Re: Another Radiometric Challenge

Joel Duff (crinoid@midwest.net)
Sat, 05 Sep 1998 09:45:12 -0700

David,

I appreciate the response and given the information I provided these arevalid concerns.  Let me try to address a few of these concerns andexpand the example some more.

At 05:07 PM 9/4/98 David Tyler wrote
>David Tyler responding to Joel Duff (Fri, 28 Aug 1998).
>
>> The Hawaiian Islands are
>> a linear chain of Islands that  is youngest at thesoutheastn most point
>> (i.e. where the active volcanoes are) and get progressivelyolder as one
>> travels toward the northwest.  Many rounds of radiometricdating have been
>> done and a very consistent relationship has been found betweenthe distance
>> of a volcano from the active island and its age. 
>
>DT:  This is an interesting set of data - which must meansomething!  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=20
>University of Tasmania).  I refer to his book "Theories ofthe 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 andshear
>zones.  The Hawaiian ridge runs parallel to the MelanesianPlateau,
>and the Emperor Ridge runs parallel to the Mariana Ridge.  Hecalls
>the gap between the the Hawaiian Ridge and the MP the"Central-Pacific
>Sphenochasm" and the gap between the Emperor Ridge and theMariana
>Ridge the "Shatsky Spenochasm".  By applying a dextralglobal torsion
>(by 34 degrees) he closes these chasms.  These changes are alllinked
>to Tethyan widening.  In this model, hot spots are not required,and
>the volcanism is mainly contemporaneous.

A lot to discuss here.  I have not read Carey's book yet but willattempt to obtain it since I am very interested in the topic.  Does Carey suggest that hot spots are not required to explain any linearvolcanic chain or just the Hawaiian Islands?  If not at least we cansee that the Hawaiian Islands appear similar to other Pacific islandchains that have active islands at the very tip of the most southeasternpoint .  More on other hot spots below.

Secondly, let me add one more bit of Radiometric correlation.  Drowned coral reefs (or whatever you believe them to be) are found in asuccessive layer of rings around the Hawaiian Islands.  On theIslands of Hawaii these reefs have been dated and a strong correlation isfound between depth and age such that a rate of subsidence (we candiscuss the evidence that is subsidence later if you wish) can becalculated very simply.  On the active end of the island this rateof subsidence is nearly the same as the measured rate of subsidence usingtide gauge data over the past 30 years and dating of drowned Hawaiianrock paintings.   Interestingly, drowned reefs on the far endof the island demonstrate a slightly lower rate of subsidence.  Thismakes sense given the  loading of new material at the active end ofthe island and thus a faster rate of "settling."

The  linear relationship between coral age and depth suggests thatthe rate of subsidence has been the same over a long period of time(100kyr).  To avoid such a conclusion either the radiometric datesmust be suspect but then why would a rate determined from radiometricdating be the same or even remotely similar to that determined frommodern tide-gauge data?
>
>And what of the radiometric dates?  Carey writes:
>"Plate tectonicists place much emphasis on the decrease in theages 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 recordthe times at
>which volcanism _terminated_.  The lavas at Hawaii are 12 kmthick.  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 EmperorRidge."
>(page 321).

It would appear that the speculation that the base of the islands aremuch older is meant for us to think that the "older" EmperorRidge seamounts are simply eroded stumps of much larger mountains and sothey have simply eroded down into much older rock and the same couldoccur to the active islands eventually.  But the older islands arenot just eroded stumps but rather the depth at which they may be found isdue to subsidence.  This is attested to by the fact that theseseamounts have the remains of ancient coral reefs on their tops.  Ifthey had been eroded to the point they are the reefs would have been thefirst to go. 

>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 haveconsidered
>Carey's explanation?

I may be way off but the description of Carey's explanation sounds likeit would fit within the alternative plate tectonic models promoted byMeyerhoff and Meyerhoff in the 1970s (American Association of PetroleumGeologists Bulletin 56(2):269-336, 1972).

>>  What is truly amazing
>> though is that this relationship was established 30 years agoand from that
>> data an estimate of how fast the Pacific plate was moving couldbe made
>> with a gradeschool knowledge of math.  Now in the last 10years satellites
>> have measured the rate of motion of the Pacific plate and thenumbers
>> 8.6-9.0 cm/yr compared to 9.0 cm/yr are the statistically thesame.   
>
>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.  Thisthrows a
>spanner in the works - the satellite measurements give a movement
>rate higher than would be expected.  My source for this=20is:
> Geology Today, 14(3), May/June 1998, 101-103.

How far off?  Sorry don't have the article on hand and won't be ableto get if for a few weeks.   I wouldn't be shocked at somedeviation.  Also, it is acknowledged that there was a different rateof at least plate movement if not hot spot movement (it is a toughproblem to separate the two variables)  for the Emperor Seamountsand Hawaiian Ridge.   Even a degree of magnitude would still beamazing given a young earth model would suggest the islands were createdover a short period of time.  Why would radiometric dates of pointscovering some horizontal distance (vs. depth related which is a separateargument) give an estimate that was anywhere even close to a satellitemeasurement? 

