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Geology (GLGY 341-UCAL) Midterm Exam

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Question 1

The pulling apart a material parallel (aka same plane) to a fracture is known as a... (also the easiest type to observe in field)

A	Mode II
B	Mode IV
C	Mode III
D	Mode I

Question 2

Given C = 100 MPa and D = 150 MPa (note they are pointing to at x-intercepts), calculate mean stress?

A	250 MPa
B	125 MPa
C	150 MPa
D	25 MPa

Question 2 Explanation: 

C = 100 MPa and D = 150 MPa are σ₁ and σ₃ respectively. Therefore, (σ₁ - σ₃)/2 = 125 MPa

Question 3

In this computer model, the differential stress is indicated by....

A	orange circle.
B	the 60-degree angle.
C	the green line between x and y.
D	large blue circle.

Question 4

When using acoustic remote sensing, salts will not appear since salts are great reflectors.

A	True
B	False

Question 4 Explanation: 

Salts will often appear clearly not because they are great reflectors, but because they are poor reflectors surrounded by great reflective materials.

Question 5

What type of strains are most common?

A	Depend on the environment, all strains are globally occur at the same rate.
B	1D
C	3D
D	2D

Question 6

What is the best indicator of structural deformation history?

A	Mineralogical and chemical composition
B	Deformed bedding surface
C	Undeformed bedding surface
D	Known fossils

Question 7

The most (ultimate) tensile strength a rock can take is indicated by which letter on the following diagram?

A	D
B	A
C	E
D	C
E	F

Question 8

By referring to the following matrix, lithostatic stress can be defined when...

A	F13, F23 and F33 have changed at the same rate.
B	F13, F22 and F31 have changed at the same rate.
C	F11, F22 and F33 have changed at the same rate.
D	F11, F21 and F31 have changed at the same rate.
E	F12, F22 and F32 have changed at the same rate.

Question 9

Under high pressure, a quartz vein has dissolved resulting collapse of a fracture. What type of fracture is described here?

A	Mode IV
B	Mode III
C	Mode II
D	Mode I

Question 10

Planes orinated __I__ to the principle stress directions experiance __II__.

A	I. parallel II. shar stress and zero normal stress
B	I. parallel II. normal stress and zero shear stress
C	I. perpendicular II. shar stress and zero normal stress
D	I. perpendicular II. normal stress and zero shear stress

Question 11

The rocks along a fault line have moved parallel but opposite to each other. What type of strain would you expect to find along the fault line?

A	Simple shear
B	Pure shear
C	Translational deformation
D	Rotational deformation
E	Super imposed strain

Question 12

Given the following, calculate the deviatoric stress;
σ₁ = 250 MPa
σ₂ = σ₃= 100 MPa

A	75 MPa
B	350 MPa
C	150 MPa
D	175 MPa

Question 12 Explanation: 

(σ₁ - σ₃)/2 = 75 MPa

Question 13

When dealing with stresses at a point, the maximum stress is in the direction that has the __I__ extension and the maximum strain is in the direction that has the __II__ extension.

A	I. least II. largest (most)
B	I. largest (most) II. least
C	I. vertical II. horizontal
D	I. largest (most) II. largest (same)
E	I. horizontal II. vertical

Question 14

Relatively, the ductile zone of a fault is ___ the center of the fault.

A	90 degree to
B	furthest away from
C	closest to
D	at the

Question 15

What is represented on the x-axis?

A	Shear stress
B	Differential stress
C	Normal stress
D	Dynamic stress
E	Compressional force

Question 16

By referring to the following matrix, if F₃₃ is the only (and one and only) one that underwent change, what type of strain is it?

A	Axial strain
B	Isotropic volume change.
C	Asymmetric strain
D	Anisotropic volume change.
E	Symmetric volume change.

Question 16 Explanation: 

aka uniaxial strain

Question 17

A thrust fault is a low angle reverse fault with about 20-30 degree in angle between the fault line and the normal.

A	False
B	True

Question 18

What is strike? (Most likely will be in the oral exam part of the lab).

A	It is the azimuthal bearing relative to true north of a vertical line on an inclined planar surface.
B	It is the azimuthal bearing to an imaginary line measured from the North.
C	It is the line that will generate at exactly 90-degrees of azimuthal bearing to an imaginary line measured from the North.
D	It is the azimuthal bearing relative to true north of a horizontal line on an inclined planar surface.
E	It is the azimuthal bearing to an imaginary line measured from the true North.

