Successes and failures in geodynamics

Journal of Geodynamics 32 (2001) 3–27 www.elsevier.com/locate/jgeodyn

Successes and failures in geodynamics: from past to future Wolfgang R. Jacoby* Institut fu¨r Geowissenschaften, Johannes Gutenberg-Universita¨t Mainz, Germany Received 15 June 2000; accepted 26 April 2001

Abstract The evolution of Earth models is reviewed and the open questions and problems are highlighted. Generally, evolution of science was not linear, but proceeded in ‘‘steps’’ of paradigms; where old ones remained within useful limits. ‘‘Geodynamic hypotheses’’, while embedded into the general concepts of space and time, were often mutually exclusive and competing until the 1900s. Wegener’s concept of continental drift was the first successful globally unifying view, but it was discarded by most Earth scientists. The ‘‘real’’ change of paradigms did not come before mid-century through geophysical observations in paleomagnetism, seismology, bathymetry, seafloor geology and dating, leading to the ‘‘New Global Tectonics’’ of seafloor spreading and plate tectonics. Although real-time plate kinematics is now firmly established through direct geodetic observations from radio-astronomy and with the aid of artificial satellites, knowledge is incomplete and must be continually ‘‘updated’’. Quite a number of problems are not yet solved. These may, some day, lead to another change of paradigms, but certainly not back to ‘‘fixism’’. Problems are related to scale, frame of reference, and especially dynamics. However, plate tectonics is presently the most successful theory in the history of geology. Further developments will be driven by discoveries of apparent dilemmas, contradictions and paradoxes. # 2001 Elsevier Science Ltd. All rights reserved.

1. Introduction In early 1960s papers on continental shelves, it was suggested that more shelf research would resolve some of the puzzles, but the answers actually came from mid-ocean studies. Understanding the evolution of the Alpine–Mediterranean belt benefited through magnetic surveys of the Atlantic Ocean; California—Pacific magnetic anomalies; seamounts, guyots and coral reefs, etc., all were reconciliated through the seafloor spreading and plate tectonics pardigm bringing * Fax: +49-613-139-4769. E-mail address: [email protected] 0264-3707/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0264-3707(01)00026-6

4

W.R. Jacoby / Journal of Geodynamics 32 (2001) 3–27

together observations on a global scale! All of us, except the younger ones, have lived to see a change of paradigms in Earth science, from ‘‘fixist’’ to ‘‘mobilist’’. A paradigm, as defined by the Oxford Concise English dictionary, is a fundamental pattern underlying a theory and a change of paradigms is something like a revolution interrupting the more usual evolutionary change of scientific views. Such a change does not mean that ‘‘all problems’’ are solved, e.g. mobilism became accepted in spite of open problems as that of dynamics. The evolution of human understanding, similar to the evolution of life, is through failures of models (Popper, 1994). A string of successes and failures forms evolution. This has been clearly expressed by Alfred Wegener (his portrait Fig. 1) in his 1912 paper (translation, this vol.): ‘‘I

Fig. 1. Portrait of Alfred Wegener.

W.R. Jacoby / Journal of Geodynamics 32 (2001) 3–27

5

consider it ... necessary to replace the old hypothesis ... by the new one, because it appears to be more successful. The inadequacy of the old hypothesis has been demonstrated by its antithesis of the permanence of the oceans. ... I call the new idea a working hypothesis ... at least until it has been possible to prove ... the horizontal displacements ... by astronomical positioning ....’’ (Ironically, the astronomical positionings Wegener found supporting his views were erroneous; only half a century later VLBI, SLR and GPS began to measure the horizontal displacements.) This paper reviews the recent Earth science revolution in the context of the past and a look to the future. Glancing back to how we got to this point, reviewing the evolution of Earth models, and taking a look at the ‘‘failures’’ is also looking forward. It is an ambitious undertaking as knowledge is exploding and everything is ‘‘in flow’’. Missing important developments and important references is unavoidable. This cannot be more than an individual’s incomplete historical view, heavily influenced by personal interpretations; it does not do justice to some arguments that have been put forward. But I wish to provoke critical reflection of the developments occurring rapidly in an ever more hectic world. I begin with general ‘‘geodetic’’ and physical models as the more specific ‘‘geological’’ or Earth models developed within these general concepts. The review of geological models begins essentially with the era of Western science and briefly reviews several 19th century hypotheses up to the early 20th century with Wegeners continental drift and the ensuing discussion. Although often reviewed, the plate tectonics revolution is reiterated briefly, leading up to a discussion of its open problems of scale, reference and dynamics. A more general discussion and a look at the human and philosophical sides of our science conclude the paper.

