THE FIRE-HARDENED ROCKS
So far we have been considering the deposits laid down, for the most
part, in a leisurely and orderly manner, by the action of air and
water; by floods, rivers, lakes, the sea, or by the slow movements
of ice. If these, however, had been the only agents by which the
earth's strata were accumulated, then it is clear that for the most
part these deposits and these strata would lie evenly, one on top of
the other, like the lines of print on this page. But as a matter of
observation the earth's strata do not lie like that. If we were to
tear this page out and crumple it up in a ball, first having torn it
in half and shredded a few irregular pieces out of it, we should get
a truer picture of the way in which many of the earth's strata are
contorted, crumpled, and displaced. They have not been so distorted
by the action of the sea, violent as are some of the sea's assaults
on the land; nor would the heat of the sun at its greatest ever
produce such effects. They must have taken place from some causes
which arise in the earth itself. These causes can be summed up in one
word--fire. Some of the strata of which we have spoken, and which
are called sedimentary strata, although they were composed of soft
materials to begin with, have become very hard since, in some cases
owing to the enormous pressure of the accumulated deposits above
them, in other cases because of chemical action. In a few cases
they have become hardened not so much by losing their water, as by
direct heat. But the hardest of them is not so hard as another class
of rocks with which we are all acquainted--rocks like granite, or
quartz, or basalt. And it will be evident to any one who thinks about
the subject for a moment that no amount of pressure would make a
rock as hard as a diamond. Now how have these rocks been made? The
answer is that they have been made in some interior furnace of heat
deep down in the earth. Sometimes they have boiled up, and we can
trace them bursting their way through the sedimentary strata above
them. We do not know very much about the furnaces or cauldrons whence
they have come; in fact, we know very little about the depths of
the earth. The deepest mine-shaft known is near Lake Superior, and
is only 5000 feet in depth. In Silesia a bore-hole has been made by
the Austrian Government of a mile and a quarter in depth. It would
be by no means an easy task to sink a great boring. The Hon. Charles
Parsons has described some of the difficulties.
The shaft would have to be sunk in a neighbourhood where it would not
be likely to encounter water on its way down, because otherwise there
would be the necessity of pumping operations. In order to be of value
for purposes of observation, the shaft would be of the size usual
in ordinary mines and coal-pits. It would be sunk in stages each of
about half a mile in depth, and at each stage there would be placed
the hauling and other machinery for dealing with the next stage
below. This machinery, in order to economise space and limit the heat
of the workings, would be electrical. Even so there would have to be
special arrangements for cooling; and the depth of each stage in the
boring would be restricted to half a mile in order to avoid great
cost in the hauling arrangements, great weight of rope, and the great
cost of keeping the machinery and workings cool. At each second or
third mile down there would be air-locks to prevent the air-pressure
from becoming excessive, owing to the weight of the superincumbent
air. For when we got between two and three miles down below the
surface of the earth the atmospheric pressure there would be double
what it is at the earth's surface, or, therefore, about thirty pounds
to the square inch. It would not be easy to work under greater
air-pressure than that, firstly because of the strain on the workmen,
and secondly because of the rise of temperature which this increased
air-pressure would cause. Therefore special chambers would have to
be constructed to relieve the pressure, as well as special pumps to
provide ventilation, and other machinery to carry the superfluous
heat to the surface. This last-named machinery would be of the nature
of brine-filled pipes, in which a freezing mixture would always be
kept circulating. (The arrangements suggested by Mr. Parsons for
keeping the underground workings cool are rather too complicated
for description here; but no doubt the means he suggests would be
effective, and it would be possible, though with great difficulty, to
keep the workings cool.)
When the borings extended to a depth of some miles it would be
necessary to freeze the bottom of the shaft. This is a thing which
is sometimes now done when a shaft is being sunk through quicksands
that may be encountered on its way down. Round the circle of the
main shaft a number of small bore-holes are driven, and into them
is poured very cold brine, which freezes the material through which
the shaft is to be driven. In the case of the great boring we are
considering this would have to be done not only at the bottom of the
shaft but also for some time on the newly pierced shaft sides, until
the surrounding rock has been cooled for some distance from the face.
What would such a shaft cost? How long would it take to build? What
would the temperature be that it encountered on the way down? The
following is the estimate offered by Mr. Parsons:--
Cost Time in Temperature
£ Years of Rock
For 2 miles depth from the surface 500,000 10 122° F.
" 4 " " " 1,100,000 25 152°
" 6 " " " 1,800,000 40 182°
" 8 " " " 2,700,000 55 212°
" 10 " " " 3,700,000 70 242°
" 12 " " " 5,000,000 85 272°
But this estimate does not include the cost of cooling the shaft or
of providing it with air-locks. Mr. Parsons in delineating the scheme
remarked on the vast amount of information with which such a boring
would furnish engineers, miners, and geologists; but the point that
we wish to make is that even with this enormous expenditure of time,
industry, and money we should be as far as ever from knowing anything
about the core of the earth. We should have only gone about a third
of the way through what geologists call the earth's crust.
