Molten rock

“ We easily fall into the habit of thinking of molten rock as we think of a molten metal, merely as a substance which has passed from the solid to the liquid condition because of high temperature. With the return of low temperature a molten metal returns to the solid state usually in the same molecular condition which it possessed before. The point of fusion and the point of solidification are the same and are rigidly fixed. If this were true of the constituents of a rock, a definite order for the solidification of the several minerals might be anticipated. ”

“ When we find so small a mass of rocks as can be contained in the bottom of a shaft, remaining red-hot for eight months, should we be incredulous on being assured by men of science that the centre of the earth, once a molten mass of rock, still remains in a molten state after untold ages? ”

“ A little later on cracks are formed, as indicated by the rise of saline and hot springs, and the issuing of carbonic acid and other gases at the surface of the earth. As the underground activity becomes greater, the temperature of the springs and emitted gases increases; and at last a visible rent is formed, exposing highly heated and glowing rock below. From the fissure thus formed, the gas and vapours imprisoned in the molten rocks escape with such violence as to disperse the latter in the form of pumice and volcanic ash, or to cause them to pour out as lava-streams. ”

“ Other causes, such as friction and crushing in the making of mountains and the chemical reactions between oxidizing agents of the crust and the unoxidized interior, have been suggested, but to most geologists they seem inadequate.
There is much difference of opinion as to the FORCE which causes molten rock to rise to the surface in the ducts of volcanoes. Steam is so evidently concerned in explosive eruptions that many believe that lava is driven upward by the expansive force of the steam with which it is charged, much as a viscid liquid rises and boils over in a test tube or kettle. ”

“ These strange outbursts, mainly of steam, but often accompanied by molten rock, appear to owe their formation to the accumulation of beds on the bottom of the ocean, which as they are formed are to a great extent filled with water. Accumulated to a thickness of many miles, the water in the lower part of these strata gradually becomes exceedingly heated. In the end it breaks forth in steam, having a temperature quite as hot as molten iron, so that it may melt ordinary rocks. ”

“ Fragmentary materials, which are sometimes discharged in enormous quantity and dispersed over a wide extent of country, the finer particles being transported by upper air-currents for hundreds of miles. These materials arise either from the explosion of lava by the sudden expansion of the dissolved vapours and gases, as the molten rock rises to the surface, or from the breaking up and expulsion of portions of the walls of the vent, or of the lava, which happens to have solidified within these walls. ”

“ At a distance from the molten masses the changes are minor, but close to the molten magmas extensive changes take place. Though not actually melted the rock near the heat center may be softened, usually is, in which case pebbles and grains or even crystals become soft and plastic, and, as a result of the great pressure, are flattened, giving the rock, when it cools again, a striated appearance. At these high temperatures the water in the rock and also some other substances vaporize, and the hot steam and vapor are active agents in making a great many chemical changes. ”

“ That the molten materials which issue from volcanic vents have absorbed enormous quantities of steam and other gases, we have the most undisputable evidence. The volume of such gases given off during volcanic outbursts, and while the lava-streams are flowing and consolidating, is enormous, and can only be accounted - 357 - for by supposing that the masses of fluid rock have absorbed many times their volume of the gases. ”

“ In any case, the earth is hot inside as proved by deep well records and igneous phenomena in general, and it is a contracting or shrinking body as proved by the many large scale zones of wrinkling or folding of rocks. If, then, highly heated solid rocks at reasonable distances down in any part of the earth are subjected to relief of pressure by an earth movement such as upward crumpling of the crust, or by readjustment of large fault blocks, such heated solid rocks would become molten. ”

“ The metamorphism produced upon fragments of different kinds of foreign material enclosed within various igneous rocks has in recent years been studied in great detail by Professor Lacroix—Les Enclaves des Roches Volcaniques, Macon, 1893.
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It is not easy to comprehend the conditions under which large masses of molten material have been injected into the crust of the earth. The two main factors in volcanic action—terrestrial contraction and the energy of the vapours in the magma—have no doubt played the chief part in the process. ”

“ We must bear in mind that although the moon's surface had become solid, its temperature may have remained high for a very long period. But the continuous radiation of the surface-heat into space would produce continuous contraction, while the convection of the interior heat would tend to increase the thickness of the outer solid shell; and this, ever pressing with increasing force on the interior molten mass, would result in frequent ruptures of the shell, and the extrusion of molten lava rising from below. ”

“ The eruptions of volcanoes afford the simplest proof. The smoke, the ashes, and the molten lava which volcanoes pour forth show us that the earth is anything but cool in the lower regions. Other terrestrial phenomena bear similar testimony. Hot springs, for instance, evince the heated condition of the deep-seated rocks. Any miner will tell you that the deeper his mine the hotter he finds his work to be. The gain in heat arises from the fact that the deeper the mine the nearer it lies to the central incandescence. We are not now referring to such heat as is produced by combustion. ”

“ As soon as the charge has been dropped on to the pool of molten material, the mass appears to spread out over the surface and float towards the skimming door, in a thin slow-moving stream which disappears when about half-way down, being usually melted within one hour. ”

“ Note, however, that we have no clear evidence, hitherto, of the time taken to produce any of them. We know that a change of temperature alters the position and the angles of the atoms of crystals, and also the entire bulk of rocks. We know that in all volcanic, and the greater part of all subterranean, action, temperatures are continually changing, and therefore masses of rock must be expanding or contracting, with infinite slowness, but with infinite force. This pressure must result in mechanical strain somewhere, both in their own substance, and in that of the rocks surrounding them ”

