Scott
I use the following terminology with introductory classes. The lithosphere is the outer part of the Earth that behaves in a relatively 'rigid' manner, compared with the underlying asthenosphere, which is able to 'flow' much more rapidly/easily. (Much of the 'stiffness' of the lithosphere is due to the abundance of olivine in the lithospheric mantle, which does not flow readily under conditions that exist in the outer ~100 km of the Earth.)
Although the entire lithosphere is 'stiff' or 'rigid', only the upper part of the crust is truly 'brittle' in the sense that it responds to stress by fracturing - generating earthquakes. Deeper parts of the lithosphere undergo slow creep, though much more slowly than the asthenosphere below.
My experience has been that many introductory students have misconceptions acquired from popular media presentations that confuse the lithosphere, the crust, and the brittle upper crust. Part of the problem is that the term 'crust' implies to the average person a layer that is both brittle and rigid, whereas neither property is actually a defining characteristic of the crust. (In actuality it is defined on the basis of a discontinuity in the velocity of seismic waves that reflects a change in bulk composition.)
Hope that helps
John Waldron
On 2011-Aug-29, at 10:56 AM, Scott T. Marshall 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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John Waldron, Department of Earth & Atmospheric Sciences, 1-26 Earth Sciences Building, University of Alberta, Edmonton, AB Canada T6G 2E3
Tel: 780-492-3892. Fax: 780-492-2030. [log in to unmask]
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