Tectonics

“ From this hypothesis of the great depth at which the granites originated, has been derived the name of "Plutonic rocks." The beginner will easily conceive that the influence of subterranean heat may extend downwards from the crater of every active volcano to a great depth below, perhaps several miles or leagues, and the effects which are produced deep in the bowels of the earth may, or rather must be, distinct; so that volcanic and plutonic rocks, each different in texture, and sometimes even in composition, may originate simultaneously, the one at the surface, the other far beneath it. ”

“ We are in danger, therefore, when we attempt to examine the influence exerted singly by each, of overlooking the modifications which they produce on one another; and these are so complicated, that sometimes the igneous and aqueous forces co-operate to produce a joint effect, to which neither of them unaided by the other could 199 give rise,—as when repeated earthquakes unite with running water to widen a valley; or when a thermal spring rises up from a great depth, and conveys the mineral ingredients with which it is impregnated from the interior of the earth to the surface. ”

“ But the springs called mineral contain an unusual abundance of earthy matter in solution, and the substances with which they are impregnated correspond remarkably with those evolved in a gaseous form by volcanoes. Many of these springs are thermal, i. e., their temperature is above the mean temperature of the place, and they rise up through all kinds of rock; as, for example, through granite, gneiss, limestone, or lava, but are most frequent in volcanic regions, or where violent earthquakes have occurred at eras comparatively modern. ”

“ If we could see back through the ages to the furthest past of geological history, we should see our world,—before any of the stratified rocks were laid down in the seas,—before the seas themselves were made,—a hot globe, molten at least at the surface. How do we know this? Because under the rocks of all the world's surface we find there is granite or some similar rock,—a rock which shows by its composition that it has crystallised from a molten condition. Moreover we have seen that the interior of the earth is intensely hot. ”

“ It is important to observe that all these subterranean operations—the formation of continents, of mountain-ranges with all their attendant phenomena of folds, faults and cleavage, and every form and phase of earthquake and volcanic activity—depend upon or originate in the interior heat of the earth. And over against the superficial or aqueous agencies, originating in the solar heat and producing the stratified or sedimentary rocks, we set the subterranean or igneous agencies originating in the central heat, and producing the unstratified or eruptive rocks. ”

“ We have seen how many of the rocks are made in the sea,—the sandstones and the clays,—but there are two other kinds of rocks, about which we must say a little. The first are the Igneous rocks, which means rocks made by fire. These rocks have solidified, most frequently in crystalline forms, from a molten mass. Lava, which flows hot and fluid, from a volcano, and cooling becomes a sheet of solid rock, is an igneous rock. Some igneous rocks solidify under ground under great pressure, and become crystalline rocks such as granite. ”

“ The Plutonic Rocks are chiefly granite of some kind or another; and though they now often appear at the surface, they are supposed to have been produced originally at a considerable depth within the crust of the Earth, “or sometimes, perhaps, under a certain weight of incumbent ocean.”14 The Volcanic Rocks have been formed at or near the surface of the Earth, and, as the name implies, they are usually ejected, in a state of fusion, from the fissures of an active volcano ”

“ In the first place, every continent, though permanent in a general sense, has been ever subject to innumerable physical and geographical modifications. At one time the total area has increased, and at another has diminished; great plateaus have gradually risen up, and have been eaten out by denudation into mountain and valley; volcanoes have burst forth, and, after accumulating vast masses of eruptive matter, have sunk down beneath the ocean, to be covered up with sedimentary rocks, and at a subsequent period again raised above the surface ”

“ The outer part of the earth is called its crust, and geology shows that this has been subject to violent changes, such as upheavals and subsidences, and that in many places sea and land have interchanged places, probably more than once. Geology also shows that the rocks of the earth's crust are filled with the remains, or fossils, of plants and animals differing from those now existing, though related to them, and that many of these must have lived millions of years ago. ”

“ The structure of the former belongs properly with the original structures of the volcanic rocks; but the latter—the true mountains—owe their internal structure and altitude or relief almost wholly to the crumpling and mashing together of great zones of the earth’s crust, being, as already pointed out, the culminating points of the plication, cleavage, and faulting of the strata. “A mountain-chain consists of a great plateau or bulge of the earth’s surface, often hundreds of miles wide and thousands of miles long. ”

“ Some rocks are altered by heat and fluids when they are invaded by hot magma in a process called contact metamorphism.
UNALTERED ROCK METAMORPHIC ROCK MAGMA
14 Dynamic Metamorphism
Still another rock-changing process is one that is associated with mountain building. When mountains are formed, heat and great pressures develop deep within the earth’s crust. The flat layers of rock are then slowly pushed and squeezed so that they bend up into arches, fracture, or slide over each other. These forces cause great changes in the rocks in widespread areas. ”

“ How do deep earthquakes stem from friction 300 miles down under the Cordillera and the west Pacific? Do earth and moon spheres as rounded tetrahedra crack similarly? Because of rotation are north-south cracks dominant? Are the core fluids changing volcanism through the ages?
The twenty-eight percent of earth surface which lies above the sea in continents is made up of siliceous sediments of shallow water basins, with quartz as the dominant mineral, their strata wrinkled, and eroded into mountain ranges. Desert and lake or river bottoms make up most of the remainder. ”

“ A considerable lapse of time must intervene between the formation of Plutonic and metamorphic rocks in the nether regions and their emergence at the surface. For a long series of subterranean movements must occur before such rocks can be uplifted into the atmosphere or the ocean; and, before they can be rendered visible to man, some strata which previously covered them must have been stripped off by denudation. ”

