Subduction

On Creationism & Plate Tectonics


What follows is the text of three posts I made in 1997, to the talk.origins newsgroup, on the matter of plate tectonics and young-earth creationism. Specifically, this was intended to address Walter Brown's severely flawed claim that subduction does not happen in plate tectonics on the earth, and indeed that plate tectonics does not happen either.

I have simply put the text here without formatting it for HTML, which I may do at a later date. just to avoid the effort. However, I have made all URLs live links. I think there is enough information here, both as direct content, and as reference to current work, to show quite clearly that all claims that plate tectonics does not happen, and specifically subduction, are obviously bogus.

Any questions regarding this post or its contents should be referred to me. Otherwise, since these are all newsgroup postings, and not private messages, I have left all identifying references and E-mail addresses in place.



From tim Mon Aug 11 15:10:09 1997
Distribution: 
Newsgroups: talk.origins
Followup-To: 
References: 
From: tim@aster.jpl.nasa.gov (Tim Thompson)
Reply-To: tim@aster.jpl.nasa.gov
Organization: NASA/JPL, Terrestrial Science Research Element
Subject: Re: Walter Brown actually responds to something... on subduction
Summary: 
Keywords: 

In article ,
"James J. Lippard"  writes:

> ---------- Forwarded message ----------
> Date: Sat, 9 Aug 1997 12:10:58 -0500
> From: Brad Anderson 
> To: Glenn Morton 
> Cc: creation@creationscience.com, walt@creationscience.com
> Subject: Re: [Creation Forum] Subduction Process
[ ... ]
> (Forwarded from Walt Brown)

   Take care to note that my reply is directed towards the comments
made by Walter Brown. I have re-shaped the post for shorter lines,
but have not omitted anything without so indicating.

[Walter Brown ... ]
> You do not know that plates are subducting down into the mantle.
> That is not an observation (as are earthquakes); that is an inference
> that overlooks the forces required.

   I disagree, it is an observation, and because the subduction of slabs
is an observed fact, all of Brown's arguments to the contrary are rendered
moot.

> Take a look at the best and most recently published efforts to "see"
> subducting plates in the mantle. [R. D. Van der Hilst, et al., "Evidence
> for Deep Mantle Circulation from Global Tomography," _Nature_, Vol. 386,
> 10 April 1997, pp. 578-584.]

   I have.

> Look at the locations in the mantle where the authors would say
> subducting plates are most apt to be (for example, Figures 3 and 4).

   I have done that too.

> Then ask yourself if that is where anyone would have predicted they
> should be. Why aren't subducting slabs where they should be?

   They are. Unlike Brown, I also bothered to find out where the slabs
"should" be. The continents move laterally, but the mantle moves more
vertically than laterally, and so a "motion gap" appears between the
subducting slab and the continent, as the continental plate moves away
from where the slab subducted. This is very clearly demonstrated in the
paper "The fate of slabs inferred from seismic tomography and 130
million years of subduction" by Lianxing Wen & Don L. Anderson, Earth
and Planetary Science Letters, 133: 185-198 (1995). See fig. 1 (p. 187)
and fig. 3 (p. 189). These diagrams reproduce several features found
in the van der Hilst et al. paper, especially for the Americas.

   In an even more elegant paper, "The influence of trench migration on
slab penetration into the lower mantle", Earth and Planetary Science
Letters, 140: 27-39 (1996), Ulrich R. Christensen shows that a fast
moving trench will cause the slab to 'lay down' on the 660 km endothermic
phase boundary, while a slow moving trench will allow the sinking slab
to penetrate. Both of these papers are in the reference list from van der
Hilst et al.

   Now go back to Brown's van der Hilst paper, and turn the page from
figures 3 & 4 to figure 5 (p. 582). Perhaps Brown will favor us with an
explanation for that thing sticking down into the mantle under Central
America, if there is no such thing as subduction.

   The relevant main-stream physics is described in some considerable
detail in the book "Global Tectonics", by Philip Keary & Frederick J.
Vine, Blackwell Science, 1996 (2nd ed); see chapter 8, "Subduction
Zones", pages 138-173. Of course, the entire book is relevant, and there
are very few books that deal with plate tectonics exclusively.

