Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

SLOW MASSIVE MOVEMENTS.

It is a far cry from the intense and inconceivably rapid oscillations of the earthquake, to the excessively slow subsidences of continents, or even the slow wrinkling of mountain folds. Not infrequently rivers wear down their channels across a mountain range as fast as it rises athwart them. The movements of continents are even more deliberate. But, far apart as these contrasted movements are, in rate and method, they are associated in ultimate causation, and the earthquake shock is often merely an incident in the formation of a mountain range or in the subsidence of a continent.

The great movements are usually classed (1) as continent-making (epeirogenic) and (2) mountain-making (orogenic). They may also be classed as (1) vertical movements and (2) horizontal movements, and dynamically, as (1) thrust movements and (2) stretching movements. It is to be understood that these distinctions are little more than analytical conveniences, for continental movements are often at the same time mountain-making movements; vertical movements are usually involved in horizontal movements, and stretching usually takes part in the processes in which thrust predominates, and vice versa. But where one phase greatly preponderates, it may conveniently give name to the whole.

Present movements.—Critical observations on seacoasts show that some shores are slowly rising and some slowly sinking relative to the ocean-level. We do not certainly know what their movements are relative to the center of the earth; very possibly all may be sinking, one set faster than the other, the ocean-surface also going down at an intermediate rate. Theoretically, all might possibly be rising, one set faster than the other, the ocean also rising at an intermediate rate, though this is extremely improbable. One set may be actually rising relative to the center of the earth, and the other sinking, while the ocean-level is stationary, or nearly so. This is the way in which we are accustomed to interpret them. A general shrinkage of the earth, however, is probably going on, carrying down land-surface and sea-surface. It has been urged by Suess[239] that the general shrinkage is so great that the local upward warpings and foldings never equal it, and that the real movements are all downward, though in different degrees. This is probably the general fact at least. Over against this is the popular disposition to regard earth movements generally as “upheavals.” There is also a predilection for regarding the rigid land as moving and the mobile sea-level as fixed. In reality, the sea is an extremely adaptive body that settles into the irregular hollows of the lithosphere, and is shifted about with every warping of the latter. Whatever change affects the capacity of its depressions affects also the sea-level. If they are increased, the sea settles more deeply into them; if they are decreased, the sea spreads out more widely on the borders of the land. The one thing that gives a measure of stability to the sea-level is the fact that all the great basins are connected, and so an average is maintained. A warping down in one part of the sea-bottom may be offset by an upward warping somewhere else in the 72% of the earth’s surface covered by the ocean, and so it is only the sum total of all changes in the sea-bottoms and borders that effects the common level. Thus it happens that, notwithstanding its instability and its complete subordination to the lithosphere, the sea-level is the most convenient basis of reference, and has become the accepted datum-plane. If there were some available mode of measuring the distance of points from the center of the earth, it would give absolute data and absolute terms, and would reveal much that is now uncertain respecting the real movements of the surface. For convenience, however, since absolute terms are impracticable, the ordinary language of geology, which represents movements as upward and downward, according to their relations to the sea-level or to the average surface, will be employed. Notwithstanding this concession to convenience in the use of terms, it is of the greatest importance to form, and to constantly retain, true fundamental views.

Fundamental conceptions.—The existence of any land at all is dependent on the inequalities of the surface and of the density of the lithosphere, for if it were perfectly spheroidal and equidense, the hydrosphere would cover it completely to a depth of about two miles. Not only are inequalities necessary to the existence of land, but these inequalities must be renewed from time to time, or the land area would soon, geologically speaking, be covered by the sea. The renewal has been made again and again in geological history by movements that have increased the inequalities in the surface of the lithosphere. With each such movement, apparently, the oceans have withdrawn more completely within the basins, and the continents have stood forth more broadly and relatively higher, until again worn down. This renewal of inequalities appears to have been, in its great features, a periodic movement, recurring at long intervals. In the intervening times, the sea has crept out over the lower parts of the continents, moving on steadily and slowly toward their complete submersion, which would inevitably have been attained if no interruption had checked and reversed the process. These are the great movements of the earth, and in them lies, we believe, the soul of geologic history and the basis for its grand divisions. The reasons for this will appear as the history is followed, and its most potential agencies are seen unfolding themselves. At the same time, there have been numerous minor surface movements in almost constant progress. While these two classes of movements have been associated, and are perhaps due in the main to the same causes, they are sufficiently different in some of their dynamic aspects to be separated in treatment.

