Environmental Engineering Reference
In-Depth Information
2.10
Earth's principal hotspots. Plotted from coordinates in
Courtillot et al. (2003).
Indo-Atlantic (including Iceland) and Pacific hotspot
families is up to 20 mm/a (Norton 2000). Composi-
tional arguments are not strong either: Humayun, Qin,
and Norman (2004) used higher Fe/Mn ratios of Ha-
waiian lavas (compared to those from mid-ocean ridges)
as proof that massive plumes are rising all the way up
from the lower mantle or core-mantle boundary, but the
difference (only 15%) is well within the scatter for the
upper-mantle ratios (C. A. Lee 2004).
Consequently, Foulger and Natland (2003) concluded
that the plume hypothesis has survived largely as a belief
and that hotspots are by-products of plate tectonics that
require a source of melt in the shallow mantle and
stretching and cracking of the crust. Only a single mode
of convection is required and plate tectonics becomes an
all-encompassing explanation of the Earth's volcanism.
In contrast, Montelli et al. (2004) presented seismic
tomographic evidence for the existence of at least six
well-resolved plumes (including the Azores, Canaries, Sa-
moa, and Tahiti) that reach into the lowermost mantle.
But interpretation of seismic data remains in dispute, and
more time is needed before the existence of plumes is un-
equivocally established (Kerr 2006). Hirano et al. (2006)
provided evidence for an alternative hypothesis: hotspot
