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Melt Down: Can Solid Rock Melt?

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Source:
Author: Janice VanCleave

Problem

Can solid rock melt?

Materials

• Cup
• Warm tap water
• Spoon
• Timer
• -by--inch (1.25-by-1.25 cm) square of milk chocolate candy
• Saucer
• Toothpick

Procedure

1. Fill the cup with warm tap water.
2. Place the spoon in the cup of water.
3. After about 30 seconds, remove the spoon from the water and place the chocolate in the spoon.
4. Set the spoon on the saucer.
5. Use the toothpick to move the chocolate around in the spoon.

Results

The chocolate melts.

Why?

Chocolate is solid at room temperature, but like all solids, it melts when heated, forming a liquid. The temperature at which a solid changes to a liquid is called its melting point. Because chocolate has a low melting point, the heat from the spoon is enough to raise the chocolate's temperature to the melting point. The hotter the chocolate gets, the more fluid (capable of flowing) it becomes.

The change in the chocolate from a solid to a liquid due to an increase in its temperature is similar to the change of solid rock to magma (liquid rock beneath the earth's surface). Rocks have a much higher melting point than the chocolate. The tremendous heat at depths of about 25 to 37 miles (40 to 60 km) below the earth's surface is great enough to melt rock. As with chocolate, the hotter magma is, the more fluid it is. Sometimes blocks of solid rock above a pool of magma break off and fall into the magma. If the temperature of the magma is below the melting point of these fallen rocks, they remain as solid chunks. These solid pieces of rock mixed with the molten (melted) rock are called xenoliths.

Let's Explore

Does all candy melt at the same temperature? Repeat the experiment, using different kinds of candy, such as caramel, white chocolate, and peppermint. Science Fair Hint: Use the results as part of a report to explain that rocks, like candy, do not all melt at the same temperature. Mention that although all rocks do not melt at the same temperature, they all eventually melt and form magma beneath the earth's surface because of the extreme heat. Magma has a temperature usually between 1,022°F and 2,192°F (550°C and 1,200°C).

What happens when magma cools slowly? Repeat the original experiment, but this time continue to move the chocolate with the toothpick for 1 minute or until it no longer moves easily. The resulting solid represents igneous rock (rock formed when hot liquid rock cools and hardens), formed by slow cooling magma beneath the earth's surface. See chapter 15, ''Transformed,'' for more information about igneous rock.

What happens when magma rises to the surface and cools quickly? Magma that reaches the earth's surface is called lava. Demonstrate the quick cooling of lava by filling a glass with ice and water. Then repeat the original experiment, but place the spoon with the melted chocolate in the glass of ice water. After about 10 seconds, remove the spoon and touch the candy with your finger. The chocolate has become solid again. This solidified chocolate represents igneous rock formed by the quick cooling of lava.

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