Nearly Constant Small Movements.

Innumerable gentle warpings have affected nearly every portion of the surface of the globe at nearly all stages of its history. Not only during the periods of great movements were there countless minor and gentler movements, but at times of relative quiescence there were slow swellings and saggings of the surface of the lithosphere. They sometimes affected small areas and sometimes large ones, and they were sometimes of upward phase and sometimes of downward. They were the immediate agencies in locating and controlling the deposition of stratified rocks, though they rest back on the great movements for their working conditions. Very slow sinkings of sea-borders have permitted deposition to go on in shallow water for long periods without being interrupted by the local filling of the sea. Very slow swellings of land tracts, relative or absolute, have permitted erosion to supply material for such sedimentation for long periods without exhausting the sources. Very slow upward warpings in one region and downward warpings in another have shifted the borders of the land and sea, and with them the areas of erosion and deposition. Thus have arisen overlaps and unconformities of strata and diversities in their distribution from stage to stage. Such movements may have amounted to a few inches, or a few feet, or a few fathoms per century. Downward movements have sometimes affected a considerable section of a continent, letting in a shallow epicontinental sea upon it, such, for example, as the North Sea upon the northwestern border of the continent of Europe, and Hudson Bay upon the northeastern part of North America. Similar movements seem to have extended the seas even more widely upon the surface of the land in times past, as attested by the great transgressions of the ocean-borders and the great epicontinental spread of strata. Notwithstanding their great breadths, the epicontinental seas were generally shallow. Similar gentle warpings of upward phase rescued the bottoms of shallow seas from submersion, and inaugurated erosion; or they bowed base-leveled lands upward, and rejuvenated their streams and inaugurated a new cycle of denudation. Often they connected continents previously separated by shallow straits, and thus inaugurated inter-continental migrations of land life, while they stopped inter-oceanic migration.

The gentleness and frequency of these movements is attested by the character of the sediments and by their relations to one another, as will be seen in the study of the sedimentary series.

Reciprocal features.—These minor warpings show a notable tendency to be reciprocal. If one area is bowed up, another near by is bowed down. If the continents settle, the oceans rise on their borders. If the land is cut down, the sea is filled up. There is an important phase of this deserving especial note. Certain tracts have been slowly bowed upwards into long land swells, the streams being rejuvenated and degradation hastened. Adjacent tracts have been slowly bowed downwards into long parallel troughs which received the wash from the adjacent swells, and thus became tracts of exceptional sedimentation. Such a tract of parallel swell and sag, if our interpretation be correct, developed along the Atlantic border of North America in the Paleozoic era. By the slow upward warping of the swells, the feeding-grounds of the streams were maintained, and the sags were filled about as fast as they sank. Thus a great depth of sediment was laid down in the course of an era measured by millions of years. So in other regions, especially near the borders of the continents, there have been similar reciprocal movements, giving at once feeding-grounds for the streams and lodgment-grounds for the sediments, side by side in parallel belts. It is a common view that these belts of deep sedimentation were the forerunners of mountain formation, and that they determined the formation of the mountains. In view of the grounds for doubting the efficiency of so superficial an agency in mountain formation, which will appear as we go on, it may be well to hold this view in abeyance, and to dwell on the reciprocal nature of the action, in which the upward bowing that gave the feeding-grounds is as vital a factor as the sagging that accommodated the sedimentation. It is important to recognize that in so far as the crust was weak enough to yield to these gentler forces, it was not strong enough to accumulate the great stresses necessary to form mountain ranges, and further, that in so far as the stresses were eased by the gentle warping, they could not be accumulated for the later work of mountain-folding. It is nevertheless probable that the conditions which located the gentle swelling and sagging also located the mountain-folding.

The Great Periodic Movements.

