The outer shells of Earth - New Theory of the Earth - page 51

Table 4.2 is a compilation of features that have
been consistently labeled as 'hotspots,' along
with their locations [ scoring hotspots ]. Some
of these features have little or no surface expres-
sion. Some are the extrapolated ends of vol-
canic chains, or were invented to extend assumed
monotonic age progressions to the present day,
assuming a fixed hotspot or plume origin. Some
volcanic regions have multiple centers that are
attributed to independent hotspots. Individual
hotspots, volcanic chains, mechanisms and inter-
pretations are discussed in Foulger et al . (2005)
and www.mantleplumes.org. Very few hotspots
have associated upper mantle or lower mantle
tomographic anomalies [ mantle hotspots ]and
most of these do not have substantial swells or
linear volcanic chains. Most hotspots, including
many with large swells and noble-gas anomalies
lack evidence for a deep origin. The most impres-
sive and geochemically distinct hotspots, includ-
ing Hawaii, Iceland, Yellowstone, the
Galapagos, Afar and Reunion are not under-
lain by lower-mantle P-wave seismic anomalies.

not conform to what is traditionally associated
with midocean-ridge basalts.
The lithosphere is cracked and variable in
thickness. The underlying mantle is close to the
melting point from the base of the plate to
about 200 km depth. Low-melting-point material
is recycled into the shallow mantle by subduc-
tion and lower continental crust delamination
or foundering. Plate boundaries are continually
forming and reforming, and being reactivated.
Under these conditions volcanism is expected
to be widespread, simply as a result of near-
surface conditions, but neither uniform nor ran-
dom. A small change in local stress can cause
dikes, extrusions and volcanoes where formerly
there were sills, intrusion and underplating that
did not reach the surface. Nothing except stress
need change. A change in water content may also
change the eruptive style.
Plate tectonics explains most volcanic fea-
tures. Mechanisms for melting anomalies along
plate boundaries or for features not obvi-
ously associated with active plate boundaries
include crack propagation, self-perpetuating vol-
canic chains, reactivated plate boundaries, incipi-
ent plate boundaries, membrane and extensional
stresses, gravitational anchors, reheated slabs,
delamination, buoyant decompression melting of
mantle heterogeneities, dike propagation, leaky
transform faults and rifting unrelated to uplift.
The lithosphere and asthenosphere control the
locations of many, if not all, midplate volcanoes
and volcanic chains (Foulger et al. , 2005).

Mechanisms for forming hotspots
Linear volcanic chains are the defining character-
istics of plate tectonics and of active plate bound-
aries. They are also expected in the waxing and
waning stages of plate boundaries and plate cre-
ation and destruction. The common characteris-
tics of features designated as hotspots or midplate
volcanism also suggest an underlying common
cause. High temperature, compared to midocean
ridges, or a common chemistry do not seem to
be requirements [ mantleplumes ]. Mechanisms
involving lithospheric architecture and
stress , and variable mantle fertility may sat-
isfy the observations for most hotspots. It is gen-
erally agreed that most melting anomalies along
ridges and midplate volcanism are not due to
deep mantle plumes and that hotspots and plumes
are not the same thing. Nevertheless, a plume is
the default explanation for a hotspot or a geo-
chemical anomaly, particularly in the field of
geochemistry where one sees reference to 'the'
Shona plume, 'the' Hollister plume and so on,
even for minor volcanic features. What is meant
is that some isotopic signature in a sample does

Plume and anti-plume tectonics

Mort de ma vie! all is confounded, all! Reproach and
everlasting shame Sits mocking in our plumes.
Shakespeare

Melting anomalies at the surface of the
Earth , such as volcanic chains, or midocean
ridges, can result from hot or fertile regions of
the shallow mantle -- hotspots or fertile spots --
or from hot upwelling jets -- plumes. They can
be the result of variations in thickness or stress
of the plates. Focusing, edge effects, ponding
and interactions of surface features with a par-
tially molten asthenosphere can create melting