Fire Rocks: What Type of Rock is Formed When Lava Cools?

PROBLEM

What type of rock is formed when lava cools?

Materials

• Small box with a lid
• Marbles

Procedure

1. Cover the bottom of the box with a single layer of marbles. The marbles should fit together loosely.
2. Close the lid on the box.
3. Using both hands, lift the box and, while holding the lid secure, shake the box vigorously up and down, then side to side.
4. Quickly set the box on a table.
5. Open the lid and observe the position of the marbles inside.

Results

Shaking the box moves the marbles, leaving them in a disorderly arrangement.

Why?

As the temperature of liquid rock within the earth increases, the movement of the molecules in the rock increases. The movement of magma molecules is symbolized in this experiment by the movement of the marbles as the box is shaken. During volcanic eruptions, liquid rock reaches the earth's surface and cools quickly in a matter of days or even hours. This rapid cooling of lava means that the molecules don't have time to move into orderly patterns before the rock solidifies. This produces igneous rock. (Igneous is a Latin word meaning "fire.") If the rock is formed by the solidification of lava poured out onto the earth's surface, it is called an extrusive igneous rock.

The irregular organization of molecules in extrusive rocks results in rocks with small crystals or no crystals. This occurs because the molecules in the liquid rock cooled so quickly that only a few or none at all had time to move into an orderly crystalline position. Extrusive igneous rocks with small crystals, such as in basalt, are described as fine-grained, and those with no crystals, such as obsidian, are described as glassy.

LET'S EXPLORE

Would slow, gentle shaking over a longer period affect the results? Repeat the experiment, but after shaking, place the box on a table and very gently vibrate the box back and forth and from side to side for about 15 seconds. The resulting orderly, single layer of marbles represents the crystalline structure of intrusive igneous rocks (igneous rock formed by the slow cooling of magma below the earth's surface).

SHOW TIME!

1. Specimens of igneous rocks can be collected or purchased at a rock shop. Examine the samples with a magnifying lens, and sort the rocks into groups labeled INTRUSIVE and EXTRUSIVE. Include the name, texture, and mineral content of each rock as part of this display.
2. The difference in crystal size in igneous rocks is due to the rate at which the liquid rock cools. Demonstrate this by asking an adult to make fudge using the following recipe and separating the mixture into two batches; one will cool slowly, forming larger, intrusive crystals, and the other will cool quickly, forming smaller, extrusive crystals.

Fudge

In a pot, combine ³/₄ cup (188 ml) of cocoa, 3 cups (750 ml) granulated sugar, and 1¹/₂ cups (375 ml) milk. Cook slowly until the chocolate is thoroughly blended, stirring gently. Boil while stirring to 234° Fahrenheit (112° C) or until the mixture forms a soft ball when dropped into cold water. Remove the pot from the heat, and add 1 teaspoon (5 ml) of vanilla extract and 2 tablespoons (30 ml) of margarine. Stir thoroughly. Pour 5 tablespoons (75 ml) of the mixture onto a sheet of aluminum foil. Immediately fold the edges of the foil together to seal the foil around the hot mixture. Vigorously beat the remaining mixture until it is thick and has a shiny appearance. Then pour it onto a buttered plate. Taste the two batches of fudge after they have cooled to room temperature. The intrusive candy will taste grainy while the extrusive candy will taste smooth.

CHECK IT OUT!

Basalt, andesite, and rhyolite basalt are the three types of fine-grained extrusive rocks. Scoria and pumice are types of glassy extrusive rocks. Find out more about igneous rocks. How are scoria and pumice formed? What are the common intrusive rocks and which is the most abundant? Why do rocks of the same composition sometimes have different-sized crystals?

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