Here, again, we are in a condition of difficulty. How thick is the
earth's crust? and what is there beneath it? Well, as we are still
such a long way from exploring it we can only give a rather doubtful
answer; and we must therefore try to show not only what is thought
about the earth's interior but why we think it. From Mr. Charles
Parsons' table it will be seen that he calculates that as the boring
went deeper it would find a higher and higher temperature among the
rocks. At two miles down it would be hotter than the hottest summer's
day at the earth's surface; at eight miles down water would boil
by itself; at twelve miles down, unless the cooling arrangements
were extremely good, the workmen would die like flies. How does
Mr. Parsons know that there would be these temperatures, seeing
that the deepest boring hitherto made is only a mile? He bases his
calculations on what we know already of the ascending temperature at
deepening levels.
For ten years Professor Agassiz took observations concerning a very
deep mine in the United States called the Calumet and Hecla Mine.
He and Professor Chamberlin, after examining all the observations
very carefully, came to the conclusion that in going down from the
earth's surface the temperature rose at a rate of about 1° of heat
(Fahrenheit) for every 125 feet.
At the North Garden Gully Mine, Bendigo, Australia, and at the New
Chum Mine a temperature of 99° F. was reached at 3000 feet, and 107°
at 3645 feet. The rate of increase of temperature was reckoned to be
1° of heat (Fahrenheit) for every 80 feet.
This rate of 1° for 80 feet was also found at a South German mine,
Maldon, as well as at a Ballarat mine, and at a mine near Port
Jackson.
In a French mine more than 3000 feet deep, at the collieries of
Ronchamp, the rate of increase was as high as 1° in 50 feet.
In the North Staffordshire mines Mr. Atkinson, H.M. Inspector of
Mines, found the increase to be on the average 1° in 65 feet; whereas
in the South Staffordshire Hamstead Colliery Mr. F. G. Meachem found
that the increase was 1° F. for every 110 feet. The same rate was
obtained at the Baggeridge Wood Colliery, South Staffordshire.
In South Wales, in the neighbourhood of Rhondda and Aberdare, the
rate is 1° for 95 feet; at Dowlais, in the Merthyr coalfield, it
was 1° in 93 feet; at the Niddrie Collieries, near Edinburgh, the
increase is at the rate of 1° in 99 feet.
It will thus be seen that all over the world there is an increase of
temperature at a rate which, on the average, is about 1° for every
100 feet. There are 5280 feet in a mile; therefore, if this rate of
increasing temperature were maintained, at a depth of 100 miles the
temperature would be perhaps 5000° F.; a temperature at which steel
would melt and boil away into vapour. At a depth of 200 miles the
heat would be greater than that of the surface of the sun.[2]
[Footnote 2: According to the calculations made by the late Mr. W.
E. Wilson, F.R.S., in Ireland, 5773° Centigrade above the lowest
temperature which is possible in space, or about 10,500° F.]
Now at temperatures like that everything we know on the surface of
the earth would melt. Something else would happen to it besides that.
Those of our readers who have ever seen experiments at the Royal
Institution in London by Sir James Dewar or Sir William Crookes will
know that if metals are made hot enough they will not only melt but
will boil away into vapour as water boils into steam. And perhaps
we need tell no one that air, if it be subjected to a low enough
temperature, can be made a solid like ice. In fact, everything in
nature, whether we generally know it as a solid (like iron), or a
liquid (like water), or a gas (like air), can be made to assume
either of the two other forms. Thus the solid iron can be turned into
a liquid or a gas, and the liquid water can be turned into a gas
by boiling, or into ice by freezing. The gaseous air can be turned
into a liquid by lowering its temperature to some 300° F. or more
below the point at which water turns into ice; while if we lower the
temperature to about 390° F. below freezing, it will turn into a
solid. At a temperature of about 490° F. below freezing everything
in nature, whether gaseous or liquid, would become a solid, and
that temperature, which is the lowest that can possibly exist, is
called Absolute Zero. But just as every gas becomes a solid at that
temperature, so there are temperatures at which every solid becomes a
gas. Gold, for instance, begins to be a liquid at about 1900° F., and
if we heat it to 2000° it will become a gas.
Therefore it will be seen that if we were to suppose that the earth
grew steadily hotter all the way down to its centre, we should
comparatively soon come to a point when everything would be trying
to turn into a gas. But there is one other thing to be thought of.
Imagine what the pressure of the weight of the rocks themselves must
be. At a depth of a mile pressure from above arising from the weight
of the overlying rock is about 6000 lb. to the square inch. At three
miles the weight has increased to 18,000 lb., at four miles to about
24,000 lb., and at five miles to about 30,000 lb. to the square inch.
