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Isostatic Rebound: Hands-on Model
Note: For the latest version, helpful notes and 5ps, photos, videos, and further informa5on, please visit this ac5vity online at h9p://polenet.org/educa5on/inquiry To illustrate Earth's viscoelas1c proper1es, we can build models that mimic how the lithosphere and asthenosphere respond to a load. This model was designed to use materials that you'll likely find in your cupboard.
This is appropriate for learners of all ages, but par1cularly middle school and high school geoscience and environmental science classrooms. You'll need:
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2 mixing bowls A variety of containers to hold flubber 1 clear rectangular container to use for model A ceramic coffee cup Plas1c Wrap Ziplock bags Measuring cup and spoons Cooking oil (or vaseline, but oil works beKer) Glue Food coloring Borax Toothpicks S1cky tack String (op1onal) The idea is to show how a weight pressing down on the flubber (Earth's asthenosphere) covered with a thin layer of plas1c wrap (Earth's lithosphere) will cause it to sink; likewise, when the weight is removed, the model will rebound. First, we need to cook up some flubber! Depending on the size of your container, you can make it in single or double batches: Flubber Recipe
Single Batch:
In a bowl, combine 3/4 cup warm water and 1 cup white glue (1 boKle of 4 oz. glue) and food coloring (if needed)
In another bowl, combine 2/3 cup warm water and 1 1/2 teaspoon borax.
Food coloring (if needed) Double Batch of Flubber:
In a bowl, combine 1 1/2 cup warm water and 2 cups white glue (2 boKles of 4 oz. glue) and food coloring (if needed)
In another bowl, combine 1 1/3 cup warm water and 3 teaspoon borax.
Mix ingredients in each container thoroughly.
Pour water/borax mixture into glue/water mixture.
Gently liZ and turn the mixture un1l only about a tablespoon of the liquid is leZ. Flubber will be s1cky for a moment or two. Let the excess liquid drop off, then flubber will be ready.
Store in air 1ght container (stores well for up to three weeks) _____________________________________________________________
LEVEL ONE: Looking from above
Requires: 1 double batch of colored flubber, 1 single batch of white flubber. 1. In large bowl, place the double batch of colored flubber. This represents Earth's mantle. Let the flubber seKle. 2. Place a piece of plas1c wrap across the top of the flubber. This represents Earth's crust. -­‐-­‐-­‐ Inquiry Ques-on: What do you think will happen when we place a load on top? 3. Roll the single batch of white flubber into a ball. This represents an ice sheet. Place it on top of the plas1c wrap in the bowl. 4. No1ce how the white flubber flows out and sinks down. Observe, then remove. -­‐-­‐-­‐ Inquiry Ques-on: What do you think will happen when we remove this load? 5. Once load is removed, observe the rebound that takes place over 1me. Interpretation:
These materials represent different parts of Earth's system in the process known as Isosta1c rebound, which is the slow rise of land masses that were depressed by the huge weight of larger ice sheets in the past, such as during past ice ages. The thin plas5c wrap represents Earth's lithosphere, which consists of the crust and the upper mantle. The thickness of the lithosphere varies widely around the world, with an average thickness of about 100 km. Let's look at the Earth from the north pole to the south pole-­‐-­‐ if we were to measure this, we would get something like 12,700 km. So rela1ve to the rest of the planet, the crust is very thin! We can think of it as Earth's skin, with several more layers of the planet beneath it. We oZen think of the lithosphere as solid rock-­‐ and it is. But this solid rock has certain proper1es that allow it to act as a membrane that encases the planet. It will sink under a load and will rebound when the load is removed due to the viscoelas1c proper1es of the Earth beneath it. The colored flubber represents Earth's asthenosphere, the layer beneath the lithosphere. It is is duc1le and can be pushed and deformed like silly puKy in response to the warmth of the Earth. It is viscous-­‐ meaning it has a thick, s1cky consistency between solid and liquid. These rocks actually flow, moving in response to the stresses placed upon them-­‐-­‐ the churning mo1ons of the deep interior of the Earth, or loads placed at the surface of the Earth. The flowing asthenosphere carries the lithosphere of the Earth, including the con1nents, on its back. The white flubber represents an ice sheet. Ice sheets are thick layers of ice that cover an extensive area for a long period. Ice sheets sit on land (unlike ice shelves, which float in water). It's not possible to replicate the forma1on of these ice sheets within this model, but we can show how ice sheets flow over 1me and cause depressions in the Earth. Again it's not possible to show the slow removal of this load (by mel1ng), but a quick removal of this load allows us to model the flowing of the asthenosphere that pushes the lithosphere back up. See video of Dr. Mike Bevis is Professor, Ohio Eminent Scholar at Ohio State University (hIp://polenet.org/?
