New-York tribune., 1902-05-14
NATURE OF ERUPTIONS.
UNIVERSITY, DESCRIBES THESE
NATURAL PHENOMENA.

Israel c. Russell, professor of geology in the
University of Michigan, has written for The
Tribune article on volcanoes. in which he
says:
When reduced to its simplest terms, vol
cano may be defined as a tube, or conduit. in
the earth's crust, through which molten rock
is forced to the surface The conduit pene-
trates the cool and rigid rocks forming the
superficial portion of the earth, and reaches its
highly heated interior.
The length of volcanic conduits can only be
conjectured, but. judging from the approXi-
mately known rate of increase of heat with
depth (on an average / degree Fahrenheit for
each 90 feeD, and the temperature at which vol
canic rocks melt Qrom 1900 to 100 degrees
Fahrenheit. when not under pressure), they
must seemingly have a depth of at least twenty
miles. There other factors to be consid-
cred, but in general terms II is safe TO assume
that the conduits of volcanoes are Irregular
openings, many miles in depth. which furnish
passageways for molten rock QaVa) from the
highly heated sub-crust portion OF the earth to
its surface.
Volcanoes are frequently arranged in well de-
fined lines as in Central America and the Les
Antilles These and other evidences may
be accepted proving that they are located on
breaks cracks in the cool and rigid rocks
composing the outer portion Of the earth. The
breaks referred to are probably not single clean
cut gashes but irregular and branching tract
ures through rocks that for the most part are
under enormous pressure. Owing to the irregu-
larities of the fissures, the molten rock pressed
up from below does not commonly rise through
Them freely, but at the most favorable localities,
as where two or more fractures intersect. There
have been times in the history of the earth
when outwelings of molten rock have occurred
throughout the extent of many long assures
and vast foundations of lava have resulted, as,
for example, when the lava plains crossed by
the Columbia River were formed. These plains,
some 2oO,OO0 square miles in area, are under
lain by sheets of once molten rock which have
an average thickness Of something like 1000
feet. Between outpourings of veritabe floods
OF lava, in the instance cited, and the erup-
tion Of the smallest volcano there is a COM
plete gradation, showing that all volcanoes have
M
Local eruptions. or volcanoes, in distinction
from 'fssure eruptiOns," present two leading
out quietly, frequently in vast quantity, but
without explosions: and in the other class the
matter extruded is generally comparatively
small in amount, but accompanied by explo-
sions, frequently Of great violence
ERUPTIONs OF QUIET TYPE
During eruptions of the quiet type. the lava
comes to the surface in highly liquid condi-
with almost the freedom of water 1t spreads
widely even on a nearly level plain, and may
form comparatively thin sheet several hun
dred square miles in area, as has been observed
in Iceland and Hawaii on the Snake River
plains. in Southern Idaho there sheets Of
manner just stated, some four hundred square
miles in area and not over seventy-tiv feet
in average thickness. When an eruption OF
highly liquid lava occurs in mountainous re-
gion the molten rock may cascade down steep
slopes and flow through narrow valleys for fifty
miles or more before becoming chilled suffi-
ciently to arrest it's progress. Instances
abundant where quiet eruptions have occurred
in the midst Of a plain, and built up lava
cones" or low mounds. with immensely expand
ed bases. Illustrations are furnished in South
Idaho, in which the cones formed only
three hundred four hundred feet high, hut
have breadth at the base of eight ten miles.
In the class Of eruption illustrated by these
amples, there is an absence Of fragmental ma
terial such explosive volcanoes hurl into
the air, and person may stand within a few
yards of a rushing stream of molten rock,
examine closely the opening from which it is
being poured. out without danger or serious
inconvenience.
WHEN ERUPTIONS BECOME VIOLENT
The quiet volcanic eruptons are attended by
the escape Of steam and 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 con-
tanned in it find difficulty in escaping, and may
be retained until, becoming concentrated IN
large volume. they break their way the sur
face, 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 Nesuvius
When steam escapes from the summit OF
volcanic conduit-which in plain terms is a tall
vessel filled with intensely hot and more or less