Fracture (geology)

“ The most obvious of these effects are the fractures with which the crust of the earth is everywhere intersected. The uplifting force upon which all volcanic phenomena depend would necessarily fracture the crust, and the wave-like motion resulting from the fracture would cause numerous secondary fractures, having a parallel direction. ”

“ Just where the lavas come from, and how they find their way through the deep-lying compact zone below the zone of fracture, may better be considered later. When they reach the zone of fracture, they usually either take advantage of fissures already formed, or force passageways for themselves by fracture. There is little evidence that they bore their way through the rocks by melting, though they appear to round out their channels in some way into pipes, ducts, and other tubular forms when they flow through them for long periods of time. ”

“ A similar interruption of continuity in the masses of Primary rocks, and in the rocks of intermediate age between these and the Coal formation, is found to occur extensively in the working of metallic veins. A vein is often cut off suddenly by a Fault, or fracture, crossing it transversely, and its once continuous portions are thrown to a considerable distance from each other. This line of fracture is usually marked by a wall of clay, formed probably by the abrasion of the rocks whose adjacent portions have been thus dislocated. ”

“ 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 the first phase of production of the schist, the irregular fractures pass into rather regular fractures, cutting the beds nearly at right angles. As the action becomes more intense in the more argillaceous beds, the angle of fracture, or cleavage, as it may now fairly be called, becomes more acute, and in the most intense phase this cleaved rock passes into a well-developed schist, the foliation of which is parallel to the bedding. ”

“ If fracture by overthrust be unknown in Egypt, another type of fracture has produced effects of a far-reaching character. In certain [8] regions of the earth, folding is no longer the conspicuous method by which the rock-components of its crust are displaced with regard to one another. In many instances small fissures have been observed in which the beds on one side have been thrown down to a lower level than they are on the other. ”

“ Oftentimes a great line of fault has been repeated, or the movements have been continued through successive periods, so that, newer deposits having covered the old line of displacement, the strata both newer and older have given way along the old line of fracture. ”

“ ZONE OF FLOW, ZONE OF FLOW AND FRACTURE, AND ZONE OF FRACTURE. We may believe that at depths which must be reckoned in tens of thousands of feet the load of overlying rocks is so great that rocks of all kinds yield by folding to lateral pressure, and flow instead of breaking. Indeed, at such profound depths and under such inconceivable weight no cavity can form, and any fractures would be healed at once by the welding of grain to grain. At less depths there exists a zone where soft rocks fold and flow under stress, and hard rocks are fractured ”

“ Fig. 46.—A series of parallel faults indicated by successive offsets in the course of an easily recognizable rock formation.
There are in addition many indications rather than proofs of the presence of faults, which must be taken account of in every general study of the geology of a district. Thus the outcrops of all neighboring formations may terminate abruptly upon a straight line which intersects all alike. Deep-seated fissure springs may be aligned in a striking manner, and so indicate the course of a prominent fracture, though not necessarily of a fault. ”

“ When the elevating force becomes greater than both the repressing forces, the crust is fractured. The strength of the strata then becomes nothing, and the repressing force is the weight alone. The elastic mass below at once expands, and the requisite space is furnished by the uplifting of the strata along the line of fracture. As the ridge of lava which fills this additional space cools, it recloses, in part, the original fracture, and the repressing force again consists of the two elements,—weight and strength. ”

“ Imagine forces of some kind, acting laterally, to compress those layers. There will be corrugations, sometimes actual fractures, the two sides of which, sliding one against the other, will be either raised or depressed. Faults is the name which we give to the fractures that penetrate the terrestrial shell and separate two masses of rock, one of which slides over the plane of fracture. The fault, therefore, reveals an edge, a lower lip produced by the subsidence of the soil, and an upper lip produced by an elevation. ”

“ Again, supposing the mammoth carving to have been made before the fracture, the carvings on the reverse of the Stone, and the apparently meaningless scratch below the perforation, which, as it were, skips the fracture, may have been made long after it. As Dr. Putnam says: "The fact that a very large number of perforated stones are broken when found is worthy of consideration, and also that in most cases the fracture is through one of the holes." ”

“ Fracture seldom or never occurs through the solid stone, in masonry, but always at the mortar joints, and generally by their loss of adhesion, to the stone at the faces of the joint. It rarely occurs through the brick, in brickwork, and only when the cohesion of the brick itself, is less than that of the cement. ”

“ Folds and faults are really closely related. In the former the strata are disturbed and displaced by bending; in the latter by breaking and slipping; and the displacement which is accomplished by a fold may gradually change to a fracture and slip. This relation is especially noticeable with monoclinal folds (Fig. 23) , in which the tendency to shear or break the beds is often very marked.
Important faults are rarely simple, well-defined fractures; but, in consequence of the enormous friction, the rocks are usually more or less broken and crushed, sometimes for a breadth of many feet or yards. ”

“ Every species of fracture, and every degree of dislocation and contortion, may be perceived in the form of mineral veins; and there is no other general principle to be observed in examining their form. ”

“ Fractures are partial or complete, the former when the bone is broken only part way through; simple, when the fracture is a mere break of the bone, and compound, when the end of one or both fragments push through the skin, allowing the air with its germs to come in contact with the wound, thus greatly increasing the danger. To be sure that a bone is broken we must consider several points. The patient has usually fallen or has received a severe blow upon the part. This is not necessarily true, for old people often break the thigh bone at the hip joint by simply making a false step. ”