   If Brown tries to tell us that continents actually do not move at all,
don't believe that either. Continental motion has beenm directly measured
for years by radio astronomical very long baseline interferometry (VLBI),
and now the global positioning system (GPS) is getting into the act as
well. We can now actually watch the continents move in real time.

   Some relevant pages for those who can't live without their WWW fix:

"This Dynamic Earth: The Story of Plate Tectonics" - online edition
http://pubs.usgs.gov/publications/text/dynamic.html

"The ABC's of Plate Tectonics" by Donald L. Blanchard
http://home.earthlink.net/~dlblanc/tectonic/ptABCs.html

VLBI Group, NASA Space Geodesy Program
http://lupus.gsfc.nasa.gov/vlbi.html

International GPS Service for Geodynamics
http://igscb.jpl.nasa.gov/

                              #####

From tim Wed Aug 13 14:38:32 1997
Distribution: 
Newsgroups: talk.origins
Followup-To: 
References: <33f0473c.258347616@news>
From: tim@aster.jpl.nasa.gov (Tim Thompson)
Reply-To: tim@aster.jpl.nasa.gov
Organization: NASA/JPL, Terrestrial Science Research Element
Subject: Re: Walter Brown actually responds to something... on subduction
Summary: 
Keywords: 

In article <33f0473c.258347616@news>, tcc@sentex.net
(Douglas Cox) writes:

> In article <5so2k1$f9k@netline.jpl.nasa.gov>, tim@lithos.jpl.nasa.gov
> (Tim Thompson) wrote:
>> [Walter Brown ... ]
>>> You do not know that plates are subducting down into the mantle.
>>> That is not an observation (as are earthquakes); that is an inference
>>> that overlooks the forces required.

[And I replied ... ]
>>    I disagree, it is an observation, and because the subduction of
>> slabs is an observed fact, all of Brown's arguments to the contrary
>> are rendered moot.

[And Cox replied ... ]
[Description of SIEM model deleted ... ]

> Now, about the subduction explanation, if it is true, it seems there
> should be not one, but two zones of earthquake activity, one above,
> and one below the slab of oceanic crust, where subduction is supposed
> to be active; these would be roughly parallel, and separated by the
> thickness of the slab of oceanic crust, whatever that is. Has this
> been observed?

   Yes. Here are two papers that describe exactly that kind of double
seismic zone for deep-focus earthquakes.

======================================================================
EVIDENCE FOR TRANSFORMATIONAL FAULTING FROM A DEEP DOUBLE SEISMIC ZONE
IN TONGA
Article (Refs:32)
by Wiens-DA (*R) Mcguire-JJ Shore-PJ
Washington Univ,Dept Earth & Planetary Sci/St Louis//MO/63130

NATURE 364(6440): pp790-793 (1993 Aug 26)

----------------------------------------------------------------------

    DOUBLE seismic zones, planes or earthquakes parallel to the dip of
a subducting slab and separated by 20-40 km, provide important clues
about the earthquake generating mechanisms and strain distribution
inside subducting slabs. Double seismic zones have been found at
intermediate depths (70-200 km) in many subduction zones [1-6] but
have not been previously reported in deep slabs. Here, by relocating
earthquakes with a hypocentroidal decomposition technique [7] and
visualizing the earthquake positions and uncertainties in three
dimensions, we identify a double seismic zone at depths of 350-460 km
in the Tonga subduction zone. Source parameters of the earthquakes
determined by waveform analysis suggest different stress orientations
for the two zones, with in-plane compression in the lower zone and
in-plane tension in the upper zone. The double zone may be due to
transformational faulting, as olivine along the edges of a metastable
olivine wedge becomes warmer and transforms to spinel [8-11].
======================================================================

======================================================================
DOUBLE SEISMIC ZONE FOR DEEP EARTHQUAKES IN THE IZU-BONIN SUBDUCTION
ZONE
Article (Refs:34)
by Iidaka-T (*R) Furukawa-Y
Univ Tokyo,Earthquake Res Inst,Yayoi 1-1-1,Bunkyo Ku/Tokyo 113//JAPAN/

SCIENCE 263(5150): pp1116-1118 (1994 Feb 25)

----------------------------------------------------------------------

    A double seismic zone for deep earthquakes was found in the
Izu-Bonin region. An analysis of SP-converted phases confirms that the
deep seismic zone consists of two layers separated by approximately 20
kilometers. Numerical modeling of the thermal structure implies that
the hypocenters are located along isotherms of 500-degrees to
550-degrees-C, which is consistent with the hypothesis that deep
earthquakes result from the phase transition of metastable olivine to
a high-pressure phase in the subducting slab.
======================================================================


   This paper describes a shallower double seismic zone off the coast
of Chile. Though not so relevant to the question of deep-focus
earthquake genesis, it does show how the double seismic zone can be
used to 'see' the subducting plate, and so this is relevant to the
question of whether or not subduction happens at all.