Mountain-forming movements.—Along certain tracts, usually near the borders of the continents, and at certain times, usually separated by long intervals, the crust was folded into gigantic wrinkles, and these constitute the chief type of mountains, though not the only type. The characteristic force in this folding was lateral thrust. The strata were not only arched, but often closely folded, and sometimes intensely crumpled. In extreme cases, like the Alps, the folds flared out above, giving overturn dips and reversed strata, as illustrated in the chapter on Structural Geology, [pp. 501–511]. In these cases there was an upward as well as a horizontal movement, for the folds themselves were lifted; but the horizontal thrust so much preponderated, and was so much the more remarkable, that the upward movement was overshadowed. It is well to note, however, that these mountain ranges are crumpled outward and not inward, as might be expected if they resulted simply from the shrinkage of the under side of a thin shell. The folds are sometimes nearly upright and symmetrical, and sometimes inclined and asymmetrical, as illustrated in the chapter referred to. Where the folds lean, the inference has been drawn that the active thrust came from the side of the gentler slope, the folds being pushed over toward the resisting side, and this seems to be commonly true. The original attitude of the beds, however, has much to do with the character of the folds.[240] By a slight change in the mode of thrust, sheets of paper may be so pushed as to lean forward or backward at pleasure. The leaning of the folds seems, therefore, a doubtful criterion for determining the direction of the active movement. Mountains of the thrust type usually consist of a series of folds nearly parallel to each other, the whole forming an anticlinorium.

Fig. 449.—The great Eurasian mountain tract. Jones Relief Globe. (Photo. by R. T. Chamberlin.)

Distribution of folded ranges.—The prevailing location of this class of mountains is so generally near the borders of the continents that the relation is probably significant. Dana[241] long ago called attention to the fact that the greatest mountain ranges stand opposite the greatest ocean-basins, and he connected the elevation of the one with the depression of the other. One of the most notable exceptions to this relation is the complex system of southern Europe, from the Pyrenees to the Caucasus, and another is the Altai and connected ranges ([Fig. 449]). The Urals and not a few minor ranges are also exceptions. It is probably better to regard the crumpled tracts as lying on the borders of great segments of the earth that acted essentially as units, and to regard the relationship to the sea as a coincidence that is only in part causal.[242]

Plateau-forming movements.—Another leading phase of crustal movement is the settling or rising of great blocks of the crust, as though by vertical rather than horizontal force. The western plateau of North America and the great plateau of Thibet are gigantic examples. The American plateau embraces numerous blocks which, while they have been elevated together, are individually tilted in their own fashion. At the surface, they are separated by fault-planes, but below, some of them, and perhaps most of them, pass into flexures. Most of these flexures are of the monoclinal type ([p. 516]), which dynamically means much the same as a fault; but some of them may be of the compressive type, without inconsistency with vertical fault-relief above. Research has not yet covered thoroughly any great plateau, and knowledge of this class of movements is less complete than that of folding by lateral thrust, and it has a less ample place in the literature of the subject. The plateau-forming movements are, however, much more massive than the mountain-folding movements, and stand next in magnitude to the continent-forming movements. Plateaus may be regarded as smaller platforms superposed on the continental platforms.

In the ocean-basins, there appear to be raised platforms of the plateau type, and there are remarkable “deeps” that have the aspect of anti-plateaus.

