Hello Scott
It seems likely that the brittle-ductile transition occurs at the
depth at which the combination of temperature and pressure is
sufficient for interstitial water of various compositions (salinity
etc) to become super-critical.
I have not, obviosly, performed the experiments necessary to prove
this, as they could be extremely dangerous.
Some of the deeper geothermal wells in Iceland have approached the
supercritical water threshold, so far without disaster.
Cheers
-Malcolm
On 29 Aug 2011, at 17
On 29 Aug 2011, at 17:56, 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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