Recent advances in geodynamics

Earth-Science Reviews - Elsevier Publishing C o m p a n y , A m s t e r d a m - Printed in T h e N e t h e r l a n d s

RECENT ADVANCES IN GEODYNAMICS ADRIAN

E. S C H E I D E G G E R

University o f Illinois, Urbana, Ill. (U.S.A.)

SUMMARY

A survey is given of recent developments in geodynamics. The first two sections describe briefly the recently accumulated pertinent geological and geophysical facts. Then, the new developments in the physical basis of geodynamics, in the theory of the earth's rotation, in our knowledge of epeirogenesis, in the proposed geotectonic hypotheses, in the theory of faulting, earthquake origination, folding and various miscellaneous phenomena, are described.

INTRODUCTION

The present review is concerned with the recent advances in geodynamics. Geodynamics is that branch of the earth sciences which is concerned with an explanation of the observed features of the earth in terms of basic dynamical principles. In this, it represents essentially an application of theoretical physics (and chemistry) to the earth. With the recent expansive growth of science, the field of geodynamics has also been growing very rapidly. The present review is essentially concerned with results that were obtained during the last few years, since the earlier status of the subject matter can be easily ascertained by a perusal of textbooks (e.g., SCHEIDEGGER, 1963a). The first two sections of this article will briefly describe the recently accumulated pertinent geological and geophysical facts about the earth, which serve as a background to geodynamics. Thereafter, we shall deal in turn with the physical basis of the phenomena involved, with the earth's rotation, with epeirogenesis, with geotectonic hypotheses, with faults, earthquakes and folds, and finally with some miscellaneous phenomena.

GEOLOGICAL DATA

As stated above, we begin with some recent geological observations that have Earth-Sci. Rev., 1 (1966) 133-153

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a bearing upon geodynamics. Here we note that, with regard to the distribution of continents and oceans throughout geological history, some further evidence bearing out continental drift has been added to that already available some years ago. Thus, PLUMSTEAD(1961) discussed the distribution of ancient plants and its bearing upon drifting continents, and CAHEN (1963) did the same with regard to glaciations. Similarly, WILSON (1962a) adduced evidence that the Cabot Fault in the Canadian Atlantic Provinces and the Great Glen Fault of Scotland might have been connected and thus support the drifting apart of these areas. Much geological work has also recently been done on the structure and evolution of continental areas. The details belong to a review of geotectonics rather than to one of geodynamics. Nevertheless, it may be said that the details of many features and areas are becoming more and more established. Particularly spectacular has been the advancement of our knowledge of marine geology. A general review of the subject has been presented by PANOV (1963). Specific areas have also been studied, such as the Arctic Ocean area by PANOV (1962), the Pacific Ocean area by MACDONALD and KUNO (1962), and the Atlantic Ocean area by HEEZEN et al. (1959). Again, the detailed descriptions of these areas belong to a review of marine geology. The main conclusions made some time ago still stand: viz. that at various times throughout geological history various stress systems have existed. There is evidence of tension in the midoceanic rifts, of compression in most mountain ranges on the Northern hemisphere, and of shear in the great fault zones of the Pacific embodying large horizontal displacements. These are important observations and have to be recognized in the theory of geodynamics.

GEOPHYSICAL DATA

The past few years have been characterized by tremendous efforts everywhere to get more and better quantitative data about our globe. Our basic ideas about the layered structure of the earth have now become fairly well established. Thus, there is the crust (extending to a depth of 5-60 kin) which is separated from the mantle by the Mohorovi6i6 discontinuity. The mantle is separated from the core by a further discontinuity at 2,900 km depth, and there may be an inner core. This picture has been accepted for many years, but many of the details are only now becoming clear. Thus, within the crust, there is the problem of the existence of a further discontinuity, usually termed the Conrad discontinuity (e.g., B~.TH, 1961). This discontinuity seems to exist in some parts of the world, but not in others. The nature of the Mohorovi6i6 discontinuity has also been subject to much controversy, which will presumably only be definitely resolved when the " M o h o l e " project (the attempt of drilling a hole into the Mohorovi6i6 discontinuity) comes to fruition. It is well known that the thickness of the earth's crust varies. Generally the Earth-Sci. Rev., 1 (1966) 133-153

