Crack (geology)

“ Cracks and fissures may be so produced in clay embankments, and the danger is that they form sliding surfaces and cavities from the presence of water, and gradually soften the 44interior until it fails from the thrust of the earth and want of uniform support; therefore, clay earths are more difficult to treat than granular soils, as local weakness is the cause of failure, a mass may be sound, but a crack or fissure may soon become large and pass through an embankment to the seat or the slopes. ”

“ Fifteen hundred feet higher up, above the top of the Plattenbergkopf, the ground began to be rifted. In 1876 a large crack split the rock across above the quarry roof, and four years later the mass thus outlined fell away. In 1879 serious signs were detected of coming ruin on a large scale. A great crack split the mountain across behind the top of the hill. The existence of this crack was well known to the villagers, who had a special name for it. It steadily lengthened and widened. By August 1881 it was over four mètres wide, and swallowed up all the surface drainage. ”

“ Vast as the force must have been which could fold the solid rocks of the crust as one may crumple the leaves of a magazine in the fingers, it is only under certain conditions that it could have produced the results which we see. Rocks are brittle, and it is only when under a HEAVY LOAD and by GREAT PRESSURE SLOWLY APPLIED, that they can thus be folded and bent instead of being crushed to pieces. Under these conditions, experiments prove that not only metals such as steel, but also brittle rocks such as marble, can be deformed and molded and made to flow like plastic clay. ”

“ The disruptions, variableness of character, and existence of fissures cause any but the most homogeneous clays to be treacherous and likely to slip, particularly the yellow and any laminated clays, as they allow water to enter by the veins which are usually frequent in the mass. Yellow clay has a greater tendency to crack upon drying than blue clay and does so much more quickly, hence its dangerous nature, and although a mass may be only damp, fissures will enable water to penetrate and reduce it to a state of instability ”

“ Igneous rocks, such as granite, are massive instead of stratified, and are usually made up of great numbers of interlocking crystals which bind one another together. Some crystalline rocks, such as schists, though not stratified, possess cleavage, which has much the effect of stratification, so far as erosion is concerned. All rocks are affected by systems of more or less nearly vertical cracks called joints. All these structures have their influence upon the rate of degradation. ”

“ Examine any average limestone quarry from Bristol to Berwick, and you will see there all that I have been describing; that is, all of it which is not soft animal matter, certain to decay. You will see the lime-mud hardened into rock beds; you will see the shells embedded in it; you will see the corals in every stage of destruction; you will see whole layers made up of innumerable fragments of Crinoids—no wonder they are innumerable, for, it has been calculated, there are in a single animal of some of the species 140,000 joints—140,000 bits of lime to fall apart when its soft parts decay. ”

“ Contraction joints or shrinkage cracks.—That many cracks in rocks are due to shrinkage, there can be no doubt. The shrinkage may result from the drying of sedimentary rocks; but more generally from the cooling of eruptive rocks. Every one has noticed in warm weather, the cracks in layers of mud or clay on the shore, or where pools of water have dried up; and we have already seen that these sun-cracks are often preserved in the hard rocks. They have certain characteristic features by which they may be distinguished from the joints of the first class. ”

“ Rocks often do not fault with a clean and simple fracture, but along a zone, sometimes several yards in width, in which they are broken to fragments. It may occur also that strata which as a whole yield to lateral thrust by folding include beds of brittle rocks, such as thin-layered limestones, which are crushed to pieces by the strain. ”

“ Again, after the formation of the beds many changes may occur in them. They may become hardened, pebble-beds being changed into conglomerates, sands into sandstones, muds and clays into mudstones and shales, soft deposits of lime into limestone, and loose volcanic ashes into exceedingly hard rocks. They may also become cracked, and the cracks are often very regular, running in two directions at right angles one to the other. Such cracks are known as joints, and the joints are very important in affecting the physical geography of a district. ”

“ As we know the chalk to have been raised from the sea, this upward movement may suggest to us the excavation of the rock by oceanic currents, and the partial deposition of sand; the general accumulation of boulders and clay demands a general disturbance affecting other, and even remote, districts; while the mass of chalk flint gravel seems the natural effect of a more local and less violent convulsion. In some instances, local gravel of this description lies both above and below the proper diluvium. ”

“ Occasionally the weight of a portion of the disintegrating rock above becomes too great for its tensile strength and the material crumbles and falls, producing caverns sometimes reaching so near to the earth's surface, as to cause sinks in its crust. These sinks, when first formed, as a rule, present clear rock fractures, and immediately after their formation there is usually a water-way beneath. In the course of time soil collects on their sides, they become cone-shaped hollows from the down-slidings of earth, and then vegetation appears on the living soil ”

“ That which occurred along this line was a differential movement and permanent displacement of the rock and earth on the two sides of a vertical crack. The principal displacement was not vertical, but horizontal. If one thinks of the land to the east of the crack as stationary, then the change may be described as a northward movement of the land west of the crack. If the land to the west be thought of as stationary then the land to the eastward moved toward the south. It is probable that both tracts shared in the movement, the eastern shifting toward the south and the western toward the north. ”

“ Where the rocks are igneous and rather uniformly dense, usually little or nothing can be accurately foretold about the underground water supplies, because in such rocks the water follows exceedingly variable and irregular cracks and fissures. In metamorphic rocks the difficulties are usually about as great. In limestone regions, with humid climate, much water travels in channels underground, but these are so exceedingly irregular that there is no way of locating them by surface studies. ”

