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support considerable structures against iso- “ that the great movements of adjustment by static tendencies; that it is not essentially rock flow or transference of material in the molten or Auidal in the ordinary sense; that earth's crust from one point to another-other molten magmas are probably local and inci- than the transference of rock in a molten condental.
dition-must take place comparatively near As to depth and distribution of the move- the surface," and that the ease of movement ments, and as to the manner of movement, “increases rapidly in proportion to their nearwhether by fracture or plastic flow or by some ness to the surface.” The mobile zone thus imunknown process, there is wide divergence of plied is inferred from experimental results to opinion. Likewise, there is doubt as to the be limited to depths within 35 miles, below laws or control under which stresses may be which a condition of no mobility seems to be transmitted. We may refer briefly to these assumed. questions.
Gilbert conceived "a relatively mobile layer Does a Zone of Weakness or Mobility Exist separating a less mobile layer above from a in the Unseen Depth?—A common conception nearly immobile nucleus," the mobile layer of the distribution of movement deep below our agreeing in depth with the depth of isostatic zone of observation confines it to a single compensation. spherical zone of weakness or mobility sur- Barrell called this weak zone the asthenorounding the centrosphere and surrounded in sphere and assigned its provisional boundaries turn by a rigid shell. This zone is supposed at depths of 75 and 800 miles from the surface. to be marked by a capacity to yield readily to This he conceived to underlie the zone of isolong enduring strains. It may be in part the static compensation, which was calculated by generating zone of magmas, which may be a Hayford to be 75 miles below the surface. factor in its supposed easy yielding. The con
Hayford assumed concentration of moveception of the existence of a weak and mobile ment within the lower part of the zone of isozone has found expression in several ways. static compensation, that is within 75 miles
The widely held belief in the existence of a of the surface. zone of rock flowage below a surficial zone of
Willis concludes that there is a zone of adfracture has commonly carried with it an as- justment below 40 miles and extending to the sumption of the relative weakness and mobile base of the asthenosphere, and that the adity of this zone. In fact “zone of rock flow- justments necessary to isostatic undertow take age" and " zone of weakness” have come to be place mainly between 45 and 100 miles from Almost synonymous in discussion of this prob- the surface. lem. Doubt as to this correlation is expressed In contrast to these conceptions of a deep later. Even if the existence of a single zone mobile zone, are the views of T. C. Chamberlin of rock flowage were proved, it does not neces- and R. T. Chamberlin, who postulate multiplicsarily follow that this is a zone of weakness. ity and irregularity of movement zones.
Van Hise assigned a depth of only six miles R. T. Chamberlino concludes that mountain to the top of this zone, though with the im- making diastrophism affects wedge shaped portant reservation that increased rigidity masses and implies steeply inclined zones of under containing pressures would greatly in- movement. crease this figure.
T. C. Chamberlin emphasizes the superficial , Adams and Bancroft,8 on the basis of ex- nature of diastrophic movements of the moun. periments with rock failure under great con- tain making kind, whether these are tangentaining pressures, conclude that the amount of tially compressive or the result of creep of tangential thrust required to produce move- continental masses under gravity. In regard ments increases so rapidly below the surface to deeper, so-called massive, movements of the 8 Loc. cit., p. 635.
9 Loo. oit,
kind reflected in major features of continental some reasons for believing that it is as good as and oceanic relief, he does not assume any any yet available to measure our course mobile substratum, but rather steeply inclined through the complex of hypotheses possible in zones of movement. As he states it:10 “In- the deep zone. Especially is it desirable to herited inequalities of specific gravity are, keep in mind the fact that cleavage, indicatperhaps more than any other agency, the gov- ing rock flowage, as observed in the deepest orning power in shaping if not actuating dias- part of our zone of observation, does not in trophic movements ”—but that “the normal general have an attitude required by the conmode of isostatic adjustment in such an earth ception of tangential shearing in a mobile zone. is thought to be wedging action in the form of This does not disprove a different attitude bemovements on the part of its constituent ta- low, but it does eliminate an affirmative bearpering prisms, conical, pyramidal, or otherwise, ing on the question which has been sometimes in response to the varying stresses imposed on implied. them. . . . They should reach to whatever Are Deep Movements Accomplished by depths may be seriously affected by differential Rock Flowage Rather than by Rock Fracstresses of an order requiring readjustment. ture?-It remains to consider the manner or No undertow in a hypothetical mobile sub- processes through which deep movements are stratum is necessarily involved and none is accomplished, whether by plastic flow, by postulated.”
