PHYSICAL GEOGRAPHY.

ey,

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig, 19, An Eruption of Mount Vesuvius,

38. Peculiarities of Craters.—The crater, as its name
indicates, is cup-shaped. The rim, though generally entire,
is sometimes broken by the force of the eruption, as in

Mount Vesuvius, where the eruption in 79 A. D.—the first"

on record—blew off the northern half of the crater. The
material thus detached, together with the showers of ashes
and streams of lava, completely buried the cities of Her-
culaneum and Pompeii, situated near its base.

The crater is often of immense size. Mauna Loa, on the
island of Hawaii, has two craters—one on the summit, and
the other on the mountain-side, about 4000 feet above the
sea. The latter—Kilauea—is elliptical in shape, and about
73 miles in circumference ; its areais nearly 4 square miles,
and its depth, from 600 to 1000 feet.

Volcanic mountains are of somewhat different
shapes, but near the crater the conical form pre-
dominates, and serves to distinguish these moun-
tains as a class. The shape of the volcanic cone
is caused by the ejected materials accumulating
around the mouth of the crater in more or less
concentric layers.

39. The ejected materials are mainly as fol-
lows:

(1.) Melted Rock, or Lava.—Lava varies, not
only with the nature of the materials from which
it was formed, but also with the conditions under
which it has cooled, and the quantity of air or
vapor entangled in it. Though generally of a
dark gray, it occurs of all colors; and its texture
varies from hard, compact rock to porous, spongy
material that will float on water. :

When just emitted from the crater, ordinary lava flows
about as fast as molten iron would on the same slope. On

steep mountains, near the crater, the lava, when very
hot, may flow faster than a horse can gallop; but it soon

 

 

cools, and becomes covered with a crust that greatly re-
tards the rapidity of its flow, until its motion can only be

' determined by repeated observations.

At Kilauea, jets of very liquid lava are sometimes
thrown out, which, falling back into the crater, are drawn
out by the wind into fine threads, thus producing what
the natives call Pélé’s hair, after their mythical goddess.

The volume of the ejected lava is often very great. Vol-
canic islands are generally formed entirely by lava streams.
Hawaii and Iceland were probably formed entirely of lava
emitted from numerous volcanic cones.

(2.) Ashes or Cinders.—These consist of minute
fragments of lava that are ejected violently from
the crater; at night they appear as showers of
brilliant sparks. When they fall directly back
on the mountain, they aid in rearing the cone.
More frequently, they are carried by the wind to
points far distant. The destructive effects of
volcanic eruptions are caused mainly by heavy
showers of ashes. The ashes, when exceedingly
fine, form what is called volcanic dust.

At the beginning of an eruption large frag-
ments of rock are sometimes violently thrown
out of the crater.

(3.) Vapors, or Gases—The vapor of water
often escapes in great quantities from the crater,
especially at the beginning of the eruption. On
cooling, it condenses and forms dense clouds, from
which torrents of rain fall. These clouds, lighted
by the glowing fires beneath, appear to be actually
burning, and thus give rise to the erroneous belief
that a volcano is a burning mountain. To the
condensation of this vapor is probably to be as-.
cribed the lightning which often plays around the
summit of the voleano during an eruption. Be-
sides the vapor of water, various gases éscape, of
which sulphurous acid is the most common.

When a large quantity of rain mingles with the ashes,
torrents of mud are formed, which move with frightful
velocity down the slopes of the mountain, occasioning con-
siderable damage. During the eruption of Galungung, in
Java, more than one hundred villages were thus destroyed.

The rock that is formed by the hardening of volcanic mud
is called tufa.

40. The Inclination of the Slopes of the vol-
canic cones depends on the nature of the material
of which they are formed. Where lava is the
main ingredient, the cone is broad and flat. The
inclination of a lava cone ranges from 8° to 10°,

Fig, 20, Lava Cone, Inclination from 3° to 10°

 

 

according to the liquidity of the lava. A very
stiff lava will form a much steeper cone.