FAILURE OF PLUME THEORY TO EXPLAIN MIDPLATE VOLCANISM IN THE SOUTHERNAUSTRAL ISLANDS

It has long been recognized that the properties of the Cook-Austral ch ain (Fig. 1) of volcanoes in the South Pacific are difficult to reconc ile with the theory that volcanic activity in plate interiors is produ ced by the drift of tectonic plates over narrow, stationary plumes(1) of hot mantle material upwelling from depth. Radiometric dates(2,3) fr om many island samples are younger or older than would be predicted if a single plume currently located at volcanically active Macdonald sea mount(4) was responsible for all of the volcanoes. Indeed, only the so uthernmost part of the Austral volcanic line has hitherto appeared to be consistent with plume activity, and then only within the past 6 mil lion years (Myr)(5,6). Here we report radiometric dates that demonstra te that these southern Austral volcanoes are actually composed of thre e distinct volcanic chains with a range of ages spanning 34 Myr and wi th inconsistent age progressions. Gravity anomalies and seafloor fabri c suggest that the volume and location of volcanism in this region is controlled by stress in the lithosphere rather than the locus of narro w plumes rising from the deep Earth.