2. Earth models 2.1. ‘‘Geodetic’’ models Earth in space and time was seen as basically flat, but seafaring people must have noticed some ‘‘curvature’’. Travelling, astronomy and human imagination widened the horizon toward a round Earth and Eratosthenes measured its radius. But ‘local’ coordinates are still Cartesian, e.g. for many practical geophysical tasks, though in principle, the change was total. With the Earth as a sphere at the centre of the world (Ptolemaic system), astronomical observations, e.g. of the planets, were awkward to describe, but if it is assumed that the Sun is at the centre (Copernican system), it becomes much more economic, elegant and ‘‘natural’’. The two coordinate systems only differ in their origin. The step towards the galaxy or the extent of the universe then became simple though in the latter step an ‘‘origin’’ could no longer be defined. Next, Keppler’s discovery of the ellipticity of the planets’ orbits, quite against his deeply rooted belief in the divine prefection of spheres and circles, led Newton to discover gravity and inertia, i.e. a dynamic model of a simple 1/r2-central force law (a direct consequence of Euclidian space geometry and gravitational flux conservation). Classical mechanics was born, but could not explain more precise astronomical observations, as the speed of light being independent from the direction of view, leading Einstein to the relativistic model of space–time. The classical theory is still a very good approximation to all we can see ‘‘close by’’. Classical physics suffered another blow, as the human horizon ‘‘extended’’ into the realm of the very small of particle and quantum

6

W.R. Jacoby / Journal of Geodynamics 32 (2001) 3–27

physics, to explain experiments contradicting classical physics. For geology, however, classical physics is generally sufficient and geologists are still struggling with quite another kind of complexity. Moreover, geology is a historical science and evolution is an essential feature of the whole universe stressing the importance of time. This became especially obvious when the evolution of life was perceived by Darwin, which succeeded the ‘‘model’’ of divine creation (still held by many). Paradigmatic changes resulted from widening of the human horizon. The old models remained adequate for ‘‘local’’ use. Some seem to fill human needs, but new paradigms give better descriptions for expanded human experience. I see no reason why this evolution will not continue into the future. 2.2. ‘‘Geological’’ models In very early times man imagined the world to be magic, animated. Earth was perceived as the provider of life, the big mother. Such a view has survived, e.g. in the Jewish–Christian religious traditions (e.g. Buber, 1950), together with myths of creation in 6 days. ‘‘Animated theories’’, exist till today, e.g. in ‘‘Gaia’’ (Lovelock, 1991). Generally held ‘‘western’’ scientific views are ‘‘materialistic’’ in the classical Greek tradition. They developed in the analytical mind through the widening of the human space and time horizon. We concentrate on physical theories, but they are extended to include the evolution of life and Earth. No exhaustive review is intended. Understanding how Earth works, requires looking at rocks, at oceans and continents, at mountain belts, at the forces shaping the Earth; studying the crust and mantle and dynamic processes interacting with the hydrosphere and atmosphere. It is interesting to investigate whether geological models evolve as the physical models, old models remaining useful or being totally abandoned. Up to the 1960s, many textbooks either ignored ‘‘geotectonic hypotheses’’ (e.g. Brinkmann, 1964) or treated them as such (Brinkmann, 1940). Holmes (1965) discusses them more seriously, but the situation was unsatisfactory as several were in competition, some in mutual diametrical opposition, with no convincingly discriminating observations. It was confusing and many observations seemed unrelated, there was no accepted unifying principle, and most hypotheses never reached the state of a theory. The dispute between the neptunists and plutonists is but mentioned; an impression of it is offered in the accompanying paper by Horn et al. (in press). Of the geotectonic models of global nature, Eduard Suess (1885) contraction hypothesis was related to cooling of the Earth (‘‘It is the differential breakdown of the Earth we watch’’), going back to Elie de Beaumont (1798–1877). It failed to explain the amount of lateral convergence in many mountain belts and particularly large scale divergence in rift zones and oceans. Opposite to contraction, expansion (Egyed, 1956, 1957), might explain some aspects of continental distribution as pieces of a total continental cover of a smaller Earth, but the extreme expansion finds no convincing physical mechanism and seems to contradict conservation of energy, angular momentum and total water (see Franck and Bounama, in press), as well as nappe and fold mountain belts. A ‘‘cosmological’’ decrease of the gravitational constant (Dirac, 1938) is generally not accepted. The 19th century produced a score of ideas and speculations on Earth dynamics, based on geology and physics. Many of these ideas vigorously survived beyond the mid-20th century. Polar wander proposed to explain past low-latitude glaciations, implies that the rotation axis moves

