product.

The expansion of the water contained in the deep-lying heated rocks
probably is by far the most efficient agent in urging them toward the
plane of escape which the fissure affords. When the motion begins it
pervades all parts of the rock at once, so that an actual flow is
induced. So far as the movement is due to the superincumbent weight,
the tendency is at once to increase the temperature of the moving
mass. The result is that it may be urged into the fissure perhaps even
hotter than when it started from the original bed place. In proportion
as the rocky matter wins its way toward the surface, the pressure upon
it diminishes, and the contained vapours are freer to expand. Taking
on the vaporous form, the bubbles gather to each other, and when they
appear at the throat of the volcano they may, if the explosions be
infrequent, assume the character above noted in the little eruption of
Vesuvius. Where, however, the lava ascends rapidly through the
channel, it often attains the open air with so much vapour in it, and
this intimately mingled with the mass, that the explosion rends the
materials into an impalpably fine powder, which may float in the air
for months before it falls to the earth. With a less violent movement
the vapour bubbles expand in the lava, but do not rend it apart, thus
forming the porous, spongy rock known as pumice. With a yet slower
ascent a large part of the steam may go away, so that we may have a
flow of lava welling forth from the vent, still giving forth steam,
but with a vapour whose tension is so lowered that the matter is not
blown apart, though it may boil violently for a time after it escapes
into the air.

Although the foregoing relatively simple explanation of volcanic
action can not be said as yet to be generally accepted by geologists,