Week 6
Microscopic Deformation. Read pages 150-192 and 199-201
in Chapter 4: Deformation Mechanisms and Microstructures.
You are expected to read all the
sections listed below. Information from the sections in italics
will be discussed in class. You are expected to read the other
sections and you may be called on in class to answer questions
based on that material.
- Crystalline Structure and the Strength of Solids
p.152-161
- Deformation Mechanisms p.161-189
- Deformation Experiments p.190-192
- The Brittle-Ductile Transition p.199-201
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You should become familiar with the
following terms during this weeks lectures and readings:
| annealing |
brittle-ductile transition |
cataclasis |
cataclastic flow |
| Coble (grain-boundary) diffusion |
crustal strength envelope |
diffusion creep |
deformation mechanisms |
| chemical concentration gradients |
deformation map |
dilatency |
dislocation |
| dislocation climb |
dislocation creep |
dislocation glide |
dislocation tangles |
| dissolution creep |
dynamic recrystallization |
edge dislocation |
grain boundary sliding |
| high-angle boundary |
intergranular microcrack |
interstitial atoms |
intragranular microcrack |
| ionic bonds |
jogs |
kinks |
line defects |
| mechanical twinning |
microcracks |
Nabarro-Herring creep |
overgrowths |
| planar defects |
point defects |
pressure shadows |
pressure solution |
| recovery |
recrystallization |
grain boundary slip |
solid state diffusion |
| strain hardening |
stylolites |
subgrains |
superplastic creep |
| theoretical yield strength |
transgranular microcrack |
unit cell |
vacancies |
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You should be able to answer the questions
below following this week:
- Why is quartz stronger than mica?
- Why don't slip planes develop in halite where the ions
are most closely spaced?
- Rank the following bonds in order of strength (highest to
lowest). Ionic, Covalent, Metallic
- Why are the actual yield strengths of minerals much lower
than theoretical yield strengths?
- What deformation conditions (temperature, pressure, rock
type etc) are necessary for the following deformation
mechanisms: cataclasis, mechanical twinning, diffusion
creep, dislocation creep, dissolution creep,
recrystallization?
- What factors would have controlled the distribution of
microscopic deformation mechanisms in the Apalachians
during the formation of the orogen? For simplicity,
assume there was a single episode of orogeny. Describe
how physical conditions and rock type varied within the
orogen, and discuss which deformation mechanisms would
have been active at different locations and why.
- Contrast deformation resulting from pressure solution and
cataclasis.
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Microscopic deformation
crystal lattice - systematic arrangement of atoms in
minerals
Atoms are held together with bonds
- Covalent bonds - adjacent atoms share electrons
- Ionic bonds - when one atom loses borrows an
electron from another atom
- Metallic bonds - electrons move freely through
material
perfect crystal - all atomic sites filled with
"correct" atoms, no gaps, no substitutions
slip plane - crystallographic plane along which the
crystal fails, typically where cations are most closely spaced
The theoretical yield strength of atoms is typically much
higher than actual strength because of the presence of defects
- imperfections in the crystal lattice. Defects come in three
flavors: point defects, line defects, and planar defects.
Point defects
The crystal lattice has an equilibrium distribution of point
defects created during lattice formation, ductile deformation, or
rapid cooling from high temperatures. For example:
- vacancies - unoccupied sites
- substitutions - "foreign" atoms occupy
atomic sites
- interstitial atoms - atoms wedge in between normal
sites in the crystal lattice.
During solid-state diffusion, point defects migrate
through the crystal lattice from areas of high stress to areas of
low stress.
Line defects (dislocations) and Planar defects
A line defect is made up of a line of atoms move through the
crystal lattice as a single unit, represents the edge of a plane
of atoms. Planar defects represent grain boundaries, or
crystallographic twin planes, or extra planes of atoms within a
lattice.
Deformation Mechanisms
Active deformation mechanisms are controlled by composition,
texture (grain size), temperature, confining pressure, fluid
pressure, differential stress, and strain rate. Five principal
deformation processes can be arranged on a deformation map:
- microfracturing, cataclasis, frictional sliding -
formation, growth, and displacement on individual
microcracks, or on systems of microcracks to create a
pervasive array of fractures;
- mechanical twinning, kinking - bending of crystal
lattice;
- diffusion creep - distortion and dilation of
crystals by movement of vacancies or atoms along grain
boundaries;
- dissolution creep - distortion and dilation of
crystals by dissolution and reprecipitation of material
with aid of fluids;
- dislocation creep - intercrystalline slip of the
crystal lattice.