W.R. Jacoby / Journal of Geodynamics 32 (2001) 3–27

7

through the planet in response to moment of inertia changes. True polar wander would, however, not change the relative positions of continents and oceans (it may interact with continental drift, though; see below). Convection in the Earth was speculated upon by Hopkins (1839) and Fisher (1881) who tried to explain the Pacific basin as the trace of a deep depression from which the Moon originated by fission setting up convection in the Earth which also created the Atlantic. He also realized the importance of isostasy. Later on, mantle convection was strongly favoured by Holmes (1928) on the basis of radiogenic heating. Griggs (1939) investigated convection with mechanical models. Plutonism, in the early 19th century, influenced later ideas of oscillation (Haarmann, 1930) or undation (van Bemmelen and Berlage, 1954) which are also related to convection. Belousov’s (1967, 1970) much later model of oceanization by mafic intrusions into continental crust was a retreating ‘‘battle’’ against drift. Early 20th century forerunners of plate tectonics are models of subfluence and subduction, proposed particularly by Alpine geologists (Argand, 1924; Ampferer, 1925; Kraus, 1936) and by Schwinner (1920) as explanation of the Andes by Pacific subduction. Vening-Meinesz’ (1962) marine gravity studies made him suggest bending down of the lithosphere south of the Sunda arc. In these models mobilistic aspects were restricted to rather small movements and in that sense they were still essentially ‘‘fixist‘‘ in character. Their failure was stopping short of the consequences. ‘‘Current wisdom’’ of the time was that continents and oceans were permanent, and ‘‘mobilism’’ failed to convince most geologists for at least 5 decades since its clear formulation, but hard evidence kept the debate alive until new evidence and a convincing mechanism appeared. Early in the 20th century, Taylor (1910) suggested large horizontal movements of India and Africa relative to Eurasia to explain the Mediterranean–Himalayan fold belt, but stopping short of general continental drift and invoking ‘‘unphysical’’ large variations of Earth flattening. Wegener (in the same year) saw the far reaching consequence of continental drift when impressed by the similarity of the continental shelf margins around the Atlantic. But he did not begin to seriously test the hypothesis before two years later when he learnt of paleontological evidence requiring connections to come and go. The landbridges proposed to pop up and down, however, contradict isostasy, and Wegener concluded that the drift hypothesis was physically more ‘‘plausible’’. When Wegener (1912) presented his ideas, they created a storm of opposition, but he was not discouraged and collected more paleontological, paleoclimatological evidence (e.g. the Permian Gondwana glaciation) and continued to work out details; after the first comprehensive publication 1912 (see translation, this issue), he published his arguments and findings in his book ‘‘Die Entstehung der Kontinente und Ozeane’’ in four editions (1915, 1920, 1922, 1930; see also translations into English 1924, 1988). The evidence included erroneous geodetic observations of longitude change across the Atlantic suggesting that Greenland moved westward away from Europe with a speed of order 10 m/a, an example of wrong evidence for a correct theory! Another failure was the lack of a convincing mechanism for continental drift. In 1912 Wegener speculated that ocean ridges are the locus of upwelling mantle material and thus anticipated seafloor spreading (Jacoby, 1981), but he gave this up for continental rafts (having finite, though small strength) plowing through a ‘‘viscous’’ substratum of sima (‘‘hard’’, but with no finite strength). This was possibly motivated by his experience with flowing ‘‘rock’’ of ice during his 1912 Greenland expedition. Frontal mountain belts with respect to the direction of motion support the notion of rafts, but Wegener saw that the forces considered (‘‘polflucht’’ and Coriolis) were insufficient. Jeffreys (as