======================================================================
AN INVERTED DOUBLE SEISMIC ZONE IN CHILE - EVIDENCE OF PHASE-
TRANSFORMATION IN THE SUBDUCTED SLAB
Article (Refs:33)
by Comte-D (*R) Suarez-G
Natl Autonomous Univ Mexico,Inst Geofis,Apartado Postal 70-172/
Mexico City 04510/Df/MEXICO/

SCIENCE 263(5144): pp212-215 (1994 Jan 14)

----------------------------------------------------------------------

    Data from two microseismic field experiments in northern Chile
revealed an elongated cluster of earthquakes in the subducted Nazca
plate at a depth of about 100 kilometers in which down-dip tensional
events were consistently shallower than a family of compressional
earthquakes. This double seismic zone shows a distribution of stresses
of opposite polarity relative to that observed in other double seismic
zones in the world. The distribution of stresses in northern Chile
supports the notion that at depths of between 90 to 150 kilometers,
the basalt to eclogite transformation of the subducting oceanic crust
induces tensional deformation in the upper part of the subducted slab
and compressional deformation in the underlying mantle.
======================================================================


   And here is one more, just to show how powerful a tool seismology
can really be in 'seeing' what goes on down inside the earth, in
detail.


======================================================================
A SHALLOW DOUBLE SEISMIC ZONE BENEATH THE CENTRAL NEW HEBRIDES
(VANUATU) - EVIDENCE FOR FRAGMENTATION AND ACCRETION OF THE DESCENDING
PLATE
Article (Refs:19)
by Prevot-R (*R) Chatelain-JL Roecker-SW Grasso-JR
Univ J Fourier,LGIT,CNRS,Bp 53X/F-38041 Grenoble//FRANCE/

GEOPHYSICAL RESEARCH LETTERS 21(19): pp2159-2162 (1994 Sep 15)

----------------------------------------------------------------------

    A shallow double seismic zone (SDSZ) has been found in the
descending Australian plate beneath the central part of the New
Hebrides island arc, directly above a large gap in intermediate depth
seismicity and between two seismic boundaries.  Ambient seismicity
occurs mostly in the upper part of the SDSZ, while earthquakes in the
lower part occur in clusters (swarms or aftershocks of large
earthquakes).  The distance between the upper and lower levels of the
SDSZ is 50-70 km, and they are joined at 80 km depth by a near-
horizontal band of seismicity.  Thrust-faulting mechanisms
predominate for earthquakes in the upper level of the SDSZ.  Those in
the lower level, however, appear to be normal faulting, despite their
being aftershocks of large thrust events.  We suggest that with the
absence of a pull from the detached lithosphere the upper part of the
Australian plate in the region of the SDSZ is resistant to subduction,
and thus the downward displacements caused by large earthquakes in the
adjoining regions result in a localized rebound.  The location of the
aftershocks within the plate suggests that a new plate boundary is
forming, which will eventually replace that outlined by the residual
seismicity in the upper level.  Thus the leading edge is decoupling,
and the boundary will eventually shift back to the lower level of the
SDSZ.
======================================================================

[See the website for more on Cox's SIEM model ... ]
> Douglas Cox
> http://www.sentex.net/~tcc

                              #####

From tim Wed Aug 27 16:13:43 1997
Distribution: 
Newsgroups: talk.origins
Followup-To: 
From: tim@aster.jpl.nasa.gov (Tim Thompson)
Reply-To: tim@aster.jpl.nasa.gov
Organization: NASA/JPL, Terrestrial Science Research Element
Subject: On Walter Brown & Plate Tectonics
Keywords: 

   I am writing this in response to posts I have seen to talk.origins,
from Jim Lippard, with regards to the discussion between Glenn Morton
and Walter Brown, on the topic of subduction in plate tectonics. I have
reviewed the material on Dr. Brown's web pages, and I have concluded
that, like his discussion of the tidal interaction between the earth
and the moon, his discussion of subduction is trivially false, and far
less interesting than his own self confidence would imply. It is my hope
that the material herein might be of some assistance to Lippard and/or
Morton in their continuing efforts. My primary intention is to show the
main arguments in outline form, with copious references to the literature,
where the detailed technical arguments can be followed in as much detail
as the reader desires.

