Nearly all active volcanoes have intervals of comparative repose,
interrupted by periods of increased activity, which terminate in a
violent ejection of matter from the interior, during which the volcano
is said to be in a state of eruption.
The phenomena which characterize these differing phases of volcanic
activity may be best made clear by describing them as actually observed
in Vesuvius, one of the most carefully studied and most active volcanoes
of modern times.
Vesuvius is a solitary mountain rising to the height of nearly 4,000
feet, from the midst of a highly cultivated plain which borders upon
the shores of the Bay of Naples. Though the mountain has a regular
conical form, two summits, very nearly equal in height, are visible
from Naples--Monte Somma on the north, and Vesuvius proper on the
south.
The Eruption begins generally with a tremendous explosion which
seems to shake the mountain to its very foundations, and hurls into
the air dense clouds of vapor and ashes. Other explosions succeed
rapidly, and with increasing violence, each sending up a white,
globular cloud of steam, or aqueous vapor. This long array of
clouds, accompanied by dark ashes, volcanic sand, and fragments of
red-hot lava of all sizes, soon forms a stupendous column.
Finally the boiling lava overflows the rim of the crater, and
descends in fiery torrents down the slopes; or, bursting the
mountain by its weight, finds a vent through some fissure far below
the summit. After the expulsion of the lava the eruption is
generally near its end, though it does not necessarily terminate at
once. Alternate phases of outbursting steam, ashes, and lava may
continue with more or less violence for weeks or even months.
The sudden condensation of the enormous accumulation of hot vapor
thrown into the air by the eruption, gives rise to striking
atmospheric phenomena. Vivid flashes of lightning start from all
parts of the column, and play about the clouds above; and often a
local thunderstorm, formed in the midst of a clear sky, pours a
heavy rain of warm water and ashes upon the slopes of the mountain.
The hot, destructive mud torrents, created by these rains, have
often been mistaken for lava streams.
The majesty of the spectacle is still greater at night. Though
flames of burning gases are of rare occurrence, the clouds and
columns of vapor are strongly illuminated by the reflection of the
white-hot lava within the crater; and fragments of this lava
constantly thrown into the air give the column all the brilliancy of
a gigantic piece of fire-work. The sky itself, far and wide,
partakes of the same vivid coloring, and the whole scene resembles a
vast conflagration.