GEOS 304 Lab 7: Microstructure/Deformation Mechanisms
Spring 2002
Name:________________
Introduction
The aim of this lab is to introduce you to the range of deformation-accomodating processes that
occur both within individual crystals and at the boundaries between crystals. These processes
occur at the atomic scale, but profoundly determine the dynamic behavior of the crust and
mantle. An understanding of these processes is useful in structural geology, but can also be
applied to a number of problems ranging from the behavior of glacial ice to the interpretation of
seismic anisotropy. There is also much overlap in the realm of material science. In structural
geology, an understanding of deformation mechanisms is useful for providing evidence of
deformation processes. This evidence in turn provides a basis for understanding rock rheology
(flow of rock and the mechanisms of flow), helps in interpretation of deformation histories, and
can provide evidence of metamorphic environments. An understanding of microstructures also
allows the types of deformation we have seen so far in class be put in a broader context.
Hopefully by the end of this lab you will have a better understanding of why brittle and ductile
deformation occur in certain environments and why increasing strain rate tends to lead to brittle
behavior.
Much of the material covered in lab today can also be found in Davis & Reynolds chapters 4
and 9. This lab handout is intended to serve as an outline of information relevant to
understanding processes addressed by the lab questions.
Mechanisms of Deformation
Diffusion Mechanisms
Diffusion processes occur by the movement of point defects (interstitials or vacancies).
Vacancies move towards 1, and material moves towards 3. The net effect is shortening in the
1 direction and lengthening in 3 direction.
Crystals can deform by lattice diffusion (diffusion within grains) or grain boundary diffusion
(like pressure solution – diffusion along grain boundaries) alone.
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Dislocation Mechanisms
Dislocation Glide: the movement of a dislocation restricted to one glide plane (one lattice
plane).
Dislocation Climb: by diffusing a few atoms above or below the glide plane, a dislocation can
“climb” out of that plane, to another. In this way it can get out of the way of another dislocation
(they don’t like to get very close to each other) or go around a large impurity in the lattice.
Climb is a mechanism by which work hardening (see glossary) is avoided.
Dislocation Creep: = Dislocation Glide + Dislocation Climb. In combination, glide and climb
can enable a grain to deform exclusively by dislocation movements.
Controlling Factors
The deformation mechanisms mentioned above interact to form unique structures and fabrics in
rocks. The activity of these mechanisms, and therefore the resulting fabrics, are governed by
circumstances such as temperature, strain rate, deviatoric stress (1 - 3), pressure (3), grain
size, and water content in a sample. In this lab, for the sake of time, we will focus only on the
first three.
Temperature: with increasing temperature, diffusion rates increase due to increased vibrational
energy in crystal lattices and mobility of defects. Increasing temperature therefore hastens
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diffusion processes including dislocation climb and thus dislocation creep. Recovery also
accelerates.
Deviatoric Stress: Drives and creates dislocations; causes deformation. More deviatoric stress
results in more dislocations, thus more deformation.
Strain Rate: Higher strain rates tend to produce higher internal stress (strain energy) if recovery
mechanisms can’t keep up. If fast enough, deformation goes back to the brittle regime
regardless of temperature. Increasing strain rate produces much the same effect in rocks as
decreasing temperature.
Recovery
When a differential stress is imposed on a rock, deformation mechanisms are activated in such a
way as to accommodate the deformation while minimizing the internal strain energy in the rock.
When a crystal is deformed, dislocations are introduced and increase the internal energy by
local changes in the distance between atoms; the increase in internal energy is proportional to
the increase in total length of dislocations in a crystal, also known as the dislocation density.
Processes of recovery or recrystallization tend to shorten, order, or destroy the dislocations.
Vacancies migrate towards and release dislocation tangles, bent dislocations straighten, and
dislocations can be arranged into networks. These processes decrease the total dislocation
length and hence the internal strain energy of crystals, thus they operate following the
thermodynamic principle of minimizing total free energy in a system. During deformation,
disordering and ordering mechanisms will compete. After deformation stops, ordering
mechanisms progress towards an equilibrium situation of the shortest possible length of
dislocations in the crystal lattice. The term recovery is used to cover these ordering
mechanisms.
Recovery Mechanisms
Dislocation Climb: (see above)
Dynamic Recrystallization: Recrystallization during deformation, produces new strain-free
grains. Dynamic recrystallization occurs by two processes, “subgrain rotation” and/or “grain
boundary migration”. Both processes occur more rapidly with increasing temperature
(diffusion rates) i.e. larger recrystallized grains are associated with increased temperatures or
slow strain rates. Dynamically recrystallized grains usually form near grain boundaries.
Subgrain Rotation: occurs when dislocation tangles arrange themselves into walls having lower
energy configurations than tangles. There is a change in lattice orientation across these
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boundaries which grows as dislocations accrue. Gradually these become actual grain
boundaries as the angle becomes larger.
Grain Boundary Migration: occurs when a large gradient of lattice energy exists across a grain
boundary. The lattice with the high dislocation density will “get eaten” by the lattice with lower
energy. This process is achieved by diffusion across the grain boundary.
Deformation Mechanism Maps
In order to establish the conditions under which deformation mechanisms are active, data from
experimental deformations have been combined with observations of rocks deformed at known
metamorphic conditions. The experiments are quite similar to our experiments in the rock
mechanics lab except that the samples are kept at elevated temperatures and deformed
continuously for a number of days. Graphs have been made from compilations of this data and
give some idea of the conditions under which the different mechanisms are active.
