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Geology 110 Syllabus

STRUCTURAL ANALYSIS

Structural Geology3 D Stress

I. THREE TYPES OF STRUCTURAL ANALYSIS

A. Geometric analysis - descriptive evaluation of structures measured in the field

B. Kinematic analysis - evaluation of structures in terms of displacements, rotations and distortions

1. Displacements

2. Rotations

3. Distortions

C. Dynamic analysis - evaluates structures in terms of forces, stresses, and rock strength

II. FORCES

A. Stress - forces acting within a body

1. Stress causes deformation, thus producing structures

2. Stress = F/A "force per unit area"

More prescisely:

a. Where area is contracted so that we have stress at a point (small area) to make stress more uniform over the area

b. stress units = F/A = (ML/T2)/L2 = M/T2L where, T=time, L=length, M=mass

B. Strain - deformation within a body in response to stress

1. Strain is the product of stress

2. We observe strain in the field (faults, folds, etc...)

C. Dimensions of force

1. F = Ma = kgm/s2 = Newtons (MKS system) where, M = mass and a = acceleration
 
MKS
CGS
English
Mass
kg
g
slug
Length
m
cm
in
Time
s
s
s
Weight
N
dyne
lb
Stress
kb
bars
lb/in2

2. Example
1 atm. ~ 0.9869 bars =
14.7 lb/in2
1 bar =
106 dynes/cm2
=
106 g/cm2
1 kb =
1000 bars

  • at 4 km depth pressure = 1 kb

  • at 8 km depth pressure = 2 kb etc.

D. Force is a vector quantity (has magnitude and direction)

Eg. Velocity - 60 mph to the east

1. Scalar quantity has magnitude only (speed in km/hr, temperature, and pressure)

E. Force can be resolved into force components and can be treated by the rules of vector algebra

1. s = Normal component, component perpendicular to the plane (sigma)

2. t = Shear component, component parallel to the plane (tau)

III. TO EXPRESS STRESS IN A BODY - must make two assumptions:

A. The distribution of forces is homogeneous over the area under consideration

B. All forces are in static equilibrium (ie. they all must cancel each other out so that there is no acceleration or torque)

IV. CONVENTIONS:

A. s yy where

1. The first y is normal to plane upon which force the force is acting

2. The second y is the direction in which is acting

B. Sign conventions

1. Compressive stress = s is positive (+)

2. Tensional stress = s is negative (-)

3. Right (dextral) shear stress (-)

4. Left (sinistral) shear stress (+)