Instances are abundant
where quiet eruptions have occurred in the midst of a plain, and
built up 'lava cones,' or low mounds, with immensely expanded bases.
Illustrations are furnished in Southern Idaho, in which the cones formed
are only three hundred or four hundred feet high, but have a breadth at
the base of eight or ten miles. In the class of eruption illustrated
by these examples, there is an absence of fragmental material, such as
explosive volcanoes hurl into the air, and a person may stand within
a few yards of a rushing stream of molten rock, or examine closely the
opening from which it is being poured out, without danger or serious
inconvenience.
"The quiet volcanic eruptions are attended by the escape of steam or
gases from the molten rock, but the lava being in a highly liquid
state, the steam and gases dissolved in it escape quietly and without
explosions. If, however, the molten rock is less completely fluid, or
in a viscous condition, the vapors and gases contained in it find
difficulty in escaping, and may be retained until, becoming concentrated
in large volume, they break their way to the surface, producing violent
explosions. Volcanoes in which the lava extruded is viscous, and the
escape of steam and gases is retarded until the pent-up energy bursts
all bounds, are of the explosive, type. One characteristic example is
Vesuvius.
"When steam escapes from the summit of a volcanic conduit--which, in
plain terms, is a tall vessel filled with intensely hot and more or less
viscous liquid--masses of the liquid rock are blown into the air, and on
falling build up a rim or crater about the place of discharge.
Pages:
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250