Modeling Crustal Folds

In this activity students will use playdough to build 3D models of Earth’s crust to observe how it bends and folds as a result of tectonic stress. These formations, when found in nature, give geologists a picture of how Earth has formed and changed over geological time.

Each group of two to four students will need at least three to four different colors of playdough.

An example of a fault line in an exposed rock.

Credit: Brandon Rodriguez

An example of a rock that has folded up, creating synclines (pointing upward from the central axis) and anticlines (pointing downward).

Credit: Bryan Wilbur

An example of asymmetrical synclines and anticlines.

Credit: Bryan Wilbur

One of the many things that makes Earth's geology unique is the presence of tectonic plates. The movement of these plates over eons of Earth’s history is responsible for the formation and breaking of supercontinents, such as Pangea. This movement occurs at tiny but measurable rates of only one to 10 centimeters per year, and can either result in the plates spreading (divergent), crashing together (convergent) or sliding past each other (transform).

Knowing how and which way these plates are moving at these boundaries between plates allows us to predict future movement, as well as risk zones, where the stress of movement can result in Earthquakes. These stress zones are what we commonly see as cracks, or faults, and are typically found in brittle rocks.

However, many sedimentary rocks, which form by deposition of eroded rocks over time, are far more pliable and deformable than their brittle counterparts. This deformation causes folds in the rocks rather than cracks. These folds give us features such as synclines and anticlines where Earth’s crust has folded up like an accordion as a result of compressional stress. In the case of synclines, the folds point upward from the central axis, or "hinge line", whereas in an anticline, the folds point downward from the central axis.

Geologists do field research to look for exposures of these faults where erosion has worn down the surface rocks over long periods of time and exposed some of the rocks deposited underneath.

While the work of field geologists remains crucial to the study of plate tectonics, NASA has also begun using satellite images to help track the subtle movements of Earth's surface over time.

For example, missions such as Terra have provided global images of high-risk erosional zones in coastal areas. The LANDSAT 8 mission, which monitored natural disasters such as lava flows from space, took hundreds of photos a day with a resolution of up to 15 meters per pixel. At this resolution, scientists were able to observe land movements as a result of natural disasters, but tracking finer movements on a geological scale was limited.

The NISAR spacecraft, slated for launch in 2024, will have those capabilities and more, mapping Earth's surface with a resolution up to a few centimeters per pixel. This is possible due to the satellite's two synthetic aperture radar bands, which will allow the mission to create a global map every 12 days at a quality never before seen in a NASA Earth science mission. This finely detailed map will allow scientists to identify areas impacted by erosion, landslides, earthquakes, and other natural hazards, monitor geological changes, and make predictions that could help communities better respond to natural hazards.

• How would you be able to determine if you had a syncline or an anticline if you only had the top view of your model, which is often all geologists can see when out in the field? Consider creating a playdough model to show students to help them make a determination.
• Again, thinking only of the top view, how could you tell if you were walking above a normal syncline or anticline (like in Model 1) versus a plunging fold (like in Model 2)? What about an asymmetrical one (like in Model 3). Hint: think about the surface area exposed after your cut.
• From your fault model (Model 4), can you determine which side of the rock moved upward or downward from the top view? What type of pressure would have caused this – pulling the pieces apart (divergent) or pushing them together (convergent)?

• See example answer key here (Google Docs) or download the answer key (.docx).
• Although student responses will vary, Model 1 should show parallel lines on the top and sides, while in Model 2, the lines should be leaning forward or backward. Conversely to Model 2, Model 3 will have symmetry on the top and side but not on the front of the diagrams, as the fold should lean to the left or right. Model 4 will show a cut in the middle of their diagram with disconnected layers throughout.

While you have the play dough out, consider these Earth science extensions:

Lesson concept by Dr. Bryan Wilbur, Pasadena City College, Pasadena, California.