Taking a Volcano's Pulse: How Does a Seismometer Record the Shaking Caused by Earthquakes and Movements of Magma Below the Earth's Surface?

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Author: Janice VanCleave


How does a seismometer record the shaking caused by earthquakes and movements of magma below the earth's surface?


  • cardboard box, measuring about 12 inches (30 cm) on each side
  • scissors
  • ruler
  • string
  • pencil
  • 5-oz (150-ml) paper cup
  • masking tape
  • small mirror about 4 inches (10 cm) square
  • rice
  • 2 books
  • flashlight
  • adult helper


Taking a Volcanos Pulse

Taking a Volcanos Pulse

  1. Turn the box on its side with the opening facing outward.
  2. Ask an adult helper to cut a circle measuring 2 inches (5 cm) in diameter in the center of the top of the box.
  3. Cut a 24-inch (60-cm) piece of string.
  4. Use the point of the pencil to punch a hole on each side of the cup just below the rim.
  5. Tie the free ends of the string in the holes in the cup, so the string forms a handle for the cup.
  6. Tape the mirror to the side of the cup.
  7. Fill the cup with rice.
  8. Push the center of the string through the hole in the top of the box so that the cup hangs with the mirror facing you.
  9. Place the pencil through the loop formed by the string and across the hole in the top of the box.
  10. Place the books inside the box.
  11. Lay the flashlight on top of the books, and position it so that the light hits the mirror and then reflects onto the right side of the box.
  12. Watch the spot of light on the wall of the box while you gently tap the left side of the box.


The light spot moves back and forth, in a zigzag pattern, on the wall of the box.


Because of inertia (resistance to a change in motion), the heavy cup resists moving as the box is shaken and tries to remain where it is. As a result, the cup lags behind and begins swinging. The swinging of the cup causes the mirror to reflect the light in different directions on the moving box's wall. A seismometer(an instrument used to measure the shaking energy of an earthquake) uses a very heavy suspended object that responds to movement in the same way. Instead of a light source, a recording pen is attached to the suspended object; if the ground beneath the seismometer moves, the weight resists the movement, but the pen touches and records the vibrations on paper. This printed record is called a seismogram.

Prior to a volcanic eruption, there may be many strong earthquakes per hour or hundreds of small earthquakes. These quakes are produced as the moving magma and hot gases push their way to the surface, causing the earth to expand and crack. Volcanologists can use seismometers to feel the pulse of the earth, aiding in the prediction of impending volcanic eruptions.


  1. Does the weight of the cup affect the degree to which the light is reflected? Fill the cup with heavier materials such as rocks or lead fishing weights.
  2. Would the direction of the earthquake affect the pattern of the light formed? Repeat the experiment, tapping the box on the top, back, and right sides.


Another seismometer that uses a light beam can be constructed by placing a bowl full of water on top of a box. Ask a helper to hold a flashlight so that its beam of light falls on the surface of the water and is reflected to a nearby paper screen. Watch the spot of light on the screen while you tap the side of the box with your hand.

Taking a Volcanos Pulse

As part of a project display, attach a diagram of a cutaway view of a volcano to the side of the box. The diagram should show the moving magma, the focus (the underground point of origin of an earthquake), and the epicenter (the point on the earth's surface directly above the focus of an earthquake). Tape the paper screen to one wall of your display, and demonstrate the light-beam seismometer.


The exact location of an earthquake's focus is very important to volcanologists when determining where a volcano might erupt. Find out more about determining the focus location of earthquakes. How are the seismometers used? How does spacing apart the seismometers aid in pinpointing these locations?

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