Objective

Here is a fascinating activity that will help you give your students a better understanding of Earth's structure and how it creates tectonic plate movement. This activity is designed for students in grades 5–8.

Background

How do you help your students visualize the huge convection cells of solid material circulating within Earth's mantle that create tectonic plate movement? This activity offers a solution: Create miniature convection cells within a desktop flow chamber so that students can see firsthand how convection cells are created and how they work. The key then to explaining how convection cells drive plate tectonics is to keep it simple. There is a vast amount of information about Earth's interior structure, all interesting, but not critical for an understanding of basic plate tectonics.

Teacher's note

Students may question why the hottest part of Earth, the inner core, is solid, and why the outer core, which is cooler, is liquid. Here is the answer. The immense pressure at the inner core causes it to remain solid even though it is hotter. The outer core is under less pressure so it remains liquid even though it is cooler.

Here are the basics. A cross section of Earth would reveal 3 major internal structures: the core, mantle, and crust. The core is made of 2 distinct regions, the inner core and outer core. The inner core is solid, and the outer core is liquid (see "Teacher's note"). For this activity, consider the core as one area located in the center of Earth producing vast amounts of heat. The mantle, an area of slowly moving convection cells of solid material between the core and the crust, is divided into several regions based on temperature and plasticity. For this activity, consider the mantle as a single contiguous region between the core and the crust where convection cells form.

The crust consists of 2 parts, the oceanic crust and the continental crust. The oceanic crust is the lower layer. It is made of basalt, a very dense, heavy material that is actually magma solidified by contact with cold seawater. All Earth's seafloors are made of basalt. The second part of the crust is the upper layer or continental crust. Less dense and lighter than the oceanic crust, it makes up Earth's continents. The oceanic and continental crusts are also known as the oceanic and continental plates or generically as tectonic plates.

Now, let's put it all together. Heat from the core creates convection cells within the mantle. The tops of these convection cells function like conveyor belts, slowly moving Earth's tectonic plates. This movement is the key concept in the theory of plate tectonics.

Materials

• Carolina's convection fluid
• French square bottle
• Ring stand
• Incandescent desk lamp with 75-W bulb

Preparation

Safety note: If you decide to use your own French square bottle, make sure it is made of clear glass. Do not use a plastic bottle.

1. Shake the bottle of convection fluid to ensure it is thoroughly mixed.
2. Fill the French square bottle completely with convection fluid, being careful not to leave any pockets of air. Screw the cap on tightly.
3. Place the bottle on the ring stand as shown in figure 1.
4. Place the lamp directly under the center of the bottle as shown in figure 1. Do not turn on the lamp.
   
   Figure 1. Fluid flow chamber set up.

Procedure

1. Begin by asking the class to share what they know about Earth's structure. Here are some suggested questions:
• How thick is Earth's crust?
• What is under Earth's crust?
• What is the inside of Earth like?
2. Describe Earth's 3 major internal structures—the core, mantle, and crust. The thickness of the core is 6,840 km. The thickness of the mantle is 2,920 km. These figures can be compared to the width of the United States, which is 4,626 km.
3. Use a local map to make the figures more relevant to your students. For example, the continental crust at its thickest point is no more than 70 km, and the oceanic crust at its thickest is about 7 km. Have your students pick a location 70 km from where they live. Now find this distance on a map of the United States and compare that distance, which represents the thickness of the crust, with the thickness of the mantle and the core.
4. Gather your students around the apparatus. Turn on the lamp and let them observe the convection cells begin to form.
Figure 2. Convection cells as observed in the flow chamber.



5. Review how the heat from the core causes the mantle to move by convection. Convection is the process of material heating and rising, then cooling and falling. This process creates the cells of circulating mantle material that act as conveyor belts pushing, pulling, and carrying the tectonic plates. Figure 2 shows what the students see in the flow chamber. Explain that this is the motion scientists believe causes tectonic plates to move. Ask your students to record their observations.
6. Emphasize to your students that the demonstration is intended only to show the pattern of movement in a convection cell. The fluid in the chamber is moving very rapidly, while the convection cells in the mantle move incredibly slowly. What students see happen in one second in the chamber may take millions of years in the mantle. Also, note that the division between cells in the chamber occurs directly over the heat source. There are many ideas to explain the uneven heating required to create such a situation in the mantle, but earth scientists are in general agreement that the convection cells do form and do move tectonic plates.
7. Switch off the light and allow the apparatus to cool. Dismantle the apparatus and return it to storage. Convection fluid is reusable. Funnel it back into its original bottle and replace the cap, or simply leave it in the French square until the next demonstration.

Final discussion questions

Lead your students through these questions. Have them refer to their notes if necessary.

• Q. What is at the center of Earth?
  A. A heat-producing core.
• Q. What is the mantle?
  A. An area of solid material surrounding the core.
• Q. What is formed in the mantle as a result of the heat from the core?
  A. Convection cells.
• Q. What are tectonic plates?
  A. Portions of Earth's crust that are moving in various directions at various rates
• Q. What part do convection cells play in the movement of tectonic plates?
  A. They act as conveyor belts, pushing and pulling the plates.