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."