Brooks Rumph Jr., age 12, of Woodland, Ga., for his question:   

 HOW DO VOLCANOES WORK?

Imagine an enormous cauldron on top of a stupendous furnace. The big pot is filled almost to the brim with a simmering mixture of steamy gases and molten minerals. Now and then a giant stoker shovels in more fuel. The furnace seethes and the pot boils over. The volcano works somewhat in this way, though naturally the whole thing happens on a much grander scale.

The average volcano is centered around a deep shaft, or conduit, that dips perhaps 20 or 40 miles down through the earth. Its roots are fed by a pool of magma, a seething furnace of minerals and gases. Now and then the explosive magma erupts up the conduit with clouds of steamy fumes, flying cinders and rivers of red hot lava. Each eruption leaves piles of debris. In time, a cone shaped mountain builds up around the volcano's throat and crater.

Nobody knows when to expect the next eruption. Obviously the mystery lies in the furnace around the volcano's roots. Since volcanoes tend to occur in groups, scientists see them in global patterns. They see the earth's rocky crust as a rather thin global skin, cracked into a dozen or so large plates and numerous splinters.

The shattered crust sits uneasily on the mantle, a hot tacky layer that goes down about 1,800 miles. Weak zones occur where two large plates rub together, often inching along in different directions. These are the fractures or fault lines where volcanoes tend to occur. In some cases, they are regions where new mountains are growing.

These crustal strains and stresses might build up enough heat and pressure to trigger a volcanic eruption. But most earth scientists now suspect that the furnace may be stoked by mighty forces deep within the plastic mantle. Some fracture zones seem to be above currents of heat and magnetic forces, rising up from the deep bowels of the earth. Others seem to be above sinking currents of energy.

Earth scientists have formed their global picture only during the past decade or so. Someday they hope to learn exactly how and when the deep furnaces are stoked to trigger eruptions.

One of the earth's most dynamic crustal faults runs down the mid Atlantic. There two major crustal plates face each other across a zone of upheaval from below. Masses of lava erupt between the two crustal plates, spreading them apart with new rocky material. And as the sea floor spreads, it gradually pushes apart the continental masses on opposite sides of the Atlantic. Underwater volcanic lavas are pushing America farther from Europe and Africa, very slowly, inch by inch.