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: email@example.com (Tim Thompson) Reply-To: firstname.lastname@example.org 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: email@example.com, firstname.lastname@example.org > 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: email@example.com (Tim Thompson) Reply-To: firstname.lastname@example.org Organization: NASA/JPL, Terrestrial Science Research Element Subject: Re: Walter Brown actually responds to something... on subduction Summary: Keywords: In article <33f0473c.258347616@news>, email@example.com (Douglas Cox) writes: > In article <firstname.lastname@example.org>, email@example.com > (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  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: firstname.lastname@example.org (Tim Thompson) Reply-To: email@example.com 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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