Caldera Demonstration Model

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Detailed Description

A caldera is a large, usually circular volcanic depression formed when magma is withdrawn or erupted from a shallow underground magma reservoir. It is often difficult to visualize how calderas form. This simple experiment using flour, a balloon, tubing, and a bicycle pump, provides a helpful visualization for caldera formation.


Date Taken:

Length: 00:02:13

Location Taken: Menlo Park, CA, US


Hi. Iím Dr. Dina. I work with the U.S. Geological Survey in Menlo Park, California with the Volcano Hazards Program. Today, weíre going to talk about calderas.

When most people think about volcanoes, they think of a triangle shape and thatís what many volcanoes look like, but there are other shapes too, and one of those is called a caldera. What weíve done is created a simple experiment to show how calderas form. The experiment uses flour, a piece of tubing with a balloon attached to the end and a bicycle pump. So what I want you to imagine is that this is the surface of the earth in an area thatís volcanically active.


Underneath the surface of the earth, weíre going to inject some magma. When we inject the magma using our handy bicycle pump, itís going to push the surface up, so letís watch.

As you can see, the flower is being moved out of the way as we inflate the balloon or add magma to the system. The Volcano Hazards Program has monitoring equipment at different volcanoes to watch ground deformation such as this. In this case, what weíre going to do is weíre going to pretend that this large amount of magma erupts, sort of like when Yellowstone erupted or Long Valley Caldera erupted.

To do that, Iím going to let the air out of the balloon and then weíre going to watch to see what happens.


So as the air comes out of the balloon, we want to imagine the magma escaping from the magma chamber. Once the magma leaves the magma chamber, thereís nothing to hold up the overlying rock and so all that rock collapses down to where the chamber used to be. So you can see in our experiment the flour has now collapsed in a somewhat circular shape. We call this shape a caldera.

For more information, please see the following web sites.

USGS Volcano Hazards Program: 
Long Valley Caldera: