Todd Burgess, age 11, of San Diego, California, for his question:

How can they tell when a volcano is extinct?

Usually, active volcanoes come in groups, accompanied by the growth of mountains. As a restless crustal zone loses its energy, they tend to die down and become extinct, one by one. Chances are, the volcanic cones in very ancient mountain ranges are all extinct However, even the experts cannot be absolutely certain. For example, west of Mexico City there are hundreds of quiet old cones and the regional volcanic activity appeared to be finished. Then in 1943, a cornfield erupted and astonished geologists watched this unexpected volcano build up a new mountain.

As a volcano grows old, it tends to erupt less often and with less violence. For centuries it slumbers in a dormant stage, even though smokey fumaroles and perhaps a few lava puddles emerge from the floor of its crater. Most dormant volcanoes gradually subside to extinction, though even those thought to be extinct may erupt with a final gasp. We cannot be sure that any volcano is extinct unless there has been no igneous activity in the neighborhood through the past million years or more.