Ductile Deformation Processes
Chapter 9 in
Van der Pluijm and Marshak
How can solid rock flow like
taffy?
Swiss Alps
Ice
is a solid
phase that
flows
when
conditions
are
appropriate
and
there
are
forces
acting
on the
material.
“g”
Ductile deformation is permanent
Accomplished by these 3 mechanisms:
• Cataclastic flow
• Diffusional mass transfer
• Crystal plasticity
Mechanism is determined by:
•
•
•
•
•
•
Temperature
Stress
Strain rate
Grain size
Composition
Fluid content
Effect of Composition
Plag.
Quartz
Olivine
Pyroxene
Calcite
Halite
Gypsum
Slow
1) Cataclastic flow:
small pieces sliding past each other
Cataclastic flow:
Is it brittle or ductile?
Ductile on large scale but brittle on small scale
2) Crystal Plasticity:
The crystal changes shape by movement of
lattice defects
Point Defects are vacancies or impurities
Dislocations form in 2 ways:
• During crystal growth
• During deformation
By moving the dislocations, the crystal changes shape
Migrating
defects
change the
lattice shape.
Occurs under
stress
Line defects in olivine
Twinning also changes the crystal shape
3) Diffusional Mass Transfer has three
mechanisms:
• Grain boundary diffusion
• Volume diffusion
• Pressure solution
High T
Low T
Grain boundary diffusion
Atoms will diffuse along the edges of the grain
to create shape changes (strain).
Volume diffusion
Atoms will diffuse through the center of the grain
to create shape changes (strain).
Pressure solution:
Material is dissolved under
stress
Dissolved rock
goes into veins
Deformed Marble
Mylonite