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
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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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