Continent-forming movements.—True continent-forming movements appear to have antedated the earliest known sediments. As far back as we can read the sedimentary record, the continents seem to have been well established, and there is little evidence that they have since been fundamentally changed. It is true that some very eminent geologists have rather freely connected formations on one continent with formations having similar faunas on an opposite continent, by a hypothetical conversion of the intervening ocean-bottoms into land or shallow water; but most such faunal relations can be explained almost equally well by migration around the coasts, or at most by mere ridge-connections. The paucity, if not total absence, of abysmal deposits in the strata of the continents, taken with the persistence of terrestrial and coastal faunas, leaves little room for assigning an interchange of position between abysmal depths and continental elevations, and vice versa. Dynamic considerations also offer grave difficulties. The doctrine of the persistence of continents probably ought not to be pushed so far as to exclude shallow water, or even land, connections between South America, Antarctica, Australia, India, and South Africa, directly or indirectly, at certain stages of geological history. Without forming final conclusions as to the measure of the change which the continents have suffered during known geological history, it is safe to conclude that the continents and ocean-basins were in the main formed very early in the earth’s history, and that subsequent changes have consisted chiefly in the further sinking of the basins and the further protrusion of the land, save as the latter has been cut down by erosion. Incidentally, the ocean-basins have probably been extended and the continents restricted. On the other hand, the continents have been built out on their borders by wash from the land, and the waters of the ocean have been somewhat lifted by the deposition of sediment in their basins. It is estimated that the cutting away of the present continents, and the deposition of the material in the ocean-basins, would raise the sea-level about 650 feet. (R. D. George.)

Relations of these movements in time.—The folding movements seem to have had extraordinary prevalence in the earliest ages, for the Archean rocks are almost universally crumpled, and often in the most intricate fashion. There is no sign that the folding was then limited to the borders of the continents; it seems rather to have affected the whole continental surface. After the beginning of the well-known sedimentary series, crumpling appears to have taken place chiefly at long intervals, thus marking off great time-divisions, and to have been confined at any given stage to certain tracts, chiefly on the borders of great segments of the earth’s crust.

Concerning the plateau-forming movements in the past, knowledge is very meager, as the detection of plateaus of ancient times is more difficult than the detection of folds. Gentle warpings have apparently been in progress at all times.

Relations of vertical to horizontal movements.—The downward movements are unquestionably the primary ones, and the horizontal ones are secondary and incidental. The fundamental feature is doubtless central condensation actuated by gravity, and the master movements are the sinkings of the ocean-basins. The great periodic movements that made mountains and plateaus, and changed the capacity of the ocean-basins, probably started with the sinking of part or all of the ocean-bottoms. In the greater periodic movements, probably all the basins participated more or less, but some seem to have been more active than others. For example, in the last great mountain-making period, the Pacific basin seems to have been more active than the Atlantic, while in the similar great event at the close of the Paleozoic, the opposite seems to have been true. The squeezing up of the continents doubtless took place simultaneously with the settling of the basins. The true conception is perhaps that the ocean-basins and continental platforms are but the surface forms of great segments of the lithosphere, all of which crowd toward the center, the stronger and heavier segments taking precedence and squeezing the weaker and lighter ones between them. The area of the more depressed or master segments is almost exactly twice that of the protruding or squeezed ones. This estimate includes in the latter about 10,000,000 square miles now covered with shallow water. The volume of the hydrosphere is a little too great for the true basins, and it runs over, covering the borders of the continents. The amount of the overflow fluctuates from time to time, and may be neglected in a study of the movements and deformations of the lithosphere.

The squeezed segments.—The great protruding segments show a tendency toward rude triangularity. They are (1) the Eurasian, now strongly ridged on the south and east, and relatively flat on the northwest; (2) the African, rather strongly ridged on the east, but less abruptly elevated on the west and north; (3) the North American, now strongly ridged on the west, more gently on the east, and relatively flat at the north and in the interior; (4) the South American, strongly ridged on the west and somewhat on the northeast and southeast.

The foregoing form the major group. The minor group embraces (5) the Antarctic segment, not as yet sufficiently known to be well defined, and (6) the Australian, broadly reniform rather than triangular. To these are perhaps to be added (7) the largely submerged platform that stretches from Sumatra and Java on the southwest to the Philippines on the northeast, and is attached to India on the northwest; and (8) Greenland, which, though closely associated with North America, is partially separated by a rather deep depression.

The depressed or master segments.—The great sunken segments show a tendency to assume roughly polygonal, rather than triangular, forms. This accords with the primary place assigned them, since, in a spherical surface divided into larger and smaller segments, the major parts should be polygonal while the minor residual segments are more likely to be triangular. The major segments are (1) the Pacific, (2) the Indian, (3) the North Atlantic, and (4) the South Atlantic. These form the principal group, while (5) the Arctic deeps (not including the shallow epicontinental portions), (6) the Mediterranean, (7) the Caribbean, and (8) the chain of deep pits between the Philippine ridge and the Bornean platform, constitute a subordinate group.

