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.
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 2022, 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.
- Remind students that Earth's crust is generally made of brittle rock, the sedimentary rocks that make up much of the surface can actually deform under stress and pressure, causing bends and folds. This is called "plastic" or "ductile" deformation as opposed to when the pressure gets too extreme or is applied to brittle rocks, causing cracking of the crust, or "faults". These folds and faults give us hints about how tectonic plates have stretched or collided over time.
- Break students into groups of two to four. Give each group three to four colors of playdough and a small plastic knife to trim the edges of their models. Have them follow along on their student worksheets. Provide colored pencils that are the same color as the playdough to help them sketch their layers.
- First, have groups stack their playdough into thin layers 0.5-1 cm thick and 5-6 cm wide on each side. The plastic knife can be used to create clean edges.
- Have students apply pressure to the short sides of the block of playdough, causing it to deform into a wavy structure. Model erosion over time by trimming horizontally roughly 1 centimeter off the top of the block, exposing the layers underneath.
- Have students draw what they see in the empty box on part 1 of their worksheet. Tell students that a syncline is the part of the structure where the arms of the fold turn upward, like a smile, while the anticline is where the arms fold downward. Then have them draw a dotted line at the center axis, or "hinge line," of their sketched structure. Prompt students to explain which of the exposed colors are younger versus older.
- Cut another centimeter off your block, this time vertically from the top to the bottom at an angle, shaving off the forward-facing side of your model. Notice that the newly exposed layers are no longer on the same plane. These are called plunging synclines and anticlines, and they occur when the folds dip in or out of the ground. Have students draw this new perspective of their model in the empty box on part 2 of their worksheet.
- Have students create a fresh set of layers and once more apply pressure from the short sides of the block, but this time making the waves lean to one side. As before, they should model erosion by trimming away the front face of their block vertically from top to bottom. The exposed layers should now look like a wave leaning to one side. This is called an asymmetrical syncline and anticline. Have students draw their observations in part 3 of their worksheet and explain how they could tell the difference between an asymmetrical and symmetrical fold just from the top view.
- Lastly, with the old block or with a new set of layers, have students cut through their block from top to bottom at an angle. This cut will represent a fault. Have students slide one side so that it's lower than the other and lightly push the sides back together. Have students erode away another centimeter from the top of their model, which will produce layers that no longer match up on any side. Students should draw their model in the empty box on part 4 of their worksheet. Have students explain how they can use the top view to determine which side of the fault was uplifted.
- 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:
Lava Layering: Making and Mapping a Volcano
Students learn about Earth processes by simulating and examining lava flows from a volcano model made of play dough.
Time 1-2 hrs
Explore Rocks Using Core Sampling
Make a sedimentary “rock” from play dough and use your geology skills to investigate it!
Time 30-60 mins
Students learn to describe meteorites and rocks using candy bars as familiar models.
Time 30-60 mins
NASA's Earth Minute: Dishing the Dirt
How can studying the soil beneath your feet help predict flash floods, grow more crops, and plan for drought?
Time Less than 30 mins
Lesson concept by Dr. Bryan Wilbur, Pasadena City College, Pasadena, California.