“ Igneous Rocks.—Rocks formed by the cooling and solidification of a hot liquid material, known as magma, that has originated at unknown depths within the earth. Those that have solidified beneath the surface are known as intrusive rocks, or if the cooling has taken place slowly at great depth, as plutonic rocks, e.g. granite. Those that have flowed out over the surface are known as effusive rocks, extrusive rocks, or lavas, e.g., basalt. Volcanic rocks include not only lavas, but bombs, pumice, tuff, volcanic ash, and other fragmental materials or ejecta thrown out from volcanoes. ”

“ But a vast amount of empty space yet remains and will continue to be added to for millions of years to come. As the earth grows older and colder, internally, the crust will become thicker [55] and more unyielding, so that as new subterranean spaces are formed by volcanic activity, fewer of these will be filled up again and the final aggregate of them will doubtless in time reach millions of cubic miles. The spaces comprising the “Pocket” continent of the moon, above the sea level, are estimated by us to amount to about 1,500,000 cubic miles.” ”

“ All those periodic disturbances which we call earthquakes, all those elevations and subsidences which they severally produce, all those accumulated effects of many such elevations and subsidences exhibited in ocean-basins, islands, continents, table-lands, mountain-chains, and all those formations which are distinguished as volcanic, geologists now regard as modifications of the Earth’s crust produced by the still-molten matter occupying its interior. ”

“ The researches of Scrope, Judd, and others into volcanic and hypogene action, and the long observations of geologists everywhere on the effects of air, rain, ice, rivers, lakes, and oceans, have resulted in putting dynamical geology on a firm basis of ascertained fact. The heated interior has been shown almost with certainty to consist of a rigid and solid mass, incandescent, but reduced to solidity under the enormous pressure of superincumbent rocks and oceans. ”

“ It follows from these considerations that the rocks which form the earth’s crust over the surface of the continents and the islands, or beneath the bed of the ocean, must have a lessening acreage year by year. These rocks must therefore submit to compression, either continuously or from time to time, and the necessary yielding of the rocks will in general take place in those regions where the materials of the earth’s crust happen to have comparatively small powers of resistance. ”

“ How far the Transfer of Sediment from the Land to a Neighbouring Sea-bottom may affect Subterranean Movements.—Little as we understand at present the laws which govern the distribution of volcanic heat in the interior and crust of the globe, by which mountain chains, high table-lands, and the abysses of the ocean are formed, it seems clear that this heat is the prime mover on which all the grander features in the external configuration of the planet depend. ”

“ After their accumulation, whether as stratified or eruptive masses, all kinds of rocks have been subject to various changes, and have acquired in consequence a variety of superinduced structures. It has been pointed out in the part of this article dealing with dynamical geology that one of the most important forms of energy in the evolution of geological processes is to be found in the movements that take place within the crust of the earth. ”

“ I continually review my own geological muddles, the controversies over steam, flames, volcano swelling, explosion craters, layers in the crust, weighting and underflowing, continental uplift, the globe’s armor plate, contraction wrinkling of basins of sediment, submarine volcanoes, linear chains of volcanoes, siliceous shell, blocks lifted or sunk, planets solar or from the sun’s binary twin, original heat or radioactive heat, thick crust or thin shell, lava reservoirs or lava core, pregeology ancestors of volcanoes, and craters on the moon. ”

“ Natural processes that change the surface features of the earth have long been the subjects of field studies by scientists. Geologists have carefully investigated the major folds formed in the earth’s crust by mountain-building forces, the clefts and depressions resulting from earthquake activity and erosion, and the vast plains leveled off by the scouring action of great ice-sheets. ”

“ An unconformity between two marine formations records the fact that between the periods when they were deposited in the sea the area emerged as land and suffered erosion (p. 227) . The attitude and structure of the strata tell also of the foldings and fractures, the deformation and the metamorphism, which they have suffered; and the igneous rocks associated with them as lava flows and igneous intrusions add other details to the story. Each formation is thus a separate local chapter in the geological history of the [292] earth, and its strata are its leaves. ”

“ During these long ages, tectonic forces, and the ever active denuding agents of the atmosphere, in all their phases, have wrought considerable transfigurations in the surface of the globe. Some portions of the earth’s crust have been swallowed by the ocean, whilst others have been wrenched from the depths by upheaving processes. ”

“ Of course they are spread out far and wide by the numerous ocean currents, some of which flow for hundreds of miles; and so the bed of the sea can only be very slowly raised by their accumulation. Still the geologist can allow plenty 169 of time, for there is no doubt that the world is immensely old; and if we allow thousands of years, we may easily comprehend that deposits of very considerable thickness may in this way accumulate on the floors of the oceans. ”

“ As geographers we are not immediately concerned with the mode of origin of those vast wrinkles, nor need we speculate on the causes which may have determined their direction. It seems, however, to be the general opinion that the configuration of the lithosphere is due simply to the sinking-in and doubling-up of the crust on the cooling and contracting nucleus. But it must be admitted that neither physicists nor geologists are prepared with a satisfactory hypothesis to account for the prominent trends of the great world-ridges and troughs. ”

“ In the same manner as aqueous and fossiliferous strata are now formed in certain seas or lakes, while in other places volcanic rocks break out at the surface, and are connected with reservoirs of melted matter at vast depths in the bowels of the earth, so, at every era of the past, fossiliferous deposits and superficial igneous rocks were in progress contemporaneously with others of subterranean and Plutonic origin, and some sedimentary strata were exposed to heat, and made to assume a crystalline or metamorphic structure. ”

“ But this is not the case, for the outer portion of the globe consists of rocks of much less density than are those of the interior, while the downward percolation of water in some instances encounters the superheated mass of the earth's interior, and is forced back to the surface as steam, as in geysers and volcanoes, or enters into mineral combinations. ”