Mapping Vertical Movements

Activity Source:

By installing GPS stations that measure the movement of Earth’s crust, UNAVCO advances geodesy, the study of Earth’s shape, gravitational field, and rotation. Each station has a receiver antenna that communicates with satellites to measure, within millimeters, how Earth is moving. Some movements are horizontal, the sliding of tectonic plates. Some movements are vertical, as when Earth’s mantle either sinks or rebounds in a process called isostatic rebound.

GPS stations in Greenland and Antarctica allow us to map glacial isostatic rebound in polar regions. As ice melts, rock rises. We measure these changes so we can create reliable models for future climate change and calculate global water availability. This activity uses “flubber” to demonstrate the visco-elastic properties of the Earth’s lithosphere and shows isostatic rebound in action.

For the teacher:

Before the activity, mix one double batch and one single batch of flubber. Single batch: In a bowl, combine 3/4 cup warm water and 1 bottle of glue; in another bowl, combine 2/3 cup warm water and 1-1/2 teaspoons borax. Double batch: In a bowl, combine 1-1/2 cups warm water and 2 bottles of glue; in another bowl, combine 1-1/3 cups warm water and 3 teaspoons borax.

Mix ingredients in each container thoroughly. Pour water/borax mixture into glue/water mixture. Gently lift and turn the mixture until only about a tablespoon of the liquid is left. Flubber will be sticky for a moment or two. When the excess liquid drops off, flubber is ready. Store in air-tight container.

Materials

• Warm water
• Three 4-oz bottles of white glue
• 4-1/2 teaspoons borax
• Measuring cup, teaspoon
• Four bowls
• Plastic wrap

Procedure

For students:

1. In large bowl, place the double batch of flubber. This represents Earth’s mantle. Let the flubber settle.
2. Place a piece of plastic wrap across the top of that flubber. The thin plastic wrap represents Earth’s lithosphere (crust and upper mantle). The thickness of the lithosphere varies around the world, averaging about 100 km.
3. The distance from the north pole to the south pole is about 12,700 km. Relative to the rest of the planet, the crust is thin. While the lithosphere is solid rock, it also has properties that allow it to act as a membrane encasing the planet. It will sink under a load and will rebound when the load is removed due to viscoelastic properties of layers underneath.
4. Discuss: What do you think will happen when we place a load on top? What will happen when we remove this load? How could we alter this model, say, in terms of the size of ice load or thickness of the “lithosphere”?
5. Roll the single batch of white flubber into a ball. This represents an ice sheet. Place it on top of the plastic wrap in the bowl.
6. Notice how the single batch of flubber flows out and sinks down. Observe, then remove.
7. Once load is removed, observe the rebound that takes place over time.

Discuss: Are there signs that a glacier has been present? (The plastic wrap would be warped.) Look at real-world examples of how scientists infer locations of former ice sheets. Erratic boulders, U-shaped valleys, drumlins, eskers, kettle lakes, bedrock striations are common signatures of the Ice Age. Isostatic rebound has caused changes to coastlines and landscapes over millennia.