Talk:Brittle-ductile transition zone
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Although it is correct that the increase in pressure with depth is an important part of defining the brittle-ductile transition zone by increasing the resistance to brittle fracture, it is equally important to consider the reduction in ductile strength with increasing depth and higher temperature. —Preceding unsigned comment added by Mikenorton (talk • contribs) 18:57, 10 October 2006
- How would increased depth/pressure reduce the ductile strength? -- Petri Krohn 17:27, 11 October 2006 (UTC)
The ductile strength of minerals is decreased by increasing the temperature for a constant strain-rate. There is generally also a pressure dependence but the effect of temperature dominates. Mikenorton 21:57, 11 October 2006 (UTC)
Petri, to elaborate, the temperature dependence of strength is shown by experimentally derived ductile flow laws of the form
where 
is the strain rate, σ is the strength, n an exponent that varies from one (a Newtonian flow) upwards, A is a constant dependent on the material, Q the activation enthalpy for the process and T the absolute temperature
This is known as power-law flow and is generally thought to be a good representation of the ductile deformation of the lower parts of the crust and lithosphere e.g. http://www.geology.ucdavis.edu/~gel219/F05_billen/lecture3.pdf or http://www.uwgb.edu/dutchs/structge/strsparm.htm see also Creep (deformation)
It is therefore not correct to say that "deep rock does not become soft", it does but this is only half of the story because the brittle strength increase with depth as the increased confining pressure prevents fracture-related dilatancy, it is the combination of the two effects that is important. The link that you provided only tells half the story and so does the extra text you put in, best to leave it all out I think. Mikenorton 20:35, 16 November 2006 (UTC)
