Grain boundaries in
a bubble raft |The
California Institute of Technology, Pasadena,
of melting anomalies at the Earth’s surface–so-called
hotspots and hotspot tracks–do not involve
high absolute temperatures or deep active upwellings.
These have been criticized on various grounds.
Propagating cracks do not provide a ready explanation
for melting, and eclogite sources require large
degrees of melting to generate basalts. However,
a combination of lithospheric properties–architecture
and stress–and asthenosperic conditions–variable
lithology and fertility–overcomes these
objections. The locations of magmatism are controlled
by the lithosphere; the magnitude of the volcanism
is controlled by the fertility and pre-eruption
ponding. If ambient mantle is close to the solidus
of peridotite then large amounts of melting can
be generated from eclogite blobs or by adiabatic
ascent of mantle at lithospheric discontinuities
or thin zones. The volume of melting depends on
the eclogite fraction in the melting domain, not
the absolute temperature. A new parameter is involved–the
scales of the fertile blobs. The fate of recycled
and delaminated eclogite depends on its origin,
composition and the amount of peridotite that
show that many so-called midplate volcanoes actually
were formed at plate boundaries, such as abandoned
ridges and triple junctions. On hindsight we know
that many “hotspot tracks” are abandoned
plate boundaries. Crustal delamination introduces
warm fertile material into the source regions
of basalts. This combined petrological and tectonic
model is called the PLATE paradigm. It is a generalization
of plate tectonics and top-down convection. It
does not rely on concentrated jets of hot mantle.
The PLATE paradigm
primarily involves the following elements and
- Lithosphere, Lithospheric Architecture
PLATE involves passive
mantle, heterogeneous shallow mantle, mantle heating,
top-down processes, cooling from above, recycling,
upper layer instabilities, stress and homologous
temperature. Plate reorganizations, plate boundary
migrations, no fixed reference system, and non-rigid
plates are all implied by the hypothesis.
PLATE is a process
(like thermal conductivity, or oxidation) and
unlike plume, which is an object.
The PLATE hypothesis is basically athermal. It
is based on geology and tectonics more than on
fluid dynamics and thermal convection. It is not
independent of plate tectonics, but a generalization
of it. In addition to variable lithospheric stress
and variable mantle fertility, ponding and lateral
transport of magma may be involved. Magmatism
may not be a steady state process, with melt being
created and extruded simultaneously and at the
The PLUME paradigm
involves thermal processes, absolute temperature,
uniformity, homogeneity, chaotic stirring, active
upwellings, core heat, bottom up, heating from
below, vagueness about depth, deep thermal boundary
layers, a fixed reference system and a pyrolite
source. Magma volumes and seismic velocities are
proxies for mantle temperature. The sources of
heat, magma and force are all the same. A plume
is a tangible object, like caloric, aether or
phlogiston. The plume hypothesis is an entirely
thermal process. It is based on a branch of fluid
dynamics and is independent of plate tectonics;
lithospheric properties are of secondary interest.
Both of the above are Paradigms. They are more
than theories or hypotheses. A paradigm is a package
of ideas, assumptions and auxiliary hypotheses
and agreements about the potentially fruitful
areas for research.
is a more powerful concept than generally assumed.
It is often considered to be a kinematic or descriptive
theory, and plates are often assumed to be rigid
and permanent. The PLATE hypothesis is more general
than textbook plate tectonics and different from
the propagating crack hypothesis, which many people
erroneously assume is the main alternative to
plumes. Just as the term “plume” implies
many things, so the term PLATE implies a variety
of tectonic and petrological processes.
Plates are not permanent
rigid things, and the surface of the Earth is
constantly “recrystallizing”. It thus
resembles more a bubble raft or a granular aggregate
than mudcracks or the patterns on the back of
a turtle. Coherent assemblies of bubbles and grains
form islands that are appropriate for the local
stress system under which they find themselves.
If stress changes, the whole system dissolves
and reforms, just like a polycrystal that recrystallizes
when stress changes. Plate reorganizations involve
more than just rigid plates growing or shrinking
and boundaries migrating and jumping. They involve
self-organization and far-from-equilibrium processes.
Once this is understood, it is recognised that
many old “hot spot” tracks e.g., 90-E
ridge and the Line Islands chain are previous
plate boundaries and presently active “hot
spot” tracks may be incipient plate boundaries
e.g., the Samoa-Easter trend or faults under tension
e.g., the Cameroon Line and Cenozoic volcanism
in western North America.
If the underlying
mantle is near its (variable) melting point, nothing
more is needed than extension to release ponded
melts. Plate reconstructions, coupled with the
concept that plate boundaries are ephemeral, suggests
a reinterpretation of volcanic chains and plateaus.
The PLATE paradigm evolved during the first few
years of the 21st century, but its roots extend
back to the earliest days of plate tectonics.
It is a synthesis of developments in petrology,
lithosphere mechanics, sampling theory, understanding
of stress and self-organized systems, and plate
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