Eugene Presnell, age 11, of Lansing, Mich., for his question:

HOW DOES A VOLCANO GROW?

Each time an active volcano erupts, countless tons of molten lava pile up around the crater. Through the ages, the mountainous cone grows higher and bulkier. The building material erupts from molten magma, miles below the surface. In some cases it comes up through a single vent and the growing volcano forms a pointed mountain. Sometimes it opens up other vents and the mountain bulges with bumps.

Volcanoes form in regions where deep cracks cause restless activity deep in the earth's crust. Usually it is a region of mountain making or fairly young mountains. In any case it is a weak crustal region, where strains and stresses generate enormous heat and pressure far below the surface.

This deep heat creates pools of buried magma  mixtures of molten minerals and steamy gases. From time to time, the magma erupts to the surface, where it cools to form solid layers of lava rock.  A volcanic eruption is a most stupendous event, though at present no one is certain of all the factors that trigger it to happen.

However, in 1943, experts had a chance to observe the birth and growth of a new volcano, some distance from Mexico City. It began when a farmer noticed smoke puffing up from a crack in his cornfield. He stuffed down some rocks, which did no good. That night the ground trembled and rocks exploded high into the air.

By morning the crack, or vent, was surrounded by a 100 foot pile of rubble. The young volcano continued to cough and choke up fumes and lava, raining down ashes and hurling rocks. Experts from far and near rushed to the scene and the growth of the young volcano was documented most thoroughly.

It was named Paricutin and its pile of volcanic debris soon buried the farm and the entire village. After a year of more or less constant activity the volcanic cone stood 1,400 feet high and rivers of lava swamped miles of surrounding farmland. Then the activity slowed down and 10 years later Paricutin was no more than a steep sided cone of volcanic debris.

This was not a major volcano and its growth was rather sudden. Most of the big, world famous volcanoes tend to erupt and rest, erupt and rest. Some remain dormant for centuries.

Eventually, as calm returns to the stressed regions below, a volcano becomes extinct and erupts no more. However, it is not easy to tell whether it is extinct or merely dormant ¬resting between shows. Sometimes, after centuries of inactivity, what seems to be an extinct volcano suddenly explodes with a new eruption  pouring forth rivers of lava and adding more debris to its mountainous cone.