Many theoretical efforts, some of considerable violence, have been
made to reconcile the supposed physical contradiction presented by
the great magnitude and area of former volcanic activity of the Moon,
and the present absence of water on its surface. So long as we accept
the generally received belief that water is a necessary agent in the
evolution of volcanic forces, the difficulties presented by the lunar
surface are rather increased than diminished by further examination and
speculation.
We know that the lava, scoriæ, dust and other products of volcanic
action on this earth are mainly composed of mixed silicates—those of
alumina and lime preponderating. When we consider that the solid crust
of the Earth is chiefly composed of silicic acid, and of basic oxides
and carbonates which combine with silicic acid when heated, a natural
necessity for such a composition of volcanic products becomes evident.
If the Moon is composed of similar materials to those of the Earth, the
fusion of its crust must produce similar compounds, as they are formed
independently of any atmospheric or aqueous agency.
This being the case, the phenomena presented by the cooling of
fused masses of mixed silicates in the absence of water become very
interesting. Opportunities of studying such phenomena are offered at
our great iron-works, where fused masses of iron cinder, composed
mainly of mixed silicates, are continually to be seen in the process of
cooling under a variety of circumstances.
I have watched the cooling of such masses very frequently, and have
seen abundant displays of miniature volcanic phenomena, especially
marked where the cooling has occurred under conditions most nearly
resembling those of a gradually cooling planet or satellite; that
is, when the fused cinder has been enclosed by a solid resisting and
contracting crust.
The most remarkable that I have seen are those presented by the cooling
of the “tap cinder” from puddling furnaces. This, as it flows from the
furnace, is received in stout iron boxes (“cinder-bogies”) of circular
or rectangular horizontal section. The following phenomena are usually
observable on the cooling of the fused cinder in a circular bogie.
First a thin solid crust forms on the red-hot surface. This speedily
cools sufficiently to blacken. If pierced by a slight thrust from
an iron rod, the red-hot matter within is seen to be in a state of
seething activity, and a considerable quantity exudes from the opening.
If a bogie filled with fused cinder is left undisturbed, a veritable
spontaneous volcanic eruption takes place through some portion,
generally near the centre, of the solid crust. In some cases, this
eruption is sufficiently violent to eject small spurts of molten cinder
to a height equal to four or five diameters of the whole mass.
The crust once broken, a regular crater is rapidly formed, and
miniature streams of lava continue to pour from it; sometimes slowly
and regularly, occasionally with jerks and spurts due to the bursting
of bubbles of gas. The accumulation of these lava-streams forms a
regular cone, the height of which goes on increasing. I have seen a
bogie about 10 or 12 inches in diameter, and 9 or 10 inches deep,
thus surmounted by a cone above 5 inches high, with a base equal to
the whole diameter of the bogie. These cones and craters could be but
little improved by a modeler desiring to represent a typical volcano in
miniature.
Similar craters and cones are formed on the surface of cinder which is
not confined by the sides of the bogie. I have seen them well displayed
on the “running-out beds” of refinery furnaces. These, when filled,
form a small lake of molten iron covered with a layer of cinder. This
cinder first skins over, as in the bogies, then small crevasses form in
this crust, and through these the fused cinder oozes from below. The
outflow from this chasm soon becomes localized, so as to form a single
crater, or a small chain of craters; these gradually develop into cones
by the accumulation of outflowing lava, so that when the whole mass has
solidified, it is covered more or less thickly with a number of such
hillocks. These, however, are much smaller than in the former case,
reaching to only one or two inches in height, with a proportionate
base. It is evident that the dimensions of these miniature volcanoes
are determined mainly by the depth of the molten matter from which
they are formed. In the case of the bogies, they are exaggerated by
the overpowering resistance of the solid iron bottom and sides, which
force all the exudation in the one direction of least resistance, viz.,
towards the centre of the thin upper crust, and thus a single crater
and a single cone of the large relative dimensions above described are
commonly formed.
The magnitude and perfection of these miniature volcanoes vary
considerably with the quality of the pig-iron and the treatment it
has received, and the difference appears to depend upon the evolution
of gases, such as carbonic oxide, volatile chlorides, fluorides, etc.
I mention the fluorides particularly, having been recently engaged
in making some experiments on Mr. Henderson’s process for refining
pig-iron, by exposing it when fused to the action of a mixture of
fluoride of calcium and oxides of iron, alumina, manganese, etc. The
cinder separated from this iron displayed the phenomena above described
very remarkably, and jets of yellowish flame were thrown up from the
craters while the lava was flowing. The flame was succeeded by dense
white vapors as the temperature of the cinder lowered, and a deposit of
snow-like, flocculent crystals was left upon and around the mouth or
crater of each cone. The miniature representation of cosmical eruptions
was thus rendered still more striking, even to the white deposit of the
haloid salts which Palmieri has described as remaining after the recent
eruption of Vesuvius.
The gases thus evolved have not yet been analytically examined, and
the details of the powerful reactions displayed in this process still
demand further study; but there can be no doubt that the combination
of silicic acid with the base of the fluor spar is the fundamental
reaction to which the evolution of the volatile fluorides, etc., is
mainly due.