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crust is about 30 km thick beneath continents (with extremes of 60 km thickness beneath some mountain ranges), but only 5 km beneath ocean bottoms. Much of the detailed geography (in three dimensions) of the crust is now becoming established because of many intensive seismic crustal studies. It has also recently come to light that the earth's upper mantle may not be geographically homogeneous. This has given rise to an international Upper Mantle Project. Problems of the upper mantle are closely related with those connected with the crust; a good review of the new geophysical techniques and of the problems involved has recently been given by PRESS (1961). Much new knowledge has also recently been accumulated in connection with the problem of ascertaining the nature of seismicJoci. It has turned out that, from the distribution of the signs of the first onsets (up or down) of earthquake waves over the globe (which is divided into quadrants), it is possible to infer the nature of the focal mechanism, and in particular, the nodal planes and the orientation of the prevalent stresses which had caused the earthquake. A summary of this problem has been given, e.g., by the writer (ScHEIDE~GER, 1963b, 1965). The main conclusions are that the stress system in the majority of earthquakes is of the shear type, i.e., of the type that would produce transcurrent faulting. However, there are areas where this is not the case and different types of stress systems seem to be prevalent. This is in conformity with similar inferences from geological observations. (See the introduction of this article.) Investigations of the gravity field of the earth have taken two main directions: (1) the collation with the details of crustal structure, and (2) the finding of the distribution of gravity "at large", particularly as it affects the motion of satellites (e.g., KAULA, 1963). A particularly significant development has been that gravimeters can now be operated from surface ships (e.g., WORZEL, 1963) and aircraft. With regard to radioactive age determinations, one may equally report a great accumulation of details. In connection with this, HURLEY et al. (1962) found much evidence for the notion of continental growth. For North America, they found a growth rate of 7,000 kmZ/million years which has been operative over most of geological history. Many measurements have also been made of the heat .[tow at the surface of the earth. The main result is that the average heat flow on continents and oceans is about the same, but that there exist local areas of high heat flow. The high values seem to be associated with tectonic features, e.g., with the crest of the East Pacific Rise (VoN HERZEN and UYEDA, 1963) and with the Austrian Alps (CLARK, 1961). Presumably, this has a geotectonic significance. Turning now to electromagnetic effects, we note that the significance of paleomagnetic measurements has now been generally accepted. A great number of individual measurements has now been accumulated. Although there still seem to be some unresolved problems with the apparently not infrequent reversals of the polarity of the earth's magnetic field, the measurements make it now almost imEarth-Sci. Rev., 1 (1966) 133-153

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possible to reject the existence of continental drift. This, in fact, is in conformity with conclusions reached from geological field observations. A different aspect of electromagnetics is concerned with telluric currents. Much work has been done to perfect the "magnetotelluric" exploration method to detect peculiarities of the crust and upper mantle of the earth (particularly by PORSTENDORFER~e.g., 1961, and RIKITAKE, e.g., 1962). The results of these investigations are very promising. Any geodynamic theory must take cognizance of the facts adduced above. As will be shown later in this review, the recently discovered facts had a profound effect on some of the standard geotectonic hypotheses.

PHYSICAL BACKGROUND In any discussion of geodynamics, an understanding of the mechanical behavior of the material involved (i.e., of the material comprising the interior of the earth) under stress is of great importance. It is a fact that, even today, one has to rely mostly on speculations in this matter since the interior of the earth is not easily accessible and the time intervals that are of interest are of the order of millions of years. Needless to say, it is impossible to make experiments that would involve such time ranges. The theory of materials under stress is commonly the subject of the field of theology. The types of behavior of matter commonly considered therein are generally termed elastic, plastic, Newtonian or generally rheological. To describe the recent advances in these fields of the theory of the behavior of continuous matter, one would need to write a book by itself. Thus, let it just be noted that BUCHWALD (1959, 1961a, b, c) studied in a series of papers new aspects o f elastic wave propagation in anisotropic media, a subject which is of considerable importance in seismological applications. Similarly, SATO and USAMI (1962 I) made a basic study of the oscillation of a homogeneous elastic sphere, a problem that is of interest in connection with the interpretation of the solid tides of the earth. Regarding the plastic behavior of matter, we may note that a new book by THOMAS (1961) has recently appeared on general problems of plastic flow and fracture in solids. A particularly important advance in this field has recently been made by BODZIONY et al. (1961) who treated the deformation of rock masses as a stochastic process. This opens a completely new vista in the theory of the behavior of the earth. A basic problem that is always haunting geologists is that of ascertaining how to account for the mechanics of failure of fracture in the interior of the earth. Faults, earthquakes and joints are all evidences or fracture processes; in order to describe such phenomena by a proper theory, it is obviously necessary to study the general mechanics of fracture first. Unfortunately, it is difficult even in ordinary engineering practice to obtain a satisfactory picture of the fracturing proEarth-Sci. Rev., 1 (1966) 133-153