8

W.R. Jacoby / Journal of Geodynamics 32 (2001) 3–27

late as 1970) objected to continental drift arguing correctly that the float model contradicts longlasting existence of ocean islands and seamounts which should flow flat in geologically short times if the sima has no strength. Convection was considered by Wegener (in the 4th edition of his book) and favored by Holmes (1928), but it was not yet well enough understood. Wegener also speculated about a relation between polar wander and drift, indeed, he anticipated a cause–effect relation similar to that given later by Goldreich and Toomre (1969). The drift model failed because it was vague and the concepts of lithosphere and asthenosphere were not yet built in. Continental drift found only few supporters, e.g. Holmes (1928), Gutenberg (1930) and DuToit (1937). Looking back, it is easy to state that it was not continental drift that failed, but the physical mechanism envisaged. Wegener clearly saw this and believed, that if observations demonstrate the kinematics, this is not invalidated by missing explanation or mechanism and driving forces (‘‘the Newton for continental drift has not yet appeared’’). Wegener’s early death in Greenland around 1930 prevented him from solving the riddle. 2.3. The plate tectonics paradigm How plate tectonics replaced continental drift (e.g. Bird and Isacks, 1972; Cox, 1973; LePichon et al., 1976) will only be briefly summarized. Since the 1950s new evidence for moving continents came from paleomagnetism (Runcorn, 1962), showing that the relative continental locations with respect to the dipole field had changed through the Phanerozoic differently from continent to continent requiring their relative drift. The new reconstructions largely agreed with Wegener’s. The consistency of paleoclimatological and paleomagnetic evidence implied that the geomagnetic field had generally been dominantly dipolar and axial. Mountain chains were interpreted as belts of convergence (e.g. Carey, 1958). The fit of the Atlantic continental margins pointed out by Wegener was substatiated by Bullard et al. (1965). Since the 1950s marine geology and geophysics advanced rapidly and magnetic surveys with the new proton magnetometers discovered the magnetic stripes symmetric about the axes of the mid ocean ridges. Improved echosounders led to a detailed description of bathymetry. Ocean floor heat flow probes began to sample data at a rapid pace. Marine geology and marine reflection seismics began to decipher the geology of the seafloor and demonstrated that the sediment thickness grows with distance from the ridge axes (Hamilton, 1959). A new global view opened up unifying observations, e.g. of the extent, ruggedness and young age of the mid-ocean ridge system, with axial rift valleys and fracture zones, chains of volcanic islands, seamounts and guyots, and the basaltic nature of the oceanic basement, as well as the system of deep sea trenches accompanying island arcs or young mountain belts and zones of deep seismicity—after these observationss had been old puzzles (Darwin, 1842; Hess, 1946). The new unifying concept was seafloor spreading (Dietz, 1961; Vine and Matthews, 1963) also discovered by L.W. Morley of Canada. Oceanic lithosphere is built by upwelling and basaltic decompression melting of mantle material (McKenzie, 1984; McKenzie and Bickle, 1988), intrusion into dykes and extrusion into pillow flows. Lithosphere plates move apart from the ridge axes, thicken and subside from the axial young hot state, explaining the bathymetry and heat flow density in oceans (McKenzie, 1967; Sclater and Francheteau, 1970). The magnetic anomaly pattern originates from remanent magnetization of basalt acquired in a ‘‘randomly’’ reversing geomagnetic field, as the rocks cool through the Curie point and the blocking temperature. Dating of the reversal series (based on flood basalt chronology, e.g. in Hawaii and Iceland) led to a magnetic