Question 19

What is the failure criterion indicated in A?

A	Griffith
B	Von Mises
C	Micro
D	Macro
E	Coulomb

Question 20

What type of geological event the following diagram represents?

A	Normal fault
B	Earthquake
C	Erosion
D	Strike-slip fault
E	Thrust fault

Question 21

According to the following diagram, what do you get if you find the distance between C and D (Note they are pointing to at x-intercepts)?

A	Failure Window
B	Normal Stress
C	Differential Stress
D	Compressional Stress
E	Shear Stress

Question 22

What is coaxial deformation?

A	It is a type of deformation where the principle stress horizontal.
B	It is a type of deformation where the principle axes of strained body changes it's orientation as deformation progress.
C	It is a type of deformation where the principle axes changes the distribution of force.
D	It is a type of deformation where the principle axes of the strained body remain in the same orientation.
E	It is a type of deformation where the volume doesn't change.
F	It is a type of deformation where the stress is a consent.

Question 23

Upon observations, you determined the sediments have few quartz veins. What type of fracture is this?

A	Mode III
B	Mode I
C	Mode IV
D	Mode II

Question 23 Explanation: 

While it could be a combination of any of the fracture modes, the simplest explanation is Mode I. The simplest one is the explanation for most geologic problems.

Question 24

What type of transformation is represented by the following diagram? (red dotted line is the original position of the rock mass)

A	Sub-simple shear
B	Rotation
C	Simple Shear
D	Pure shear
E	Translation

Question 25

How is stress different from strain?

A	Stress is when you pull apart rock units and strain is when rock units are pushed against each other.
B	Stress is force per square inch (it can be treated as pressure). Strain is the deformation cause by the applied stress.
C	Strain is a force while stress is a the cause of the force that generate strain.
D	Stain is force per square inch (it can be treated as pressure). Stress is the deformation cause by the applied strain.

Question 26

Unfortunately as a Geologist, you were asked to determine which mathematical model should be entered into the modeling software(used for plotting the field data). Knowing that you are in an area with extreme tensile pressure, which model do you choose?

A	Von Mises model
B	Envelope model
C	Coulomb model
D	Griffith model

Question 26 Explanation: 

Computers are great tools. I have worked with several Engineering companies and found that it is difficult, if not impossible for a software to predict the best course of action for specific project. This is where professionals like Geologists, Engineers and Mathematicians come in.... No matter how many computers you have or what software you use, you will come across situations where automated systems screwed up calculations due to human errors. For example, if you pick Coulomb model from the software menu, it will come back with a great number, but it will be WRONG. The point of this question is to demonstrate that computers are NOT a replacement for a Geologist!

Warning: I did work for a company where the chief Engineer was fired and Alberta Government charged several people with offenses due to mathematical modeling errors result in an outcrop collapse near a bridge killing a person.

Question 27

What is the difference between translation and distortion? (or are they the same?)

A	Translation require no change in principle elongation axes.
B	Translation only can occur in brittle materials while deformation can occur in both brittle and ductile materials.
C	They are the same.
D	Distortion only occur when a body of materials shifted from one position to another.

Question 28

What is the failure criterion indicated in E?

A	Macro
B	Griffith
C	Micro
D	Von Mises
E	Coulomb

Question 29

In this computer model, the most left hand circle (blueish-green) between σ₃ and σ₂ is drawn based on what envelope/principle?

A	Griffith
B	Von Mises
C	Coulomb
D	Confining

Question 30

Generally what type of stresses would you expect in lithosphere?

A	extensional
B	deviatoric
C	compressional
D	biaxial
E	uniaxial
F	lithostatic

Question 31

What type of transformation is represented by the following diagram? (red dotted line is the original position of the rock mass)

A	Simple shear
B	Pure shear
C	Sub-simple shear
D	Translation
E	Rotation

Question 32

In solids it is possible to have variations in the magnitude of the stress field. This is because solids can support stress gradients. This gradient can be best described as...

A	differential stress.
B	normal stress.
C	deviatoric stress.
D	shear stresses

Question 33

What is the difference between magnitude and net of a force? (general physics-structure)

A	Magnitude a measure of force regardless of the direction(net force depends on the direction).
B	Magnitude a measure of force in international standards (net force is often use to simplify force distribution).
C	Net force a measure of force regardless of the direction(magnitude depends on the direction).
D	Net force a measure of force in international standards (Magnitude is often use to simplify force distribution).