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cess, let alone in such an inaccessible material as is represented by the interior of the earth. Nevertheless, KUZNETSOVA (1962) and ADLER (1963) have recently put forward new theories of fracturing in the earth. The last-mentioned author in particular developed the well-known theory of Mohr so that the latter could somehow be linked with the Griffith theory of fracture-initiation from small cracks. In an application of basic physics to geodynamics, it is important to distinguish various time-scales: short (stress cycles of up to 4 h duration), intermediate (stress cycles of 4 h to 15,000 years' duration) and long (stress cycles with a duration of more than 15,000 years). Recently, it has been pointed out by CARE¥ (1962b) that similar considerations of the scale of geotectonic phenomena should not only refer to their temporal scale, but also to their geometrical scale. [n the temporal field, the shortest seismic oscillations may have periods of a few milliseconds, and the recurrence of orogenic cycles may have periods of hundreds of millions of years. Similarly, geometrically, one is concerned in the study of petrofabrics with distances of a few microns, and in the study of the influences of the sun and moon on the earth, with distances comparable to the dimensions of the solar system. It is clear that no single acceptable model can apply to the whole range of the orders of magnitude involved. While it has been known for some time that the rheological behavior of a substance differs in different time ranges, one is generally less aware of the fact that similar observations may be made if a change of the geometrical (rather than temporal) dimensions is involved. One instance where this problem has been studied on a small scale is a paper by MoGI (1962), who investigated the influence of the dimensions of specimens on the fracture strength of rocks. Turning to the rheological behavior of the earth in the various temporal ranges indicated above, we note that most recent investigations are concerned with what we called the short time range. What has mostly been studied is the attenuation of seismic waves (e.g., LOMNIYZ, 1962), the elastic behavior of rock specimens (e.g., USHER, 1962) and the free oscillations of the earth (e.g., CALOI, 1962). Unfortunately, the time range involved does not have too many geodynamic applications. As noted above, in the long time range, experiments are extremely difficult to perform. However, since it is conjectured that in their time range the earth exhibits the phenomenon of creep, any rock-creep investigations (see e.g., a review by MURELL and MISRA, 1962) are of importance. In conclusion, it may be said that the picture regarding the rheology of the earth has changed but little during the past few years. It is still held that in the short time range the crust and mantle behave like an elastic solid of high rigidity (the core like an elastic fluid and the inner core possibly being again solid); in the long time range, creep is the predominant phenomenon with a viscosity of some 1022 c.g.s, units.

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GEODYNAMICS AND THE EARTH'S ROTATION

The earth's rotation has an influence upon geodynamic phenomena, because the equilibrium figure of a rotating celestial body is not that of a perfect sphere. Hence, any changes in the speed of rotation or in the position of the poles cause stresses within the earth's interior. In this connection, a new summary of the theory of the figure of the earth has recently been published by BROVARet al. (1961), presenting mainly the Russian viewpoint on this subject. In this group, PARIISKII (1963a) discovered a resonance effect between earth tides and a new diurnal nutation of the earth. The effects of gravity upon finite deformations in the interior of the earth have been studied by JAIN and SRINIVASAN (1963). Most of the investigations of geodynamic effects of the earth's rotation, however, are concerned with tidal forces. Thus, BUCHHE1Mand SMITH (1961) discussed the earth's free oscillations as observed on earth-tide instruments in Germany. SL1CHTER(1963) gave a theoretical account of the secular effects of tidal friction upon the earth's rotation. HIERSEMANN(1962) described a new determination of the tidal number 1 from strain seismometer readings, PARIISKn (1963b) discussed the connection of earth tides with the internal structure of the earth, and VERBAANDERT (1963) described some new instruments. A great number of observations on earth tides and a large amount of theoretical work on the subject has been reported from the observatory at Uccle in Belgium, under the directorship of Melchior. Summaries of this work have been published on several occasions (MELCHIOR, 1962; MELCHIORand P~QUET, 1962, 1963). The Uccle group has, in particular, automated tidal measurements and their harmonic analysis. It has turned out that there are seven tidal waves in the solid earth, four being semidiurnal and three diurnal. However, correlation coefficients exist between the amplitudes of all these seven tidal waves. There are systematic differences in the relative phases of the tidal waves at different stations, a fact which cannot yet be fully explained; they are perhaps due to properties of the earth's upper mantle. Fairly accurate estimates, however, can now be made of the coefficients that characterize the elastic properties of the earth.

