Resources Saskatchewan Evergreen CurriculumGrade 9 Science: The Atmosphere Living and Learning in the Space Age

# The Effects of the Ice Age on the Earth

Watching the video:

Locate the part in the video where the narrator says:
" Although it seems like the events of 20,000 years ago no longer affect us, in fact there is one important effect still going on today". Watch carefully the graphic sequence that follows this introduction. It illustrates how the earth and the oceans responded to the advance and retreat of the ice sheets during the period of glaciation.
• What analogy is used in this sequence of the video to describe how the earth behaves?
Trying an experiment: Take a large, ziplock plastic bag and fill it about half full of water. Lay it flat in a sink.(See figure 1) Now take a rolling pin and gently place it on the bag of water and push down. Slowly lift the rolling pin back up. Try this a couple of times.
• What happens when you push downwards?
• Can you see where the displaced water goes? Look again and this time watch the air pocket as you try pushing on the rolling pin again. It will help you see how the material under the load is pushed away and where it goes.
• What happens when you slowly remove the load?
Figure 2 illustrates what happens.

Figure 1: Here is the ziplock freezer bag half full of water and lying in the sink. This is the form of the bag when there is no weight or load being placed on the bag. This is the original form and the form to which the bag returns after the load is gone.

Figure 1 - Before and after the load

Figure 2: Here you see how the load deforms the bag and how the water is pushed aside. Note the bulging affect of the displaced water. Can you see this in the real demonstration?
Even though we know that the earth's crust is not plastic and the mantle beneath the crust is not water, this demonstration can help us understand how the earth behaves. When a load is placed on it, the earth's mantle behaves partly like a viscous material such as water. The upper mantle remains solid rock, but over a long period (geologically) it can be thought of as flowing like very thick molasses. And remember - the earth's components move very slowly, so slowly that we do not notice the movement.

Figure 2 - During the load