Isostasy: Exploring why continents are high and ocean floors are low

Aurora Pun
University of New Mexico
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A self-paced tutorial applies students' understanding of density in combination with analogies to apply principle of isostasy to explaining Earth's surface relief. The tutorial is bookended by online discussions that prepare students for the assignment and then formatively assess their learning.

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This is an introductory course in physical geology for majors and non-majors (mostly) intended for first-year undergraduate students. There are no prerequisites but this course is a prerequisite for other geology courses.

Skills and concepts that students must have mastered

Basic understanding of density, erosion, weathering, deposition, formation of metamorphic rocks at depth, and the fundamental differences between continental and oceanic crust.

How the activity is situated in the course

This is a stand alone exercise or tutorial


Content/concepts goals for this activity

Use of isostasy to understand elevation differences of parts of Earth's surface in terms of differences in continental and oceanic crust.

Higher order thinking skills goals for this activity

  • Make and test hypotheses of isostatic behavior of crust and mantle
  • Apply analogies to geologic processes
  • Synthesis of relationships between processes (e.g. link of isostasy with the exposure of metamorphic rocks at Earth's surface).

Other skills goals for this activity

Working with fellow students.

Description of the activity/assignment

The use of analogies of wood blocks floating in water are concrete examples students can use to infer similar processes in the abstract principle of isostasy. In preparation for the activity, students will participate in an online discussion of isostasy in order to resolve points of confusion. To actively engage students to develop and understanding of isostasy, they will then complete the online tutorial in a self-paced manner with the ability to "work with" fellow students using the discussion board to pose and address questions about the activity. The tutorial prompts students to transfer what they observe in the analogies to predicting variations in Earth's surface elevations that can be linked to variations in material properties, erosion, and mountain building. The tutorial will be followed by a follow-up discussion as a formative assessment with questions to evaluate their understanding of isostasy. Exam questions will then be used as a summative assessment tool. After completing this exercise, it is an expectation that students will be able to explain the role of isostasy as the reason why continents have a higher elevation than ocean floors, an outcome listed for the course.

Teaching Tips

Adaptations that allow this activity to be successful in an online environment

Elements of this activity that are most effective

The student's ability to make predictions of responses based on their understanding of isostasy and to explain their choices. Experiences of using a similar in-class activity exploring isostasy with analogies in face-to-face classes indicate students ability to explain their reasoning for their predictions gives them confidence of their understanding of isostasy.

Recommendations for other faculty adapting this activity to their own course:

This basic activity can be adapted for use in small or large face-to-face classes as a short online tutorial or homework. Clicker questions can be used as a follow up to evaluate student understanding.

Determining whether students have met the goals

Formative assessment with discussion of follow-up questions to determine general understanding. Summative assessment with exam questions.

More information about assessment tools and techniques.

Teaching materials and tips

Other Materials

Supporting references/URLs

This activity, authored by A. Pun is part of the Instructor's Resources for "How Does Earth Work?" 2e, a textbook by G. Smith and A. Pun, published by Pearson.

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