LAYER CAKE GEOLOGY

John R. Wagner
Department of Earth Sciences
Clemson University
Clemson, S.C. 29634-1908


Level: Grades 4 - 6 Can be modified for use with Grades 2 - 3.

Estimated Time Required: 50 - 60 minutes

Anticipated Learning Outcomes

Background

No prior knowledge of geology is required. However, the teacher should introduce the two basic concepts of layered rock: superposition (oldest layer at bottom, youngest at top) and original horizontality (sediments deposited in horizontal layers until some force changes their tilt or orientation).

Materials

Procedures

  1. Display the cake in a central location so all students have a clear view. Explain to the class that the cake represents a portion of the earth's crust with the top of the cake representing the surface of the earth. Each different color layer (including icing layers) represents a separate layer of sedimentary rock within the earth's crust. Ask the following questions and have student volunteers answer them by using the cake:
  2. If the cake is large enough (or if multiple cakes are available), cut two slices of cake for each group of up to four students. Each slice should be no more than 5 cm wide and should be as long as possible. Each group of students should follow the instructions listed below, answer the questions, and record their results on a separate piece of paper. [Or the teacher may do this as a demonstration.]

    Place a slice of cake on a sheet of paper on a desk or table. How many ways can you think of to change either the shape of your cake slice or the orientation of individual layers within? Do not try out any of these ideas yet, but write them all down to share with the class. Predict the visible changes which would be seen in the cake layers because of each of these ideas. Which ones are possible on the real earth? What forces might cause such shape changes in the real earth? Is there a difference in behavior of the cake layers versus the icing layers? Why or why not?

    Assign each group two of the suggested ideas to try out so that every idea is tested by at least one group. Each group should record observable changes in visible structure and make special note of any differences in behavior between the icing layers and the cake layers.
  3. Compile the results of all groups. Assign results to one of two categories:

FOLD = layers have different orientation but are still in contact with each other and in same sequence as before.

FAULT = layers have broken and moved apart relative to each other.

Ask if there are any cases where some of the layers showed fault characteristics and others did not. Can you think of a reason why this might have occurred? What characteristics of layers do you think are most responsible for whether folds or faults result from applied forces (stress)? Can you imagine a real world example where different types of rocks would behave differently under the same conditions of force?

 

Results & Discussion

  1. Students will normally conclude that the same flat sequence of layers exists at every location for a given cake. The rationale may vary but should include reference to all visible sides having the same sequence. Students may want to cut open the cake to look at the center. Such a test will work but has the disadvantage of destroying the structural integrity of the cake. The value of road cuts, railroad cuts, and natural canyons to geologists can be mentioned, but in general we do not have the option of "slicing open" sections of the earth to view the inside (especially when dealing with a very flat land surface). Hopefully, students will come up with the alternative idea of taking a small core out of the center of the cake to view its cross-section. Student volunteers can test their prediction by taking and comparing three (or more) random core samples of the cake using the transparent plastic tubes. Actually, no specific number of samples is sufficient to "prove" that the layered sequence is the same everywhere, but the more samples taken, the stronger the likelihood of that statement being true. The procedure of drilling holes into the earth to collect core samples is a very practical way of determining what lies below and is used extensively in such diverse fields as oil exploration, mineral exploration, and ground water monitoring for pollution.
  2. Students will probably think up at least a dozen ways to alter the cake structure. Some common methods of alteration along with possible forces within the earth which produce them are listed below.

     PROCESS

    RESULT 

    FORCE RESPONSIBLE

    turn cake on edge  vertical layers  tectonic uplift & tilt 
    turn cake upside down overturned layers tectonic uplift & tilt 
    compress cake vertically by squeezing layers from above thinner layers   gravity loading (weight)
    compress cake horizontally by squeezing cake sideways folds

     

    tectonic plate collision

    pull ends of cake apart creating break fault   tectonic rift zones
    push ends of cake sideways in opposite directions  fault tectonic transform fault 
    push top of cake sideways relative to bottom of cake  sliding layers  weakness allows sliding 
    push up on center from underside of cake  dome (fold)  igneous intrusion 


    The biggest differences in behavior of cake and icing layers should be observed during vertical and horizontal compression and in the sliding of layers over each other. The icing layers are much more flexible and will "flow" without any noticeable fracturing, but they do not show noticeable change in thickness. The cake layers, on the other hand, tend to break in places under stress, but are easily compressed from thick to thin layers. Sliding tends to occur within the icing layers and not the cake layers.
  3. Results will vary depending on exactly how much force (stress) was put on the cake. In general, only "pulling apart" or "pushing laterally in opposite directions" will produce faults. Simple compression is likely to produce folding, while tilting or overturning tends to leave layers pretty much in their original relative condition. Several methods may show differential behavior between icing layers and cake layers, depending on the level of force (stress) applied. By comparing several examples from the class data, it should be possible to point out that faults (breaking) are the end result of forces (stresses) that exceed the ability of the material to bend. Some materials reach that critical point at lower force (stress) levels than do other materials. As a simple geologic example, sandstones are more rigid than shales. Under similar stress conditions, sandstone may crack in several places while shale layers appear undisturbed.

Conclusions

  1. Multiple core samples provide information about layered structures beneath the surface.
  2. Core sample data can be collected from several locations to determine structure over a wide area.
  3. Increasing the force (stress) on layers can cause folding at first, then eventually faulting as layers break apart.
  4. The type and direction of stress applied to layers determine the structures that result.
  5. Brittle layers break sooner than more ductile layers affected by the same force (stress).
  6. Sliding of layers is more likely to occur within a ductile layer.

Additional Activities

With advanced students, the effect of rock structure and erosion on topography can be modeled and investigated by creating folds in the cake and then cutting off (eroding) parts of the cake which are higher in elevation (see reference below). Sketches can be made of the "geologic map view" of the top of the cake after erosion takes place. "Stream channels" can likewise be cut into the cake surface with a knife to illustrate geologic map patterns in an area dissected by streams.

Layer Cake Geology can also be used as a teaching tool for younger students. To adjust this activity for grades 2 - 3 use only Procedure #1, and make following changes:

Reference

WAGNER, J.R., 1987. Using Layer-Cake Geology to Illustrate Structural-Topographic Relationships, Journal of Geological Education, v. 35, p. 33-36.

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