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Easy Flow: How Does Pressure Affect the Rock in the Asthenosphere?

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Author: Janice VanCleave

Problem

How does pressure affect the rock in the asthenosphere?

Materials

  • Measuring cup (250 ml)
  • Tap water
  • 9-oz (270-ml) plastic drinking glass
  • 10 tablespoons (150 ml) cornstarch
  • Spoon
  • Bowl

Procedure

Easy Flow

  1. Prepare simulated "putty rock" by following these steps:
    • Pour 1/4 cup (63 ml) of water into the plastic glass.
    • Add 1 level tablespoon (15 ml) of cornstarch and stir well. Continue adding cornstarch, 1 tablespoon (15 ml) at a time. Stir well after each addition. NOTE: The mixture should be thick enough that it is very hard to stir. Add a few drops of water if all of the starch will not dissolve, or add a little starch if the mixture looks thin.
  2. Set the bowl on a table.
  3. Hold the glass containing the putty rock in one hand, and tilt the glass slightly so that about one-half of the material flows slowly into the bowl.
  4. Observe how the material flows.
  5. Use the spoon to scrape the rest of the material out of the glass and into the bowl.
  6. Observe how the material behaves when forced to move.

Results

The material flows easily out of the glass when not forced, but cracks and breaks if pressure is applied.

Why?

The earth can be divided into three main sections: core, mantle, and crust. The innermost and hottest section is the core, with the mantle above it, topped by the thin outer covering called the crust. The crust and the upper portion of the mantle make up a layer that is called the lithosphere. Below the lithosphere is a portion of the mantle called the asthenosphere. In this zone, the rock making up the mantle behaves like both a liquid and a solid. Rock in the asthenosphere is thought to behave like the simulated putty rock prepared in the experiment; it flows easily if moved slowly, but thickens and breaks if pressure is applied. This ability of a solid material to flow is called plasticity.

LET'S EXPLORE

Would the rate at which the pressure is applied affect the results? Use the prepared simulated putty rock from the original experiment. First, apply pressure over a longer time period by placing the tip of the spoon against the surface of the putty rock. Allow the spoon to slide downward slowly. Do not push the spoon. Then, very slowly scoop out a spoonful of the material. Repeat the action, this time pushing the spoon in and lifting it back out quickly. Notice the difference in response when more pressure is applied to the material.

Easy Flow

SHOW TIME!

The crust of the earth is growing at areas called mid-ocean ridges; these ridges are divided by cracks in the crust that extend into the mantle. At the mid-ocean ridges there is decreased pressure on the magma (molten rock material beneath the surface of the earth). With less pressure, the magma flows more easily and moves upward through the cracks. The rising magma cools at the surface, forming a new layer on both sides of the crack. This new material pushes against the old layer of ocean floor, causing it to spread by about 1 inch (2.5 cm) each year. As the ocean widens the continents of Europe and North America move apart.

Yet, the earth is not expanding like an inflated balloon. There are places on the crust that are sinking down into the mantle, allowing the earth's size to remain constant. You can demonstrate the rise of magma and the sinking of the crust by asking an adult to help you build two conveyor-belt models. Follow these steps for each model:

  • Place one thread spool at each end of a 2-inch × 4-inch × 6-inch (5-cm × 10cm × 15-cm) wooden board.
  • Insert a nail through the hole of each spool, and hammer the tips of the nails into the board. Leave enough space between the spools and the head of the nails so that the spools turn easily.
  • Connect the two spools by wrapping a strip of paper around the center of each spool and taping the strip together.
  • Use a pencil to mark a starting line across each strip.

Turn the two models on their sides, end to end, with the spools facing you. Move each paper strip as needed to bring the starting lines directly above the two inner spools. Using your fingers, move the lower part of the paper strips toward the center, so that the paper strips move up and over the inner spools. This action causes the lines to move apart, representing the separation of the ocean floor as magma rises. To represent the sinking of the crust into the earth, move the paper strips in opposite directions.

Easy Flow

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