Experimentally Deformed Quartzite: An Example
Close inspection of deformed rocks is partially motivated by the idea that evidence of
deformation conditions such as temperature, strain rate, and stress directions can be found in
microstructures and fabrics. Below is a description of a series of quartzite samples deformed at
nearly the same strain rate, but at different temperatures. We label the samples by increasing
temperature from 1 through 4:
Sample 1 is cold (room temperature). As stress is applied, the sample accommodates a small
amount of strain (mainly elastic). The sample fails catastrophically along faults.
Microscopically, the rock is filled with a series of cracks. Undulose extinction is not seen.
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Interpretation: Dislocation glide is active only to a very small degree, however diffusion and
therefore dislocation climb are not. Accommodation of strain prior to failure is due to minor
amounts of dislocation glide and crystal lattice stretching. Work hardening occurs quickly,
as dislocations pile up and dislocation climb and recovery are not active. Patchy or
undulose extinction is not seen because relatively few dislocations have been created.
Sample 2: No fractures are formed. Undulatory extinction is strong and patchy. A mantle of
extremely small grains may be visible around original grains.
Interpretation: Diffusion occurs, but remains too low for dislocation climb to operate as a
significant recovery process. The small amount of diffusion however may allow
dynamically recrystallized grains to form at grain boundaries. The patchy and irregular
extinction is due to dislocation tangles (work hardening).
Sample 3: Quartz grains are flattened and show strong undulose extinction. There are a large
number of small, newly recrystallized grains mantling the larger, very deformed original
grains.
Interpretation: Dislocation climb can occur allowing tangled dislocations to reorganize
themselves in lower energy configurations, namely subgrain boundaries, which then become
grain boundaries (after rotation of more than about 15 degrees).
Sample 4: Equant grains with 120 degree triple junctions, weak undulatory extinction.
Interpretation: Recovery is widespread. Grain boundaries migrate very quickly, so newly
formed grains (low-strain) also grow quickly at the expense of strained grains.
Glossary
Deviatoric Stress- 1 –  3
Internal Strain Energy- Proportional to the dislocation density; at a minimum when a crystal lattice
is free of dislocations; reduced by recovery processes.
Interstitials- Point defects in a crystal lattice; an additional lattice element in between regular lattice
units (See figure page 4)
Undulose or Undulatory Extinction- Irregular or patchy extinction of a single crystal under crossed
polars due to a distorted crystal lattice with a high concentration of defects.
Vacancy- Point defect in a crystal lattice; a missing lattice element in between regular lattice units
Work hardening- Same as strain hardening. The tangling of dislocations into densities which
prevent movement. This has the effect of making crystals “harder”.
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Question 1:
On the Clue Lab Macs can be found a file entitled recrystallizationstatic.mov. This short film
shows a material recrystallize under static conditions. Static recrystallization (also known as
annealing) occurs in the absence of differential stress. Watch the movie carefully several times,
concentrating on changes in grain boundaries and grain sizes. Static recrystallization is driven by
the need to lower the overall strain energy in a rock. In a few sentences, describe the changes
occurring during recrystallization and speculate on how these processes lower overall energy in the
sample.
Also on the Clue Lab Macs are a number of thin section photographs. Compare slides 13 and 14.
Both of these quartzite samples were deformed in an experimental apparatus under nearly exactly
the same conditions, however one of the samples was later statically annealed (allowed to
recrystallize statically after deformation). Annealing is accomplished in the laboratory by heating a
sample for a period of hours after an experiment. Based on your observations of the static
recrystallization film, which sample was statically annealed? How does the annealed fabric differ
in appearance from the fabric in the other sample? Note at least 2 differences- labeled sketches
may be useful. What do these visible changes represent at the atomic or crystal lattice level?
Compare sample 14 (deformed quartzite) to sample 5 (undeformed quartzite). What differences
between slides 5 and 14 would allow a geologist to tell in which sample deformation has occurred?
Hint: Undeformed quartzite is formed by the precipitation of quartz (or other cementing material)
onto and between original quartz grains in a sandstone. Do you see any evidence of this process in
slide 5?
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Question 2:
Watch the film recrystallizationdynamic.mov a few times. The material in this film is
recrystallizing as a result of applied differential stress. Look at the movie several times,
concentrating on changes in grain boundaries. In a few sentences describe the visible changes that
occur in grain C as it is deformed.
What happens to grain B as it is deformed? Make a few sketches showing this progression.
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Question 3:
Structural geologists are often interested in determining the conditions of deformation, e.g.
temperatures, stresses, strains, and strain rates. Fabrics developed in deformed rocks are the direct
consequence of these and other factors, and so it is often possible to extract useful information
regarding deformation conditions by studying microfabrics. The above slides and films have
shown the effects of processes that compete during deformation. Differential stress applied to rocks
leads to the production of dislocations evidenced by wavy (undulose) extinction. Differential stress
increases internal strain energy. Annealing or recovery (seen most clearly in the static
recrystallization movie) reduces these effects – lowers energy- by reducing the volume of grain
boundaries, growing undeformed grains at the expense of strained grains (reducing dislocation
density), and seeking 120 degree grain boundary intersections. Take a close look at samples 4,8,
and 12. These samples were all deformed at nearly the same strain rate but at different
temperatures. Based on your observations of the films and the information in the introduction, rank
the samples in order from lowest to highest deformation temperature and explain the logic behind
the ranking.
Question 4:
Slide 22 shows a naturally deformed quartz-mica aggregate. What is the relationship between the
foliation defined by the quartz grains and that defined by the mica grains? Make a small sketch
showing this relationship. What does this relationship suggest about the type of shear (pure or
simple) involved in the deformation of this rock? If simple shear, what sense of shear is indicated?
Hint: It may help to realize that rotation and recrystallization of mica and quartz occur at different
rates- quartz recrystallizes relatively quickly and may represent the incremental strain ellipse, mica
recrystallizes quite slowly and represents the finite strain ellipse.
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