Geology Reference
In-Depth Information
Morgan proposed a second hypothesis, one that gained his ideas a lot of attention,
though it is not required by the idea of a plume, and it has led to some confusion
in later debates about plumes. Morgan proposed that volcanic hotspots do not
move relative to each other. His idea was soon encapsulated in the term
fixed
hotspots
. An important reason for this hypothesis gaining early attention was that it
potentially provided a framework against which plate motions could be measured,
the so-called
fixed hotspot reference frame
. Morgan had been one of the first to
determine the relative motions of many of the plates [45], and he was actively
interested in using the hotspots to refine plate motions, a project that was eagerly
taken up by others as well. At the time it was still widely believed that the lower
mantle had such a high viscosity that it would be essentially immobile, and this
provided a plausible reason for hotspot 'fixity' (and Morgan actually argued that
plumes represented a way in which the lower mantle could convect in spite of its
stiffness). However, this view of the lower mantle had just been challenged [46],
and the modern view that the lower mantle has a moderate viscosity and convects
quite actively, as discussed in Chapter 4, soon took over. This left the hotspots with
no clear reason for being fixed, though they might move more slowly than plates
because the viscosity of the lower mantle is greater than that of the upper mantle.
Indeed, by now some slow relative motions among hotspots are plausibly resolved
[47, 48]. The (approximately) fixed hotspots have nevertheless been very useful in
determining plate motions. For our purpose here, which is to examine the science
of mantle plumes, it is important to realise that whether plumes are fixed or slow-
moving is an issue quite peripheral to the physics of plumes that we are about to
discuss.
7.1 Inferring plumes from surface observations
The definition and identification of volcanic hotspots have been debated. Not every
intra-plate volcano necessarily qualifies as a hotspot, or at least one to which
Morgan's plume hypothesis might relate. Though over 100 have been proposed,
around 40 have gained general acceptance, and a representative selection is shown
in Figure 7.2.
The strongest and clearest hotspots and hotspot tracks allow some straightfor-
ward inferences about the mantle process that gives rise to them, even without the
physical theory that we will soon get to. However, there are hotspots and tracks that
are more complicated or less clear-cut than the classic case of Hawaii. For example,
some of the volcanism in the South Pacific does not form simple linear chains, and
a few linear chains do not seem to have a clear age progression. Such examples may
well involve additional processes, as will be discussed later. Nevertheless, many
hotspots do manifest the relative simplicity of the Hawaiian hotspot.
