HOTSPOTS, MANTLE PLUMES, FLOOD BASALTS, AND TRUE POLAR WANDER

Persistent, long-lived, stationary sites of excessive mantle melting are called hotspots. Hotspots leave volcanic trails on lithospheric plates passing across them. The global constellation of fixed hotspots thus forms a convenient frame of reference for plate motions, through the orientations and age distributions of volcanic trails left by these melting anomalies. Hotspots appear to be maintained by whole-mantle convection, in the form of upward flow through narrow plumes. Evidence suggests that plumes are deflected little by horizontal flow of the upper mantle. Mantle plumes are largely thermal features and arise from a thermal boundary layer, most likely the mantle layer just above the core-mantle boundary. Experiments and theory show that gravitational instability drives flow, beginning with the formation of diapirs. Such a diapir will grow as it rises, fed by flow through the trailing conduit and entrainment of surrounding mantle. The structure thus develops a large, spherical plume head and a long, narrow tail. On arrival at the base of the lithosphere the plume head flattens and melts by decompression, producing enormous quantities of magma which erupt in a short period. These are flood basalt events that have occurred on continents and in ocean basins and that signal the beginning of major hotspot tracks. The plume-supported hotspot reference frame is fixed in the steady state convective flow of the mantle and is independent of the core-generated (axial dipole) paleomagnetic reference frame. Comparison of plate motions measured in the two frames reveals small but systematic differences that indicate whole-mantle motion relative to the Earth's spin axis. This is termed true polar wander and has amounted to some 12-degrees since early Tertiary time. The direction and magnitude of true polar wander have varied sporadically through the Mesozoic, probably in response to major changes in plate motions (particularly subduction zone location) that change the planet's moments of inertia.