>Misinterpretation in the Pacific?
>
>The Hawaiian-Emperor chain of volcanic islands and seamounts inthe
>Pacific Ocean (Fig. 7) is famous, because it was in thatconnection
>that many of the ideas on mantle plumes and hotspots were workedout.
>The conventional wisdom, reached after much early debate, is thatthe
>islands/seamounts are the product of a mantle plume, fixed in
>(lateral) position relative to the mantle, that gave rise to aseries
>of volcanic hotspots at the surface as the Pacific lithosphericplate
>moved above it. The chain is also famous for its bend, interpretedas
>a sudden change in the direction of motion of the Pacific Plateabout
>43 million years ago.
>
>This story is now so widely accepted that it almost has the statusof
>`fact'. But what if it's not true? What if it's merely wishful
>thinking? The alternative is perfectly obvious, of course, if notmuch
>thought about. Could the bend in the chain be due not to a changein
>the motion of the Pacific Plate but to a sudden movement of themantle
>plume itself? Possible movements of mantle plumes have been=20much
>considered in the past and not always ruled out, but envisagedmotions
>have generally been small, barely invalidating the general conceptof
>a `fixed hotspot frame of reference'. Needless to say, anyproved
>deviation from that would be devastating, requiring a major rethinkon
>a number of fronts.

True, it is difficult to establish the absolute fixity of the spots and amajor deviation would require some new thinking.  This still doesnot get us out of the correlation between age and distance even in theEmperor Seamounts.   If we have dramatic movement of thehotspot then how are the radiometric dates to be understood? Paleomagnetic positions are notoriously difficult to determine and theerror bars on most of the dates determined for the seamounts are huge(see Clague and Dalrymple, 1987).   With the new technology fordetermining paleomagnetism coming on line this fall it will beinteresting to see what happens when new and much more extensive studiescan be made.     Still outlying data points are notunexpected in the current data and are much easier to explain than theradiometric dates.    But I am willing to allow that thereappears to be contradictory data regarding the exact origin of theEmperor Seamounts.  Tarduno and Cottrells data are very interestingand I hope further data is gathered to test their proposal.  I wouldnote that the error bar around the paleomagnetic  measurement ishuge (Fig 4) and the traditional view of the origin of the seamountsappears to encompassed within that value though is on the fringe. Further the studies of the other island chains in the Pacific that alsohave an apparent bend have either not been studied enough to drawdefinitive conclusions or do not have bends that only loosely correlatewith the Emperor-Hawaiian Ridge bends.   I willing to leave thechallenge solely on the data of the Hawaiian Ridge.  

>Yet that is what Norton proposed a few years ago (Tectonics,v.14,
>p.1080, 1995) and is what Tarduno and Cottrell consider in moredetail
>now (Earth & Planetary Science Letters, v.153, p.171,=201997).
>Specifically, Norton suggested that the hotspot moved southwards upto
>43 million years ago, creating the Emperor seamount chain, andthen
>became fixed in the mantle, the Hawaiian part of the chain havingbeen
>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 theEmperor
>chain should be the same as the present latitude of Hawaii,under
>which the supposed mantle plume now rests. Moreover, thanks tosamples
>collected from Detroit seamount (see Fig. 7) during the OceanDrilling
>Program, the palaeomagnetic test can now actually be carriedout.
>The results are troubling. For one thing, there is an age
>inconsistency. Hotspot-plate motion models suggest an age of75-65
>million years for Detroit seamont, but new 40Ar/39Ar data indicatean
>age of about 80 million years. More seriously, thepalaeolatitude
>given by the Detroit rocks is about 36=B0N, whereas the presentlatitude
>of Hawaii is only about 19=B0N. Taken at face value, that means thatthe
>plume could not possibly have remained stationary before 43million
>years ago - i.e. the Emperor chain cannot be explained entirely, ifat
>all, by plate motion above a stationary plume. However, the
>palaeomagnetic data do not have the resolution to allow decisionson
>whether the Emperor chain is entirely due to plume movement orpartly
>to plume movement and partly to plate motion.
>
>Either way, the palaeomagnetic data, if confirmed (and confirmationis
>necessary because of the limited sampling of the Emperor chainthus
>far and the possibility that movement of the Earth's pole mayhave
>biased the data to some, unknown, extent), would appear to demolisha
>cherished hypothesis and thus require `a major change in how weview
>mantle dynamics and the history of plate tectonics'.
>
>[end of quote]
>----------------------
>
>Hope this feedback is of interest.
Yes, it is - thanks.  The Geology Today issue is at the binders (itseems everything I ever want is!).  I am surprised I missed that onein my literature search was researching the subject recently.

Cheers,

Joel Duff

<http://scribers.midwest.net/crinoid/origins.htm>
<http://scribers.midwest.net/crinoid/hawaii.htm>

Relevant refs:


Johnson, H. P. and D. Van Patten. 1996.  Age-dependent variation inthe magnetization of seamounts.  Journal Geophysical Research101(B6):13701-13714.

Larson, K. M., J. T. Freymueller, and S. Philipsen. 1997.  Globalplate velocities from the global-positioning system.  JournalGeophysical Research 102(B5):9961-9981.

Ludwig, K. R., B. J. Szabo, J. G. Moore, and K. R. Simmons. 1991. Crustal subsidence rate of Hawaii determined from234U/238Uages of drowned coral reefs.  Geology 19: 171-174.

Moore, J. G., B. L. Ingram, K. R. Ludwig, and D. A. Clague. 1996. Coral ages and island subsidence, Hilo drill hole.  JournalGeophysical Research 101(B5):11599-11605.

Sharp, W. D., B. D. Turrin, and P. R. Renne. 1996.  the40Ar/39Ar and K/Ar dating of lavas from the Hilo 1-km core hole, HawaiiScientific Drilling Project.  Journal Geophysical Research101(B5):11607-11616.

Szabo, B. J. and J. G. Moore. 1986.  Age of -360-m reefterrace, Hawaii, and the rate of late Pleistocene subsidence of theisland.  Geology 14: 967-968

Wessel, P. and L. W. Kroenke. 1998.  The geometric relationshipbetween hot spots and seamounts:  implications for Pacific hotspots.  Earth and Planetary Science Letters 158:1-18.