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Your teacher will show you a simple model of a volcanic eruption. You can demonstrate this to your parents at home, if you have their permission and they realize that a tube of toothpaste will be destroyed.

Your teacher will have a partly used tube of toothpaste, with the cap screwed on tightly, and the tube flattened so that the toothpaste is evenly distributed inside the tube.

The outside of the tube represents the hard crust of the earth, and the toothpaste inside the tube represents the molten rock (magma) which lies in a "magma chamber" far beneath the earth's surface.

When your teacher presses on the lower end of the tube (away from the cap end), you can see that the upper end of the tube starts to swell because of the pressure inside the tube. If there were some way to heat the toothpaste and make it expand, that would show another part of the reason that magma comes up and out. (It is forced up and out by heat and pressure.)

If there is a weak place (or a hole) in the wall of the toothpaste tube, the toothpaste will start to flow out that weak place or hole. This is how magma sometimes comes out from the chamber. Usually, the very top of a mountain has a weak spot or a crack (fissure, pronounced "FISH-er) in it, and the magma pushes its way out at that place. However, sometimes the weakest spot is on the side of the mountain, like on Mt. St. Helens where there was an area in which the crust of the earth was thinner than in other places on the mountain, and the magma blasts its way out there.

Your teacher can illustrate this by puncturing the tube, either near the cap end or along the side of the tube, with a pin. A little more pressure against the tube will force the toothpaste (pretend magma) out through the "fissure" made with the pin.

The toothpaste flowing up through the hole due to pressure is similar to magma flowing from the earth and forming volcanoes. It is important to remember that heat plays a
role in vulcanism (volcanic activity) that is just as important as pressure, if not more important.

Another way to illustrate the eruption of a volcano is to make a cone from a sheet of white paper. The white color, of course, is like snow. Clip off the very top of the cone, put the toothpaste tube (without its cap on) inside the cone, put a little pressure on the lower end of the tube, and watch the pretend magma flow out the top of the paper mountain. It will flow out the top, and make a "river" down the side of the mountain. When it cools and hardens, it will be called "lava."

There are basically two types of lava; Pahoehoe (pronounced "Pa-HOY-hoy"), and Aa (pronounced "AH-ah"). Pahoehoe lava is basaltic lava that looks somewhat billowy or ropy, and almost smooth, whereas Aa lava (also basaltic) is sharp and splintery. If you walk bare-footed over cooled Aa lava, it hurts your feet, and you say, "Ah! Ah!"

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