Geoscience Reference
In-Depth Information
Hotspots
sources of lithospheric stress that may account
for linear volcanic chains.
Volcanic chains and midplate volcanism moti-
vated an amendment to the plate tectonic
hypothesis. Wilson (1973) suggested that time-
progressive volcanism along the Hawaiian chain
could be explained by the lithosphere mov-
ing across a
'jetstream of lava' in the
mantle
under the island of Hawaii. Volcanic
islands appeared to be thermal anomalies so
the term
hotspot
was coined to explain them,
the assumption being that high absolute tem-
perature was the controlling parameter; magma
volume was used as a proxy for mantle temper-
ature. Mantle fertility, magma focusing, passive
heterogeneities, cracks and lithospheric stress
can also localize melting, but the term
hotspot
is well established. Hotspots are also called
midplate volcanism, melting anomalies
and
plumes
.
These are all somewhat misleading terms since
anomaly
implies background homogeneity and
constancy of stress, mantle composition and
temperature that is inconsistent with plate
tectonics. Hotspots, in common with other
volcanic features, occur in extensional regions
of the lithosphere, either plate boundaries or
intraplate. Many 'midplate' volcanoes started
on ridges which have since moved relative to
the
Summary
Complexity theory is a recent phrase but an
ancient concept. It is usefully applied to plate
tectonics. Plate tectonics is a slowly driven,
interaction-dominated system where many deg-
rees of freedom are interacting. Stresses in plate
interiors can change much more rapidly than
plate motions since the latter are controlled by
the integral of slowly varying buoyancy forces.
Apparent changes in orientation and activity of
volcanic chains are therefore more likely to be
due to changes in stress than to changes in
plate motions. The
stress-crack hypothesis
seems to
apply to most volcanic chains while the
core-
plume hypothesis
accounts for only a fraction
of intraplate magmatism. Dramatic changes in
plate configurations in part of the world (e.g.
Pangea breakup) are likely to be accompanied
by a global reorganization (e.g. formation of
new plate boundaries and triple junctions in
the Pacific and Farallon plates) and creation of
new plates. Minimization of dissipation appears
to be a useful concept in global tectonics. Obvi-
ously, formation of a new crack, ridge or suture
may reduce TF or collisional resistance, and may
change or reverse mantle convection.
Plates, like other dynamical and complex
systems, organize themselves into structures that
interact. They are provided with energy from the
mantleandtheyevolveundertheinfluenceof
driving forces and interaction forces. There is
no single characteristic plate size or characteris-
tic time scale. Such systems organize themselves
without any significant tuning from the outside.
The fundamental question in mantle dynamics
and thermal history is whether the mantle is pri-
marily a
top-down
(plate tectonics),
inside-out
(nor-
mal mantle convection with simple boundary
conditions) or
bottoms-up
(pot-on-a-stove, plume)
system. In these kinds of problems, the out-
come can be entirely the result of the assump-
tions, even if a large amount of computation
intervenes. One gains intuition in complexity
problems by exploring large numbers of cases; a
single experiment or calculation is usually worth
less than none.
anomaly
or
been
abandoned.
Some
may
be
expressions
of
incipient
or
future
plate
boundaries.
Plates, Plumes and Paradigms
provide
catalogs and maps of those volcanic, tectonic and
geochemical features that have become known as
hotspots
, including those that may have a shallow
plate-tectonic or asthenospheric origin [see also
mantleplumes, global mantle hotspot
maps, scoring hotspots
]. Many proposed
hotspots, including isolated structures and the
active portions, or inferred ends, of seamount
chains, do not have significant swells, substan-
tial magmatic output or tomographic anomalies
[
hotspots tomography
]. A
melting anomaly
,or
hotspot
, may be due to localized high absolute
mantle temperature, or a localized fertile, or
fusible patch of the asthenosphere. Some have
been called
wetspots
and some have been called
hotlines
. The localization may be due to litho-
spheric stress or architecture.
