FRIDAY, MARCH 4, 1921

THE STRUCTURAL FAILURE OF THE

LITHOSPHERE1

As a foundation for ordinary human activities it is but natural that the lithosphere or solid earth should be a popular symbol of strength and permanence; but the geologist sees abundant evidences that it has fared badly in the contest with environmental forces, past and present. It has been weak and incompetent; it has bent, crumpled, broken and mashed; structurally it has failed; in considerable part it now consists of structural ruins.

The problem of the structural geologist includes the restoration of these ruins and a determination of the conditions and causes of failure. His problem is not rendered easier by the fact that it is seldom possible to see the structures in three dimensions, and that he must base his restoration on fragments of evidence seen at the surface or on the very limited outlook of underground openings or on inferences from environmental conditions. Furthermore, the geologist seldom sees rock failure in actual progress. If he does he may not recognize it because the movement is sc slow. He arrives after the disturbance is over and must infer the nature of the forces and processes from the results. In attempting to picture conditions in the inaccessible deer zones, he must make long range inferences from the few available facts.

The study of structural geology naturally begins with the mapping and description o: separate structures like folds, faults, joints and cleavage. Too often this has been re garded as the end and not as a step toward the understanding of the structural conditions as :

1 Address of the retiring vice-president an chairman of Section E, the American Associatio: for Advancement of Science, Chicago, Decembe 28, 1920.

whole. The necessity of integrating evidence and information from scant observations requires an understanding of the interrelations of structures and of great group characteristics of a given environment or of a given kind of rock. I would like to comment briefly on some of these broader considerations, not exhaustively, and certainly not with full understanding, but with a view to indicating some of the salient facts now known and the manner in which these facts have been built into certain generalizations and hypotheses as to movements of the lithosphere.

I. STRUCTURAL FAILURE IN THE ZONE OF

OBSERVATION

We may direct our attention first to the structural failure of rocks extending downward only a few miles from the earth's surface. The characteristics of this region are disclosed to us by deformed rocks, some of which were once far below the surface, but now brought within our range of observation by the erosion of overlying rocks. This may be conveniently referred to as our zone of observation.

Heterogeneous Nature of Movement.-In this zone, some of the rocks have been deformed by rock flowage and some by rock fracture, both kinds of deformation often resulting in folding and tilting of beds. By rock flowage we mean "solid," "plastic," (6 massive," or "viscous " movement under great containing pressures during which the rock and its constituent minerals retain their properties of elasticity and rigidity. No one of these descriptive terms may be technically accurate and comprehensive, but the movement partakes of the characters expressed by all of them. The movement is not necessarily slow and continuous; there is geologic evidence that it is periodic. Rock flowage is essentially characterized by the parallel dimensional arrangement of minerals, like mica and hornblende, developed by recrystallization during the process. These minerals are present abundantly after the process, not before. Rock flowage is intimately associated with fracture, including the minute granula

tion and slicing of mineral p cluding larger fractures, es shearing type. While rock fl fracture constitute two distin formation, there is almost con between the two, and much not accurately described by displacement may take place fracture, or along a fracture has been local rock flowage, o of closely spaced parallel frac flowage affecting all of t masses, or along a zone of which evidences of fracture distinct or altogether lacking. plane may show all of these large way a considerable zone often be interpreted, in its re placement and stresses, in m manner as a fracture plane.

The difficulty of a precise de two phenomena of fracture an illustrated in the so-called flow experimentally. Shearing, thru and slicing are here strongly while the parallelism of mir brought about through recrys conspicuous in schists, slates which are the principal evidenc age in nature, is almost lacking mental results. Deformation i cially is plastic flow, but the deformation observed in nature fracture. With a longer time perimental flowage would pres closely approximate that of nati

Structural failure within our vation, whether by fracture or been confined to any particu formation, but is so distribute cate that adjustment of rock deforming stresses has been ac movement in many zones, in tions, in all directions, and wit tions. Rocks in this zone as not yielded to stresses as homoge In fact, even down to compa units of volume the rule is heter matter how homogeneous the f

seem, rock movement discloses zones of inherent weakness along which the movement is largely concentrated.

Causes of Movement.-Rock failure is evidence of overpowering stresses, but the causes and directions of these stresses are not so clear.

Failure on a mountainous or continental scale points to great earth stresses of the kinds which have been variously ascribed to adjustments under gravity between earth masses of differing densities and topographic relief, to adjustments under gravity of a solid shell to a shrinking centrosphere, a comception based on the supposed transfer of heat and magmas from the centrosphere outward, to tidal strains, to changing centrifugal pressures caused by changes in rate of the earth's rotation, or to combinations of these causes.

So clear is the evidence that great earth forces of this kind have been operative that other causes of movement have been perhaps underestimated or ignored in explaining local failure. Such are the pressures and changes of temperature attending the extrusion and intrusion of igneous rocks, in the vicinity of which there is often clear evidence of local failure, the recrystallization of rocks during long periods causing local changes of volume, the leaching of substances near the surface causing voids and weakness and consequent slump under gravity, and other volume changes under weathering. When rocks are in a soft and incoherent condition, they are especially susceptible to local stresses. Mud, marl, sand and salt deposits crumple and slip as the deposits are slowly built up, either under air or water. Local loading by water and ice or rock materials may cause them to fail.

Unconsolidated glacial deposits show a variety of joints, faults, and folds. In the settling, consolidation, and desiccation of soft deposits, stresses are set up resulting in local failure. When the deposits are seen later as hard rocks it is difficult to determine the extent to which the failures are to be attributed to these early and local causes acting during the soft formative stages, and to what extent they are the result of regional deformation after the rocks are strong and hard.

The part played by the forces of crystalli zation in initiating earth stresses is yet bu little understood. Growing crystals have bee found experimentally to exert considerabl linear forces. There seems to be evidence i rocks that these forces have been sufficient t widen openings or to expand the rock mass Crystallization may also contract the roc mass. The impressive manner in which cry stal habit asserts and maintains itself unde most intense conditions of metamorphism seems to indicate the reaction of considerable forces of crystallization to external environ ment. It is the custom usually to explair such facts on the basis of adaptation to en vironment, and to put the emphasis largely on the environmental conditions as determining the outcome. It is clear, however, that these conditions have not been sufficiently intense to interfere with or overcome the tendency of crystals to take whatever form best suits their atomic structure-in other words, to de velop their own habit. The philosophy of the precise relations between inherent crystallizing power and environmental forces is not understood, but enough is known to warrant the suspicion that the cumulative effects of the forces of crystallization crystallization may themselves initiate earth stresses of a high order of magnitude.

In my own structural field work, I have become impressed with the necessity for better criteria for the separation of rock structures due to local causes of the kind above indicated from the results of failure under the greater regional earth stresses. Of course there is no clean-cut separation between the two. An accumulation of minor and local causes may cause relatively large earth movements, and conversely major earth movements are resolved into a complex of minor related structural phenomena.

Angular Relations of Rock Structures to Causal Stresses.-Just as structures in themselves do not indicate all the causes of failure, neither do they indicate clearly the directions of application of stress. On the whole the geologist's attempt to relate specific structures with specific stress systems has not been