How a Volcano Really Works

     As it rises, the pressure of the rock around and above it becomes less and less. At a certain point, that pressure drops so low that it cannot keep the rock balloon solid any more. The balloon melts. It becomes an underground liquid that we call magma - a liquid imprisoned in an underground chamber of the solid rock around it.

       Ordinary steam, heated to only 212° f  (100 °C), can drive a locomotive. Now try to imagine the power of super-steam, heated to 3000°F (nearly 2000 °C).

       Early explanations for the awesome power of volcanoes involved stories of gods and giants. But today we have a much better idea of what makes a volcano erupt.

       Magma is more than a liquid rock. It also contains gas. Most of the gas is super hot steam - super-steam heated to thousands of degrees, steam so hot that it glows. Other gases include carbon dioxide (the gas that makes the bubbles in soft drinks,) and gases containing sulphur. But the main gas is super-steam. And now that the balloon is liquid, the super-steam can escape.

       The super-steam and other gases bubble out from the magma. Pressure builds up in the magma chamber. enormous amounts of pressure. Something has to give. A weak area of rock in the ground cracks or a plug in the mouth of the volcano gives way. The magma explodes out into the air. The volcano is erupting.

       Deep in the Earth, a region of very hot rock starts to rise. It is as hot as lava, but it is not yet a liquid. The gigantic pressure of the Earth all around it keeps it solid. Nevertheless, it slowly pushes its way up through  the slightly  cooler rock around it. It is a balloon of solid rock, lifting up slower than a snail's pace toward the earth surface.

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