James Barnett, age 13, of Denton, Tex., for his question:

WHY DOES A GEYSER ERUPT?

A geyser is a hot spring that erupts in a column of steam and hot water intermittently. A geyser erupts when the base of a column of water resting in the earth is vaporized by hot volcanic rock. The force with which the water column is expelled depends on its depth.

The weight of the water column increases with its depth. The weight, in turn, increases the pressure exerted on the base of the column, thereby increasing the boiling point of the water there.

When the water finally boils, it expands, driving some water out into the air. With the weight of the column reduced, the pressure correspondingly drops and the boiling point of the water remaining in the column falls below its actual temperature. Then, the entire column is instantly vaporized, causing the geyser to erupt.

Almost all known geysers are located in three countries of the world: the United States, New Zealand and Iceland.

The most famous geyser in the world is Old Faithful in Yellowstone National Park, which expels about 10,000 to 12,000 gallons at each eruption. Old Faithful erupts at intervals of between 37 and 93 minutes, its column rising to a height of between 125 and 170 feet.