   Walter Brown presents his explanation for why subduction of plates into
the lithosphere cannot occur, in one of his technical notes, in his web
page collection:

http://www.creationscience.com/onlinebook/technicalnotes/subduction.shtml

The model presented on this page consists of a single inequality that
relates the downward force on the plate to the friction resisting the
plate's downward motion. The forces, he reasons, must be greater than
the frictional resistance, or subduction cannot occur. He presents
his equation, and a numerical evaluation of it, shows that the net
force is less than the frictional resistance, and so 'proves' that
subduction cannot occur. But the inequality he presents is wrong,
and therefore his results must be re-examined in light of a correct
solution to the problem.

   Brown makes two major errors at once. First, he uses a model of
solid-to-solid friction. But the mantle and plate must be treated
as viscous fluids, not as a solid-to-solid system, so his chosen
friction model is unphysical. To compound the error, Brown uses
a (fluid) lithostatic pressure for the mantle, which is inconsistent
with the (solid) friction model. Second, he evaluates his unphysical
inequality with the wrong numbers anyway. Brown uses mechanical
properties for rock from an engineering handbook (not surprising
for an engineer) to provide numerical values, but those values are
inappropriate for materials under mantle conditions. Brown says,
for instance, that the properties are independent of both composition
and temperature, up to 350 degrees Celsius. However, temperatures
far higher than that are encountered in the mantle, and Brown makes
no mention of pressure at all. It is well known, and has been for
many years, that mantle properties are very much temperature and
pressure dependent, and that rock creep under mantle conditions is
very much dependent on composition and grain size. Both Brown's model,
and its numerical evaluation, are wrong.

   Although the falsification of Brown's model is a trivial
exercise, we should go further, and decide whether or not plate
tectonics and subduction really are physically reasonable, and whether
or not it is fair to say that subduction is actually an observed,
rather than an inferred process. My answer in both cases is "yes".

   First, consider whether or not subduction is physically reasonable.
That it is, should be fairly obvious. A correct view, that a fluid
plate is falling through a surrounding fluid medium, does not violate
any of our basic physical sensibilities. The plate is more dense,
and more viscous, because of its lower temperature, and will continue
to sink through the less viscous mantle until buoyancy forces reverse
the trend. Research shows that this does indeed happen; plates
subducted at a shallow angle do not penetrate into the deeper mantle,
but rather reverse course as their temperature equilibrates with the
surrounding mantle, while plates subducted at a sharp angle, do
penetrate the deeper mantle [there is a discontinuity in the mantle
at 660km depth that the plates must penetrate] (Christensen, 1996).

   Next, consider whether or not it is fair to say that subduction is
an "observed" phenomenon. It is, and the key lies in the observations
that density variations in the mantle, implied by seismic velocities,
show us the subducting plates in a manner very similar to that in
which an X-ray shows up the bones of a skeleton, and that double
planed earthquake zones not only outline the subducting plates,
but also indicate differing earthquake mechanisms above and below
the plate, as one would expect (McGuire & Wiens, 1995; Compte & Suarez,
1994; Prevot et al., 1994; Wiens, McGuire & Shore, 1993).
Note also that deep focus earthquakes occur only in subduction zones,
and that earthquakes of all varieties are strongly concentrated at
plate boundaries.

   Brown's specific criticism of van der Hilst, Widiyantoro & Engdahl,
1997, that the subducted slabs are not where they are supposed to be,
or not where one might expect them, is also easily falsified. As the
plates plunge downward (i.e., subduct), the continents continue their
lateral motion and override the subducting slab. This is very well
illustrated by Wen & Anderson, 1995. Although pre-dating the van
der Hilst et al. paper by two years, Wen & Anderson figures 1 & 3
nicely predict the arrangement of deep subducted plates observed
by van der Hilst at al., and in their figures 1c & 1d. Agreement
here is quite pronounced for the Americas, Indonesia, and the area
east of Australia. Furthermore, Brown seems to ignore altogether
the dramatic demonstration in van der Hilst et al., figure 5a,
which certainly appears to show a subducting slab beneath central
America. Christensen, 1996, also discusses the general topic of
the effect of plate motion on subduction.

