What does the core/mantle boundary look like?

Suzanne Baldwin

Syracuse University

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This activity was selected for the On the Cutting Edge Reviewed Teaching Collection

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- Scientific Accuracy
- Alignment of Learning Goals, Activities, and Assessments
- Pedagogic Effectiveness
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This activity has benefited from input from faculty educators beyond the author through a review and suggestion process. This review took place as a part of a faculty professional development workshop where groups of faculty reviewed each others' activities and offered feedback and ideas for improvements. To learn more about the process On the Cutting Edge uses for activity review, see http://serc.carleton.edu/NAGTWorkshops/review.html.

Initial Publication Date: January 15, 2011 | Reviewed: October 31, 2012 (see revision history: 2 events)

Summary

This activity explores how earth scientists infer what materials are present at the core mantle boundary and what this boundary might look like. It provides students with the opportunity to contribute to the scientific debate regarding what the core mantle boundary looks like, it's composition, structure, and properties.

Geophysics, Geochemistry, Mineralogy, Igneous and Metamorphic ...(more) | College Upper (15-16)

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Context

Audience

This activity can be used in an undergraduate Mineralogy course.

Skills and concepts that students must have mastered

Before beginning this activity the students will have learned and be able to answer the questions 'what minerals are in the core and how do we know?" and "what minerals are in the mantle and how do we know?" See attached file for specific topics to cover prior to assigning this activity.

How the activity is situated in the course

Introductory Geology is a prerequisite for mineralogy and introductory chemistry is a corequisite. This activity follows discussions of minerals of the core and mantle, approximately one third through the semester. It is assigned as a take home exam question and prior to discussions of the crust and surface minerals.

Goals

Content/concepts goals for this activity

Goals of this study are to integrate data sets from cosmochemistry, experimental petrology, and tomography to develop a model for the structure and composition of the core-mantle boundary. Students will gain an appreciation of how we learn about this boundary within the deep Earth when we have no direct evidence for what exists at the core-mantle boundary.

Higher order thinking skills goals for this activity

These skills include formulating a hypothesis, and formulating ideas for testing the proposed hypothesis. Students will also contribute to a debate for which the answer is unknown. They will learn that they are able to contribute to this debate.

Other skills goals for this activity

Reading the literature, drawing schematic figures, working in groups and presenting results are other skill goals.

Description of the activity/assignment

We will begin with a review of what minerals are inferred to be in the core and what minerals are inferred to be in the mantle. Students will then sketch what they think the core-mantle boundary looks like. Students will make observations of samples of pallasite (identify texture and compositions). Next using phase diagrams (e.g., Fe, olivine, post-perovskite) determine the temperatures and pressures at which these minerals might be stable at the core-mantle boundary. What is the coordination number for these inferred phases at this boundary? Are the phases liquid, solid or both? Next assess and describe how P and S wave velocities change at the core-mantle boundary.

Determining whether students have met the goals

Students will submit two drawings of what they think the core-mantle boundary looks like ( the drawing that they started off with and the one they revised following this learning activity). They will submit a paper (no longer than 2 pages) discussing the indirect evidence they have used to generate their final figure. They will share with the group their results.

More information about assessment tools and techniques.

Teaching materials and tips

Activity Description/Assignment:
In this exercise we will explore what the core-mantle boundary might look like. No one actually knows what this boundary looks like, so this is an opportunity for you to contribute to this debate. Since we can't observe materials at the core-mantle boundary directly, how can we know what's there? What minerals might be there? Are there solid and liquid phases? You will have an opportunity to explore the latest research on this topic to develop your own model for what the core-mantle boundary might look like.

1) To begin, draw a sketch of what you think the core-mantle boundary looks like. When you are finished hand this sketch in. (This will be an in class assignment to be turned in during lecture)

2) Next we will explore the recent literature to ascertain what phases might be present at the core-mantle boundary. Indirect evidence for what might be at the CMB comes from the fields of cosmochemistry, experimental petrology and seismic tomography. Some references to get you started are listed below.

Indirect evidence from cosmochemistry.Pallasites are a type meteorite that has often been used to infer what the CMB might look like. What minerals can you identify in the pallasite samples? For those who do not have access to pallasite samples, students can check out the pallasite samples for sale on the web (e.g., http://www.gizmag.com/the-fukang-pallasite--extraterrestrial-gemstones-for-sale/9134/picture/43667/)

Indirect evidence from experimental petrology. Using the phase diagrams of post-perovskite and perovskite, and Fe describe the P-T conditions inferred at the Earth's core mantle boundary. http://tristan.ferroir.free.fr/Post-perovskite.php

Indirect evidence from seismic tomography. (to be added) discussion of fate of subducted slabs, etc.

As you dive deep into your research on what materials might be at the core-mantle boundary, feel free to see me to discuss any questions you might have.

3) Finally, empowered with the results of your literature search, and from compiling all indirect evidence, make a revised sketch of what you think the core mantle boundary looks like. Your revised sketch is to be accompanied by a written description of its features with references to back up your inferences. This accompanying description should be no longer than 2 pages.