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derived mainly from a plume tail, since they assumed that plumes are part of an
upper-mantle convection system and that plumes therefore derive from no deeper
than 670 km. In this case the plume heads would have diameters of no more than
about 300 km and volumes less than about 5% of a plume head from the bottom of
the mantle [68].
The second part of White and McKenzie's model encounters the difficulty that
a number of flood basalt provinces are said, on the basis of field evidence, to have
erupted mainly before substantial rifting occurred (e.g. Deccan Traps) or in the
absence of any substantial rifting (e.g. Siberian Traps, Columbia River Basalts)
[120]. It also fails to explain the very short timescale of flood basalt eruptions, less
than 1 Ma in the best-constrained cases. The third part of their model implies that
a sufficient volume of warm mantle would take about 50 Ma to accumulate, but,
at the time the Deccan Traps erupted, India was moving north at about 180 mm/yr
(180 km/Myr) so it would have traversed the extent of the flood basalts in only
about 10 Ma. It is implausible that sufficient warm mantle could accumulate from
a plume tail under such a fast-moving plate.
The latter two difficulties are avoided by the plume head model of flood basalts,
since the flow rate of the plume head is much greater than that of the tail and much
of the melting is inferred to occur from beneath the intact lithosphere upon arrival of
the plume head, as we saw in Chapter 7. On the other hand, White and McKenzie's
discussion of rift-margin volcanism is quite reasonable, and also compatible with
the plume head model.
8.4.2 Superplumes
The term
superplume
has been used in many contexts, usually without any clear
explanation of what the term might refer to, other than something larger than
or different from a 'normal' plume. The question is whether there might be any
phenomenon for which the term might be appropriate. There may be one such
possibility, but the other uses of the term seem to be either unjustified or so ill-
defined as not to be useful.
The term seems first to have been used by Larson [121]. Larson argued that there
had been a burst of volcanism in the Pacific during the Cretaceous, and that it cor-
related with a number of other phenomena of the period. There were indeed several
large hotspots active during this time, and possibly other less localised volcanism
as well. Larson proposed that the period might be explained by a 'superplume',
but offered no definition or physical basis for the term. Because the phenomena
persisted over tens of millions of years and at several loci, it seems unlikely that a
single mantle upwelling would be responsible. A slightly unusual burst of plume
activity seems quite adequate to account for the observations, and Davies and
