Superplumes may develop. One appears to
have done so beneath the Pacific Ocean during the
middle of the Cretaceous period (Larson 1991).
It rose rapidly from the core–mantle boundary
about 125 million years ago. Production tailed off
by 80 million years ago, but it did not stop until
50 million years later. It is possible that super -
plumes are caused by cold, subducted oceanic
crust on both edges of a tectonic plate accumu -
lating at the top of the lower mantle. These two
cold pools of rock then sink to the hot layer just
above the core, and a giant plume is squeezed out
between them. Plume tectonics may be the
dominant style of convection in the major part of
the mantle. Two super-upwellings (the South
Pacific and African superplumes) and one superdownwelling
(the Asian cold plume) appear to
prevail (Figure 5.7).