Tilt: How Does a Tiltmeter Give Clues to Where a Volcano is Likely to Occur?

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Author: Janice VanCleave


How does a tiltmeter give clues to where a volcano is likely to occur?


  • Pencil
  • 2 5-oz (150-ml) paper cups
  • Drinking straw
  • Modeling clay
  • Shallow baking pan
  • Tap water



  1. Use the point of the pencil to make a hole through the side of one paper cup near the bottom edge. The hole must be small enough so that the straw will fit tightly in it.
  2. Remove the pencil and insert about 8. Fill both cups halfway with water. 1/2 inch (1.3 cm) of one end of the straw into the hole.
  3. Use a small piece of clay to form a seal around the hole.
  4. Poke a hole near the bottom edge of the second cup.
  5. Place the other end of the straw in the hole.
  6. Seal the opening with a piece of clay.
  7. Place the connected cups in the center of the pan.
  8. Fill both cups halfway with water.
  9. Lift one end of the pan so that it is about 2 inches (5 cm) above the table.
  10. Observe the contents of each cup.


The water in each of the two cups is at the same height before the pan is raised. Raising the pan causes the amount of water to decrease in the elevated cup and to increase in the lower cup.



The instrument in this experiment is called a tiltmeter. A tiltmeter measures the tilting of the ground on the sides of a volcano. The tilting is caused by magma and gases pushing the ground upward. Volcanologists place the tiltmeter on a volcano, with one end pointing toward the volcano's cone and the other end pointing away. A swelling in the volcano is detected when the water content in the end pointing toward the cone decreases. A specific amount of tilt does not tell scientists when an eruption is about to take place, but the rate at which the volcano swells provides clues. An unusually large swelling in a short period of time tells them that an eruption is most likely on the way.



  1. Does the length of the straw affect the results? Repeat the experiment two times, first using a straw cut in half, and then using two straws connected by pushing the end of one straw into the end of the second straw.
  2. Do the sizes of the cups affect the results? Repeat the original experiment twice, first using smaller cups, and then using larger cups.


A carpenter's level is like a tiltmeter. When the bubble in the liquid-filled glass cylinder is in the middle, the carpenter's level is exactly level or horizontal. Use a carpenter's level as a tiltmeter to demonstrate the tilt of the earth as magma and gases cause the earth's crust to expand. Cut a 2-inch (5-cm) hole in the bottom end of a large shoe box. Place a deflated balloon inside the box with its mouth pulled through the hole. Fill the box with soil, making the surface as level as possible. Place the carpenter's level on top of the soil. Ask a helper to blow up the balloon as you watch the movement of the bubble in the level. Photographs and diagrams of the position of the bubble can be used as part of a project display.


Some volcano tiltmeters are so sensitive that they can detect the slightest movement in the earth's crust produced by the pull from the moon's gravity. Find out more about tiItmeters. How do the different types vary?

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