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390 VOLCANIC AND EARTHQUAKE ACTION
“When reduced to its simplest terms, a volcano may be defined -
as a tube, or conduit, in the earth’s crust, through which the molten
rock is forced to the surface. The conduit penetrates 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 approximately known rate of increase of heat
with depth (on an average one degree Fahrenheit for each sixty
feet), and the temperature at which volcanic rocks melt (from 2,300
to 2,700 degrees Fahrenheit, when not under pressure), they must
seemingly have a depth of at least twenty miles. There are other
factors to be considered, but in general terms it is safe to assume
that the conduits of volcanoes are irregular openings, many miles
in depth, which furnish passageways for molten rock (lava) from
the highly-heated sub-crust portion of the earth to its surface.
ERUPTIONS OF QUIET TYPE
“During eruptions of the quiet type, the lava comes to the
surface in a highly liquid condition—that is, it is thoroughly fused,
and flows with almost the freedom of water. It spreads widely,
even on a nearly level plain, and may form a comparatively thin
sheet several hundred square miles in area, as has been observed in
Icelandand Hawaii. On the Snake River plains, in Southern Idaho,
there are sheets of once molten rock which were poured out in the
manner just stated, some four hundred square miles in area and not
over seventy-five feet in average thickness. When an eruption of
highly liquid lava occurs in a mountainous region, the molten rock
may cascade down deep slopes and flow through narrow valleys for
fifty miles or more before becoming chilled sufficiently to arrest its
progress, Instances are abundant where quiet eruptions have