> The Hawaiian Islands are
> a linear chain of Islands that is youngest at the southeastn most point
> (i.e. where the active volcanoes are) and get progressively older as one
> travels toward the northwest. Many rounds of radiometric dating have been
> done and a very consistent relationship has been found between the distance
> of a volcano from the active island and its age.
This is an interesting set of data - which must mean something! The
problem for earth scientists is: how should this trend be
interpreted?
As an alternative to the Plate Tectonic model, consider that
developed by Warren Carey (Professor Emeritus of geology in the
University of Tasmania). I refer to his book "Theories of the Earth
and Universe", Stanford University Press, 1988.
His analysis is one which promotes the concept of tectonic control.
He reconstructs the Pacific Floor in terms of age provinces and shear
zones. The Hawaiian ridge runs parallel to the Melanesian Plateau,
and the Emperor Ridge runs parallel to the Mariana Ridge. He calls
the gap between the the Hawaiian Ridge and the MP the "Central-Pacific
Sphenochasm" and the gap between the Emperor Ridge and the Mariana
Ridge the "Shatsky Spenochasm". By applying a dextral global torsion
(by 34 degrees) he closes these chasms. These changes are all linked
to Tethyan widening. In this model, hot spots are not required, and
the volcanism is mainly contemporaneous.
And what of the radiometric dates? Carey writes:
"Plate tectonicists place much emphasis on the decrease in the ages of
the volcanic rocks along the Emperor and Hawaiian ridges, and
interpret them in terms of motion of the Pacific "plate" over a
"hotspot" fixed in the mantle. But these ages record the times at
which volcanism _terminated_. The lavas at Hawaii are 12 km thick. I
suggest that the lava at the base is very much older than those
measured at the surface, perhaps as old as any in the Emperor Ridge."
(page 321).
The least that can be said about this is that there is here an
alternative model - and I am a great believer in the evaluation of
multiple models! I wonder how many geoscientists have considered
Carey's explanation?
> What is truly amazing
> though is that this relationship was established 30 years ago and from that
> data an estimate of how fast the Pacific plate was moving could be made
> with a gradeschool knowledge of math. Now in the last 10 years satellites
> have measured the rate of motion of the Pacific plate and the numbers
> 8.6-9.0 cm/yr compared to 9.0 cm/yr are the statistically the same.
It is an impressive correlation. But is it soundly based? In recent
years, it has become apparent that the alleged hot spot under Hawaii
is NOT stationary. The data suggests it is moving. This throws a
spanner in the works - the satellite measurements give a movement
rate higher than would be expected. My source for this is:
Geology Today, 14(3), May/June 1998, 101-103.
Misinterpretation in the Pacific?
The Hawaiian-Emperor chain of volcanic islands and seamounts in the
Pacific Ocean (Fig. 7) is famous, because it was in that connection
that many of the ideas on mantle plumes and hotspots were worked out.
The conventional wisdom, reached after much early debate, is that the
islands/seamounts are the product of a mantle plume, fixed in
(lateral) position relative to the mantle, that gave rise to a series
of volcanic hotspots at the surface as the Pacific lithospheric plate
moved above it. The chain is also famous for its bend, interpreted as
a sudden change in the direction of motion of the Pacific Plate about
43 million years ago.
This story is now so widely accepted that it almost has the status of
`fact'. But what if it's not true? What if it's merely wishful
thinking? The alternative is perfectly obvious, of course, if not much
thought about. Could the bend in the chain be due not to a change in
the motion of the Pacific Plate but to a sudden movement of the mantle
plume itself? Possible movements of mantle plumes have been much
considered in the past and not always ruled out, but envisaged motions
have generally been small, barely invalidating the general concept of
a `fixed hotspot frame of reference'. Needless to say, any proved
deviation from that would be devastating, requiring a major rethink on
a number of fronts.
Yet that is what Norton proposed a few years ago (Tectonics, v.14,
p.1080, 1995) and is what Tarduno and Cottrell consider in more detail
now (Earth & Planetary Science Letters, v.153, p.171, 1997).
Specifically, Norton suggested that the hotspot moved southwards up to
43 million years ago, creating the Emperor seamount chain, and then
became fixed in the mantle, the Hawaiian part of the chain having been
generated by the method outlined in the first paragraph above. As
Tarduno and Cottrell now point out, this is testable by
palaeomagnetism, for if the hotspot has always remained fixed, the
original palaeolatitudes of the now-extinct seamounts of the Emperor
chain should be the same as the present latitude of Hawaii, under
which the supposed mantle plume now rests. Moreover, thanks to samples
collected from Detroit seamount (see Fig. 7) during the Ocean Drilling
Program, the palaeomagnetic test can now actually be carried out.
The results are troubling. For one thing, there is an age
inconsistency. Hotspot-plate motion models suggest an age of 75-65
million years for Detroit seamont, but new 40Ar/39Ar data indicate an
age of about 80 million years. More seriously, the palaeolatitude
given by the Detroit rocks is about 36¡N, whereas the present latitude
of Hawaii is only about 19¡N. Taken at face value, that means that the
plume could not possibly have remained stationary before 43 million
years ago - i.e. the Emperor chain cannot be explained entirely, if at
all, by plate motion above a stationary plume. However, the
palaeomagnetic data do not have the resolution to allow decisions on
whether the Emperor chain is entirely due to plume movement or partly
to plume movement and partly to plate motion.
Either way, the palaeomagnetic data, if confirmed (and confirmation is
necessary because of the limited sampling of the Emperor chain thus
far and the possibility that movement of the Earth's pole may have
biased the data to some, unknown, extent), would appear to demolish a
cherished hypothesis and thus require `a major change in how we view
mantle dynamics and the history of plate tectonics'.
[end of quote]
----------------------
Hope this feedback is of interest.
Best wishes,
David J. Tyler.