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Sinker: How is a Volcanic Caldera Formed?

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Author: Janice VanCleave

PROBLEM

How is a volcanic caldera formed?

Materials

• scissors
• ruler
• poster board
• 2-liter soda bottle
• 2 cups (500 ml) of soil
• large bowl
• tap water
• teaspoon (5-ml spoon)

Procedure

1. Cut two 3-inch × 8-inch (7.5-cm × 20-cm) strips of poster board.
2. Ask an adult helper to cut the top from the soda bottle.
3. Ask your helper to cut two horizontal slits, one on each side in the center of the bottle and large enough for the poster-board strips to slide through.
4. Pour the soil into the bowl and add water, 1 teaspoon (5 ml) at a time, while mixing with your hands. Continue adding the water until the soil is slightly moist and begins to stick together.
5. Place half of the moist soil inside the bottle.
6. Press the soil against the sides of the bottle up to the slits, leaving a large, empty cavity in the center.
7. Place the strips of poster board, one on top of the other, through the slits in the bottle.
8. Place the remaining soil on top of the strips of poster board and mold it into the shape of a volcano top, as shown in the diagram.
9. Hold the ends of the strips, one in each hand, and slowly pull the strips in opposite directions.
10. Observe the top of the volcano as the strips separate.

Results

When the supporting strips are removed, the top of the volcano falls into the cavity below.

Why?

The soil falling into the cavity below simulates the formation of a volcanic caldera (a large, roughly circular crater with steep walls formed when the top of a volcano collapses). The rapid ejection of magma during a large volcanic eruption can leave the magma chamber empty or partially empty. The unsupported roof of the empty chamber can slowly sink under its own weight, forming a caldera.

LET'S EXPLORE

1. Would the size of the cavity affect the results? Repeat the original experiment using twice as much soil inside the bottle.
2. Does the size of the volcano above the cavity affect the results? Repeat the original experiment using two cups of soil to build the volcano on top of the paper strips.

SHOW TIME!

Simulate the use of a radar altimeter by placing small blocks of wood against a wall to represent land features with different altitudes (see diagram). Roll a small rubber ball along the floor and record the time it takes for the ball to return to you. The rolling and reflecting of the ball off the blocks of wood simulates the sending of radio signals from a radar altimeter and their return after bouncing off a surface.

You can demonstrate the development of rilles by filling a small box about three-fourths full with dry sand or salt.

Punch a hole in the side of the box and insert a pencil so that it lies flat on the surface of the sand. Sprinkle a thin layer of sand over the pencil. Slowly pull the pencil out of the hole. Photographs of the surface before and after removing the pencil can be displayed, along with pictures of the moon's surface showing the rilles.

1. Before the Pioneer-Venus space craft began its orbit around Venus in 1978, only a limited amount of information was known about the planet The craft had many instruments, but its radar altimeter (the instrument that uses echo signals to determine profile of a land surface) allowed scientists to peek through the thick blanket of clouds that covers the planet in order to collect information about its surface. The altimeter readings revealed that Venus has lowlands, rolling plains, and highlands. In the highlands, the readings indicate possible volcanic structures similar to shield volcanoes (volcanoes composed of layers of solidified lava, a wide base, and a large, bowl-shaped opening at the top) on earth. There is some evidence of the presence of a caldera.
2. An interesting volcanic feature on the earth's moon is the presence of rilles. Rilles are long, straight or winding valleys cut in the surface. These channels may have formed by lava flowing on the surface or possibly by lava that flowed underground. When the lava drained away, the roof of the underground tunnel collapsed, forming a valley. This happens on earth, too, or anywhere there is volcanic action with lava flow. These underground tunnels are known as lava tubes.

CHECK IT OUT!

1. The formation of a caldera does not always indicate that the volcano will not become active again. Find out more about the formation of calderas and how the collapsed floor of the caldera can be lifted. What is a resurgent dome? How was Wizard Island formed in the middle of Crater Lake?
2. Some scientists believe that volcanoes on Venus are actively erupting right now. Find out more about the volcanoes on Venus and other celestial bodies in our solar system. Are there volcanoes on Mercury's smooth plains? Are there volcanoes beneath the thick atmosphere covering Saturn's largest satellite, Titan?