EPEIROGENESIS

The part of geodynamics which concerns itself with phenomena on a continental scale is often referred to as epeirogenesis. The question of the origin of continents needs to be tied up with our notions of the origin of the earth. The various theories of the origin of the earth are well-known (e.g., SeHEIDEGGER, 1963a). There are still no further definite facts that would unmistakably advance the acceptability of one theory over another; the times during which the Earth-Sci. Rev., 1 (1966) 133-153

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pertinent phenomena occurred are simply too far off for one to be certain about them. Thus, it is still not clear whether the earth started as a hot or as a cold body, whether the planetary system started as part of a catastrophe or through a slow evolutionary process, whether the moon was created out of the Pacific Ocean crust or some other way. Several papers and reviews have appeared (CAMERON, 1962; DE JAGER, 1962; UREY, 1962a) discussing these questions, but their results can hardly be considered as final. Of particular importance in the geodynamic contest is the problem of the earth's thermal history, viz. whether the earth is heating up or cooling down. It is well-known that during the past decades much work has been done on this question and the last few years are no exception to this (e.g., SHNEIDEROV, 1961a; LU~IMOVA, 1962), but again, the question cannot be regarded as settled as of yet. In one regard, however, it appears that some more definite conclusions can now be arrived at with regard to epeirogenetic phenomena. It seems that the fact that continents have moved relative to each other during geologic history cannot be any longer seriously doubted. Unless the interpretation of paleomagnetic measurements is somehow completely faulty (cf. section on geophysical data), it appears that the drifting of continents must be regarded as an established fact. The more extreme views of this phenomenon assume that the oceans have expanded between the continents rather than that the latter have drifted apart (cf. also section on geotectonic hypotheses), so that the earth would have greatly expanded throughout geological history. This induces a big difficulty with regard to the paleodensity of the earth, which, for a four-fold increase in surface area since the Paleozoic would have to have been around 45 g/cm 3 at that epoch. Such a four-fold increase in surface area would be required to produce the present continental crust from an ancient " n o r m a l " crust, the oceans being assumed as "new" features of our globe that were formed since the Paleozoic. In accepting continental drift as a truly real phenomenon, the problem is that of suggesting an acceptable mechanism for its occurrence. At present, convection currents (see also the section on geotectonic hypothesis) appear to be the most likely cause. G u s s o w (1962a), on the other hand, assumes that erosion can cause a disequilibrium in the rotational equilibrium of the earth's crust and hence the necessity for a readjustment by horizontal movements of the continents.

GEOTECTONICHYPOTHESES Preliminaries

We now turn our attention to the actual mountain-building processes. As is well known (e.g., SCHEIDEGGER,1963a), a number of theories of mountain building have been proposed, and recent work published during the past few years concerns Earth-Sci. Rev., 1 (1966) 133-153

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several of them. We shall discuss here the various theories and the new developments therein one by one.

Fundamentals Before one can actually deal with individual geotectonic hypotheses, one must attain clarity with regard to various fundamental problems, such as the volumes involved in orogenesis, the formation of geosynclines, the energy requirements etc. The scale of geotectonic phenomena has been investigated by CAGEY (1962b) (see also above), while some questions of the energy requirements have been analyzed by SHIMAZU (1962). Several publications deal with the theory of formation ofgeosynclines (SAITO, 1962; VAN BEMMELEN,1962; DIEVZ, 1963). In particular, SmTo (1962) investigated a model of the earth where the crust is regarded as a thin elastic plate superimposed upon an incompressible viscous fluid, the continents constituting a load upon the free surface of the elastic plate. In this fashion, Saito arrived at an explanation of mid-oceanic rifts and intercontinental geosynclines. VAN BEMMELEN(1962) proposed that the whole evolution of the earth must be regarded as a superimposition of motions of different orders of magnitude. This would then lead to concepts of the undation theory (see below). The basic principles of the type of stress field evident in the earth are becoming more and more certain. There is little doubt that in some parts of the world crustal shortening actually does occur, whereas in others, the existence of tension or shear is undeniable. It appears that a geosyncline must have been formed by some active downwarping; if a mountain range was later created, there does not seem to be a way around the hypothesis that somehow a lateral compression took place.

