>Dear Scott,

Far from being a naive or foolish question, you have hit upon and 
asked one of the central and most important questions in tectonics 
and the material science of olivine. There are two splendid 
(opposing) publications that address this question head-on in the GSA 
magazine, one by Watts and Burov, the other by James Jackson. I have 
been working on and worrying about this problem for at least ten 
years and am gradually moving towards a "conclusion" but I am 
sufficiently far from that "conclusion" to prevent my giving you a 
definitive "solution" This is a seriously difficult problem that has 
occupied better minds than mine. The best starting point is to think 
of the lithosphere as a boundary conduction layer that may deform but 
does not internally convect, then think about the 
rheology/deformation mechanisms  of olivine and quartz for a PTt 
spectrum. It is not simple.

Best wishes,
John  Dewey

>Hello Geo-Tectonics Folks,
>
>I have run across a conundrum of sorts concerning my understanding 
>of the lithosphere, asthenosphere, and the brittle-ductile 
>transition. I hope that folks on this discussion list can be of 
>help. Hopefully, I haven't said something foolish in here...
>
>In my intro geology course here at Appalachian State University, I 
>teach about the layers of the Earth (crust, mantle, and core). I 
>also discuss the lithosphere and asthenosphere and the 
>brittle-ductile transition. I define the lithosphere as the layer of 
>the earth that undergoes brittle behavior and the asthenosphere as 
>the layer that undergoes ductile flow. Later on in the course I talk 
>about the LVZ and stuff like that, but in the beginning, I just want 
>them to realize that there is more than one way to subdivide the 
>layers of the earth and that the tectonic plates are lithospheric 
>not crustal.
>I then talk about the brittle-ductile transition because this marks 
>the approximate depth of the base of seismicity (and the strongest 
>portion of the lithosphere/crust) and it therefore controls the 
>magnitude potential of a given active tectonic region. E.g. 
>subduction zones can make the biggest earthquakes because they have 
>the largest potential rupture area. I think this is a useful concept 
>to teach intro students because I often hear the students saying 
>things like..."I heard that the next earthquake in California could 
>be a M9.5 or more."
>This is of course not possible (unless we are way off in our 
>understanding of basic earthquake physics). I think that teaching 
>about the brittle-ductile transition is therefore useful, even for 
>intro-level students. The problem is that, as I have defined 
>lithosphere, the entire lithosphere should all undergo brittle 
>failure.
>
>
>So, here is my question (Finally!): If the brittle ductile 
>transition lies within the lithosphere, does this not contradict the 
>definition of lithosphere? How do folks on this list define these 
>terms to intro-level students? Should I define 
>lithosphere/asthenosphere as Fowler's text does (i.e. non-convecting 
>vs. convecting)?
>
>Any thoughts or advice list members may have on this would be very 
>enlightening.
>Cheers,
>-Scott
>
>--
><><><><><><><><><><><><><><><>
>Scott T. Marshall
>Department Of Geology
>Appalachian State University
>572 Rivers St.
>Boone, NC 28608
>
>http://www.appstate.edu/~marshallst/
>ftp://pm.appstate.edu/pub/prog/marshallst/


-- 
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Prof. John F. Dewey FRS, M.R.I.A., FAA, Mem. Acad. Eur., Mem.
US Nat. Acad. Sci., Distinguished Emeritus Professor University of 
California, Emeritus Professor and Supernumerary Fellow, University 
College Oxford.

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