When and How Did Continental Crust Form?

David Mogk
Montana State University
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Many models have been proposed regarding the timing and mechanisms that first formed the continental crust. The purpose of this exercise is to help students explore the question of crustal genesis and evolution through guided discovery of the primary scientific literature to find and critically evaluate the major lines of evidence that address these various models for crustal genesis and evolution.

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This activity can be used in upper division courses for geology majors (petrology, geochemistry, historical geology, tectonics).

Skills and concepts that students must have mastered

Students should have a basic knowledge of petrology (i.e. composition of continental v. oceanic crust), and be able to navigate around the periodic table (e.g. be familiar with LILs, REEs, HFSEs, etc.). A basic understanding of petrologic processes such as partial melting, differentiation, etc. will also be useful.

How the activity is situated in the course

This is a stand alone exercise that can be done in a laboratory or class setting. I suggest using one of the following active learning approaches: 1) Jigsaw Method, 2) Role Playing or Debates, 3) Reading From the Primary Literature, or 4) Problem-Based Learning.


Content/concepts goals for this activity

Content and concept goals are focused on interpretation of geologic data to answer a contemporary scientific question. This exercise requires students to be able to integrate:
1) the composition of the mantle (depleted and undepleted),
2) whole rock and trace element data of continental crustal rocks,
3) data from numerous isotopic systems: U-Pb Nd-Sm Lu-Hf Re-Os etc., and
4) estimates of the heat budget of the Earth and how this has been distributed throughout Earth history.

Higher order thinking skills goals for this activity

Higher order thinking skills include critical thinking about the claims made from articles from the primary scientific literature, and the ability to integrate across many lines of evidence. Students must prioritize and evaluate the evidence that is presented, and develop their own conclusions based on the evidence.

Other skills goals for this activity

1. Communication skills--whether through jigsaw, role playing, debates, or report-outs, students must be able to clearly articulate the scientific arguments made by the authors to their peers in the class.
2) Quantitative skills--students must be able to understand the masses represented by differentiated continental crust compared with the composition of the mantle or whole earth, and they must also be able to place rates of differentiation into a global context.

Description of the activity/assignment

Given the extensive literature on the composition and evolution of continental crust there are a number of teaching strategies that can be employed to encourage active learning by students. A critical reading of this collection of articles will provide students with a good opportunity to evaluate the chemical isotopic and physical evidence that has led to the development of these models of continental crustal growth. These instructional approaches build on recommendations from Project 2061, Science for all Americans:
1) Start with questions about nature.
2) Engage students actively.
3) Concentrate onthe collection and use of evidence.
4) Provide historical perspectives.
5) Use a team approach.
6) Do not separate knowing from finding out.
A compilation from the primary literature has been provided (see the reference list at the end of this web page: http://serc.carleton.edu/NAGTWorkshops/earlyearth/questions/crust.html), along with guiding questions for deeper exploration and discovery. Recommended instructional methods include: jigsaw method, role playing or debates (have each student play the role of Richard Armstrong, Ross Taylor, William Fyfe...), reading the primary literature, or problem-based learning (which is purposefully ambiguous and addresses questions that require independent discovery).

Determining whether students have met the goals

I recommend using a Scoring Rubric to determine if students have mastered the concepts and content, and to assess the overall effectiveness of communication of the central ideas of each model to their peers.

More information about assessment tools and techniques.

Teaching materials and tips

Other Materials

Supporting references/URLs

This activity was developed as part of the On the Cutting Edge workshop on Understanding Early Earth. The full exercise can be accessed at: http://serc.carleton.edu/NAGTWorkshops/earlyearth/questions/crust.html.