Overview
Using foam, wood, sponges, aluminum foil and a tray that can hold water we will explore the nature of how things float, learn about density (how mass is distributed over surface area) and become familiar with the movement of the Earth's tectonic plates. It is important to give background information about density before begining this activity.

We covered the introduction to the Isostasy activity as a group. The students' answered the preliminary questions on their own, then reviewed all of the questions as a group. After the students completed the experiments, we reexamined the answers and concepts with the students. We discussed surface area and why certain boats float better than others do. The students enjoyed this activity and remained engaged.
We reviewed the Isostasy lab questions, density, and surface area. Students completed summary questions, then drew pictures of plate interactions. We talked about the isostatic readjustment that has occurred since the ice ages (Earth's lithosphere rebounding from continental glaciers.)

Goals:
Using various materials from around the house we can discover how and why things float. This discovery leads us to ask questions about how the Earth behaves.

Central Question:
Why do lithospheric plates behave as they do?

Materials for each team:
a block of Styrofoam (roughly 4" by 5")
a block of wood (roughly 4" by 5")
a thin rectangular sponge (roughly 4" by 5")
2 roughly square sheets of aluminum foil
a tray holding water of approximately 6 inches deep.
Other resources:
Work sheets

Setting the stage:
Go over the introduction and preliminary questions as a class. Break into groups of approximately three to four students and let the student exploration begin with Predictions. It is important to remind the students that the water represents the mantle or the asthenosphere, not the ocean.

Exploration/Investigation
Have students predict which material they think is the most and least dense. They should also predict which they think will float the lowest and which will float the highest.

Put the materials into the water. Using the foam, wood and sponge, determine which of the materials actually does float the highest and the lowest. They should review their predictions and see if they were correct or incorrect (and why.) They should answer the corresponding questions on their activity sheets.
Part 2 lets the students investigate how continental and oceanic plates interact. They will learn about subduction and collision zones. They should anser the corresponding questions on their activity sheets.
Part 3 reviews the property of density. The students will learn through boat design about mass distributed over surface area (density.) They should answer the corresponding questions on their activity sheets.
The summary questions can be used as a class discussion and assessment tool.

Bringing it together:
Class discussion-can answer the summary questions together. Did any of the groups get their ball of aluminum foil to sink? Why they were successful. Why to metal boats float? Talk about icebergs and their unseen underwater "roots." A talk about isostatic rebound of continents from the melting of continental glaciers during the last ice age was also very interesting to the students.

Assessment:
Activity Sheets
Summary questions as a class discussion

Background:
Isostasy is the process by which the lithospheric plates, which are "floating" on the mantle, adjust vertically to achieve an equilibrium (the crust and upper mantle have a uniform density.) Because oceanic crust has a higher density than continental crust, it adjusts lower into the mantle. This condition is augmented by the fact that oceanic crust is thinner and therefore has less mass to compensate for through buoyant displacement of the mantle. Continental crust is thicker and has a lower density than oceanic crust, therefore, it floats higher and has a deeper "root" than oceanic crust. The thickest continental crust is found under large mountain chains (such as the Himalayas.) The thickest continental crust also has the deepest "root." This phenomenon can be compared with the behavior of floating wood blocks, all with the same density (see figure). The thicker blocks stand higher but have more root than the thinner ones.

Activity Sheets:
Student Activity sheet can be downloaded and printed from HERE.