Plumes and Supercontinents

Introduction

Supercontinents have aggregated and dispersed several times during geologic history, although our geologic record of supercontinent cycles is only well documented for the last two cycles: Gondwana–Pangea and Rodinia (Hoffman 1989; Rogers 1996). It is generally agreed that the supercontinent cycle is closely tied to mantle processes, including both convection and mantle plumes. Pangea 200 Ma was centered approximately over the African geoid high (Fig. 2.28(a)), and the other continents moved away from this high during breakup of Pangea. Because this geoid high contains many of the Earth's hotspots and is characterized by low seismic-wave velocities in the deep mantle, it is probably hotter than average, as discussed in Chapter 2. Except for Africa, which still sits over the geoid high, continents seem to be moving toward geoid lows, which are also regions with relatively few hotspots and high lower mantle velocities, all of which point to cooler mantle (Anderson 1982). These relationships suggest that supercontinents may affect the thermal state of the mantle as the mantle beneath continents becomes hotter than normal, expands, and produces the geoid highs (Anderson 1982; Gurnis 1988). This is followed by increased mantle plume activity, which may fragment supercontinents or at least contribute to dispersal of cratons.