fracture or by some combination of these These are only a few of the views that kinds of deformation. The widely current might be cited to indicate the wide range of hypothesis is that deformation in the deep hypotheses possible as to depth, number, and zone is mainly by rock flowage. The deattitude of deep mobile zones. The very di- formed rocks have not been seen, nor have versity of these views emphasizes the restricted the environmental conditions been accurately range of known facts. The requirement of measured; yet there are weighty consideraproof naturally rests most heavily on hypoth- tions favoring this view: eses which most precisely restrict the locus of Experimental work has shown that rock movement. So many assumptions must enter flowage requires containing pressures equal at into this proof that in our present state of least to the crushing strength of rocks, and knowledge it can not be rigorous. The safest these pressures surely exist in the deep zone. scientific attitude for the time being would Within the zone of observation even the seem to be one of rigid adherence to the known strongest rocks have locally suffered rock facts, and the recognition of the possibility of flowage and hence have locally, even at that more than one hypothesis to explain them. shallow depth, been under containing presThis is not incompatible with a sympathetic sures sufficiently in excess of their crushing attitude toward the efforts of those attempting strength to permit flowage. With greatly inproof of a single hypothesis.
creased pressures at greater depths it is logical Until the time comes when it is possible to to argue that conditions for flowage would be furnish definite proof of any specific localiza improved. Under these conditions the resisttion of movement, my own inclination is to ance to deformation is a function of the keep clearly in mind the distribution of move internal friction or viscosity of the rock. ments within the zone of observation, already This property does not of necessity bear any summarized, as perhaps the best guide to the relation to the compressive strength or comcondition that may be assumed at least for petency of the rock-qualities which detersome distance below our lowest observations. mine its behavior in the absence of great conThis measuring stick is short, but there are taining pressures. Quartzite or granite, so
10 Chamberlin, T. C., “Diastrophism and the far as we know, may have no greater viscosity Formative Processes,” Jour, Geol., Vol. 21, 1913, than marble or slate. Adams' experiments p. 520; Vol. 26, 1918, p. 197.
show diabase and marble in a composite specimen behaving similarly. In fact marble evidence of rock flow. Presumably with actually penetrated the harder diabase. Like longer time and proper conditions of temperawise, gypsum penetrated steel. While there ture and mineralizers, parallelism of newly are probably differences in the internal fric
fferences in the internal fric- developed minerals, characteristic of rock tion or viscosity of different rocks under these flow, would result. So far as the experimental conditions, the results are nevertheless homo- results go, however, they fail to exhibit strucgeneous in approximating rock flowage-in tures which in ordinary geologic field intercontrast to the heterogeneous results under pretation would be classed as typical rock less containing pressures where competency flowage. They would be called fracture or and strength of rocks play a part.
combined fracture and flowage. They would Earth temperatures increase with depth. be described as shear planes and faults. They Increase in temperature aids and accelerates might suggest rupture of the kind that origirock flowage. This is evidenced by flowage of nates earthquake shocks. hard rocks at moderate depths at batholithic R flowage has been widely assumed to contacts. Also facts of physical chemistry indicate weakness and mobility. The correlashow that increase of temperature increases tion of rock flowage with weakness may arise molecular activity, hastens endothermic re- from the fact that certain soft rocks such as actions (anamorphic reactions are largely shales, which are inherently weak, may often endothermic), increases solution, both liquid be observed to have undergone rock flowage, and solid, and hence recrystallization, and de- while adjacent strong rocks have been uncreases viscosity or internal friction.