page_id=137): ”If you imagine a very wide ice sheet, like the one that happened in Canada. That's so wide compared to the thickness of the lithosphere that the lithosphere will just flex to accommodate whatever the ice wants to do. And so in some sense the lithosphere is just a membrane between the ice and the asthenosphere. The asthenosphere's moving away from the load-­‐ it's moving away from the pressure of this load and the ice is sinking down-­‐ the lithosphere's sort of just moving with it, it's a substrate between the ice and the asthenosphere. In effect, the sinking will stop when the ice is floaFng in the asthenosphere and the lithosphere's just something that's in between the two. Once you get to a smaller load, it's a bit more complicated. There'll be some support from the lithosphere itself. The lithosphere just collapses under a very wide load. But under a small load, it bends to give it some support. So you'll have a bit more complicated story where it's part supported by the asthenosphere and it's part supported by the flexural rigidity of the lithosphere." Questions & Discussion:
1. How could we alter this model? (Larger/smaller ice load, thinner/thicker lithosphere) 2. Are there any signs that a glacier has been present? (The plas1c wrap will be slightly warped). Look at real-­‐world examples of how scien1sts infer loca1ons of former ice sheets. Erra1c boulders, U-­‐shaped valleys, drumlins, eskers, keKle lakes, bedrock stria1ons are just some common signatures of the Ice Age. Isosta1c rebound has caused significant changes to coastlines and landscapes over the last several thousand years. ____________________________________________________________
LEVEL TWO: Looking from the side
This more in-­‐depth look at isosta1c rebound requires more prepara1on, but shows in further detail how the lithosphere and asthenosphere are deformed under a load. 1. Mix up 2 batches of flubber (single or double, depending on the size of your container). 2. Smear oil along the inside of container (sides only). This will ease flow and prevent s1cking to the sides. 3. Place a piece of plas1c wrap on the boKom of the container. 4. Place a layer of one color on top of the plas1c wrap. Fold the edges over to cover the flubber. Place another layer of plas1c wrap with sides s1cking up. 5. Add a layer of the alternate color. Fold the sides of the plas1c wrap over the flubber. You're trying to create layers of flubber without leing them mix. Add a clean piece of plas1c wrap and top with alternate color. Repeat un1l you have 4-­‐8 layers. Aim for roughly 1/2 inch layers. 6. Place toothpicks with small pieces of s1cky tack at the top on the far edges of the container. Push into at least 2 layers of the flubber. -­‐-­‐-­‐ Inquiry Ques-on: What do you think will happen to the layers when we place this load? What will happen to the toothpicks? 7. Place a mug on the top layer-­‐ You want it to be flush with the side of the container. You may need to guide it if it slips back or away. Depending on the bowl, you may need to add extra weight to achieve the desired effect. Don't worry about what's happening on the other side-­‐ this model only focuses on the front side. 8. Watch how the load causes the flubber to sink. Note how the layers move. 9. Remove the load and observe the rebound. This does take several hours to complete, again depending on the size of the container and the number of layers. Check back every hour or leave siing on a flat surface overnight. (Just remember these kinds of drama1c changes take thousands of years in geologic 1me!) Interpretation:
Again, the layers of colored flubber represent the asthenosphere and the plas5c wrap represents the lithosphere. The toothpicks represent a GPS sta5on. GPS sites are installed on hard rock. Because most of the polar regions are covered with a thick, icy blanket, these sta1ons tend to be established on nunataks. Nunataks are isolated peaks of rock projec1ng above a surface of ice or snow. The GPS sites register incoming microwave signals from satellites. Over 1me, whether it is a result of plate tectonics or glacial isosta1c adjustment (crustal movement caused by the mel1ng of and consequen1al release from the weight of the polar ice), the GPS data measures the slightest movement of the surface from its original loca1on. Note how the toothpicks not only move up and down but they lean towards the weight. Scien1sts take this movement into considera1on when processing their data. You can use the op1onal string by taping a taut piece across the length of the container. This will help illustrate how the toothpicks move over 1me. Ideally, the model should show a movement like the one illustrated above. Bear in mind that you will not get a perfect curve ( just as the Earth does not produce a perfect curve). But the general trends should be clear. The curve will vary depending on the size of your container in rela1on to the weight of your load. Questions & Discussion:
1. How important is it that POLENET's GPS sta1ons be poin1ng straight up when they are installed? 2. How could you tweak the model to get different results? (ie, larger/smaller weight, less/more load-­‐bearing 1me, thinner/thicker lithosphere, etc) 3. Imagine you have a sea that surrounds your GPS sta1on. What will happen to sea-­‐level when the load is placed? When it is removed? The ice sheets at the last Glacial Maximum were so massive that global sea level fell by about 120 meters. Thus con1nental shelves were exposed and many islands became connected with the con1nents through dry land. This was the case between the Bri1sh Isles and Europe, or between Taiwan, the Indonesian islands and Asia. Most important is the existence of a sub-­‐con1nent between Siberia and Alaska that allowed the migra1on of people and animals during last glacial maximum. 4. How fast and how much isosta1c rebound occurs is determined by two factors: 1, the viscosity (flow) in the asthenosphere, and 2, the ice loading and unloading histories on the surface of Earth. Viscosity of the asthenosphere is important in understanding mantle convec1on, plate tectonics, dynamical processes in Earth, the thermal state and thermal evolu1on of Earth. However viscosity is difficult to observe because creep experiments of mantle rocks take thousands of years to observe and the ambient temperature and pressure condi1ons are not easy to aKain for a long enough 1me. Thus, the observa1ons of postglacial rebound provide a natural experiment to measure mantle viscosity. How would this model behave differently if its asthenosphere was more viscous? Less viscous?