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be a likely source of thermal plumes. Some even argue that the core would start out
hotter than the mantle, rather than locally equilibrating with the mantle, as assumed
in Figure 7.3. Such a superheated core would strengthen this argument. Estimates
of the present temperature of the core, based on extrapolated material properties
of iron alloys, commonly require it to be 1000
◦
C or more hotter than the lower
mantle [51], in which case a hot thermal boundary layer would exist at the base of
the mantle.
This line of argument implies that thermal plumes in the mantle are virtually
inevitable, quite apart from whether they are detectable at the surface. In an Earth-
like planet cooling from the surface, heat will conduct from the core, forming a hot
thermal boundary layer at the base of the silicate mantle. With sufficient heat flow,
the boundary layer will become unstable and generate hot, buoyant upwellings.
The favoured form for such upwellings is columns, as we will see shortly. Thus we
should expect mantle plumes if there is sufficient heat flow from the core. These
would not necessarily be detectable, but volcanic hotspots are a very plausible
consequence of such upwellings.
Thus, with few assumptions and no theory, it is possible to infer the presence of
hot, narrow columns rising from the base of the mantle under the more prominent
volcanic hotspots such as Hawaii. This does not prove the existence of mantle
plumes, but it does establish them as a plausible hypothesis with significant evidence
in its support. Further arguments will be encountered as we look at experimental
and theoretical understanding of buoyant upwellings.
7.2 Hotspot swells, plume flows and eruption rates
As well as the narrow topography of the Hawaiian volcanic chain, a broad swell
in the sea floor surrounding the chain is evident in Figure 7.1. This swell is up to
about 1 km high and about 1000 km wide. Such a swell might be due to thickened
oceanic crust, to a local imbalance of isostasy maintained by the strength of the
lithosphere, or to buoyant material under the lithosphere. Seismic reflection profiles
show that the oceanic crust is not significantly thicker than normal [52]. Nor can
such a broad swell be held up by the flexural strength of the lithosphere [53].
The straightforward conclusion is that the Hawaiian swell is held up by buoyant
material under the lithosphere.
Swells like that in Figure 7.1 are evident around many of the identified volcanic
hotspots. Other conspicuous examples are at Iceland, which straddles the Mid-
Atlantic Ridge, and at Cape Verde, off the west coast of Africa (Figure 2.4). The
latter is 2 km high and even broader than the Hawaiian swell, apparently because
the African plate is nearly stationary relative to the hotspot [50].
Hotspot swells provide us with important information. They can be used to
estimate the rate of flow of buoyancy in the plumes, and from that the flow of