affected. Or, a zone of flowage passing Notwithstanding these and other considera- through a homogeneous formation unquestions, any conclusions as to the existence of tionably indicates movement along the flowage a deep zone in which all rocks flow when de- zone, and, therefore, indicates the weakness of formed is hypothesis, not proved fact, and this zone relative to adjacent undeformed perhaps will always remain so. The environ- parts of the mass. But it would be equally mental conditions are not accurately known; valid to argue that where fracturing has been and even if each of the factors were measured, concentrated along a zone between undetheir conjoint effect is still speculative. Vari- formed rocks it too indicates movement, and ations in the time factor alone may determine therefore relative weakness. It is a long step whether a rock flows or fractures. Rock flow- from this to the conclusion that flowage indiage which has occurred in rocks now cates greater weakness than fracture. It is cessible to our observation fails to indicate entirely conceivable that it might require increase with depth with sufficient clearness more energy to make rock flow than to make and definiteness to warrant confident down- it fracture. Indeed there is some reason for ward projection.
believing, both from experimental work and Experimental evidence has been construed from observations in areas of combined fracto indicate that under great containing pres- ture and flowage, that relief actually takes sures, of the kind probably existing at depth, place first and most easily by fracture and the movement under thrust or shear is of that flowage occurs only when it is possible the nature of rock flowage, but this is partly to concentrate much more energy into the a matter of definition. The rock breaks and rock. Both structures show weakness relative granulates, often along definite planes, but to adjacent undeformed masses, but in relathe parts are still held together; it really tion to each other degree of weakness is a flows. Displacements along these planes may much more complicated problem. partake of the nature of faults, and there is . Our question, then, as to the extent to no development of true flow cleavage deter- which deep movements are accomplished by mined by a parallel arrangement of minerals rock flowage can not be simply and definitely under recrystallization, the common geologic answered in the present state of knowledge.
The preponderance of environmental evidence ences are such as to require it, there is seems to indicate that rock flowage is the dis- movement in the direction of easiest relief. tinctive kind of movement, but so many The stress as reflected by the movement qualifications, definitions and assumptions would seem to have been transmitted in a enter into this conclusion that my present in definite direction, and yet the pressures may clination is to keep firmly in mind the com have remained hydrostatic. If we were to plex facts of deformation in our zone of ob
imagine a volume of liquid deep below the servation as a possible key to the interpreta
surface subjected to differential stress suffition of unseen movements. This attitude will
cient to deform its containing walls, it is clear require us to pay more attention than here
that the movement would be in the direction
of easiest relief, notwithstanding the hydrotofore to the possibilities of heterogeneous structural behavior at great depths. Particu
static conditions within the liquid. Periodic
ity of movement is possible under this conlarly should we keep in mind the fact that
ception. Rock structures indicate movement the kind of rock flowage accomplished experi
only, not necessarily the inherent stresses. mentally produces structures which in the
Movement of rocks under the conditions supearth have sometimes been called fracture or
posed to obtain deep below the surface seems combined fracture and flowage. We may as
likely to be at least in part a matter of relief sume a downward extension of combined frac
of materials so contained between rigid memture and flowage, as observed in the field, and
bers that the direction of escape is definitely still meet the conditions of flow implied by oriented. Of course this supposition assumes experiment.
