Ductile Deformation Processes

Chapter 9 in

Van der Pluijm and Marshak

How can solid rock flow like taffy?

Swiss Alps

Ice is a solidphase thatflowswhen conditionsare appropriateand there are forcesacting 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

Slow

Halite G

ypsum

Calcite

Quartz

Plag.

Pyroxene

Olivine

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 Low T

High T

Grain boundary diffusion

Atoms will diffuse along the edges of the grainto create shape changes (strain).

Volume diffusion

Atoms will diffuse through the center of the grainto create shape changes (strain).

Pressure solution:Material is dissolved under

stress

Dissolved rock goes into veins

Deformed Marble

Mylonite