GEOS 304 Lab 1: Introduction to Deformed Rocks
Fall 2002
Name: ___________________
Part I: Introduction
Structural geology is the study of how and why rocks deform. The rocks we confront range from
barely disturbed outcrops that might be right at home in Iowa (boring!) to the geologic equivalent
of a train wreck - rocks so twisted that it may be impossible to fully understand their history. In all
cases, however, the questions that we ask are more or less the same: What is the structure? What
was its initial geometry? What are the materials involved? How have the materials been
deformed? Digging a little deeper, we may want to know about the temperature and pressure
conditions at the time of deformation, when and how fast the deformation took place, and
ultimately why it took place. The task is often complicated by the presence of multiple generations
of deformation, each overprinting the previous ones.
Today’s lab is an introduction to some geologic structures and to the types of questions that
structural geologists ask. The lab spans a wide range of scales, from hand-samples and
photographs to orogenic-scale cross-sections. The goals of the lab are fourfold:
1. To learn the scientific method and how to apply it to structural geology.
2. To make some careful observations of geologic structures.
3. To gain an understanding of how each rock/system of rocks has deformed.
4. To begin learning how to quantify the strain, i.e., how much the rock has deformed.
The Scientific Method: The scientific method is simply a tool for answering scientific questions.
It can be thought of as the following four steps:
a) Define the question/problem. This is often the hardest step but is crucial. You can’t
answer a question without knowing what the question is! In this lab, we will generally
define the question/problem for you.
b) Make observations. Keep your eyes open, and look around for any information that may
be pertinent to the problem. Ask lots of questions!
c) Formulate multiple hypotheses. Brainstorm for possible answers to the question based on
your observations. At this point, don’t worry too much about whether your ideas are
reasonable. Most major scientific advances have been a result of ideas that seemed
completely unreasonable at the time.
d) Test the hypotheses. Are there certain observations that can rule out one or more of the
hypotheses you generated? This is the time to worry about whether ideas seem plausible.
You may have to return to step two for further observations.
Using the scientific method will not always result in a unique answer but it will narrow the range
of possibilities. Keep in mind that it is also an iterative process. To fully answer a question, you
may have to cycle through the steps several times.
Part II: Hand Samples
1) (20 pts) Red Mudstone. This sample is a piece of the Triassic Moenkopi Formation from
eastern Utah, and is typical of rocks that reside at very shallow depths near the front of a
mountain belt.
1
The question: What causes the color variation in the rock?
Observations: Use the space below to write down observations you make about the sample.
We have listed a few questions that may help to guide your observations.
a) What are the two main colors in the rock?
2
b) Are there any interesting structural features in the rock?
c) How would you describe the geometry of the features?
d) Are these features associated with the color changes? How?
Other observations:
Hypotheses: Based on your observations, generate a couple of ideas that attempt to answer the
question.
Test the Hypotheses: List observations that exclude one or more of your hypotheses.
Summarize: What is the most plausible hypothesis? List observations that point towards it.
3
2) (20 pts) Black Schist (SG-12). This is a sample typical of the deeper levels (higher pressures
and temperatures) of mountain belts. The sample comes from a major shear zone in the
Appalachians. Like faults, shear zones are structures that accommodate offset. The difference
is that a shear zone distributes the motion across a tabular zone rather than a single discrete
break in the rocks as a fault does.
The Question: What is the sense of shear on the large polished face (i.e., which way did the
shear zone move)? In order to determine the sense of shear, it is important to know the
orientation of the sample from within the shear zone. In this case, the side with the sample ID
is up and we want to know whether the top of the sample moved to the right or to the left with
respect to the bottom of the sample. As an analog, it may help to imagine a peanut butter and
jelly sandwich. If the top piece of bread moved to the right (relative to the bottom piece of
bread) the PB & J might display a fabric that shows the right-handed (dextral) motion. We will
look for the same sort of fabric in the rock.
Observations:
a) Is there a planar fabric (schistosity) in the rock? Describe the fabric (“fabric” means
systematic grain size, shape, and/or orientation).
b) What are the reddish grains in the rock (examine the largest cut face)?
c) Look closely at their shape and structure. Are they simply round grains? Draw a detailed
sketch of one. You should see that the grains have spiral inclusion tails. Make sure you
include these tails in your sketch. Does the schistosity bend around the grains?
Hypotheses: What are the possible senses of shear? This is easy, as there are only three
possible answers: no shear, right-handed (dextral) shear, or left-handed (sinistral) shear.
4
Test the Hypotheses: List observations that exclude one or more of your hypotheses or that
point towards the most plausible one. Hint: think about winding string into a ball and how that
may relate to the grains or, alternatively, think of the grains as little ball bearings within the
sample. As the top of the sample moves relative to the bottom, the grains rotate.
Summarize: Write a brief answer (one or two COMPLETE sentences) to the question.
3) (20 pts) Northern Snake Range Marble: There at least two types of structures developed in
this rock: folds and fractures. There are two questions associated with this sample.
Question 1: Which structures came first, the folds or the fractures?
Observations:
a) Describe the folds.
b) Describe the fractures
Hypotheses: What are the three possible answers to the question?
Test the Hypotheses: List observations that exclude one or more of your hypotheses or that
point towards the most plausible one.
5
Summarize: Write a brief answer (one or two COMPLETE sentences) to the question.
Question 2: What is the percent shortening of the rock? Knowing that this was a sedimentary
rock, we can assume that the white and gray layers were once flat. Using the marked endpoints
(A and A’) measure the initial length of that layer (i.e. follow it around the folds from A to A’).
Now measure the final length of that layer (the straight line distance from A to A’). Using this,
calculate the percent shortening.
Initial length (Li):
Final length (Lf):
L  L f 
  100 :
Percent Shortening =  i