Now the average strength required to crush rocks has been shown to
be about 25,000 lb. to the square inch for granite, for limestones
about 16,000 lb. to the square inch, and for the sandstones about
6000 lb. to the square inch. At a depth of five miles, therefore, the
weight above must be equal if not greater than the resisting power
of the rock. What will happen lower than that? An experiment shown
some years ago by Sir William Roberts Austen at the Royal Institution
gives us some idea of what might happen. He subjected iron to very
great hydraulic pressure, and he arranged the experiment in such a
way that the spectators could see an image of what was happening
projected by a beam of light on to a kind of magic-lantern screen.
The iron began to move like slowly melting pitch, or very thick
gum. In fact, at depths of about six, seven, or eight miles, it is
supposed by many geologists that if the lower rocks had room to move
they would have a tendency to flow.[3]
[Footnote 3: _Geology: Earth History_, p. 127. Chamberlin and
Salisbury.]
Suppose, however, they cannot flow, that there is no room for them to
flow, and that the pressure is not merely thirteen or fifteen tons to
the square inch, as it would be at depths between five and six miles,
but a hundred times that amount, as it might be between five and six
hundred miles down. What would happen then? We can only imagine what
does happen by stating what does not happen. It used to be supposed
as late as half a century ago that the earth consisted of a crust of
hard rocks perhaps thirty to fifty miles in thickness, and that below
this crust the whole earth was a mass of red-hot or white-hot molten
stuff with flaming gases mixed with it. If that were the case it
would explain a good deal of what we see around us. It would explain
the volcanoes, for instance, which belch out fire and lava and ashes
and molten rock, and sometimes great fragments of rock. Perhaps some
of our readers may remember the great eruption of Mount Pelée, which
took place in Martinique some years ago. At one stage of the eruption
a great obelisk of rock a thousand feet high was pushed upwards out
of the crater, and eventually sank back again. It came out of the
depths of the earth. It was like a vent-peg plugging some boiling
mass below. Similarly we might suppose that all volcanoes were
vent-holes for the tremendous commotion of boiling fiery rocks below
the earth's surface. The only thing we can urge is that they do not
seem big enough for the purpose, if the earth were indeed all molten
except for a thin crust--thirty miles thick. For that would leave a
molten ocean more than 7900 miles across any way it was measured:
7900 miles deep, 7900 miles broad, 7900 miles long, if we take the
diameter of the earth to be 8000 miles. We all know what great tides
the Moon and Sun by their attraction raise in the earth's outer ocean
of water. Think what tides they would raise in this inner ocean of
molten rock and metal. The earth's crust would not be able to hold
such tides in. The molten stuff would be always breaking through the
flimsy thirty miles of outer solid rock as if it were egg-shell.
Twice a day there would be outbreaks of lava vast enough to submerge
continents.
No, that will not do. We will not confuse our readers by telling them
all the theories that have been formed, but will only state what
the late Lord Kelvin believed, and most of the present generation
of geologists believe. It is that the heat of the earth's crust
continues to increase only for a certain distance of the way down,
and that owing to pressure the earth is solid (though very hot
except towards the surface) for two thousand miles down. There
remains a thickness of another four thousand miles on either side
of the earth's centre to be considered. That might be molten, but
the pressure would be so great that it would behave as if it were a
solid. We know the earth cannot be solid all through because it does
not weigh enough. The earth cannot, of course, be weighed in any
scales, but there are methods of weighing it nevertheless. One of
two methods is by seeing how strongly it attracts bodies to itself.
But these things belong rather to the romance of astronomy than to
that of geology. We need only trouble ourselves at present about the
results.
One word more about the deep interior of the earth. Dr. J. J. See,
an American astronomer, has found how heavy and how hard the earth
is, taken as a whole. He finds that if it were built from surface
to surface of hardened steel it would be just about as heavy and
as hard--or as rigid. The steel would be like that used for the
armour-plate of battleships. Dr. See is not prepared, however, to
discard the idea that the earth has a large fluid interior. If it
were fluid, yet it would be subjected to such enormous pressure by
its own weight, that if there were a moderately thick earth-crust,
its tidal surgings would be so "cabin'd, cribbed, confined," that
they would be comparatively ineffectual. We must not run away with
the idea (against which Dr. See specially warns us), that there is
any free circulation of currents within the fluid interior. The
rigidity produced by pressure (or weight) is too great for that.
Indeed, this pressure is so great that, as another scientific
authority, Professor Arrhenius, has pointed out, the matter at
the core of the earth might even be gaseous; and yet would be so
compressed by pressure that it would possess a rigidity equal to the
hardest steel. The earth may be partly solid, partly liquid, partly
gaseous, but for all practical purposes Professor See would have us
regard it as a solid sphere having an average hardness and weight
and "rigidity" greater than that of ordinary steel.
We are still some way off an explanation of how the many igneous
rocks which were and are being "boiled up" in some inner molten
cauldron came to the surface; but the better to understand that we
must ask our readers to carry their imagination back to the very
beginning of the world when it was "without form and void."