Class Notes:
Rock Deformation
Some major
concepts include:
* Stress
* Shear
*
Strength
* Strain
STRESS:(force per unit area)
Types of Stress:
:Oppositely directed forces
(stresses) acting towards each other
:Oppositely directed forces
(stresses) acting away from each other
:Oppositely directed forces
(stresses) acting parallel to each other across a surface or plane.
__________________: Equal compressive stresses
on all sides
:The ability of an object to
resist deformation
:A change in shape or size due to
an applied force (stress); = Deformation.
Examples of
strain in response to different types of stresses.
Types of Strain:
:
:
:
:a temporary change in
shape or size that is recovered when the applied force is removed.
:a permanent change in
shape or size by bending or flowage that is not recovered when the applied force is removed.
:a permanent change in
shape by the lose of cohesion that is not recovered when the applied force is removed.Ð Òit breaksÓ
:A fracture along which some
movement has occurred.
Q. What is
the Relationship Between Stress, Strength, and Strain?
A.
Stress and
Resulting Strain (fig. 10.2)
Example: Fig
10.5
Factors Determining
the Style of Deformation
:
:
:
:
Effect of Pressure:
High Pressure - Deformation
Low Pressure - Deformation
Effect of
Temperature:
Hi Temperature - Deformation
Lo Temperature - Deformation
Effect of
Strain Rate:
Hi Strain Rate - Deformation
Lo Strain Rate - Deformation
Effect of
Composition:
ÒHardÓ(e.g.
granite) - Deformation
ÒSoftÓ(e.g.
marble) - Deformation
How can you
describe an inclined bedding plane in space?
:bearing of a horizontal line on
an inclined plane
:the maximum inclination of an
inclined plane, measured from horizontal
Examples
(see GEODe & Fig 10.7))
Q. How do
you represent the orientation of a 3-dimensional surface and a 2-dimensional map?
A. Strike and Dip Symbols
Strike and
Dip Symbols for Inclined Beds:
Examples:
Strike and
Dip Symbols for Horizontal Beds
Strike and
Dip Symbols for Vertical Beds
Strike and
Dip Symbols you should know.
Q. What do
geologic maps show? how do they
differ from geologic cross sections?
Geologic Map
and Cross Section (GEODe & fig. 10.8)
Q. How can
you recognize folded rocks from strike and dip symbols on a geologic map?
A.
Anticlines
and Synclines (fig. 10.10)
: a fold where the older rocks on the inside of the fold
: a fold where the younger rocks
on the inside of the fold
Fig. 10.11
A fold is
classified on the geometry of its:
- the sides of the fold
- the plane of symmetry
- line connecting points of maximum curvature along a folded
bed
Fold Symmetry
is based on: ______________________________________________
A vertical
axial plane indicates a ___________________________ fold (fig. 10.9a)
An inclined
axial plane indicates an ___________________________ fold
A horizontal
fold axis indicates a _________________________ fold
An inclined
fold axis indicates a ______________________
fold
(Fig.
10.9)
Q. What do
horizontal folds look like on a geologic map and cross section?
A.
Map
view
Cross
Section
Q. What are
the geologic map symbols for horizontal anticlines and synclines?
A.
Q. What do
plunging folds look like on a geologic map and cross section?
A.
Map
View
Cross
Section
Q. What are
the geologic map symbols for plunging anticlines and synclines?
A.
Q. How would
you tell a Dome from a Basin? (fig. 10.16)
You should
be able to define and recognize the geologic symbols for the following terms.
You should also be able to recognize them from a geologic map or cross section.
Lastly, why are they significant?
*
Anticlines vs. Synclines
*
Symmetrical vs. Asymmetrical Folds
*
Horizontal vs. Plunging Folds
What
types of geologic structures form the Valley and Ridge Province of the
Appalachian Mountains? How did they form? How do you know?
Compressional,
tensional, and shear stresses can form brittle as well as ductile deformation (Press
& Siever, 2005)
: a fracture along which some
movement has occurred. It is an example of brittle failure.
Types of faults
(animation)
The block
above the fault plane is called the _________________________
(figure
10.20)
The block
below the fault plane is called the _________________________
Like a bedding
plane, a fault plane can be described by its strike and dip!
: Faults where the movement (slip) between
adjacent blocks was in the dip direction.
:a dip-slip fault where the
hanging wall moved down relative to the footwall. (fig. 10.21)
: a dip-slip fault where the hanging wall moved up relative to the footwall. (fig. 10.23)
Examples:
Q. How can
you tell which side moved up relative to the other side? Arrows?
A.
Q. Under what
type of stresses would you expect to find normal faults formed? (fig.
10.21)
A.
Q. Under what
type of stresses would you expect to find reverse faults formed? (fig.
10.23)
A.
Q. What is
a thrust fault?
A.
Large-scale
Thrust Sheet (fig. 10.24)
: Faults where the movement (slip) between adjacent
blocks was in the strike direction.
: a
strike-slip fault where the opposite block has moved to the left, relative to
the observer. (GEODEe)
: a
strike-slip fault where the opposite block has moved to the right, relative to
the observer.(fig. 10.27)
Q. Strike
slip faults form under what types of stresses?
A.
Examples:
Q. How do
you determine the sense of movement (relative offset) along strike slip faults?
A.
________________: a scarp formed by the offset of
adjacent blocks along a fault. It can be thought of as an exposed fault plane that
may or may not have been modified by subsequent erosion.
Fault
Scarp in California (fig. 10.19)
Fault scarp
3+ m high formed by the 1964 Alaska Earthquake. (GEODe)
Grand Tetons
fault scarp 3000 meters high formed over the last 9 million years.
Long Point
Fault Scarp in Houston
Folded and
Faulted Sedimentary Rocks Exposed on Wave-cut Platform
What do different
types of faults look like on a geologic map and in cross section?
Q. What
is the relationship between the
type and orientation of a fault and the type and orientation of the stresses
that caused the fault?
A.
What
types of structures form the Basin & Range Province of the Southwestern US?
What type
of stresses formed the Basin and Range? How do you know?
You should
be familiar with the following geologic map symbols:
Strike and
Dip of inclined, vertical, and horizontal beds
Horizontal
and plunging anticlines and synclines
Normal, reverse,
and strike slip faults
Why is understanding
deformation of the Earth important?
1) Major part
of the history of the earth
2)
Explains the origin of mountain belts
3) Major factor
in the trapping of hydrocarbons and the concentration of ore deposits
4)
Faulting causes earthquakes
Examples of
some structural hydrocarbon traps.
Faults and
earthquake risk.