A Quantitative Visualization of Mantle Melting

Content/concept goals for this activity

The goals of this activity include:

• Students will practice visualizing increasing pressure as increasing depth in the earth.
• Students will practice visualizing the mantle adiabat as a phase boundary between solid and liquid.
• Students will evaluate the role temperature, pressure, and water play on mantle melting.

Higher order thinking skill goals for this activity

Students will engage spatial skills by orientating where the Earth's surface and interior are on this diagram, what a temperature-pressure profile of the ambient mantle looks like, and how to illustrate a geologic process quantitatively on a diagram. Students will make a spatial prediction, receive feedback, and modify their prediction based on the feedback.

Audience

Students in an upper-level petrology course.

Skills and concepts that students must have mastered

Students at this point will have been taught a qualitative overview of the three ways mantle melting can occur (i.e., flux melting, decompression melting, melting in the presence of a mantle plume) by means of geologic cartoons (ie., cross-section cartoons of a subduction zone, mantle plume, and ocean island) and videos. This activity is meant to be a quantitative visualization of why and how mantle melting occurs using a pressure-temperature profile of the Earth.

How the activity is situated in the course

This activity is used as a formative assessment question both prior to and after a lecture about mantle melting. Displaying the results after administering the question provides students and instructor immediate feedback about what students know prior to the lecture and how their understanding has changed after a lecture is given.

Example of how to implement this exercise in the classroom

1. Students are shown the first diagram and are asked the following, "Click on the diagram where the surface of the Earth is represented."
This is to emphasize the orientation of this particular diagram (some pressure-temperature diagrams are reversed).

2. Students are shown the second diagram and asked the following, "Click on the diagram where the mantle is liquid" ; "Click on the diagram where the mantle is solid"; "Click on the diagram where the shallowest point of melting occurs"; "Click on the diagram where the deepest point of melting occurs."


3. Students are asked a predictive question before being shown the third diagram; "What will happen to the solidus if we add water?", or "Indicate to your neighbor how the solidus would change in the presence of water on this diagram."

4. Students are shown the third diagram and asked a follow-up question such as, "Click on the diagram where the shallowest melting occurs in a wet setting."

This exercise can be used to assess student understanding both before and immediately after lecture about mantle melting. An example of a mantle melting lecture should include schematics of the different tectonic settings and processes that lead to mantle melting (e.g., decompression melting, flux melting, melting in the presence of a mantle plume).

Students often get hung-up on the concept of an "adiabat" so this concept will need to be taught explained thoroughly throughout the lesson and term.

Note you can include end-member values for pressure and temperature on this diagram for more context. Although keeping it simple may simplifying the lesson.

This question is useful for students to self-assess where their answer fits relative to other students in the class. The web-based system (e.g., Top Hat) can display student responses in a heat map image that highlights the most common answers. In most systems it is possible to designate a region for the correct answer, but receiving a right-wrong answer is likely less useful than engaging students in peer discussion if the students' responses do not converge on one region. Follow-up discussion will occur between students after the heat map is displayed as a greater class. Then concepts will be reviewed in response.

 

 

 

There are several systems that offer click-on-diagram questions. Examples includes:https://www.polleverywhere.com/app, https://tophat.com

LaDue, N.D. and Shipley, T.F. (2018). Click-on-Diagram Questions: A New Tool to Study Conceptions using Classroom Response Systems. Journal of Science Education and Technology, 27(6), 492-507.

Winter, J. D. (2010). Principles of igneous and metamorphic petrology. New York: Prentice Hall.

Helpful video resources:

• Teach the Earth "Three Ways to Melt the Mantle"
  https://serc.carleton.edu/NAGTWorkshops/video/minutevids/237239.html
• Arizona State University, The CLASroom, "The Science of Magma"
  https://www.youtube.com/watch?v=nDmD0lx3cGs