Geology Reference
In-Depth Information
Figure 7.2. Volcanic hotspots (dots). Dashed lines are contours of the residual
geoid [49], which is sensitive to long-wavelength gravity variations. From Duncan
and Richards [50]. Copyright by the American Geophysical Union.
The occurrence of isolated, persistent and relatively slow-moving volcanism
far from plate margins implies an independent and persistent source of magma
that is not directly involved with plate motions. The localisation of the volcanism,
generally within a radius of a few tens of kilometres, implies that the source is
laterally narrow, no more than about 100 km in diameter. The persistence of the
volcanism for tens of millions of years (at least 90 Myr in the case of Hawaii)
implies a large or self-renewing source. The slow lateral motion implies that the
source is below the zone in which the mantle moves with the surface plate, as both
Wilson and Morgan inferred. The persistence and slow motion together suggest
a vertically extensive source, or in other words a column. The column is around
100 km in diameter but would extend at least 1000 km into the mantle, based on
the kinds of convective flow shown in Chapter 6. Thus we have inferred a picture
of a tall and remarkably narrow mantle column.
The volcanism of hotspots of course requires melting, and this could be due to
either higher temperature or a different composition within the column. If it is due
to being hotter, then a persistent supply of heat is available at the base of the mantle,
where heat from the core is inferred (on independent evidence) to conduct into the
mantle, forming a hot thermal boundary layer. Such a hot thermal boundary layer