“ But, as I have said, we do not clearly know why great reservoirs of melted rock occur from time to time deep under our feet. We may perhaps one day find the clue from the fact that nearly all, if not all, volcanoes occur near to the water's edge, either on the coast of the great oceans or of some enormous inland sea or lake. But at present all we can say is, that in certain parts of the globe there must be from time to time great masses of rock heated till they are white-hot, and having white-hot water mingled with them. ”

“ When molten lava is only thrown up a short distance into the air from a crater it is still partially molten when on falling it again reaches the earth, and therefore clings to any surface on which it falls. ”

“ If, therefore, a large quantity of rock be melted up in the bowels of the earth by volcanic heat, the denser ingredients of the boiling fluid may sink to the bottom, and the lighter remaining above would in that case be first propelled upward to the surface by the expansive power of gases. Those materials, therefore, which occupy the lowest place in the subterranean reservoir will always be emitted last, and take the uppermost place on the exterior of the earth’s crust. ”

“ Now the wind blows away these crumbling particles, and the solid unchanged rock beneath is again exposed to the crumbling agencies.
The changes in temperature between day and night cause rocks to contract and expand, and these changes put a strain upon the mineral particles that compose them. Much scaling of rock surfaces is due to these causes. Building a fire on top of a rock, and then dashing water upon the heated mass, shatters it in many directions. ”

“ There still remains an enormous mass of molten materials that has been shut up beneath that crust since the surface of the globe assumed its present condition. The mineral matter that formed the globe had converged towards its centre of gravity, and the arrestment of the momentum of the coalescing particles resulted in intense heat. ”

“ Again, the melting-point is probably a great deal higher for substances under great pressure. The weight of the crust is a burden the under-rocks bear. Probably the pressure on every square inch reaches thousands of tons. Could any substance become liquid with such a weight upon it, whatever heat it attained? Nobody can answer this question.
The theory that volcanoes are chimneys connecting lakes of burning lava with the surface of the earth is discredited by geologists. The weight of the overlying crust would, they think, close such chambers, and reduce liquids to a solid condition. ”

“ The disintegrated materials, produced by chemical and mechanical actions of the atmospheric waters upon rock-masses, are by floods, rivers, and glaciers, gradually transported from higher to lower levels; and sooner or later every fragment, when it has once been separated from a mountain-top, must reach the ocean, where these materials are accumulated and arranged to form new rocks. ”

“ The general conclusion of a temperature sufficient to melt the mineral substances of which rocks are composed, at no considerable distance below the surface, is confirmed by the fact that portions of the interior of the earth—at least, at the volcanic centres—are in a melted state. ”

“ In the first place, the disrupting effects of changes of temperature are slight or nil where the solid rock is protected by soil, clay, sand, gravel, snow, or other incoherent material. If the constituent parts of the loose material are coarse, like bowlders, their surfaces are affected like those of larger bodies of rock. The color of rock, its texture and its composition, also influence its range of daily temperature by influencing absorption and conduction. Dark-colored rocks absorb more heat than light-colored ones, and compact rocks are better conductors than porous ones. ”

“ I began to run wildly, hurrying through the inextricable maze, still descending, still running through the substance of the earth's thick crust, a struggling denizen of geological 'faults,' crying, shouting, yelling, soon bruised by contact with the jagged rock, falling and rising again bleeding, trying to drink the blood which covered my face, and even waiting for some rock to shatter my skull against.
I shall never know whither my mad career took me. After the lapse of some hours, no doubt exhausted, I fell like a lifeless lump at the foot of the wall, and lost all consciousness. ”

“ In their drawing-room home they roll themselves up and sleep as dormice do in cold weather, but with warmth and sunlight they expand their petal-like tentacles, and as flies in summer air, now float on the surface of the water, now gem the fronds of the sea-blooms, hang like blossoms from a spray, or deck the brown rock with life and beauty. ”

“ No; man alone had produced those reddish vapours, those gigantic flames worthy of a volcano, those tremendous vibrations like the shock of an earthquake, those reverberations, rivals of hurricanes and storms, and it was his hand which hurled into an abyss, dug by himself, a whole Niagara of molten metal! ”

“ Where is the girl so angelic as not to feel a slight curiosity to know how she shall look in a new and strange costume? Mary bent over the rock, where a little pool of water lay in a brown hollow above the fluctuations of the tide, dark and still, like a mirror,—and saw a fair face, with a white shell above the forehead and drooping wreaths of green seaweed in the silken hair; and a faint blush and smile rose on the cheek, giving the last finish to the picture. ”

“ An artist would have gone into raptures at the splendour and majesty of the spectacle, unfolded every evening, at the dazzling mass of colours shading off from one cloud to another, from deep violet at the zenith, to vivid crimson at the horizon, at the glittering cascade of molten gold, showered on the aerial rocks, but in this case the rocks were clouds, and these clouds, drifting across the sun's disk, absorbed its last rays, and completely hid them from the eyes of the anxious observers. ”

“ Or the mournful surges
That ring the dead seaman's knell.
When hearts have once mingled,
Love first leaves the well-built nest;
The weak one is singled
To endure what it once possest.
O Love, who bewailest
The frailty of all things here,
Why choose you the frailest
For your cradle, your home, and your bier?
Its passions will rock thee,
As the storms rock the ravens on high:
Bright reason will mock thee,
Like the sun from a wintry sky.
From thy nest every rafter
Will rot, and thine eagle home
Leave thee naked to laughter,
When leaves fall and cold winds come. ”