The seismic structure of the transition-zone discontinuities was studied beneath the forty-nine hotspot locations of the catalog of Courtillot et al. (2003), using a global data set of SS precursors. Some of these hotspots are proposed to originate from plumes rising in the upper mantle or from the core-mantle boundary region. I found thin transition zones in approximately two-thirds of the twenty-six hotspot locations for which precursor observations could be made. This observation agrees with the expectation for the olivine phase transition of a systematically thin transition zone in high-temperature regions. Other hotspot locations showed a clear deepening of...