Each member of the minor group is an irregular chain of depressed pits rather than a single continuous deep, unless the Arctic depression, of which little is now known, proves an exception. They lie between the greater segments at what may be conceived to be points of critical working relations, and are accompanied by small elevated blocks. The Caribbean, the Mediterranean, and the Bornean regions are the seats of the greatest present volcanic and related activities.

In a general view, there are then four great sunken quadrilaterals and four great elevated triangles, with minor attendants in each class. Lest fondness for simplicity and symmetry lead too far, we must hasten to observe that the dimensions are not alike in either class. The Pacific segment is more than twice the size of any other basin segment, and four times that of the North Atlantic. The Eurasian triangle is more than twice the average size of the other land segments, and nearly three times that of the South American. Nor is there any large common divisor of approximate accuracy. This is not at all strange if the earth be regarded as a body of somewhat heterogeneous composition which naturally shrank in rather irregular segments. On the other hand, this irregularity is somewhat strange if the earth has evolved from a very homogeneous and symmetrical, primitive, fluid state. It is also a serious consideration in any theory that appeals to crystalline form, or analogy, as in the doctrine of a tetrahedral earth.

Roughly approximated in millions of square miles, the major depressed segments are as follows: the Pacific, 60, the Indian, 27, the South Atlantic, 24, and the North Atlantic, 14, leaving 8 for minor depressions. The elevated segments are Eurasian, 24, African, 12, North American, 10, and South American, 9, leaving 10 for the minor blocks.

If these segments be regarded as the great integers of body-movement, two-thirds of them taking precedence in sinking and the other third in suffering distortion, it is easy to pass to the conception of sub-segments, moving somewhat differently from the main segments, so as to aid in their adjustment to one another, and thus to the conception of plateaus and deeps. It is easy also to pass to the conception of mutual crowding and crumpling at the edges of these segments, accompanied by fracture and slipping. These conceptions perhaps represent the true relations between the massive movements of the abysmal and continental segments, as well as the less massive plateau-forming movements and the mountain-forming distortions. The mountains and plateaus are probably the incidental results of the great abysmal and continental readjustments.

The great movements are probably to be attributed to stresses that gradually accumulated until they overcame the rigidity of the thick massive segments involved, and forced a readjustment. In accumulating these stresses, some local yielding on weak lines and at special points was an inevitable incident in distributing more equably the accumulating stresses. So, also, the first great readjustments probably left many local strains and unequal stresses which gradually eased themselves by warpings, minor faultings, etc., so that some minor movements were a natural sequence of the great movements. But there were doubtless many local and superficial causes, such as irregular gains and losses of heat, regional loading and unloading, solution, hydration, etc., that have caused local or regional movement, and which have little to do with the great deformations of the earth’s body. As implied above, the gentle, nearly constant movements probably fall mainly into a different category from the great periodic movements. Both will be considered further.

The differential extent of the movements.—Between the highest elevation of the land and the lowest depth of the ocean, there is a vertical range of nearly twelve miles. There may have been higher elevations, relatively, in past times, but probably not deeper depressions; and so, if we assume that the surface was once perfectly spheroidal, this may be taken as a maximum expression of differential movement, not absolute vertical movement. From the Thibetan plateau, where a considerable area exceeds three miles in height, to the Tuscarora deep, where a notable tract exceeds five miles in depth, the range is eight miles, which may fairly represent the vertical range of rather massive differential movement. From the average height of the continents to the average abysmal bottoms of the oceans the range is nearly three miles, which may be taken as the differential movement of the great segments. Under certain hypotheses of the origin and early history of the earth, to be sketched later, the surface is not assumed to have been perfectly spheroidal originally, and hence the present irregularities do not necessarily imply so great differential movement.