“ The upper part of the curves seen in this diagram, Figure 73, and expressed in fainter lines, has been removed by what is called denudation, to be afterwards explained.
FRACTURES OF THE STRATA AND FAULTS.
Numerous rents may often be seen in rocks which appear to have been simply broken, the fractured parts still remaining in contact; but we often find a fissure, several inches or yards wide, intervening between the disunited portions. These fissures are usually filled with fine earth and sand, or with angular fragments of stone, evidently derived from the fracture of the contiguous rocks. ”

“ In Plate V, page 112, are given six figures representing various cracks and fissures. No. 1111 represents the cracks in the glaze of Japanese pottery, magnified. No. 2 shows the mud cracks on the edge of a lake, to the extent of two feet. No. 3 is a series of cracks in an asphalt pavement, covering about two feet. No. 4 shows the form of cracks in the surface of a mesa in Arizona, the result of the summer heat, the length being about ten feet. No. 5 is a tracing from a drawing by Professor W. H. Pickering showing cracks in the lunar crater Eratosthenes, with an extent of fifty-five miles. ”

“ An entirely different kind of shrinkage-crack is that which occurs in certain carbonised and flattened plants, and which sometimes communicates to them a marvellous resemblance to the netted under surface of an exogenous leaf. Flattened stems of plants and layers of cortical matter, when carbonised, shrink in such a manner as to produce minute reticulated cracks. These become filled with mineral matter before the coaly substance has been completely consolidated. A further compression occurs, causing the coaly substance to collapse, leaving the little veins of harder mineral matter projecting. ”

“ The first formed rocks no doubt became at once the prey of denudation and disintegration, and on their surface would be accumulated the products of their own destruction: newer strata would entirely cover up portions of the original land, to be in their turn succeeded by still later deposits. There is reason to believe that in the remotest ages of the Earth’s history, the forces of denudation and igneous activity were more potent than in later times, and thus the oldest rocks could hardly retain their original structure through the long ages of geologic time. ”

“ The quantity of such deep-seated (so-called “plutonic”) igneous rock material which has been intruded into the earth’s crust within known geologic time, is far greater than that which has been forced to surface, that is the so-called “volcanic” material. The plutonic rocks are always thoroughly crystallized, and they are generally coarser grained than the volcanic rocks.
Where molten materials have been forced into cracks or fissures in the crust of the earth and there congealed, we have a very common mode of occurrence called “dikes” (Plate 9) . ”

“ It has squeezed into fissures forming dikes; it has burrowed among the strata as intrusive sheets; it has melted the rocks away or lifted the overlying strata, filling the chambers which it has made with intrusive masses. During all geological ages molten rock has found way to the surface, and volcanoes have darkened the sky with clouds of ashes and poured streams of glowing lava down their [276] sides. The older strata,—the strata which have been most deeply buried,—and especially those which have suffered most from folding and from fracture, show the largest amount of igneous intrusions. ”

“ The more active and tangible processes by which surface rocks are broken up, such as wave wear, river wear and glacier wear, are processes of corrasion. The mechanical wear effected by wind-driven sand might be considered either as corrasion or as weathering. It is more likely to be regarded as corrasion if the amount of wear is considerable enough to be obvious. Rock is sometimes decomposed by the chemical action of hot vapors, gases, and waters rising to the surface from considerable depths. ”

“ They fracture with an astonishingly sharp edge, which cuts, like a knife, the fingers and clothes of the climber. Notwithstanding their apparent hardness, which seems to promise for them an unusual durability, they crack up with great rapidity under the action of frost or of blows, and rapidly subdivide into small angular debris. The smoothness of the fractured surfaces, when fresh, reduces the friction between the fragments much below that normal to the debris of ordinary rocks, so that slopes of volcanic debris are very unstable. ”

“ Serpentines, for instance, are frequently so crushed and cracked that [254] one may search in vain over a whole mountain for a place where the rock is sufficiently free from cracks to allow of a hand-specimen of the ordinary size being extracted without breaking up into fragments under the hammer. In the more acid rocks, like the granites, crushing has been a less powerful aid to disintegration, and here the diurnal temperature-range has been the principal factor ”

“ It is a species of indurated clay, harder or softer according to the pressure to which it may have been subjected. When protected from the action of the elements it retains its hardness, but when exposed to them it gradually softens and crumbles away. After a time the superstratum of rock, which is full of cracks and seams, is undermined and precipitated into the chasm below. If the stratum of shale lies at or near the bottom of the channel below the Falls, it will be measurably protected from the action of the elements. In this case retrocession will necessarily be very gradual. ”

“ If the stones are of thickness correctly proportioned to their area, the danger of cracking under the press is fairly remote; still, it does happen occasionally that a stone incurs damage through clumsiness of workmen. It can occur also through careless warming, or through sharp frost. In such cases even a strong stone will crack, especially if the workmen apply undue pressure. ”

“ A stream might cut its valley much more rapidly in such a sandstone than in a compact limestone, though the latter is, as a whole, the more soluble. The constituent minerals of crystalline rocks resist solution and decay unequally, and when any one is dissolved or decomposed the rock crumbles and the less soluble constituents are ready for removal by mechanical means. So long as the material loosened by disintegration is removed, chemical heterogeneity favors erosion; but if the loosened débris is not removed erosion is not favored by chemical heterogeneity. ”