Why is there Lithosphere?

STUDENT PAGES

Procedure

1. Your teacher will provide each student group with two small, square chocolate candy bars and a graham cracker. Do not eat the materials (yet!), and leave the wrappers on the chocolate bars.
A volunteer must agree to sit on one of the chocolate bars (wrapper on) for about 10 minutes.
1. What is the purpose of sitting on one of the candy bars?
1. Why don’t we just heat the candy bar in the microwave?
1. Before removing the candy bar from under the volunteer student, consider how these three items (cracker and two chocolate bars) can be arranged to model the outer portion of the earth (i.e., excluding the outer and inner core). Remember that the two chocolate bars have the same composition.
1. Draw your model in your science notebook. Shade and label the different layers, according to the different compositions.
1. The cracker represents the earth’s oceanic crust and both chocolate bars are the mantle – because they have the same composition.
1. Of the three layers, where would the warmest layer be located?
1. Why?
1. Where should the room temperature chocolate bar be located?
1. The room temperature chocolate bar represents the uppermost portion of the mantle, or the “upper mantle.” The warm chocolate bar will be referred to as a portion of the asthenosphere. The asthenosphere is also part of the mantle. So, now you know what each of the 3 layers of your model represents.
1. Revise and label your initial drawing to reflect these new observations. Labels should show the earth layer represented (not the type of candy bar).
1. The lithosphere is the outer portion of the Earth comprised of the crust and upper mantle.
1. Which layer or layers represent(s) the lithosphere?
1. Place a bracket } on your drawing next to the layers that represent the lithosphere.
1. After about 10 minutes, the chocolate bar should be warm enough for the activity. With clean hands, unwrap both chocolate bars and construct your proposed model.

READ THIS NEXT PARAGRAPH BEFORE YOU BEGIN TO APPLY STRESS TO YOUR MODEL!

Your teacher will show on the overhead projector the proper way to work with your model. One student should hold the model in two hands in front of him/her so everyone in the group can observe. Slowly and carefully apply downward stress to the edges and upward stress to the center.
When any portion of the model breaks – or fractures –STOP applying the stress.

1. Draw what you observe in your science notebook. Label this drawing with appropriate terms for the layers and title it as “Post-Stress.”
1. Use your observations of the model to answer the following questions.
1. Which Earth layer or layers fractured when stress was applied (NOTE: Use the Earth layer terms, not “cracker” or “chocolate bar”)?
1. Which layer or layers did not fracture?
1. Which layer or layers would you consider to be rigid and brittle?
1. The term plastic texture of solid rock means that it has the ability to move or flow when stress is applied. In other words, the rock is not liquid, but it is still able to flow, because it has a plastic texture. The layer of the mantle that is plastic is called the asthenosphere. The top of the asthenosphere lies approximately between 100 and 200 km below the earth’s surface. Geologists are uncertain where the base of the asthenosphere is located, but estimates are that it extends to a depth of between 600 and 700 km. In other words, the asthenosphere is NOT the remainder of the mantle, but is just the hot, plastic portion of the mantle below the lithosphere.
1. Which layer or layers would you consider to be the asthenosphere with its plastic texture?
1. Compare your group’s results with results of other student groups. Were any other results different?
1. Why do you think geologists group the crust and upper mantle together as the lithosphere, when each layer is made of different materials?
1. Stress causes fracturing of the brittle lithosphere. These broken pieces of lithosphere are called plates, or lithospheric plates. Continued stress (and other factors that won’t be described here) cause plate motion. Because of the rigid nature of the lithosphere, plates move independently of the underlying plastic asthenosphere.
1. How many lithospheric plates are illustrated in your Post-Stress drawing?
1. Review figures of the lithosphere and asthenosphere from texts, web sites, or CD-ROMs and compare them to your model.
1. Discuss the results of applying stress on the lithosphere and asthenosphere, based on your observations of the model.
1. If your teacher gives permission, and you have been handling the model with clean hands, it is now okay to eat the Earth model.

Summarize your observations and discoveries by answering the questions on the next page.

COASTeam Program, Project Oceanica, College of Charleston

Why is there Lithosphere?

STUDENT PAGES

Questions

1. Draw a side-view (called a cross-section) of the lithosphere and asthenosphere. Label the tectonic layers. Also, label the individual layers of the cross-section.
1. Why does the upper mantle fracture differently from the mantle below it?
1. How do the differences in textureof the mantle and the crust relate to the formation of lithospheric plates?
1. Are the upper mantle and mantle below it the same composition?
1. Write a one-paragraph summary of how the different textures of the mantle affect the formation of lithosphere and asthenosphere. Include why the mantle has different textures.

COASTeam Program, Project Oceanica, College of Charleston