Hi Scott,
I think of the lithosphere as the part of the earth that generally sustains stresses greater than a few MPa; the asthenosphere does not. This doesn't therefore rely on defining the deformation mechanism or dominant rheological behavior. I'd be interested to hear if anyone else supports this definition.
Cheers
Virginia
On 30/08/2011, at 4:56 AM, "Scott T. Marshall" <[log in to unmask]> wrote:
> 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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