Deformation Maps
- show temperature vs. differential stress vs. strain rate
for a specific grain size of a specific mineral
- maps are divided into deformation fields representing the
dominant deformation mechanisms
Microfractures, Cataclasis, Frictional Sliding
- grain-scale brittle processes
- microcracks are present in all rocks and may be
- intragranular - within a single
grain
- intergranular - around grains,
along grain boundaries, especially in fine
grained rocks
- transgranular - cut across adjacent
grains
- cataclasis - brittle granulation of rock, angular
fragments, progressive decrease in grain size with
deformation, occurs at low temperatures and pressures
- cataclastic flow - slip on discrete
surfaces that separate undeformed areas
- usually in association with fault zones, e.g.
Bellefonte, Pennsylvania fault rock sample
cataclasis in moderate to high porosity rocks results in a
decrease in rock volume
- cataclasis in low porosity rocks results in an increase
in rock volume (dilatency)
Mechanical Twinning & Kinking
- mechanical twinning - bending of crystal lattice,
by shearing parallel to a crystallographic plane e.g.
calcite, plagioclase feldspar
- only planes oriented at suitable angle to shear stress
will form mechanical twins
- not strongly influenced by temperature, i.e. form at
relatively low temperatures
- twin planes thicken with increasing strain and higher
temperatures
- kinking - bending of crystal lattice in platey
minerals, e.g. micas
Diffusion Creep
- diffusion - movement of atoms through the interior
of grains, along grain boundaries, and through pore
fluids, thermally activated
- creep - time-dependent strain that occurs at low
differential stress
- diffusion creep is most efficient in fine,
grained rocks
volume-diffusion creep (Nabarro-Herring creep)
- diffusion within grains, only effective at high
temperatures & low-moderate differential stress
- slowest across grain interiors, more rapid where
vacancies are abundant
- vacancies migrate towards maximum stress, atoms migrate
towards minimum stress
- crystal grows parallel to minimum stress, becomes smaller
parallel to maximum stress
- process becomes less effective as grain elongates
grain-boundary diffusion creep (Coble creep)
- diffusion of material along grain boundaries from areas
of high compressive stress to areas of lower stress
- relatively fast in comparison to volume-diffusion creep
- more efficient, occurs at lower temperatures
Dissolution Creep (Pressure Solution)
- dissolution creep - distortion and dilation of
crystals by dissolution under stress and reprecipitation
of material in the presence of water
- residues of insoluble material will remain along jagged
dissolution seams (stylolites), e.g. see slabs of
polished limestone in the lobby of the Martin Center
- transport occurs along fractures or through pore spaces
- reprecipitation occurs in areas of low stress
Dislocation Creep
- shearing of the crystal lattice along a crystallographic
plane
- dislocation - line that separates deformed and
undeformed parts of crystal lattice
- dislocation glide - propagation of a dislocation
through a crystal lattice
- dislocation tangle - forms where dislocation
planes interact to prevent slip in crystal lattice, in
effect this introduces more defects into the crystal
lattice
Recovery & Recrystallization
- recovery - "heals" lattice by
rearranging or destroying dislocations
- recrystallization - converts old grains with
defects into "new" grains
- dynamic recrystallization -
recovery/recrystallization during deformation
- counteracts strain hardening, allows dislocation creep to
continue
- rate of dislocation creep controlled by rate of
recovery/recrystallization
- annealing - recovery/recrystallization after
deformation
Recovery
principal recovery mechanism is dislocation climb
- allows dislocations to climb to next crystallographic
slip plane and thus avoid tangles
- may result in arrangement of dislocation into low-angle
boundaries, causing adjacent parts of a crystal
to have slightly different orientations (~10o) - subgrains
- continued deformation may create high-angle boundaries
separating distinct grains
Recrystallization
- boundary-migration recrystallization occurs when
Brittle-Ductile transition
- zone within the earth where the dominant deformation
changes from brittle to ductile
- continental crust - 10-15 km on average
crustal strength envelope
- composite of Byerlee's law for upper crust and quartz
flow law for lower crust
lithosphere strength envelope
- shows an increase in strength below the Moho where
olivine flow law replaces quartz flow law
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