Contraction theory The first geotectonic hypothesis which we shall consider is the contraction theory. As is well known (e.g., SCHEIDEGGER, 1963a), this theory assumes that the earth started out as a hot celestial body, and has been contracting since its beginnings. The uppermost shell of the earth cooled first to equilibrium with solar radiation and is therefore contracting no longer. Because it has to adjust itself continually to a shrinking interior, it is in a state of internal compression which gives rise to mountains. One of the chief difficulties with the contraction theory is that it is not at all certain that the earth is contracting. A new investigation of this question was made by CAGNIARD (1961), who came to the conclusion that the core is expanding, but the crust is contracting. Since, however, these questions are tied up with the earth's birth and early thermal history, and the latter is uncertain, it is difficult to arrive at unequivocal results.

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A big difficulty is also the fact that the geotectonic stress field in the earth's crust is not everywhere an internal compression. It is easy to visualize mountains being formed through contraction (LYTTLETON, 1963), but the other types of features are difficult to explain by it. in this instance, CHANG (1961) postulated a mechanism for the block faulting in China, based upon an alternating contraction and expansion pattern in the earth. Contraction alone cannot produce block faults. All in all, it may be said that in recent years the contraction hypothesis of orogenesis has been losing favor among scientists, chiefly because it produces a homogeneous stress state in the earth's crust which does not seem to be supported by the observations.

Expansion theoo, As was noted above, it is still to this day not certain whether the earth has been cooling down or heating up since its creation. Hence, it is possible to speculate that the earth might have been heating up and therefore expanding. As was mentioned earlier, the distribution of continents could indeed be explained by such a hypothesis, if one were willing to accept a very high rate of expansion (and hence a high rate of density decrease). Recent discussions of the tectonic implications of the expansion hypothesis have been published by EGYED (1961), IVANENKO and SAGITOV(1961), NEIMAN and KIRmLOV (1961), SHNEIDEROV(1961b, 1962), BARNETT (1962), BROSSKE(1962), HILGENBERG(1962), and NEIMAN (1962). Again, as with some of the theories discussed above, the difficulty with the expansion hypothesis is that it implies a very homogeneous stress state over the earth's surface; viz. one of peripheral tension. The large horizontal displacements, whose existence has recently come to light, remain therefore essentially inexplicable. Furthermore, although the occurrence of a certain amount of expansion is certainly within the realm of possibility, the large values usually postulated are difficult to bring into accord with the commonly accepted principles of physics. A possibility might perhaps exist if some completely new views on gravitation (see e.g., DICKE, 1962; WITTEN, 1962 for reviews on this subject) or radiation fields (SHNEIDEROV,1962) were adopted.

Continental drift We have already mentioned above on several occasions that evidence for continental drift has been accumulating. It appears that the relative position of continents on the earth's surface has shifted throughout geological history, with a corresponding shift of the pole of rotation of the earth. Several recently published books and papers have concerned themselves with the general aspects of continental drift (BEEKHUIS, 1962; GIRDLER, 1962; MUNYAN, 1962; RUNCORN, 1962a; WEERTMAN, 1962 and WYATT, 1962). It may be noted, however, that, by itself, the continental Earth-Sci. Rev., I (1966) 133 153

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drift hypothesis is not actually a geotectonic theory, but it is either a fact or it is not. It becomes a geotectonic theory only if a mechanism for its occurrence is supplied. This leads us to the convection current hypothesis. Convection current hypothesis