   The combination of tomography, double seismic zones, the
distribution of tensional & compressional earthquake mechanisms,
and the fact that deep focus earthquakes occur only in subduction
zones, makes it very difficult to avoid the conclusion that subduction
is an observed process.

=================
REFERENCES CITED:
=================

Compte, D. & G. Suarez
"An Inverted Double Seismic Zone in Chile - Evidence of Phase
Transformation in the Subducted Slab"
Science, 263:(212-215), 14 January 1994


Christensen, Ulrich R.
"The influence of plate migration on slab penetration into the
lower mantle"
Earth and Planetary Science Letters, 140:(27-39), 1996


McGuire, J.J.& D.A. Wiens
"A Double Seismic Zone in New Britain and the Morphology of the
Solomon Plate at Intermediate Depths"
Geophysical Research Letters, 22:(1965-1968), 1 August 1995


Prevot, R.; J.L. Chatelain, S.W. Roecker & J.R. Grasso
"A Shallow Double Seismic Zone Beneath the Central New Hebrides
(Vanuatu) - Evidence for Fragmentation and Accretion of the
Descending Plate"
Geophysical Research Letters, 21:(2159-2162), 15 September 1994


van der Hilst, R.D.; S. Widiyantoro & E.R. Engdahl
"Evidence for deep mantle circulation from global tomography"
Nature, 386:(578-584), 10 April 1997


Wen, Lianxing & Don L. Anderson
"The fate of slabs inferred from seismic tomography and 130 million
years of subduction"
Earth and Planetary Science Letters, 133:(185-198), 1995


Wiens, D.A.; J.J. McGuire & P.J. Shore
"Evidence for transformational faulting from a deep double seismic
zone in Tonga"
Nature, 364:(790-793), 26 August 1993

=====================================================
ALSO SEE THESE UNCITED REFERENCES FOR FURTHER READING
=====================================================

Green II, Harry W.
"Solving the Paradox of Deep Earthquakes"
Scientific American, 271:(64-71), September, 1994


Hynes, Andrew; Jafar Arkani-Hamed & Reinhard Greiling
"Subduction of continental margins and the uplift of high-pressure
metamorphic rocks"
Earth and Planetary Science Letters 140:(13-25), 1996


Iidaka, T. & Y. Furukawa
"Double Seismic Zone for Deep Earthquakes in the Izu-Bonin
Subduction Zone"
Science, 263:(1116-1118), 25 February 1994


Jacobs, J.A.
"Deep Interior of the Earth"
Chapman and Hall, 1992
Vol. 6 in the series "Topics in the Earth Sciences"
ISBN 0-412-36570-7      QE509.J27
[University College of Wales, Aberystwyth,
and University of Cambridge]


Keary, Philip & Frederick J. Vine
"Global Tectonics"
Blackwell Science, 2nd ed., 1996
ISBN 0-86542-924-3      QE 511.4.K34


Poirer, Jean-Paul
"Introduction to the Physics of the Earth's Interior"
Cambridge University Press, 1991
Vol. 3 in the series "Cambridge Topics in Mineral Physics
and Chemistry"
ISBN 0-521-38097-9 (H)
ISBN 0-521-38801-5 (P)
QE 509.P64
[Institut de Physique de Globe, Paris]


Shimenda, Alexander I.
"Subduction: Insights from Physical Modeling"
Kluwer Academic Publishers, 1994
Vol. 11 in the series "Modern Approaches in Geophysics"
ISBN 0-7923-3042-0      QE 511.46.S54
[Laboratoire de Geophysique et Tectonique,
Universite de Montpellier II, Montpellier, France]


Wang, K.L. & G.C. Rogers
"An Explanation for the Double Seismic Layers North of the Mendocino
Triple Junction"
Geophysical Research Letters, 21:(121-124), 15 January 1994

                                   ===
                                   END
                                   ===



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