along-axis crustal magnetization. Off-axis, the magnetization contrast is on average higher for volcanic seafloor than for smooth nonvolcanic topography, indicating that the contribution of the basaltic upper crustal layer to the production of magnetic anomalies remains important. However, magnetic anomalies that record past magnetic polarity events are found almost everywhere in the survey area, even over domains that lack a volcanic upper crustal layer, arguing thus for the contribution of other sources. We propose that both gabbros and serpentinized peridotites contribute to these anomalies. Although not systematic, and weak over most parts of the survey area, an induced component of magnetization is clearly present in some nonvolcanic seafloor domains. Serpentinized peridotites are the likely carriers of this induced magnetization component.