The existence of subcrustal currents had already been speculated upon some 60 years ago, but it has only been fairly recently that more serious consideration has been given to their actual existence. Presumably, such currents take place in the mantle of the earth as the latter may have a finite viscosity so that creep phenomena could occur (see also the section on physical background). However, the validity of the postulated convection mechanism is still not unequivocally established. It is easy to be convinced that the ranges for the various theological parameters are rather narrow for convection to be possible. In this connection, KOPAL (1963) studied the general possibility of convection in planetary interiors, COOK (1963) investigated viscosity-depth profiles based on certain rheological equations, RIKITAKE(1963) investigated the problem of convection from the standpoint of the thermal state of the earth, and STACEY(1963a, b) analyzed the theory of creep in rocks and postulated a silica-differentiation mechanism for convection currents. It is generally assumed that convection currents occur in cellular patterns in the earth's mantle. The cells are generally correlated with the topography of the earth. A harmonic analysis (in terms of spherical harmonics) is made of the deviations on the earth's surface and the latter correlated with a spherical distribution of convection cells (VENINGMEINESZ, 1961a, b, 1962; GmDLER, 1963). RUNCORN (1962b, 1963a, b) postulated that the convection pattern in the earth's mantle has been changing throughout geological history; every time a change-over from one mode to the next higher took place, an orogenic cycle is supposed to have occurred. A similar view has been adopted by SUTTON (1963). Some geological consequences of the convection current hypothesis have been followed up by WH.SON (1962b, 1963a, b, c). Furthermore, WEERTMAN(1963) postulated a mechanism by which continents are constantly levelled by erosion and constantly renewed by compressive stresses produced by convection. In this, continents are assumed to play an active role in the convection cycles rather than a passive one as is usually thought. Undation theory

According to the undation theory, one surmises that all tectonic phenomena can be explained in terms of up-and-down movements of the earth's crust. This view of orogenesis is presently held primarily in the Soviet Union (BEeousov, 1960; LYUSTIKH, 1961), but elsewhere it has also been taken up, (e.g., HESS, 1962). Earth-Sci. Rev., ! (1966) 133-153

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One of the problems in this theory is to find a mechanical cause for the up-and-down cyclic movements. Generally, one advocates some process of magma differentiation (see e.g., SHIMAZU, 1961a, b; 1962). Earlier, VAN BEMMELEN(1949) assumed a volcano-tectonic process (collapse of magma chambers) as being able to produce vertical movements of the crust, but recently COTTOY (1962) questioned this assumption. Another cause for the supposed up-and-down movements has been advocated by ODHNER, who recently adduced some new evidence for his constriction hypothesis (ODHNER, 1962). The latter supposes that climatic temperature changes are sufficient to cause constrictions in the earth's crust (through lateral thermal expansion and contraction) which, in turn, would cause the up-anddown movements. The chief difficulty in the undation theory is that it is not easy to apply it to the large horizontal displacements within the earth's crust, whose occurrence seems to have been established pretty well beyond any doubt. Rotation fields' An interesting and fairly new geotectonic hypothesis is that which postulates that regular horizontal displacement patterns exist on the earth's surface. This idea leads in particular to the postulate of regular rotation fields; this has recently been followed up, particularly by PAVONI (1962). Accordingly, the surface of the earth can be divided into three rotation fields. Similar ideas had been postulated earlier by several people (see SCHEIDEGGER, 1963a). The cause for the horizontal displacements is probably to be envisaged in the presence of convection currents, but the existence of the latter is not necessarily an integral part of the rotation-field hypothesis as it is with the continental drift hypothesis. It is conceivable that some other mechanism causing the displacements could be postulated. Irregular rotation of the earth If there is an acceleration or deceleration in the speed of the earth's rotation, it is clear that this must produce a change in the equilibrium configuration, i.e., a change in the earth's ellipticity. It is conceivable that this can produce geodynamic forces and effects, an idea which has recently been revitalized by STOVAS (1957, 1961). Meteorite impact hypothesis Finally, we may mention that it has also been proposed that geodynamic effects might be due to the impact of large meteorites (DACHILLE, 1962, 1963). An impact of large enough meteorites could conceivably cause a change in the direction of the Earth-Sci. Rev., 1 (1966) 133-153

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earth's axis or in its speed of rotation with attendant geophysical effects as indicated above.

Evaluation It remains to make an evaluation of the various presently available geotectonic hypotheses. During the past few years, the inhomogeneity of the tectonic stress field has become almost well established, and this presents great difficulties to any theory that postulates a fundamentally homogeneous (uniform) stress field on the earth. Of all the theories available, it appears that only the convection current hypothesis (in connection with continental drift or rotation fields) has enough flexibility to produce the required variety of stresses. It should be kept in mind, however, that not even the theological possibility of convection in the earth's mantle has been proven beyond doubt, so that it may be that the solution of the geotectonic riddles lies in some mechanism that has not yet been thought of.