A corresponding evolution of gases takes place in cosmical volcanic
action, whenever silicic acid is fused in contact with limestone or
other carbonate, and a still closer analogy is presented by the fusion
of silicates in contact with chlorides and oxides, in the absence of
water. If the composition of the Moon is similar to that of the Earth,
chlorides of sodium, etc., must form an important part of its solid
crust; they should correspond in quantity to the great deposit of
such salts that would be left behind if the ocean of the Earth were
evaporated to dryness. The only assumptions demanded in applying these
facts to the explanation of the surface configuration of the Moon are,
1st, that our satellite resembles its primary in chemical composition;
2d, that it has cooled down from a state of fusion; and 3d, that the
magnitude of the eruptions, due to such fusion and cooling, must bear
some relation to the quantity of matter in action.
The first and second are so commonly made and understood, that I need
not here repeat the well-known arguments upon which they are supported,
but may remark that the facts above described afford new and weighty
evidence in their favor.
If the correspondence between the form of a freely suspended and
rotating drop of liquid and that of a planet or satellite is accepted
as evidence of the exertion of the same forces of cohesion, etc.,
on both, the correspondence between the configuration of the lunar
surface, and that of small quantities of fused and freely cooled
earth-crust matter, should at least afford material support to the
otherwise indicated inference, that the materials of the Moon’s crust
are similar to those of the Earth’s, and that they have been cooled
from a state of fusion.
I think I may safely generalize to the extent of saying, that no
considerable mass of fused earthy silicates can cool down under
circumstances of free radiation without first forming a heated solid
crust, which, by further radiation, cooling, and contraction, will
assume a surface configuration resembling more or less closely that of
the Moon. Evidence of this is afforded by a survey of the spoil-banks
of blast furnaces, where thousands of blocks of cinder are heaped
together, all of which will be found to have their upper surfaces (that
were freely exposed when cooling) corrugated with radiating miniature
lava streams, that have flowed from one or more craters or openings
that have been formed in the manner above described.
The third assumption will, I think, be at once admitted, inasmuch as it
is but the expression of a physical necessity.
According to this, the Earth, if it has cooled as the Moon is supposed
to have done, should have displayed corresponding irregularities,
and generally, the magnitude of mountains of solidified planets and
satellites should be on a scale proportionate to their whole mass.
In comparing the mountains of the Moon and _Mercury_ with those of
the Earth, a large error is commonly made by taking the customary
measurements of terrestrial mountain-heights from the sea-level. As
those portions of the Earth which rise above the waters are but its
upper mountain slopes, and the ocean bottom forms its lower plains
and valleys, we must add the greatest ocean depths to our customary
measurements, in order to state the full height of what remains of the
original mountains of the Earth. As all the stratified rocks have been
formed by the wearing down of the original upper slopes and summits, we
cannot expect to be able to recognize the original skeleton form of our
water-washed globe.
If my calculation of the atmosphere of _Mercury_ is correct, viz.,
that its pressure is equal to about one seventh of the Earth’s, or
4¼ inches of mercury, there can be no liquid water on that planet,
excepting perhaps over a small amount of circumpolar area, and during
the extremes of its aphelion winter. Thus the irregularities of the
terminator, indicating mountain elevations calculated to reach to
1/253 of the diameter of the planet, are quite in accordance with the
above-stated theoretical consideration.
There is one peculiar feature presented by the cones of the cooling
cinder which is especially interesting. The flow of fused cinder
from the little crater is at first copious and continuous; then it
diminishes and becomes alternating, by a rising and falling of the
fused mass within the cone. Ultimately the flow ceases, and then the
inner liquid sinks, more or less, below the level of the orifice. In
some cases, where much gas is evolved, this sinking is so considerable
as to leave the cone as a mere hollow shell; the inner liquid having
settled down and solidified with a flat or slightly rounded surface, at
about the level of the base of the cone, or even lower. These hollow
cones were remarkably displayed in some of the cinder of the Henderson
iron, and their formation was obviously promoted by the abundant
evolution of gas.
If such hollow cones were formed by the cooling of a mass like that
of the Moon, they would ultimately and gradually subside by their own
weight. But how would they yield? Obviously by a gradual hinge-like
bending at the base towards the axis of the cone. This would occur
with or without fracture, according to the degree of viscosity of the
crust, and the amount of inclination. But the sides of the hollow-cone
shell, in falling towards the axis, would be crushing into smaller
circumferences. What would result from this? I think it must be the
formation of fissures, extending, for the most part, radially from the
crater towards the base, and a crumpling up of the shell of the cone by
foldings in the same direction. Am I venturing too far in suggesting
that in this manner may have been formed the mysterious rays and rills
that extend so abundantly from several of the lunar craters?
The upturned edges or walls of the broken crust, and the chasms
necessarily gaping between them, appear to satisfy the peculiar
phenomena of reflection which these rays present. These edges of the
fractured crust would lean towards each other, and form angular chasms;
while the foldings of the crust itself would form long concave troughs,
extending radially from the crater.
These, when illuminated by rays falling upon them in the direction of
the line of vision, must reflect more light towards the spectator than
does the general convex lunar surface, and thus they become especially
visible at the full Moon.
Such foldings and fractures would occur after the subsidence and
solidification of the lava-forming liquid—that is, when the formation
of new craters had ceased in any given region; hence they would extend
across the minor lateral craters formed by outbursts from the sides of
the main cone, in the manner actually observed.
The fact that the bottoms of the great walled craters of the Moon are
generally lower than the surrounding plains must not be forgotten in
connection with this explanation.
I will not venture further with the speculations suggested by the
above-described resemblances, as my knowledge of the details of the
telescopic appearances of the Moon is but second-hand. I have little
doubt, however, that observers who have the privilege of direct
familiarity with such details, will find that the phenomena presented
by the cooling of iron cinder, or other fused silicates, are worthy of
further and more careful study.