De-Plumeing the Depths

One of the more romantic ideas associated with plate tectonics has been the deep mantle plume - streams of hot rock arising thousands of miles down on the Earth's core/mantle boundary and bobbing to the surface like colored oil in a lava lamp, producing great floods of lava or chains of islands dotted across the sea. Alas, this idea now seems as dated as those lava lamps of my childhood.

Shoot. First the tooth fairy, then momentum conservation, and now this. If anybody tells me there isn't really a Hogwarts School of Witchcraft and Wizardry, I will probably just lose it.

Gillian Foulger outlines the original idea here:
An important contribution to this debate came
hot on the heels of the newly accepted plate tectonic
theory. Morgan (1971) suggested that
“hotspots”, i.e. areas of exceptionally intense
volcanism such as Hawaii, Yellowstone and
Iceland, are fuelled by plumes of buoyant, hot
material that arise in the deep mantle and punch
through the mobile, convecting, shallow mantle
to reach the surface (figure 1). This theory was
developed in order to explain the timeprogressive
volcanic trails associated with some
hotspots, and their apparent fixity relative to
one another. If the sources of the volcanism are
rooted in a relatively immobile deep mantle,
they will not move relative to one another and
the plates at the surface will drift passively above
them, bearing away trails of volcanism. Hot
plumes are unlikely to form spontaneously in a
gradational layer, but would rise from a thermal
boundary layer. This implies that their source
would have to be the core–mantle boundary,
which is the largest thermal boundary layer in
the Earth apart from the surface itself.


Unfortunately, it seems that none of the conditions implied are really quite so - there aren't always trails, they aren't really quite fixed wrt each other, and, worst of all, there doesn't seem to be a hot deep mantle plume below any of the hotspots.

Iceland is a famous example: supposedly a hot spot sitting right on a mid ocean ridge. Coincidentally (according to plume theory) or not at all coincidentally (according to the new ideas) Iceland also sits right on the Caledonian suture - the ghost of the Iapetus Ocean that vanished 400 million years ago when the continents collided to form a supercontinent. The Foulger paper cited above is very good.

A more general introduction is here, with a number of good links. Tomographic seismography experiments planned for Hawai'i should settle the issue.

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