- 3 small plastic bottles of seltzer of soda
- 1 small bottle of food coloring
The teacher should conduct the following experiment. Wearing safety goggles and old clothing is advised. The experiment could ruin clothing and hurt unprotected eyes. Follow the steps below having the students write down what they observe and hear.
- Show the students the bottle before it is opened explaining that the gases dissolved in the liquid are under much more pressure than gases outside the bottle. As the bottle is opened the gas (Carbon Dioxide) will be visible as it escapes creating bubbles and a hissing sound.
- add 3 drops of red food coloring
- put the plastic cap back on the bottle
- shake the bottle profusely hold the bottle over a sink or drain- tell the students that the liquid inside the bottle represents magma, which is molten rock and gas inside the Earth.
- turn the cap slowly allowing the "lava" to erupt-tell the students that as magma escapes to the surface it is then called lava.
The teacher should explain that the liquid has dissolved gases in it (Carbon Dioxide), just as magma has many dissolved gases in it.
When the bottle is not open the students will not be able to see the gas because the liquid has the gas disolved in it. Because of the higher pressure in the bottle you can not see the gas bubbles. When the bottle is opened the students will see the gas escape. The liquid will erupt out with the gas because it is under more pressure than the outside environment. When magma rises in the conduit the pressure falls as it nears the surface of the Earth. The lava will escape violently as the pressure drops for the same reasons that the soda water escaped with the carbon dioxide gas. When a volcano erupts the lava may be very frothy from the escaping gases. This is true especially if the magma has a high gas content. The most violent eruptions are due to a great build up of pressure from magma that has a high gas content. Magmas with little dissolved gas usually do not erupt violently.
- Toothpaste in a tube
Cut a hole in the cardboard so that the neck of the opened toothpaste tube fits into the hole. Squeeze the tube lightly so that a little toothpste comes out then stop. Explain that the toothpaste is very thick and pasty like dactitic lava is. This is the same lava that has built the lava dome in the crater in Mt. St. Helens. Squeeze the tube again and stop, explaining that the dome was built very slowly with these same starting and stopping motions. The dome grew for seven years and has basically halted its growth as of 8/20/95.
The dome that is in the crater in Mt. St. helens today is not the only dome that has occupied this space. Another dome grew during the first month after the original eruption but blew up in June of 1980.
Continue the same pattern of squeezing and stopping until the students understand the concept of dome growth.