Jeff Hegedus, age 11, of Highland Park, N. J., for his question:

What causes a volcano to erupt?

We cannot predict when a volcano will erupt. But we know more or less where to expect it. A volcano is part of a chain of volcanoes, a long curve of earthquake faults and growing mountains. The study of one volcano does not tell us much about what causes it. Modern science tends to take a global view of eruptions and other activities in the earths crust.

The facts gathered during the igy have given us our first global picture of the earth. We must., it seems, know a lot more about the over all structure of the earth's crust before we can understand single events, such as an earthquake or a volcanic eruption. Many scientists think we must go even deeper for our answers,

Volcanoes erupt in the earth's crust which is a layer of minerals from 20 to 40 miles deep. Below the crust is the dense mantle, reaching down some 1500 miles. The mantle is under terrific pressure from the rocky layers above it; and the deeper we go into the earth., the hotter it becomes. The thin crust rests on the hot, dense mantle,, and the mantle rests on the still hotter and denser core of the earth.

Some scientists suggest that volcanic zones are caused by convection currents deep within the earth. A small convection cell occurs in a pot of boiling water as hot water from the bottom rises up and cooler water at the top swings around to replace it. Vast convection cells may occur as heat from the earth's interior rises to the surface and cooler currents sink. Where two such currents meet and sink, we may get a weak area in the earth's crust  a volcanic belt of earthquake faults and growing mountains.

The coats of a volcano are cracks and vents reaching miles down into the crust and perhaps into the mantle. Tensions in the weak zone build up to a critical point. The seething interior suddenly bursts through the pressure above its and the volcano erupts,

Flaming gases steaming smoke and searing cinders are thrown high into the air. The earth quakes with underground explosions. Rivers of molten lava flow dawn from the crater of the volcano. We have to know far more about the deeps underground causes of the earth's volcanic zones. Then perhaps we can predict when the pressures will build to a critical pointy resulting in a volcanic eruption. If we knew enough to predict an eruption, people could be warned and removed from the disaster area.

A volcano will erupt and rest tame after tunes but at last it dies. Other eruptions may break out along the volcanic zone. The region remains restless through millions of years while new mountains are building. Then at last the entire volcanic zone becomes quiet. In the remote past volcanoes seethed in maine and new england and vast flows of molten lava spread through Colorado.