Download Folding, Faulting and the Brittle/Ductile Transition Zone

Folding, Faulting and the Brittle/Ductile Transition Zone
Overarching concept: Fractals
Key concepts:
Fold
Fault
Stress
Strain
Mantle
Crust
Plastic deformation
Brittle deformation
Instructional Objective
Compare and contrast the processes of faulting and folding
Engage
Active participation demonstration using layers of different colored clay (both pliable and
brittle). Show students that some of the clay layers are stiff, while some are flexible.
Have them write out a hypothesis filling in the following statement: If I apply stress on
either side of these layers of clay…
Then teacher places two boards, or books, on either side of the layers and apply pressure
in the horizontal direction. The pliable layers should fold, while the brittle layers should
fracture, or fault.
Explore
Have students use their textbook and other resources they deem appropriate to relate the
demonstration to geology. Using their textbooks to be able to use the correct terminology
and relate it to the earth, have students design two of their own demonstrations: one that
would represent folding, one representing faulting.
Investigative questions to be answered in all experiments:
What, in the Earth would behave like the flexible clay? (answer: rocks below the
brittle/ductile transition zone)
What would behave like the brittle clay? (answer: rocks located at a shallower depth)
Is there a relation of depth in Earth as to where rocks may be more ductile than brittle?
Explain
Have students swap experiments with each other. Once they have performed each others'
experiments they should have a strong concept of the difference between the two
processes. Ask for a student volunteer to draw three columns on the board. Label one
brittle, one ductile, and one brittle/ductile. Have students call out individual examples or
characteristics that fit into each column.
After they have supplied examples and characteristics regarding depth and heat, label the
ductile column with Fold, brittle column with Fault, and leave the third column as
Brittle/Ductile.
Teacher then goes over the defining characteristics of faults and folds, using the
information the students supplied and other missing components. This discussion should
stem from an introduction (or revisiting) of the concept of fractals in the earth system.
Using fractals to group concepts in geology emphasizes that geologic processes are
complex at all scales. Large scale features mimic smaller scale features, and so on. Folds,
for example, can be seen from space (see image below, of the Appalachian Mountain belt
in Pennsylvania) and are the result of massive tectonic events. However, look at the
image of Massanutten Mountain below, this is an example of a fold on a smaller order.
Next, look at the image above the evaluate section, this of a fold in outcrop scale, at an
even smaller scale yet. If examined closer still, this outcrop-scale fold would have the
same structures (called z or s-folds) that can be seen in the image of the Appalachian
Mountains. This concept of the fractal nature of geology is crucial to understanding
geologic systems.
In addition, make sure to mention the brittle/ductile transition zone in regards to where
rocks behave more plastically versus where they tend to break and fracture.
Also, in the column that has characteristics of both faults and folds, add notes about faultrelated folding.
Image of Appalachian Mountain Belt (Tectonic scale)
Outcrop Scale Image
Massanutten Mountain (regional scale)
Extend
Using Google Earth or WhirlWind and pictures of outcrops in cross-section view, supply
students with coordinates of exposed faults and folds.
Ask them to fly to the coordinates and work together to classify which coordinates or
images are faults and which are folds.
Typical locations would be Massanutten Mountain (see image above), the San Andreas
Fault, the fault block mountains in California, and the Appalachian Mountains. These
locations are exceptional exposures of features that are typically subsurface and have
been uplifted or tilted. A discussion of how they came to be should accompany students'
exploration of each location.
Evaluate
Using the activities outlined above, have each student come up with a written definition
of folding and one of faulting. These definitions must be extensive, covering the depth
and nature of each process. Students must hand these two definitions to the teacher and
either get them approved or have to revise them before leaving the classroom.
Connections
Throughout this lesson the teacher will emphasize the fractal nature of folding and
faulting. Specifically, that these are smaller scale geologic processes that are a result of
larger scale tectonic processes like those discussed in previous lessons. Also, this lesson
will be connected to future lessons concerning earthquakes and metamorphism.
Reflections
This lesson is akin to my constructivist nature. Students are using examples and their own
characteristics to develop a definition of the terms folding and faulting. Also, I am a
strong proponent of concept-based teaching. As you may have noticed, many of my
lessons mention systems and complex system principles like fractals or cycles. I believe
that it is important to view the earth as a system with many defining and interrelated
parts. Teaching in this manner allows students the ability to continually relate individual
concepts back to a broad overarching concept, the earth system.
The final summative project will assess the success of this lesson. It is very inquiry based
and open-ended, therefore it may be difficult to tell how well the students understood the
material until the results of the final summative project are in.