1. GEO 5/6690 Geodynamics 10 Oct 2014 Last Time: RHEOLOGY
Laboratory studies & mineral physics suggest two main
types of thermally-activated, stress-driven creep (flow) in
“solid” rock:
• Diffusion creep can occur as diffusion of exotic atoms or
vacancies through a grain or movement of atoms along grain
boundaries (“Coble creep”). The relationship of stress to
strain rate is linear, viscoelastic, and sensitive to grain size:
• Dislocation creep describes movement of dislocations
through a lattice, is nonlinear, & dominates unless grain size
<< 1 mm:
Read for Wed 15 Oct: T&S
© A.R. Lowry 2014

2. Next Journal Article Reading:
For Monday Oct 13: Watts & Burov (2003)
Lithospheric strength and its relationship
to the elastic and seismogenic layer
thicknesses. Earth Planet. Sci. Lett.
213(1-2)
(Xiaofei will prep discussion materials)

3. Laboratory studies & mineral physics suggest two
dominant “flavors” of non-recoverable strain:
(1) Linear viscoelastic creep: “Diffusion”
where viscosity
Here:
R = gas constant
T = temperature Ea = activation energy
P = pressure Va = activation volume
d = grain diameter D0 = frequency factor
m = 2 in crystal interiors (rock mat’l prop’s)
3 on crystal boundaries

4. Laboratory studies & mineral physics suggest two
dominant “flavors” of non-recoverable strain:
(2) Nonlinear Viscoelastic:
“Dislocation creep”
where effective viscosity
Here:
R = gas constant
T = temperature
P = pressure Ea = activation energy
b = dislocation density Va = activation volume
n ~ D0 = frequency factor
 = shear modulus (rock mat’l prop’s)
Edge dislocation
Screw dislocation

5. Laboratory studies of rock strain use roughly the same
equations as those derived from first principles in mineral
physics, but collapse them to observable constant params
depends on:
• Lithology
(pyroxene > olivine > feldspar > quartz)
• Water fugacity fH2O
• Temperature T
(and to a lesser extent)
• Strain rate 
• Grain size d
• Pressure P .
Ideally, we would like to use geophysics to
determine each in situ! But it’s not so simple.

6. Brittle-field (Amonton’s or
Byerlee’s law) assumes
elastic-plastic
constitutive law
In Yield
Strength
Envelopes,
we essentially
assume a
steady-state
(i.e., constant
strain rate)
That assumption
is valid for
problems in
which time-
scales are long
and stress is
~ constant.
Ductile assumes Newtonian
or non-Newtonian viscous flow

7. Moho temperature TMoho from Pn phase:
Pn velocity variation
Moho temperature from
Pn & mineral physics
Buehler & Shearer, JGR 2010
Schutt et al., Geology in prep

8. Wait… What? Temp under ND > NV-UT?
Moho temperature from
Pn & mineral physics
(Partly, but not entirely,
because the Moho is
deeper in the stable part
of the continent…)
Schutt et al., Geology in prep

9. Dry Wet These temperatures are sufficiently high to ensure lower
crustal flow for
all likely crustal
lithologies,
wet or dry…
Viscosity is
very sensitive
also to lithology
& water!
Pyroxene
Feldspar
Quartz
1019
1022