If the protruding portions of the lithosphere were graded down and the basins graded up to a common level, this level would lie about 9000 feet below the ocean-surface. This equated level is the best basis of reference for relative segmental movements. Referred to this datum plane, the continents, having an area about half as great as that of the ocean depths, have been squeezed up relatively about two miles, and the basins have sunk about one mile from the ideal common plane. The total downward movement, representing the total shrinkage of the earth, is quite unknown from observation. It is probably very much greater than the differential movement, as will appear from theoretical considerations as we go on.

The extent of the lateral movements has a peculiar interest, for it bears theoretically on the shrinkage of the earth. Every mile of descent of the crust represents 6 miles (6.28) shortening of the circumference. If the vertical movements were limited to the relative ones just named, the mile of basin descent would give but little more than 6 miles of surplus circumference for lateral thrust and crumpling. How far does this go in explaining the known facts? By measuring the folds of the Alps, Heim has estimated the shortening represented by them to be 74 miles.[243] Claypole estimated the shortening for the Appalachians in Pennsylvania, not including the crystalline belt on the east, at 46 miles;[244] McConnel placed that of the Laramide range in British America at 25 miles,[245] and LeConte that of the Coast range in California at 9 to 12 miles.[246] These estimates must be corrected for the thickening and thinning of the beds in the process of folding, for the composite character of the folds, and for the effects of shearing and faulting. These will in part tend to increase and in part to decrease the estimates. The first effect of horizontal thrust is to close up all crevices and compact the beds as much as they will stand without bending. A part of the unusual thickness which the beds of folded regions commonly show is probably due to this edgewise compression. In experiments on artificial strata made to illustrate foldings ([Fig. 449a]), the thickening of the layers is a very appreciable part of the process, though probably natural beds do not thicken in equal proportion. After the beds have been closely folded and the thrust is athwart them, they are thinned and stretched on the limbs of the fold. How far this and other causes of extension offset initial compression is undetermined, and is differently estimated. It seems highly probable from the nature of the case that the edgewise compression which resulted from sustaining the full stress before the beds bent, was much greater than the crosswise compression on the limbs of the folds, which came into action only after the stress had been largely satisfied by folding.

Fig. 449a.—Illustrations of Willis’ experiments in the artificial representation of mountain folding. The sections were formed of layers of wax of different colors, and were mechanically compressed from the right. The upper section shows the original state, and the offsets of the succeeding sections at the right indicate the amount of shortening. (Thirteenth Ann. Rep. U. S. Geol. Surv.)

Whatever the correction, and whatever the probable errors of the above estimates, the amount of shortening involved in folding is large. The estimates given are merely those for certain periods of folding, and represent only that portion of the compression of the circumference which was concentrated in a given mountain range. The whole shortening of a circumference is to be found by adding together all the transverse foldings on a given great circle, following it about the globe at right angles to a given folded tract. In so doing, it will be seen that the belt does not usually cross more than one or two strongly folded tracts of the same age, from which it is inferred that the shortening on each great circle was largely concentrated in a few tracts running at large angles to each other, to accommodate the shrinkage of the globe in all directions. If the folding in a main range crossing any great circle is doubled, it will probably represent roughly the shortening for that entire circle for that age. If one is disposed to minimize the amount of folding, the estimate may perhaps be put roundly at 50 miles, on an entire circumference, for each of the great mountain-making periods. If, on the other hand, one is disposed to give the estimates a generous figure so as to put explanations to the severest test, he may perhaps fairly place the shortening at 100 miles, or even more. For the whole shortening since Cambrian times, perhaps twice these amounts might suffice, for while there have been several mountain-making periods, only three are perhaps entitled to be put in the first order, that at the close of the Paleozoic, that at the close of the Mesozoic, and that in the late Tertiary. The shortening in the Proterozoic period was considerable, but is imperfectly known. The Archean rocks suffered great compression in their own times, and probably shared in that of all later periods, and if their shortening could be estimated closely, it might be taken as covering the whole. Assuming the circumferential shortening to have been 50 miles during a given great mountain-folding period, the appropriate radial shrinkage is 8 miles. For the more generous estimate of 100 miles, it is 16 miles. If these estimates be doubled for the whole of the Paleozoic and later eras, the radial shortening becomes 16 and 32 miles, respectively.