Julian Martinez, age 11, of Kansas City, Kan., for his question:
HOW DOES A GEYSER ERUPT?
A geyser is a spring that throws hot water into the air with explosive force. Sometimes the water shoots up from a geyser in great columns that are cloudy with steam. Most erupt at irregular intervals and no one knows when they will go off.
Geysers are formed near rivers and lakes where water drains through the earth deep below the surface. A deep channel reaches from the surface far into the earth. Cold water seeps down this channel until it reaches rocks that are very hot.
When the channel is filled with water, the liquid deep within the rocks cannot boil because of the weight of the column of water above it. But gradually the heat at the bottom of the column rises far above the boiling point for water.
When the water becomes extremely hot, steam begins to form. The rising bubbles lift the column of water a little, pushing some of the water in the channel over the opening onto the surface of the earth. This makes the column of water lighter, and more water is able to turn into steam.
Slowly, more water is pushed out of the channel, making the column of water still lighter and turning more of the water at the bottom into steam.
As more water is pushed out at the top of the channel, greater pressure is built up by the steam and suddenly all of the water near the bottom of the channel expands into steam and forces out the rest in a steam explosion.
After the water and steam settle back on the earth, some of the water seeps back into the earth and fills up the channel again. The crevices that feed the channel usually contain constrictions or sharp bends that prevent convections which cause the water to mix to a uniform temperature.
This mixing keeps the water from becoming superheated enough to immediately explode into steam again. It takes time for the pressure to build up for another eruption.
There are at least 200 active geysers in Yellowstone National Park. Other noted geyser groups can be found in Iceland about 70 miles from the capital, Reykjavik, in the midst of barren lava fields, and in the southern part of New Zealand.
Probably the world's most famous geyser is "Old Faithful," in Yellowstone Park. It erupts for about four minutes once every 65 minutes. The eruptions are usually 120 to 150 feet high. The regular pattern of eruptions has not been interrupted in over 80 years.
The regular pattern of "Old Faithful" is most unusual. Other geysers erupt on irregular schedules. Some go off several times in an hour, while others do not go off for days, weeks or even months.
In some geysers the water only bubbles over the ground. In others, it soars in fountains well over 100 feet high.