Question 34

Triaxial test is used to measure ___ of soil types and rock units.

A	strength
B	compaction strength
C	history of deformations
D	porosity
E	faulting levels

Question 35

Calculate the lithostatic stress under 10 km of a rock unit composed of olivine which has a density of 3600 kg/m³ (use g = 9.81 m/s²)

A	0.35 Gpa
B	98100 Gpa
C	35316 Gpa
D	98.1 Gpa
E	35 Gpa

Question 35 Explanation: 

Given, stress =
stress = (3600 kg/m³)(9.81 m/s² )(10,000 m)

Question 36

11. What type of transformation is represented by the following diagram? (red dotted line is the original position of the rock mass)

A	Translation
B	Pure shear
C	Simple shear
D	Sub-simple shear
E	Rotation

Question 37

What type of transformation is represented by the following diagram? (red dotted line is the original position of the rock mass)

A	Translation
B	Pure shear
C	Rotation
D	Simple shear
E	Sub-simple shear

Question 38

Deformation which will not result in physical change in the structural unit in question has underwent a... (there is more than one possible answer, but choose the MOST suitable one)

A	rotational deformation.
B	translational deformation.
C	pure shear deformation.
D	shear deformation.

Question 39

Even through the principle stress can be in any direction, due to the strong force of gravity, it is usually oriented vertically to a body.

A	True
B	False

Question 39 Explanation: 

But it still can be at an angle to a plane, a grain, a bed or a feature or a fracture.

Question 40

What is the fault separation distance between a stratigraphic section?

A	unit separation
B	fault separation
C	stratigraphic separation
D	fault distance
E	unit distance

Question 41

Where on a stereonet a pole to plain will plot?

A	90 degrees away from the dip.
B	45 degrees away from the dip.
C	180 degrees on either direction.
D	180 degrees towards the dip.

Question 42

What is represented on the y-axis?

A	Compressional force
B	Dynamic stress
C	Differential stress
D	Normal Stress
E	Shear stress

Question 43

If you are working in an environment with high tensile geologic forces, which of the following failure envelope would you enter into the computer?

A	Coulomb
B	Mohor
C	Von Mises
D	Griffith

Question 44

Which of the following can be classified as a coaxial strain?

A	Simple Shear
B	Pure Shear
C	Rotation
D	Sub-simple Shear
E	Translation

Question 45

Why is the Coulomb failure envelope is not perfect?

A	It under-predicts uniaxial compressive strength at higher confining pressures.
B	It predicts unrealistic strength by over predicting the failure envelope in tensile regions.
C	Past practical experiments indicates that it will not comply with 90% of the lab tests.
D	It under-predicts strength in tensile regions.

Question 46

In the following matrix only one force value (F) has changed. What type of volume change would you expect from the rock body?

A	isotropic
B	anisotropic
C	deviatoric
D	differential

Question 47

In this strain ellipsoid, where is the maximum strain?

A	σ 1
B	σ 3
C	σ 1 and σ 2
D	σ 2 and 3
E	σ 2

Question 48

Rocks are weaker in tension because of micro defects.

A	False, they are weaker due to macro defects
B	True

Question 49

"A discontinuity with wall-parallel displacement dominated by brittle deformation mechanisms." This is the...

A	general definition of a fault.
B	kinematic definition of a fault.
C	specific definition of a fault.
D	dynamic definition of a fault.
E	traditional definition of a fault.

Question 50

When will a structure become unstable?

A	2 θ > 90
B	2 θ < 90
C	2 θ = 90
D	2 θ < 45
E	2 θ > 45

Question 51

What is the failure criterion indicated in F?

A	Von Mises
B	Coulomb
C	Griffith
D	Micro
E	Macro

Question 52

In this computer model, the most right hand circle (orange) between σ₂ and σ₁ is drawn based on what envelope/principle? (choose the best)

A	Confining
B	Griffith
C	Von Mises
D	Coulomb

Question 52 Explanation: 

While more than one answer is possible (Coulomb and Von Mises), they all can be explained by confining principle.

Question 53

Triaxial test is used to determine the deformation of materials, which then is used to determine the strength.

