Geology Reference
In-Depth Information
The fact that hotspot locations do not correlate strongly with the current config-
uration of plates (Figure 7.2; [85]) indicates that the plume and plate modes are not
strongly coupled. The implication is that plumes rise through the plate-scale flow
without substantially disrupting it. In fact, experiments have shown that plume tails
can rise through a horizontal background flow, bending away from the vertical but
retaining their narrow tubular form [86]. However, there is a correlation between
plume locations, broad geoid highs and slower seismic wave speeds in the deep
mantle, indicating that plumes form preferentially away from deeply subducted
lithosphere (Figure 7.2, [85, 87]).
Stacey and Loper [88] were apparently the first to appreciate that, if plumes
come from a hot, lower thermal boundary layer, then their role is to cool the core,
in the sense that they are the agent by which heat from the core is mixed into the
mantle. In this interpretation, the role of plumes is primarily to transfer heat from
the core
through
the mantle, but not
out of
the mantle. Plates transfer heat
out of
the
mantle. Plumes bring heat to the base of the lithosphere, which is mostly quite thick
and conducts heat only very slowly to the surface. For example, no excess heat
flux has been consistently detected over the Hawaiian swell [89]. While in some
cases, like Iceland, the lithosphere is thin and a substantial part of the excess plume
heat may be lost to the surface, more commonly much of the plume heat would
remain in the mantle, presumably to be mixed into the mantle after the overlying
lithosphere subducts.
Thermal plumes do not explain all non-plate magmatism. It is plausible, but not
yet clearly demonstrated, that some volcanism that is less regular than the classic
age-progressive linear hotspot track may be due to thermochemical plumes, whose
behaviour is more complicated and erratic than thermal plumes. It is also plausible
that some volcanism has other sources, and several hypothesised mechanisms will
be discussed in the next chapter. However, such other mechanisms as may exist
evidently account for only a small proportion of the heat transport through and out
of the mantle, otherwise their existence would be easier to demonstrate.
