Magma Lamp: What Happens When a Less-Dense Fluid Interacts with a Denser One?

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Updated on May 08, 2013

Magma is a mixture of molten rock and solids, found deep underneath the surface of the earth. It's most commonly associated with volcanoes, which shoot lava,which is magma that has come up from underground, when they erupt. But while magma is still underground, liquids of different densities interact with one another. What does it look like? How do they behave?This project seeks to find out.

What happens when a less-dense fluid interacts with a denser one?


  • Two empty two-liter bottles
  • Tornado Tube (available for about $2)
  • Food coloring
  • Large sink or plastic tub
  • Cold water
  • Hot water


  1. Fill one bottle with very cold water—ice water is great, but don’t include any ice. Set it aside.
  2. Fill the second bottle with very hot water(handle it carefully!). Put a few drops of the food coloring in, screw the top on the bottle, and give it a good shaking. Keep doing this until you’re happy with the color.
  3. Take the cap off of the bottle of hot water and replace it with the Tornado Tube. Set this bottle down in the sink or tub.
  4. Now pick up the bottle of cold water. Put your hand over the opening so no water gets out and turn it upside down. Hold it over the bottle of hot water, then quickly and carefully pull your hand away while putting the mouth of the bottle into the Tornado Tube. Screw the cold water bottle into the tube.
  5. Keep the bottle with the colored hot water on the bottom and watch what happens. The hot water, which is less dense than the cold water, will be forced upward as the denser cold water displaces it, or makes it move by taking its place. This is like the movement of magma under the Earth’s surface: the hot molten stone gets gradually pushed upward by the pressure of the denser, cooler stone around it. Sometimes it cools slowly under the surface and becomes a pluton, and sometimes it erupts and becomes lava!
Michelle Formoso is a mom and library sciences student at San Jose State University.

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