The ability of gas to force a liquid through a constriction is easily demonstrated with a bottle of soda. The demonstration works best with a clear plastic container with the label removed. This demonstration can be messy and is best done outside or over a sink. The students need to be close by, sitting in a half-circle, if possible.
Gas (carbon dioxide) is dissolved in the soda. A close look in the bottle by the students reveals that no bubbles are present. The pressure within the bottle is enough to prohibit bubble formation. Like the soda in the unopened bottle, magma deep in the Earth contains dissolved gas and no bubbles because the weight of the overlying rocks exerts high pressure.
Small bubbles appear if the bottle is opened slightly, then closed. The slight reduction in pressure within the bottle allowed some bubbles to form and grow. As magma approaches the surface, the reduction in pressure allows gas bubbles to form and grow.
Shake the bottle vigorously to force the gas out of the soda. Unscrew the cap quickly to release the pressure. The gas pressure is quickly released as the gas forces the soda through the opening. When magma rises near the surface, the low pressure allows the bubbles to expand rapidly. The rapid expansion forces the magma from the vent and fragments the lava.
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