Volcano Types: How are Composite Volcanoes Formed?

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Author: Janice VanCleave


How are composite volcanoes formed?


  • 4 or 5 ice cubes
  • 2 wide-mouthed, clear-glass, quart (liter) jars
  • cold tap water
  • small baby-food jar
  • warm tap water
  • red food coloring
  • spoon
  • modeling clay
  • pencil
  • scissors
  • ruler
  • drinking straw


  1. Place the ice cubes in one of the quart (liter) jars. Fill the jar with cold tap water.
  2. Fill the baby-food jar to overflowing with warm tap water.
  3. Add five drops of food coloring to the baby-food jar and stir.
  4. Cover the mouth of the baby-food jar with a ball of clay.
  5. Use a pencil to punch two holes through the clay stopper.
  6. Cut a 4-inch (10 cm) piece from the drinking straw and insert it into one of the holes made in the clay stopper. The top of the straw should extend slightly above the clay.
  7. Push the clay around the straw to seal the opening.
  8. Place the baby-food jar inside the empty quart (liter) jar.
  9. Remove any unmelted ice cubes from the first large jar, and pour the chilled water into the second jar (the one containing the baby-food jar). Completely fill the second jar with the chilled water.
  10. Observe the contents of the second large jar for two or three minutes.


The warm, colored water rises upward until it reaches the surface of the cool water. After a while, the layer of colored water at the surface starts to fall.


Water molecules, like all matter, are spaced closer together when cold and move farther apart when heated. The density, which is a scientific way of measuring "heaviness," is less for the warm, colored water than the colder, clear water because of this spacing. The less dense warm water is more buoyant (able to float) than is the denser chilled water; thus, the warm, colored water rises to the top and forms a layer at the surface above the chilled water. As the warm water cools, its molecules move closer together, the water becomes denser, and it begins to sink.

Volcano Types

The slow movement of the warm water can be compared to the movement of hot magma during what is called a quiet eruption (see Experiment 7 for the different types of volcanic eruptions). The hotter the magma, the farther apart are its molecules and thus the less dense it is. The hot, buoyant magma rises from the magma chamber (pool of magma deep within the earth) to the surface through a central opening known as a vent (the channel of a volcano that connects the source of magma to the volcano's opening). In your model, the straw represents the vent. like the warm water in the straw, hot magma rises through the vent to the surface. The colored water began its fall as it cooled, but during most quiet volcanic eruptions the magma first fills a crater (the bowl-shaped depression at the top of a volcano) before flowing out as lava (magma that has reached the surface). The lava eventually cools, forming a hard layer at the top and down the sides of the volcano; thus, the volcano grows larger with each eruption.

Composite volcanoes are coneshaped volcanoes formed by alternate layers of solidified lava and rocks. Each layer is made by a different type of volcanic activity. The lava layer is formed as previously described during quiet eruptions, but the rock layers are the result of violent eruptions that expel rock fragments.

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