FAULTS, EARTHQUAKES AND FOLDS

Faults and folds are smaller features on the earth's surface which are also, like the larger-scale features considered above, an expression of the response of the earth's crust to the tectonic stress field. Faults are fracture phenomena. The actual mechanics of faulting is still rather imperfectly understood. This applies to fracture processes even in engineering materials, let alone in materials under geological conditions. One of the problems that has recently become of importance, is the question of the role which pore-water pressure played in faulting processes. The first to raise this question seems to have been HILLS (1954); it was later taken up by Hubbert and coworkers (HUBBERT and RUBEY, 1959). HANDIN et al. (1963) made experimental studies of the deformation in sedimentary rocks in dependence of the pore pressure. In this connection, a particular problem is the determination of the correct fracture criterion. It seems that the same criterion as that used in soil mechanics is adequate; it is commonly assumed that an "effective stress", being the difference between the total stress and the pore water stress, uniquely determines the deformation of the rock. Subjects related to faulting have also been studied by RALEIGH and GR1GGS (1963), who investigated the effect of the toe in over-thrust faulting, and by SPRY (1962), who investigated the mechanism of columnar jointing, especially in basalts. A special type of faulting is evidently represented by earthquakes. We have touched upon such problems earlier. We may note that the first motion in a seismic shock, in general, seems to give rise to a quadrant-type distribution of push and pull. While this enables one to find nodal planes, the actual source mechanism is Earth-Sci. Rev., I (1966) 133 153

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not thereby elucidated. A recent review of the problems involved has been published by the writer (SCHEIDEGGER, 1965). In addition, CALOI (1958) showed that there is a connection between earthquakes and slow movements in the earth's crust. The radiation patterns of various types of sources and various types of waves have been studied by various people. The general aspects of the questions were discussed by HONDA and E~URA (1958), CHINNERY (1960) and MITRA (1960). BRUNE (1961) investigated the origination of Rayleigh waves, TEISSEYRE(1958) and YANOVSKAYA (1958) analysed the implications of the dislocation theory on the generation of seismic waves. In the latter connection, SCHXVFNER (1961) considered asymmetrical dislocations, and KNOPOFV and GILBERT (1960) showed that a displacement dislocation yields identical radiation patterns as the double couple model for the source. A particularly important series of investigations was made by Scholte and coworkers (ScHOLTE, 1962; SCnOLTE and RITSEMA, 1962) who investigated extended volume sources. This obviates one of the objections that has been commonly levelled against the customary models of earthquake foci. The models generally assume an infinitesimally small singular seismic source, whereas it is quite certain that, in reality, a seismic focus has a finite extent whose volume can even be estimated in many cases. Thus, Scholte's model is the most realistic that has yet been postulated. Another problem in the faulting process is the question of friction at an earthquake fault. Evidence has recently come to light (DIETRICHSON, 1960) indicating that a fair amount of heat may be generated during faulting. This has a bearing upon the problem of friction near the fault zone. Finally, we note a few new publications that have a bearing upon the phenomenon of folding. The general aspects of this phenomenon, based upon his rheidity concept, have recently been discussed by CAREY (1962a). Experiments to produce folds have been reported by MCKEE et al. (1962) and by PATERSON and WEISS (1962). A particularly interesting series of papers has recently been published by RAMBERG (1961; 1963a, b, c). The first of these (RAMBERG, 1961) deals with the instability created when a sequence of competent and incompetent layers is in a state of compression. The next paper (RAMBERG, 1963a) further investigates the geometrical characteristics of folds, particularly in connection with the distribution of strain in the rock. RAMBERG(1963b) then also investigates the evolution of drag folds. The last paper in the series (RAMBERG, 1963c) treats again instability problems, particularly in connection with buckling phenomena in a viscous layered material. It may thus be seen that the theory of faults, earthquakes and folds has made some progress during the past few years. In principle, one has arrived at a mechanical understanding of these phenomena, although many of the details are not yet entirely clear. There is no doubt that a further application of the methods of mathematical physics will be required. Earth-Sci. Rev., I (1966) 133 153

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MISCELLANEOUSPHENOMENA In the last section of our review, we deal with a variety of miscellaneous geodynamic phenomena. These concern certain effects of meteorites, the problem of the origin of diapirical structures, some questions of volcanology and, finally, the socalled postglacial-uplift problem. Meteorite craters

