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# Riser: How Does Density Affect the Movement of Magma?

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Source:
Author: Janice VanCleave

### PROBLEM

How does density affect the movement of magma?

### Materials

• Tap water
• Quart (liter) jar with lid
• Red food coloring
• Spoon
• 1 cup (250 ml) vegetable oil
• Timer

### Procedure

1. Pour the water into the jar.
2. Add ten drops of the food coloring and stir.
4. Secure the lid.
5. Hold the jar so that the light from a window or desk lamp shines through the liquid in the jar.
6. Slowly turn the jar until it is upside down, and then return it to its original position.
7. Observe and record the movement of the contents inside the jar for about 30 seconds.

### Results

When you first pour the oil into the jar, it floats on top of the colored water. After you tip the jar, most of the oil immediately rises again to rest above the colored water, and small bubbles of oil continue to rise for a short period of time.

### Why?

The separation of the two liquids is due to their being immiscible, meaning they do not mix. The differences in the densities (a comparison of the "heaviness" of materials) of the water and oil result in the denser water sinking to the bottom and the less dense oil floating to the top. like the oil, magma, which is less dense than the rock around it, tends to rise to the surface. Magma begins its upward movement from depths of 35 miles to 50 miles (56 km to 80 km) beneath the earth's crust. This upward journey can be caused by pressures within the earth, but more often magma rises because its density is lower than that of surrounding material.

### LET'S EXPLORE

Does shaking the bottle longer affect the results? Repeat the original experiment, shaking the jar vigorously for 5 seconds. Record your observations every 5 minutes for 30 minutes, and then continue checking every hour until no further changes occur. Shaking the oil and water can be used to simulate mixing magma with denser materials. How does mixing the materials affect the material of lesser density? Science Fair Hint: Diagrams showing the contents of the jar at different time intervals can be made and used as part of a project display.

### SHOW TIME!

1. Unless restricted by pressure, hot materials expand (get larger) and cold materials contract (get smaller). All rock materials do not expand at the same rate; thus, within the earth some of the heated rocks expand and become less dense than the surrounding rock material. You can demonstrate the difference in the densities of a compressed material and the same material when it is expanded. To do so, roll a lemon-sized piece of clay into a ball. Then, shape another similar sized piece of clay into an open box; be sure to make the box as large as possible. Place both pieces of clay on the surface of a container of water.
2. To demonstrate the way one material will rise through another due to differences in density, fill a plastic dishwashing liquid bottle halfway with vegetable oil and secure the lid. Fill a small aquarium with water. Add a few drops of blue food coloring to the water and stir, making the water lightly colored. Turn the dishwashing liquid bottle on its side and push it down to the bottom of the aquarium. Open the spout and gently squeeze the bottle. Use photographs of the experiment as part of a project display.

### CHECK IT OUT!

At times it appears that magma moves through solid rock. By a process called stoping, blocks of solid rock in the path of magma are broken, melted, and added to the flowing hot, liquid rock. Find out more about magma. Does magma always reach the earth's surface? Learn about the effects that the movement of magma has on the earth's crust, such as the production of earthquakes.