Will Sager presents a heart warming
history of his epiphany [Ed: See What
the Hell is Hawaii? page].
My petrology friends often use the parable of a visitor
from another planet and her inability, according to
them, to understand rocks unless she is taken aside
by an experienced petrologist who explains the mysteries
to her. She then understands that the mantle is composed
of homogeneous isothermal refractory pyrolite and that
all melting anomalies are due to the importation of
high absolute temperature material from deep inside
the Earth, heated by the core. The canons of Bowen are
revealed to her. The trick of "adding back" olivine
to infer the temperatures of melt sources, assuming
the composition of a primitive parent,
are explained to her delight.
Her less indoctrinated planetary colleagues
would be unable to understand Sager’s laments.
Why should age progressions represent convective velocities?
What is the matter with volcanism and fissures on a
stationary plate? What about the thousands of small
volcanoes not in a line? How can anything besides the
inertial frame be "fixed" in a spinning
convecting deforming planet? When James Dwight Dana
visited the Hawaiian islands, he was like a first-time
extra-terrestrial visitor. Dana (1849), developed
the giant fissure hypothesis to explain the age progressions
and parallelism of the island chains in the Pacific.
This possibility was critiqued, along with others,
Clague & Dalrymple (1989)
just 140 years later. The absolute fixity of hotspots
and their very high temperatures were regarded as absolute
constraints that ruled out all options except the fixed
Giant fissures were found wanting
because there was no obvious way to have melt, unless
it was already there–the partially molten and
inhomogeneous asthenosphere hypothesis. The assumption
of most marine scientists was, and is, that the upper
mantle is subsolidus and uniform and that melting requires
importation of heat from the deep mantle or frictional
heating of rapidly moving plates. The perceived requirements
of fixity and high temperature, and an homogeneous
shallow mantle, favored the plume idea and other ideas
were shoved onto the back burner. Subsequently, many
volcanoes were dated and few showed the expected age
progressions or lack of motion with respect to others.
Since it was now known that volcanic islands were fixed,
and were spots, the observations were interpreted in
this light. It was too late to say “never mind” and
start afresh. New hotspots were placed at places where
the age progressions along a chain were irregular;
this explains the proliferation of new plumes, all
in a row, in the south Pacific. Other irregularities
were explained by numerous changes in the directions
and velocities of the plates or the poles of rotation.
Hotlines were proposed. The radii of plume-spots were
increased to retain the plausibility of fixity. Plume
juice was allowed to flow for long distances to fix
up other problems. All islands became plumes.
Regions where the fissure hypothesis
was most plausible, based on the tectonics (e.g., Samoa,
Juan Fernandez, San Felix, Afar, Kelvin fracture zone)
were also treated as plume tracks. Most hotspots are
currently on, or started on, ridges or fractures zones.
In fact, many hotspots are named after the fracture
zones they are on (e.g., Canaries, Azores,
Jan Mayen). Some are clearly reactivated abandoned
ridges. But these observations or concerns were also
As problems mounted, the concepts
of mantle roll, mantle wind, group motions, westward
plate drift, Euler pole jerks, long-distance lateral
transport, deformable plates, hidden plate boundaries,
and true polar wander (TPW) were introduced. There
is nothing wrong with TPW but it has often been used
with hotspots as a known fixed reference frame, just
to fix up discrepancies with the plume hypothesis.
The other complications removed the elegant simplicity
of the fixed plume hypothesis, in which knowledge of
the plate motions and single hotspots could predict
all the island chains in the world. Modeling now requires
a convection and a viscosity model, and a starting
depth, radius and time for each plume, plus adjustable
plate motions and deformations. Hotspot motions can
be satisfied but at enormous sacrifice in simplicity
Some volcanoes on the Earth’s
surface are now attributed to features on the core
some 7000 km away and in a different ocean. Most hotspots
are not hot and do not have high heat flow, but these
problems have been rationalized away. The most common
method to rationalize problems is the Texas
Sharpshooter Method, well known to statisticians. Re-label the hotspots
that fit as Real Plumes or Primary Plumes, and relegate
the others to secondary or tertiary status; they are
not really plumes. This is also known as the Pick-and-Choose
Meanwhile the melting problem has
been solved. The upper mantle is heterogeneous and
contains substantial low melting blobs. The solidi
of parts of the mantle are well below the minimum melting
temperature of refractory peridotite. Small volcanoes
are being found that have sources that underlie old
lithosphere and in what was thought to be particularly
cold mantle. Flexure of the plate allows magma to access
the surface. The problem
is not "why are there volcanoes in
some places" but "why
are there not volcanoes everywhere?" Relative
fixity can be understood with asthenospheric counterflow
models, and the stability of the global stress pattern.
We know that some, perhaps many, oceanic islands and
plateaus are underlain by continental crust and we
know that the lower continental crust can delaminate
and enter the mantle as mafic blobs. We know that the
deep mantle geotherm is unlikely to be an adiabat.
Because of radioactive heating and subduction cooling
it must have a maximum at depth.
negatives of the plume hypothesis outweigh the
positives, it has been difficult for many in
the Earth science community to abandon the idea.
It keeps getting "fixed up". We now
know that the hallmarks of the plume hypothesis–fixity,
parallelism, and high heat flow–are not
valid, but this has not been enough to abandon
what has become an albatross. Even the basic
assumptions are now known to be wrong–an
homogeneous, isothermal upper mantle. When these
assumptions are dropped, there may be no need
for the plume hypothesis, but these amendments
have also have been incorporated into the hypothesis.
The visitor from another planet would
look at the evidence and would come up with a sensible
hypothesis. If a marine geophysicist were to take her
aside and present her with the plume theory, the visitor
would certainly say “What the Hell is that?” (e.g., Anderson,
2007). Such a strange idea would not be publishable
on the home planet.
Dana, J.D., 1849, Geology, in Wilkes, C., ed., United States Exploring Expedition,
with atlas: Philadelphia, C. Sherman, New York, Putnam,
p. 756 pp.
Clague, D. A., and Dalrymple, G. B., 1989, Tectonics,
geochronology, and origin of the Hawaiian-Emperor
volcanic chain, in Winterer, E. L., et al., eds., The
Geology of North America, The eastern Pacific and
Hawaii, Decade of North American Geology,
vol. N, Geol. Soc. Amer., Boulder, CO, p. 188-217.
last updated 26th