Extensional tectonics and global volcanism

Most of the world's hotspots occur on or near plate boundaries. Proposed "hotspot tracks" are primarily along pre-existing linear features, and volcanism is controlled by extensional stress. These linear features are transform faults, fracture zones and abandoned ridges. Almost half of the "midplate" volcanoes occur on young (< 30 My) lithosphere. The correlations of volcanoes with seafloor age, topography, fracture zones, gravity and focal mechanisms show that tensional tectonics is essential for volcanism, supporting the hypothesis that volcanism is controlled by lithospheric architecture and stress, not by hot narrow jets. Non-thermal explanations for focused magmatism are consistent with absence of heat flow, uplift and subsidence anomalies. Extensional stresses and lithospheric fabric are the controlling influences on the timing and location of "midplate" volcanism. Island and seamount chains provide maps of stress and fabric, not plate motion.