“ The rocks, as we have seen, are cracked across their stratification. When a large sheet of gneiss is gradually sliding down, there comes eventually upon some part of it a strain sufficient to produce a fracture. ”

“ The contortions and fractures bear always such relation to each other as appears positively to imply contemporaneous formation. Through all the lowland districts of the world the average contour of the waves of rock is somewhat as represented in Fig. 16 a, and the little cliffs or hills formed at the edges of the beds (whether by fracture, or, as oftener happens in such countries, by gradual washing away under the surge of ancient seas) are no higher, in proportion to the extent of surface, than the little steps seen in the centre of the figure. ”

“ Joint cracks are generally very irregular in direction and spacing, while fault fractures are usually fairly regular and straight. Many cracks are not wide enough to allow anything like good passageways for water, while others are sufficiently open to allow water to travel along them for hundreds, or even thousands of feet. In canyons of the West, springs not rarely emerge from the bottoms of great, nearly vertical ledges of granite and other hard crystalline rocks, the waters certainly having entered the rocks hundreds, or even some thousands of feet, higher. ”

“ Again, let any one try to collect specimens of granite, porphyry, or other varieties of trap-rock, and he will find it often next to impossible to get one with a fresh fracture. He strikes with his hammer an apparently solid block, and he finds it break into small pieces, each surface of which (except that of the original exterior) is coated with dust which has somehow penetrated into little joints and fissures quite invisible to the eye. When my son and I were engaged in collecting specimens from the ridge of Samrat Fiddân, we had frequent instances of this kind ”

“ Any blow delivered at or near the basic level tends primarily to involve the weaker area, the base, passing secondarily upwards on to the vault. One may go even so far as to say that in most combined vault and basic fractures, the vault fracture is a secondary development, the basic fracture being the primary lesion. ”

“ Fractures are generally the result of great force applied directly to the shaft of a bone, or to its extremity; but they are also not unfrequently caused by twisting of the limb whilst the muscles are in a powerful action. Bones are broken transversely; but more frequently there is a degree of obliquity in the fracture, and the fragments are generally detached. A bone may be split longitudinally, as from a musket-ball striking its shaft in the centre ”

“ Some enumerate also the capillary fracture, but it is a very narrow fissure which eludes the senses, and therefore, being often overlooked owing to the symptoms of it not being obvious, it occasions death. An indentation is not a division of a bone, and, therefore, such an injury is not properly called a fracture, but is, as it were, a protrusion and bending of the skull inwardly, forming a hollow without a solution of continuity, as when vessels made of copper, or the raw hide of an ox, are struck on the outside. ”

“ That in time the user learns that the stone is made more effective for severing tissues by fracturing it in such way as to give a sharp edge, the fracture being originally accidental and afterward designed ”

“ The existence and situation of a fracture can often be recognized long after the accident, by the callus or slight unevenness due to projection of the edges or ends of the fragments. Where the bone lies deeply covered by soft parts, it is difficult and often impossible to tell, long after union has taken place, whether or where a fracture has occurred. ”

“ Below the zone of fracture, joints cannot be formed, for there the rocks tend to flow rather than break. The rocky crust, as it slowly accommodates itself to the shrinking interior of the earth, is subjected unceasingly to stresses which induce jointing by tension, compression and torsion. ”

“ A break or rupture through very close pack ice, compact pack ice, consolidated pack ice, fast ice, or a single floe resulting from deformation processes. Fractures may contain brash ice and/or be covered with nilas and/or young ice. The length of a fracture may vary from a few meters to many miles. A large fracture is more than 500 meters wide- a medium fracture is 200 to 500 meters wide- a small fracture is 50 to 200 meters wide, and a very small fracture is 0 to 50 meters wide. ”

“ It may be important to determine whether a bone was fractured before or after death. This point can not always be decided. Fractures inflicted during life are, as a rule, attended with more extravasation of blood and evidences of reaction in the surrounding tissues; but fractures produced within a few hours after death may resemble these very closely. Usually a greater degree of force is necessary to fracture bones in the dead than in the living body. ”

“ Wedge-shaped fractures of this bone were apt to show multiple secondary wave fissures concentric with the main line, and consequently free comminution. I saw several examples, the loose fragments being remarkably numerous. Plate XIX. is an example of an oblique fracture produced by a bullet which has ploughed across the bone, displacing large fragments anteriorly, but finely comminuting the bone in its course, and leaving small fragments of the mantle on its way. Plate XX. is an example of the rare condition of transverse fracture. ”

“ The bottom of the cave, on first removing the mud, was found to be strewed all over like a dog-kennel, from one end to the other, with hundreds of teeth and bones, or rather broken and splintered fragments of bones, of all the animals above enumerated; scarcely a single bone has escaped fracture, with the exception of some of the more solid and hard bones of the foot ”

“ The practical importance of a given fracture depends upon whether it is simple or compound, rather than upon the exact nature of the damage done to the bone. Compound fractures which open the dura mater are the most serious. Simple fractures result, as a rule, from diffuse forms of violence, and are liable to spread far beyond the seat of impact. Compound fractures result from severe and localised violence—for example, the kick of a horse or the blow of a hammer—and tend to be limited more or less to the seat of impact. ”