Calderas and craters are common volcanic features.
This simple demonstration conveys many of the concepts about how these features form.
The following items are needed:
a small box
a balloon (red is best)
a clamp for the plastic tubing
Line the box with newspaper. Punch a hole through the center of the bottom of the box and the newspaper. Pass the tubing through the hole. Tape the balloon to the tubing. Blow through the tubing to inflate the balloon to a few inches in diameter. Clamp off the plastic tubing.
Bury the balloon under a cone of flour.
Sculpt the flour into the shape of your favorite volcano. This teacher is making her favorite Alaskan stratovolcano. The gas pressure in the balloon holds up the top of the cone. In volcanoes the gas dissolved in the magma exerts pressure on the surrounding rocks. For most of the history of the volcano the pressure is great enough to hold up the summit of the volcano.
Open the clamp and let the balloon deflate. The flour at the top of the cone collapses because there is not enough force to hold it up. In volcanoes, a large ash eruption or removal of magma to a deeper level reduces the pressure and causes the rocks at the summit to collapse. A large collapse associated with an eruption forms a caldera.
A smaller collapse associated with removal of magma to a deeper level forms a pit crater.
Note: this demo shows a crater or caldera forming at the summit of a stratovolcano. To demonstrate how pit craters form on shield volcanoes use a smaller box and enough flour to make the surface almost horizontal.
How this demonstration is like real calderas and pit craters:
gas pressure holds the overlying material in place
removal of gas pressure causes collapse
the collapse feature is circular
size of the collapse feature is related to the amount pressure released.
How this demonstration is NOT like real calderas and pit craters:
real calderas and pit craters won't fit in a classroom
calderas form from large explosive ash eruptions
water vapor, sulfur dioxide, and carbon dioxide, not air, are the common volcanic gases.
Steve Mattox wishes to thank the participants in Volcanology for Earth Science Teachers for sharing their ideas and enthusiasm.