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# Plate Tectonics: Floating Crustal Sections

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Author: Janice VanCleave

The Earth's lithosphere is made up of sections called plates that move in relation to each other. This movement is made possible because the plates float on the asthenosphere, which has a thick mudlike texture.

In this project, you will demonstrate seafloor spreading at midocean ridges. You will use models of seafloor spreading to indicate the changes in the Earth's magnetic field over long periods of time. You will learn about the theory of plate tectonics. You will also model the movement of lithospheric plates at divergent boundaries, convergent boundaries, and transform boundaries.

### Materials

• Sheet of typing paper
• Scissors
• 42-ounce (l.19-kg) empty, round oatmeal box
• Serrated knife (use with adult approval)

### Procedure

1. Fold the paper in half lengthwise with the long edges together.
2. Unfold the paper and cut it in half along the fold line.
3. Use the knife to cut a 1/4 × 5-inch (0.63 × 12.5-cm) slit in the side of the box.
4. Put the paper strips together, one on top of the other, then push the papers down through the slit in the box. Keep about 2 inches (5 cm) of the strips on the outside and fold them back on opposite sides of the slit.
5. Hold the ends of the strips, one in each hand, and slowly pull about 6 inches (15 cm) of the papers in opposite directions along the surface of the box (see Figure 19.1).

### Results

The paper strips emerge from the box and move along the box's surface in opposite directions.

### Why?

Where the papers exit, the box represents a midocean ridge (one of a number of ridges forming a continuous chain of underwater mountains around the Earth). In the center of the midocean ridge is a rift valley. A rift valley is a deep, narrow crack in the Earth's crust, like the slit in the box. Molten rock rises to the surface through this crack. About half of the lava rising out of the rift valley spreads on either side of the midocean ridge. The portion of crust on either side of the ridge is moved apart by the addition of the new material. The lava hardens and forms new ocean floor. This process of the creation of new oceanic crust that moves slowly away from the midocean ridges is called seafloor spreading.

Evidence of seafloor spreading is a pattern of parallel magnetic "stripes" that are identical on each side of a midocean ridge. The magnetic stripes came about because, before the lava solidified into rock, the mineral grains of magnetic iron in the lava aligned in the direction of the Earth's magnetic field (region in which magnetic materials are acted on by magnetic forces). When the lava solidified, the grains of the rock were permanently fixed in the direction of the Earth's magnetic field. But because the Earth's magnetic field has reversed itself many times over millions of years, stripes of rock next to each other may have grains aligned in different directions.

### Try New Approaches

1. Represent the formation of magnetic stripes on the crust beneath the ocean by pulling the paper strips out of the box so that about 2 inches (5 cm) of the paper comes out of the slit on both sides. Use a colored marker to make a colored stripe about 1/2 inch (1.25 cm) wide across the paper strips where they come out of the box. Pull the paper strips so that another 1/2 inch (1.25 cm) comes out the slit. Use a different-colored marker to color the new white paper above the slit. Continue to pull out 1/2 inch (1.25 cm) of new paper from the slit, alternating the colors until six to eight matching stripes are made on each side of the paper. Science Fair Hint: Display the model of the magnetic stripes along with a diagram, similar to Figure 19.2, showing a midocean ridge with magnetic stripes on either side of the ridge. Use arrows to indicate the reversed directions of the stripes.
2. While the seafloor may spread from 1 to 5 inches (2.5 to 12.5 cm) or more per year, the total amount of crust stays the same. This is because as new crust is being formed at the midocean ridges, old crust is sinking into the asthenosphere, where it melts and is absorbed into the mantle. Represent this movement by repeating the original experiment, but cut three slits, 4 inches (15 cm) apart, in the box. Tape both strips of paper, one on top of the other, to a pencil, near its point (see Figure 19.3A). Wind all but about 6 inches of the. paper around the pencil. Position the box on its side, with the center slit on top and a slit on each side. Place the pencil and paper strips inside the box. Separate the strips, pushing one strip out through each of the side slits. Put the ends of the-strips together and push them down through the center slit. From inside the box, pull the papers down as far as possible without pulling the strips off the pencil. Slowly turn the pencil to wrap the paper around it as you observe the movement of the paper on the outside of the bolt (see Figure 19.3B). For more information about seafloor spreading, see David Lambert and the Diagram Group, The Field Guide to Geology (New York: Facts on File, 1988), pp. 40–43.
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