Esther Kattula, age 10, of Butte, Mont., for her 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 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 very hot rocks.
When the channel is filled, water 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 and steam begins to form. The rising bubbles lift the column a little, pushing some of the water in the channel over the opening onto the surface of the earth. This makes the column lighter, allowing more water to turn into steam.
Slowly, more water is pushed out of the channel, making the column still lighter and turning more of the water at the bottom into steam.
As more water is pushed out at the top, pressure builds and suddenly all the water near the bottom of the channel expands into steam and explosively forces out the rest.
Some of the water seeps back into the earth and refills the channel. The crevices that feed the channel usually contain constrictions or sharp bends that prevent convections that cause the water to mix to a uniform temperature.
This mixture 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.
In Wyoming's famed Yellowstone National Park, there are more than 200 active geysers.
Another famous group of geysers can be found on barren lava fields in Iceland about 70 miles from the capital, Reykjavik. Still another major group of geysers is located in the southern part of New Zealand.
Probably the most famous geyser in the world is "Old Faithful." It is located in Yellowstone Park and erupts on a regular schedule for about four minutes once every 65 minutes.
Old Faithful's eruptions rise 120 to 150 feet high. Their regular pattern has not been interrupted in about 90 years. This frequency and regularity is most unusual. Other geysers erupt sporadically. Some go off several times 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 shoots up in fountains well over 100 feet high.