n. "For example, we do not know exactly what causes magma to form.
Magma is, in simplest terms, molten rock. Some event takes place far
below, where the earth's crust ends and the mantle begins, and the rock
melts."

"How far below?" Rick asked.

"The distance varies. Under the ocean trenches, for example, the mantle
may begin only four miles down. Under some of the mountainous land
masses it may be closer to forty miles."

Scotty whistled. "That's a whale of a distance. How can you tell how far
down it is?"

"By the seismic traces from earthquakes, or from explosive shots like
the ones we are shooting. When the shock waves have reached the zone
between the earth's crust and the mantle, we see the results on our
tracings."

"Is it really a sharp line?" Rick queried.

"Probably not. No one is sure yet. It may be a kind of transitional
zone, from one kind of material to another, or it may be a distinct
layer. We call it the Mohorovicic Discontinuity, after the Yugoslav
scientist who discovered it by analysis of seismic tracings. At any
rate, it is somewhat above this discontinuity that magma is formed. We
don't know how."

"Then it rises?" Scotty asked.

"It forces its way up, by expansion. Sometimes the magma strikes water
and there is an explosion--a steam explosion. But generally the magma
rises through a fairly small channel. It forms a pool under the volcano.
The pool is actually a reservoir of molten rock. Generally it is shaped
like a lens. The magma gathers. Eventually it forces its way to the
surface, again through channels."

"What kind of channels?" Rick asked.

"It depends on the kind of volcano. Sometimes the channels are
weaknesses in the whole surrounding earth structure, and the magma flows
through cracks and emerges as sheets of lava. Sometimes there is a
central channel through which the magma can rise."

"Which do we have?" Scotty wanted to know.

"Probably neither or perhaps both. There was once a central channel in
El Viejo. It is closed now, and we do not know if it is weaker than the
rest of the mountain. There is a weak fissure under the hot springs. So,
El Viejo can vent either way."

Rick shook his head. He had learned enough of natural forces to know
there are often no definite answers to questions, but this was critical.

"So the volcano could blow off on top or side, and we can't guess
which?"

"That is correct. However, explosive action in a volcano usually comes
when the