The Effect of Postglacial Rebound on Melt Estimates for the Greenland Ice Sheet

Overview

This lesson was created by 2017 PolarTREC teacher Steve Kirsche who took part in the Dynamic Observations of the Microstructural Evolution of Firn expedition. The lesson is intended to introduce students to the concepts of isostasy and postglacial rebound through a demonstration and related instruction. Students will then see how postglacial rebound is an important factor when assessing the extent of the ice melt on the Greenland ice sheet.

Objectives

At the completion of this lesson:

• Students will gain a greater understanding isostasy and postglacial rebound.
• Students will be able to see how complex systems in nature require analyzing a wide range of factors.

Lesson Preparation

Prior to the lesson

• Students should understand the layers of the Earth (crust, mantle, outer core, and inner core) as well as the layers of the mantle (lithosphere, asthenosphere, and mesosphere).
• Students should have a general knowledge of Greenland including the fact that most of it is covered with an ice sheet.
• The teacher will need to create the model used as a demonstration in the lesson.

Creating the model

• A clear glass container such as a glass baking pan or a small aquarium will be needed. This should be filled about ¾ full with water. It is a good idea to add a few drops of blue food coloring to the water to make it more visible.
• A piece of a 2 x 4 board will be used to model the continental crust (Greenland). If you mark each centimeter on the side of the board, it will be easier to measure how much remains above the water.
• A piece of 1 x 4 board will be used to model the ice sheet. It can be left natural or painted white. The white will help students better see it as the ice sheet when it is placed on top of the 2 x 4.

Teaching the Lesson

• The teacher will review the prerequisite knowledge.
• The demonstration will be completed.
• Students will read/analyze the ScienceDaily release to see how postglacial rebound is important when estimating ice melt.
• Students will write a brief explanation of how the demonstration models the effect of postglacial rebound and its effect on estimating ice melt in Greenland.

Procedure

1. Begin by reviewing the layers of the Earth (crust, mantle, outer core, and inner core) and the layers of the mantle (lithosphere, asthenosphere, and mesosphere). It is important to emphasize the concept that the lithosphere “floats” on the asthenosphere, allowing for tectonic plate movement.
2. Conduct the demonstration.

• Show the students the clear container with the water in it and explain that this represents the asthenosphere.
• Take the piece of 2 x 4 board and explain that this represents the lithosphere. Ask what they expect to happen if the board is place in the water. Lead a discussion with the students to remind them that the lithosphere consists of the upper mantle and the crust and that it “floats” on top of the asthenosphere.
• Place the board in the water and let students see how it floats, but part of it sinks down into the water. Explain the concept of isostasy where pieces of the lithosphere, such as Greenland, push down into the asthenosphere a bit. You can use the example of a piece of memory foam to illustrate this. If you lay on a memory foam mattress you sink down in a bit. Measure how much of the board is above the water.
• Place the piece of 1 x 4 on top of the piece of 2 x 4 to model an ice sheet on top of the land. Students should note that the combined weight of the two boards causes the board to sink a bit more into the water. Explain that the ice sheet on top of Greenland causes it to sink down even more in the asthenosphere. Measure the amount of the board that is above the water.

• In the attached photos, the board with the attached ice sheet had 2.7 cm above the water. When the 1.8 cm board was removed, there was only 1.6 cm above the water line. Your measurements should be similar (or you can use the attached photos). Lead a discussion of how the “elevation” above the water changed by 1.1 cm when there was a loss of 1.8 cm of ice. In Greenland, a similar phenomenon is occurring and it is called postglacial rebound. It means that just looking at the change in elevation due to ice melt is not telling the whole story – there is actually more melting going on than what is measured this way.
  3. Provide students with a copy of the ScienceDaily news release (https://www.sciencedaily.com/releases/2016/09/160923123732.htm). They should read through this article, highlighting or underlining the important points. After the students have read it, discuss as a group how postglacial rebound can affect the accuracy of the measured ice loss on the Greenland ice sheet. Please note that, for Greenland, the postglacial rebound is slower than the ice melt. So, although it affects the accuracy of the ice melt readings, it could be worse.
  4. Conclude the lesson by having the students write 2-3 paragraphs to respond to the following writing prompt:

• Explain what postglacial rebound is and why it is happening as the Greenland ice sheet has shrunk.

• Explain how the demonstration we did in class models this phenomenon.
• What are the limitations of this as a model?

Extension

• The demonstration can be easily modified to make it into a lab/activity that the students can do themselves.
• For some students, the ScienceDaily release will provide enough information to understand the concepts being taught. For other students, they should use the full University of Buffalo paper (http://advances.sciencemag.org/content/2/9/e1600931.full).
• A component on the densities of ice and continental crust can be added. A small piece of granite can be used. Students can determine the volume using displacement in a graduated cylinder and determine the mass using a triple beam balance. This can be compared to the average density of ice which is approximately .92 g/cm3. Using this information students can hypothesize the relative postglacial rebound compared to the thickness of ice loss.

Resources

• Site for the expedition to Summit Station (can also be reached at: polartrec2017.com): https://www.polartrec.com/expeditions/dynamic-observations-of-the-microstructural-evolution-of-firn
• ScienceDaily release which provides an overview of a 2016 University of Buffalo paper which helps examine the effect of postglacial rebound on ice melt estimates for the Greenland ice sheet: https://www.sciencedaily.com/releases/2016/09/160923123732.htm
• Full text of the University of Buffalo paper: “Geodetic measurements reveal similarities between post–Last Glacial Maximum and present-day mass loss from the Greenland ice sheet”: http://advances.sciencemag.org/content/2/9/e1600931.full

Assessment

Students will be assessed by evaluation of the paragraph(s) they write at the end.

Author/Credits

Steve Kirsche, 2017 PolarTREC Teacher, steve [at] redotter.com