that on some scale, small or large, there are How Are Stresses Transmitted in the Deep units of mass competent to act as retaining Zone?-In our zone of observation stresses walls for materials acting under hydrostatic are clearly transmitted by the competent pressure. If all the mass in the deep zone members of the lithosphere. In any area of were under hydrostatic pressure, the retaining deformation evidence may usually be found walls might be regarded as the solid shell of the control of the structure by one or more above, inequalities in the competence of which competent members. When the notion was would control the movements in the direction widely held that the interior of the earth was of easiest relief. However, rock structures, molten or fluidal, hydrostatic stress condi- such as cleavage and folds, with vector artions were naturally assumed. With the later rangement of the sort observed near the surknowledge that the earth acts essentially as a face and of the sort supposed to exist below, solid throughout, this view was largely aban- tell us only of the direction of movement and doned in favor of the view that rocks in the fail to indicate whether the stresses are hydrodeep zone act as rigid competent members static or otherwise. capable of transmitting stresses in definite directions. The vector properties of cleavage
CONCLUSION and other structures supposed to develop in Within the zone accessible to observation this zone were cited to indicate the definite movements of rock masses are accomplished orientation of stresses. It does not follow by fracture and flowage. These processes may from this, however, that pressure conditions be distinct and separate, or so interrelated as were or are not hydrostatic, especially under to make definition difficult. The zones of slow movements. Rocks under compression movement are many, their positions and attifrom all sides greater than their crushing tudes diverse. In general they indicate shearstrength seem to transmit stresses in a man ing or grinding movements between rock ner suggesting approach to hydrostatic con- masses, accomplished both by fracture and ditions of pressure. When the stress differ- flowage, and caused by stresses inclined to the zones of movement. This conception is ment, and even if it does, that this condition taken to afford the best initial basis for the is not upset by what might be called a heterointerpretation and correlation of observed geneity of the time factor as represented by rock structures. There is no certain evidence differing rates of deformation. If homogeneof increase or decrease of movement toward
ous environmental and time conditions are the bottom of this zone. Beyond a shallow assumed, it is yet to be shown that these are surface zone, there is no certain evidence of sufficient to overcome the heterogeneity of the increase of rock flowage and decrease of rock physical properties of the rocks and to cause fracture with depth. There is no certain evi- homogeneous behavior through any consider
able zone. dence that rock flowage means greater weak
It is not even certain that they
may not fix and accentuate the heterogeneous ness than rock fracture. There is no certain evidence in rock flowage that pressures are
properties of rocks. Certainly in the zone of
observation there is comparatively slight evidominantly hydrostatic or dominantly those
dence of their efficacy in causing more uniof competent solid bodies.
form deformation with depth. Movements are known to occur in the zone
In short, as between alternative conceptions below our range of observation, but their
as to the conditions in the deep zone, the burnature and distribution are the subjects of
den of producing affirmative evidence would varied hypotheses based on a few known con
seem to rest heavily on any conception involvditions. Much of the sharper diastrophism ing radical departure from the known irreguseems to be confined to a thin surficial zone.
lar distribution and manner of movement Deeper movements, of a more massive type, within our zone of observation. We come, periodic, and possibly slower, seem to be im- therefore, to the Chamberlin conception of a plied by the relative movement of great earth heterogeneous structural behavior of the earth. segments as represented by continents and
C. K. LEITH ocean basins. Their depth is unknown. Most UNIVERSITY OF WISCONSIN of the current hypotheses agree in assuming a single mobile zone in which rocks move dominantly by rock flowage. The basic re
SCIENTIFIC EVENTS quirements of reasonable hypothesis, however,
DINNER IN HONOR OF THE RETIRING may be equally well met by a conception of
SECRETARY OF AGRICULTURE movement much like that of the zone of ob
The success of Secretary E. T. Meredith in servation. This does not require or postulate interesting the public in the investigational the conception of the existence of any single
work of the U. S. Department of Agriculture mobile zone, or zone of slipping, or zone of
has been unique. His prompt recognition of flowage, or of an asthenosphere. It supposes
the needs of the department and his activity movement irregularly distributed in many
in behalf of the investigators there, have zones, with any inclination, and
attracted the attention of scientific men plished by both fracture and flowage as far
throughout the country. Coming to the secrebelow the surface as movement extends taryship at a time when the morale of the always remembering that some of the struc- scientists in many government departments tures geologically described as fractures, may was being seriously impaired through disbe expressions of mass movement of the kind couragement as to the possibility of securing defined as flow in experimental results. adequate support for investigation, his cam
Conditions of temperature and pressure paign of education had the effect both of and vulcanism become more intense with awakening the public to the extent and imdepth, but it remains to be shown that their portance of the work, and of heartening the conjoint action results in a uniform environ- workers.