Rock Under Stress

By Laura Hospitál on May 15, 2017
Modeling shear stress with playdoh.

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Tectonic plates are giant slabs of Earth's lithosphere (the outer layer of the Earth ) that move in relation to each other. This movement can occur in one of three ways, toward one another (at convergent boundaries), away from one another (at divergent boundaries) or past one another (at transform boundaries). Please see this link for a related activity on convergent, divergent, and transform boundaries. Students should understand the content in the above activity ideally before this activity is competed. When the plates move in these ways, stress is placed on the rock.  Stress in geological terms is the force applied to rock per unit area. 

Three types of stress will be modeled in this activity:

  • Compression
  • Tension
  • Shear stress

Key terms:

  • Plate tectonics: The scientific theory that the Earth's crust is made up of plates that slowly shift position
  • Deformation: Tilting, breaking and bending of rock
  • Compression: Stress which causes rock to squeeze or push against other rock 
  • Tension:  Stress which occurs when rock pulls apart or gets longer 
  • Shear Stress:  Stress which occurs when tectonic plates move past each other causing rock to twist or change shape
  • Fault: break in rock
 

Preparation:

  • Play-doh should be molded into two pieces of fairly flat rectangular solid for each student.
  • Gather the other materials. Fruit leathers are usually available at larger grocery stores or natural foods stores. 

Materials

  • 2 rectangular solid pieces of Play-doh for each student
  • Fruit leather - 1 per student.  Be aware of any food allergies or restrictions
  • 1 thick rubber band for each student

Procedure

After the introduction of tectonic plates, students will learn about the types of stress which can be placed on rock. Students will be instructed to model the 3 types of stress placed on rock as follows:

Part 1 - Compression

  1. As students enter the classroom, let them know that today's activity will include consuming a tasty model of rock. Have students wash their hands or use hand sanitizer.
  2. When students inquire about this, pass out the fruit leathers.  Tell the students that they will model what happens at  boundaries when tectonic plates compress against each other.
    • Ask - What type of boundary is this?  Convergent Boundary - Discuss
  3. Have students hold one side of the fruit leather with one hand and the other side with their other hand.  As students are instructed to "compress the fruit leather"
    • Ask - What happens to the rock - " It bends." Discuss.  
    • Ask - What would happen it this was harder like rock when it bent a certain amount?  - It would break (faults would form)
  4. Tell students that some rock is more firm and will break more easily than other portions of the lithosphere. 
  5. Allow the students to eat the fruit leathers. 

Part II - Tension

  1. Say, "Where 2 plates are moving apart from each other
    • Ask - What kind of boundary is this?  - Discuss - divergent boundary
  2. Tell students that  another type of stress occurs at divergent plates. This stress is called tension.
  3. Pass out the rubber bands for students to stretch.
    • Ask - What happens when we stretch the rubber bands?  Do they become longer or shorter? - Longer - Discuss
  4. When the stress becomes too much, faults can occur where the rock breaks. 

Part III - Shear Stress 

  1. Pass out the sets of 2 blocks of Play-doh to each student.  
  2. Explain to students that the blocks of Play-doh represent 2 tectonic plates sliding past each other.
    • Ask - What type of boundary is this? - Transform boundary
  3. Have students model this using the Play-doh.
  4. Notice that the Play-doh will change shape and twist.

Closure

Have students record the different types of stress and the corresponding plate boundaries in a document or review orally if time does not allow for this.   

Resources used in preparation of this activity included: 

8th Grade Science Fusion, cK-12 website http://www.ck12.org/earth-science/Geological-Stresses/lesson/Geological-...

NGSS Standards:

Middle School:  Forces and Motion
 
PS2.A: Forces and Motion
The motion of an object is determined by the sum of the forces acting on it; if the total force on the object is not zero, its motion will change. The greater the mass of the object, the greater the force needed to achieve the same change in motion. For any given object, a larger force causes a larger change in motion. (MS-PS2-2)
 
Middle School:  History of Earth
 
ESS2.B: Plate Tectonics and Large-Scale System Interactions
Maps of ancient land and water patterns, based on investigations of rocks and fossils, make clear how Earth’s plates have moved great distances, collided, and spread apart. (MS-ESS2-3)
 
Collage of Rock Under Stress