John V. Tuichner, age 12, of Columbus, Ohio, for his question:

 How did Old Faithful start?

In Yellowstone National Park there are many hot springs and geysers. Old Faithful is the star performer of the geysers. Every year, millions of tourists wait to see him spit his plume of steamy water into the air. He acts like the spout of a boiling kettle, which is what he is. The kettle is a series of underground cracks and hollows. The water is ordinary ground water. The rocks below ground are hot enough to boil the water.

Geysers occur in regions of old volcanic activity. Long ago, the region of Yellowstone seethed with volcanic activity. Hot lava poured out from countless vents in the ground. The hot lava spread and its surface cooled in the air. It became solid rock. This prevented the heat from escaping from the lava under the surface.

Heat was trapped below ground. Ground water seeps into the crevices among these hot rocks. The water boils and becomes steamy. Where it finds a vent to the surface, it spits forth a white plume. Down below, the kettle fills up again and gets ready to spit up another plume of steamy water.