VOLCANOES.

 

 

31, Laplace’s Nebular Hypothesis agrees very well
with the idea of a former igneous fluidity, since, at the

time of its separation from the nebulous sun, the earth .

must have had a temperature sufficient not only to fuse,
but even to volatilize, most of its constituents.

32. Proofs of a Present Heated Interior—The
following considerations show that the inside of
the earth is still highly heated:

(1.) The deeper we penetrate the crust, the
higher the temperature becomes. Moreover, the
rate of increase, though varying in different lo-
calities with the character of the materials of the
crust, is nearly uniform over all parts of the sur-
face, the average value of the inerease being 1°
Fahr. for every 55 feet of descent.

This would seem to indicate that the entire
inside of the earth is heated, and that the heat
increases as we go toward the centre.

We cannot, however, estimate the thickness of the crust
from this fact—

1. Because we have never penetrated the crust more
than a few thousand feet below the level of the sea, and
therefore we do not know that this rate of increase of
temperature continues the same;

2. Even if it did continue uniform, since the melting-
point of solids increases with the pressure, we do not
know what allowance should be made for this increase.

(2.) In all latitudes: prodigious quantities of
melted rock escape from the interior through
the craters of volcanoes. The interior, there-
fore; must be hot enough to melt rock.

83. Condition. of the Interior—We do not
know the condition of the material which fills
the interior of the earth. It might be supposed,
since rock escapes from the craters of volca-
noes in a fluid or molten condition, that the in-
terior is filled with molten matter; but this is
not necessarily so, since the enormous pressure
to which the interior is subjected would prob-
| ably be sufficient to compress it into .a viscous

. or pasty mass, or, possibly, even to render it solid.
The lava which issues from the crater of a vol-
cano is necessarily more mobile than the interior
of the earth; for, coming, as it does, from great
depths, it must grow more and more liquid as it
approaches the surface and is thus relieved of its
pressure. Indeed, the most viscous rock conceiv-
able, if highly heated when ejected from pro-
found depths, would become comparatively fluid
on reaching the surface.

34, Views Concerning the Condition of the
Interior —Considerable difference of opinion ex-
ists as to the exact condition of the interior of
the earth. The following opinions may be men-
tioned ;

 

 

(1.) That the earth has a solid centre and
crust, with a heated or pasty layer between.

(2.) That the crust is solid, but the interior
highly heated, so as to be in a fused or pasty

condition.

(8.) That the earth is solid throughout, but
highly heated in the interior.

Of the above views, the second is perhaps the
most tenable, and will be adopted as serving in
the simplest manner to explain the phenomena
of the earth arising from the presence of a highly
heated interior. Admitting the crust to be suf-
ficiently thin, and in such a condition as to per-
mit of but a small degree of warping, then all

the phenomena can be satisfactorily explained.

35. Thickness of the Crust—We cannot as-
sign a definite limit to the thickness of the crust,
since the portions that are solid from having
cooled, most probably pass insensibly into those
that are nearly solid from the combined influence
of loss of heat and increasing pressure. It seems
probable that the portion solidified by cooling is
thin, when compared with the whole bulk of the
earth; in other words, the heated interior lies
comparatively near the surface.

36. Effects of the Heated Interior—As the
crust loses its heat it shrinks or contracts, and,
growing smaller, the materials of the interior are
crowded into a smaller space, and an enormous
force is thus exerted, both on the interior and on
the crust itself, tending either to change the shape
of the crust, to break it, or to force out some of
the interior. The following phenomena are there-
fore caused by the contraction of the crust:

(1.) Volcanoes ;

(2.) Earthquakes ;

(3.) Non-voleanic igneous eruptions ;

(4.) Gradual elevations or subsidences of the
crust.

—0594 00 —_

CHAP DER «Ji

V oleanoes.

37. Voleanoes.—One of the most striking proofs
of the existence of a heated interior is the ejection
of enormous quantities of melted rock through
openings in the crust.

A volcano is a mountain, or other elevation,
through which the materials of the interior escape
to the surface. The opening is called the crater,
and may he either on the top or on the sides of
the mountain.