 Li 

Part III: Quantifying Strain
4) (20pts) Initially Circular Objects: We often use a construction called the strain ellipse to
portray how much a rock has been deformed. If we have a marker (usually an imaginary one)
on the rock that was circular prior to deformation, it will be elliptical after deformation
(assuming that strain is homogeneous). The shape of the resulting ellipse tells both the
directions and magnitudes of compression and extension. The photo below shows a Cambrian
slate from North Wales that has been deformed. The light patches are reduction spots that were
initially circular. Assuming that the area of the spots is unchanged, we can calculate the initial
size of the spot and quantify the strain. Pick any two of the spots to examine, then label them
and fill in the table.
Spot 1
Spot 2
Length of Longest
Radius (A)
Length of Shortest
Radius (B)
Initial Radius (R)
Minimum Stretch (S3)
Maximum Stretch
(S1)
Hints:
1. Remember that radius is half the diameter
2. Initial radius can be calculated using the formula R  AB .
6
Lfinal B

Liniti al R
L
A
4. Maximum stretch (S1) can be calculated using the formula S1  final 
Liniti al R
Are the stretch values the same for the two spots? Does this make sense? Explain.
3. Minimum stretch (S3) can be calculated using the formula S3 
Picture from Ramsay & Huber, 1983. The Techniques of Modern Structural Geology, Volume 1: Strain Analysis.
Academic Press, New York. p. 173.
7
5) (10 pts) Fry Method: Unfortunately, it is actually quite rare to find a rock with initially
circular markers in it. Another technique for generating a strain ellipse is the Fry method,
named after its inventor, Norman Fry. If we have a rock with a number of points (such as the
ooids in the photo below) that were initially randomly distributed, we can find the strain ellipse
by measuring the distances between neighboring points (approximated here by the centers of
the ooids). Create a strain ellipse on a piece of tracing paper by following the steps below.
A. Draw a cross (about 3-4 cm long) at the center of a piece of tracing paper. For all of the
following steps, the axes of the cross must remain parallel to the edges of the ooid photo.
In other words, the tracing paper has to maintain a constant orientation relative to the
“outcrop”.
B. Place the cross on the center of one of the ooids.
C. Without moving the tracing paper, place a dot at the center of each of the ooids near the
cross. Once you have finished that, go back to step B and repeat the process with the cross
centered on a new ooid.
D. When you think you have enough data (more is better), trace the outline of the region
containing few or no dots. This is your approximate strain ellipse.
Picture from Ramsay & Huber, 1983. The Techniques of Modern Structural Geology, Volume 1: Strain Analysis.
Academic Press, New York. p. 112.
8
6) (10 pts.) Cross-Section: So far, all of the examples in this lab have been of relatively small scale (hand samples and even thin-section in the case
of the oolites) but the same concepts can be applied at any scale. This last figure is a cross-section through the Himalayan fold-thrust belt from
DeCelles et al., 1998. Using the Lakarpata Group as a marker, calculate the percent shortening from X to X’. (See question 4 if you need a
reminder on how to do it).
Li:
Lf:
% shortening:
Note: In reality, this is a bare minimum estimate for the shortening since we are not taking into account either the parts of the Lakarpata Group
that have been eroded away or any internal strain within that group.
9