A	True
B	False

Question 54

The following image is a graphical representation of a strain ellipse during deformation. What term would best describe this image?

A	non-coaxial strain
B	pure shear
C	translation
D	rotation
E	coaxial strain

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Strong skills in geometry, trigonometry, calculus, spatial visualization and sometimes structural engineering is very helpful in this field of study.

Use of Stereonet

Key terms:

Strike: azimuthal (bearing) relative to true north of a horizontal line on an inclined planar surface

Dip: angle of inclination of the surface of interest in the direction perpendicular to strike

Trend: of a line is the imaginary horizontal line of a vertical plane (similar to strike)

Plunge: angle of inclination in the vertical from the horizontal

Apparent dip: inclination from horizontal of any line on an inclined plane that is not perpendicular to strike

Pitch (rake): angle measured on an inclined plane, between the strike and any other line those lines in the plane. The line of greatest slope in any plane must have a pitch of 90-deg. The pitch of a line on a surface is measured by placing a protractor on the surface of the line

Basic Field Work

Strike and Dip

Best method: using brunton compass

Dumb method

Identifying Unstrain State

With the advancement in computerized databases on geotechnical data, today most companies use software programs to determine the original state of a geological feature. However, all of these programs are based on fundamental principles of structural geology and a geologist has to identify the features first before enter the data in to a computer system.

There are several ways to identify the original state of deposition, which include the following principles or laws:

• Principle (law) of Original Horizontality: due to the force of gravity acting on sediments, layers of sediments are always originally deposited horizontally.
• Principle (law) of Superposition: “Sedimentary layers are deposited in a time sequence, with the oldest on the bottom and the youngest on the top.”
• Principle (law) of cross cutting relationships: an igneous intrusion is always younger than the rock it cuts across.

If a structure do not follow these principles, it can be explained using geological events that effected the structure such as a fluid flow across a crack of a sediment after the sediment was deposited. This is used to compile a list chorological events in order to understand the nature of the sample.

Deformation

The deformation play a major role in how structures are form in nature. By definition, deformation is the transformation of a geological structure from an initial to a final geometrical shape by means of rigid body translation, rigid body rotation, strain and/or volume change. Understanding this definition is critical to further the knowledge in the field of structural geology.

Rotation is caused by the geologic structure rotated in a circle where center moves in a slower speed resulting in a short distance compared to the edges the speed is higher with greater distance.

Translation is caused by shifting the structure from an initial point to a final point. In translation, every single particle of a geologic unit or a structure moves in the same direction for same distance and at the same speed.

Strain and Stress are related to each other, but they are two different concepts. Stress is a force that is applied to a unit area where strain is the deformation of a geologic unit/structure as a result of strain. A geologist cannot observe stress, but rather deduce the characteristics of stress using the strain recorded in rocks and geologic structures. Strain can be classified in to three different categories; pure shear, simple shear, and subsimple shear..

– pure shear is caused by compressional forces into the geologic material from two sides.

– simple shear occurs when a structure is subjected to stresses in opposite direction.

– subsimple shear is complicated because it is caused by a combination of simple shear and pure shear.

Please read more on this at: Shear Strain

Remember that deformation can only be identified using relative theory. In other words, deformation is the difference between known state of a structure to a final state of a structure. Most of the time geologists use known fossils to identify changes.

One dimensional strain involves only either stretching or shortening, where elongation, e is defined as:

e = (l-l_o)/l_o and l is the final length and l_o is the initial length.

The stretching, s of a structure is defined as:

s = 1 + e OR s = l/l_o

Two dimensional strains involves changes in structure in 2D on a plane, where angular shear, shear strain and strain ellipse is described using relative angles. This can be confusing since all angles are given relative to the normal of along a line; usually line A or line B. In angular shear, Ψ represents the angle between two originally perpendicular (the normal line of the undeformed original is perpendicular) lines.

Mohr Circle

Mohr circle or Mohr diagram is named after Christian Otto Mohr is a 2D representation of a 3D stress on a structure on a point, often a crack.

Triaxial Test

Triaxial test is a conventional testing method used in laboratory testing for obtaining shear strength parameters for different soil and rock types under a controlled stresses. The information obtained from such tests are used to determine the history of deformation events and the current strength conditions. The current strength conditions are used for constructing dams, building, roads and such. Not only the Geologists but also Engineers use information obtained from triaxial test.

Here are some videos of real life triaxial tests;