Geology Reference
In-Depth Information
Emperor
Seamounts
Hess Rise
Hawaii
Mid-Pacific Mountains
Figure 7.1. Topography of the sea floor near Hawaii, showing the Hawaiian
seamount chain (white), the Hawaiian islands (black) and the Hawaiian swell.
Contours are at depths of 3800 m and 5400 m.
[25]. Wilson called the mantle source a mantle 'hotspot', and he imagined it as a
volume of hotter mantle located near the centre of a convection 'cell', which would
account for it not moving with the lithosphere at the surface. Wilson's idea did
not immediately catch on, perhaps because his idea of a hotspot in the middle of
the mantle seemed
ad hoc
, and also because the age progression of the Hawaiian
chain was only just being accurately established in 1963 [26, 27]. Nevertheless
we can recognise Wilson's observations and ideas as a seminal contribution to
understanding mantle convection.
In 1971 Jason Morgan proposed an alternative mechanism for the volcanism
[43, 44]. Instead of a hot 'spot' somewhere in the middle of the mantle, Morgan
proposed that a thin, hot column of mantle material rises from the core-mantle
boundary. Such buoyant columns are known in other fluid-dynamical contexts
(such as the smoke column rising from a candle), and he adopted the existing term
plume
for his proposed mantle columns. Morgan supposed that the heat carried by
the plume comes from the core. This mechanism, though novel at the time, had
the merit of being based on a plausible fluid-dynamical process and of having an
energy source that could persist for the tens of millions of years required by some
of the seamount chains. Thus, through Wilson's and Morgan's insights, the concept
of a mantle plume was born.
The term 'hotspot' used by Wilson referred to a hot volume somewhere deep
in the mantle, but its usage began to shift after Morgan's hypothesis. Wilson's
version was quickly dropped, so it seems useful to use the term to refer to the
volcanic centres at the Earth's surface. To avoid any confusion, one might speak
of a
volcanic hotspot
. The chains of islands and seamounts extending away from
active volcanic hotspots have become known as
hotspot tracks
.