There seems to be little doubt that several notable holes in the earth's crust have been caused by meteorite impact. For example, it is clear that the Arizona Crater was caused by some impact, but even greater depressions may be due to the same cause. The Manicouagan-Mushlagan Lake area in Canada was recently investigated with this hypothesis in view, but it turned out that the impact hypothesis is probably not tenable in this case (WILLMORE, 1963). Nevertheless, it appears that a constant stream of meteorites is hitting the earth and produces small and large craters. The statistics of the rate of arrival of such meteorites have recently been studied by M1LLARD(1963); general problems of meteorites have been reviewed by HEIDE (1963). The hypothetical and proven meteorite craters on the earth are generally likened to the craters on the moon. Therefore, the study of the surface of the moon is of a certain importance in this connection. Because of recent space programs, there has been considerable activity in this regard resulting in a series of publications to which the reader may be referred (FIEDLER, 1961; FIRSOFF, 1961; Mc GILLEM and MILLER, 1962; BALDWIN, 1963). A problem connected with that of meteorites regards the origin of tektites. The question of whether these strange objects are of terrestrial origin or not is still not entirely settled, and a large amount of literature has recently been published on the subject (O'KEEFE, 1960, 1963; SCHILLING, 1961; ADAMS and HUFFAKER, 1962; BARNESand PITAKPAIWAN, 1962; LOWMAN,1962; MARTIN,1962; SCHWARCZ, 1962; TAYLOR,1962; UREY, 1962b, 1963; CHAPMAN and LARSON, 1963; O'KEEFE and SHUTE, 1963). There seems to be a preponderance of opinions advocating that tektites originated from the moon, but this can by no means be accepted as final. Diapirical structures

The origin of diapirical structures on the earth, such as salt domes, plutonic intrusions, etc., has long been a great puzzle. It is generally contended that at least salt domes are due to differential rheid flow caused by the weight of the overburden. A similar origin is now also being postulated for various types of igneous intrusions by Gossow (1962b), whose contention is that gravity can provide sufficient energy to cause such phenomena to take place. Ordinarily, heavy material rises Earth-Sci. Rev., I (1966) 133-153

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only to such a level where density differentials cease, unless there are fissures. This view has recently been confirmed by RAMBERG (1963d), who made model experiments of gravity tectonics by means of a centrifuge (to provide a possibility of changing the gravity acceleration). He obtained many impressive pictures and was able to experimentally reproduce diapirical and other intrusive structures. By a discussion of the possible density differences, RAMBERCexplained many features of the geology of intrusive masses.

Some quesioons of volcanology Turning now to some specific questions of volcanology, we note that some new investigations of the heat flow in volcanic areas have been made by BANWELL (1963) and by DONALDSON (1962). A particular problem is that of the flow of lava in a crater, especially as related to the former's viscosity. Studies of this question have recently been published by YOKOYAMA (1961), MURASE (1962), and RITTMANN (1962a). In addition, RITTMANN (1962b) also made a new investigation of the mechanical processes involved in the activity of volcanoes.

Postglacial uplift Finally, we shall briefly review new investigations of the vertical movements in various parts of the world that are commonly ascribed to isostatic adjustment since the melting of the ice after the end of the last ice age. In this connection, CAPUTO (1962) published some tables for the deformation of various earth models, BROECKER (1962) postulated that during the uplift not only a rebound might take place but also a phase change (specifically between eclogite and basalt) in the layers in question. Further, TAKEUCHI (1963) made an analysis of the various possible timescales that are involved in the isostatic adjustment and their connection with data from satellite observations, marine transgressions, etc. Finally, it should not be overlooked that the view of isostatic adjustment as the cause of uplift has been severely criticized by LYUSTIKH (1963) who maintains that the vertical motions in Finland could be due to other effects.

CONCLUSION

Looking back over this review, it can be noted that progress is being steadily made towards an explanation of the earth's surface features in terms of a mathematicalphysical description of the phenomena. Because of the time-scales involved, difficulties still remain, but as more things are being measured and greater efforts are being expended in developing appropriate theories, the ultimate aim of providing an understanding of the mechanics of the earth in terms of basic physics and chemistry is gradually being approached. Earth-Sci. Rev., 1 (1966) 133 153

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