W.R. Jacoby / Journal of Geodynamics 32 (2001) 3–27

9

time scale, substantiated by deep sea drilling; stratigraphic and radiometric ages were coordinated. The success of resolving the kinematics of seafloor spreading was sensational (e.g. LePichon, 1968). Seafloor spreading was corroborated by the volcanic chains as ‘‘hotspot’’ traces with a corresponding age succession, and by the identification of the ridge offsets as active transform faults supported by the earthquake focal mechanisms (Sykes, 1969; Isacks et al., 1968) and with traces in older crust as inactive fracture zones (Wilson, 1965; Molnar and Atwater, 1973). Seafloor spreading had to be ‘‘completed’’ by seafloor ‘‘annihilation’’ if the Earth does not expand. In the 1960s, the new global seismological network WWSSN showed that this occurs in the deep sea trenches. The ‘‘complete’’ ‘‘New Global Tectonics’’ or plate tectonics features: . the narrow belts of seismicity, identified as deforming plate margins and thus defining the plate boundaries (Isacks et al., 1968; Barazangi and Dorman, 1969); . the focal mechanisms of earthquakes along plate boundaries confirming the predicted relative plate movements; . the definition of the Wadati–Benioff zones (earlier discovered by Wadati, 1935; Benioff, 1954) as descending inclined lithosphere slabs deforming internally (subduction zones); sediments on oceanic plates are ‘‘scraped off’’ (or subducted) forming accretionary wedges and new continental crust; orogeny occurs in continental collision (Dewey and Bird, 1970; for a new review see Brown, in press); continental crust will not subduct because of buoyancy, and if convergence continues (India–Asia collision) thrusting, folding and extreme deformation ensue (indentation of heated ‘‘soft’’ Asia by cold hard India; Dewey and Bird, 1970; Tapponnier and Molnar, 1979); . the recognition of the Wilson cycle of opening and closing oceans (Wilson, 1966, 1968; Dietz and Holden, 1970); . the vertical rheological structure, discovered earlier through glacial isostasy, as an essential model aspect allowing lithosphere sufficient mobility on the asthenosphere for relative plate movements and other relaxation effects to vertical loading (Walcott, 1970). The essential aspect is the rheology difference (effective viscosity ‘‘contrast’’ by many orders of magnitude with a minimum in the asthenosphere) between the plates and the substratum. In this sense the plates can be considered rigid. The rigid plates move on a ‘‘sphere’’ by rotation, and this assumtion permits a great simplification of kinematic description and transfer of information to distant points. The needed parameters are a rotation vector ! and one of location r: the linear velocity is: !
r as demonstrated by interplate earthquake mechanisms around the North Pacific (McKenzie and Parker, 1976) and by the seamount chains (e.g. Hawaii-Emperor), as hotspot traces of (Wilson, 1963, 1973). Relative motion is predicted even where not definable, e.g. in a complex mountain range, from distant data. The important point is that the plate tectonics model can be tested and is ‘‘falsifiable’’ by geological observations in the widest sense, triggering a host of new aspects as the evolution of triple points, where three plates meet, and the geological consequences (McKenzie and Morgan, 1969; Atwater, 1975), response of the lithosphere–asthenosphere system to glacial, erosional and sedimentary (un)loadings (Walcott, 1970), redistribution of water, etc. The tests were basically ‘‘all’’ positive, the unifying principle revolutionized the Earth sciences and deepened understanding of the Earth. Global systems of plate movements could be derived from sets of geophysical and geological data as spreading rates, transform fault orientations and focal mechanisms (Minster et al., 1974;

10

W.R. Jacoby / Journal of Geodynamics 32 (2001) 3–27

Minster and Jordan, 1978). The change of paradigm was very fast as in all revolutions, the new model was established within a couple of years, or less. After long opposition to mobilism, nearly everybody ‘‘jumped on the bandwagon’’. Some aspects were added to the basic idea, e.g. the concept of exotic terranes proposed for the North American Cordillera with characteristic and distinct units fault- separated from neigbouring units (Irwin, 1972; Brown, in press) with ‘‘erratic’’ paleomagnetic poles not fitting contemporaneous poles from the North American craton (Irving et al., 1996); terranes have now been identified ‘‘everywhere’’ and are viewed as the building blocks of the complex continental crust. Plate tectonics concentrates on kinematic aspects without considering dynamics, analogous to Wegener’s view of continental drift (see above). While this was an advantage for the acceptance of the concept its dynamics remains an open question till today. This and several other problems lessened the euphoric mood. It is time to assess the problems and failures of plate tectonics. 2.4. Problems Open questions lead the way to the future of geodynamics. Plate tectonics has definitively replaced fixism, but it has shortcomings and is not the ultimate Earth theory. There are problems of scale, reference frame, dynamics, Earth evolution through time and features or processes beyond plates, i.e. extrinsic to plate tectonics sensu stricto. Some are related to idealization or approximation which can be modified easily. Others are serious and unsolved leading to substantial modifications of plate theory. 2.5. Scale Idealized rigid plates are defined by sharp boundaries drawn on a global scale. Plates are not truly rigid, and on the field scale, instead of sharp boundaries, multiple complex fault zones form broader non-rigid plate margins. But the approximations facilitate mathematical treatment. Internal plate deformations have been taken into account (Sykes and Sbar, 1973; Smith, 1977) and will more so in the future. Some aspects remain open. Active oceanic ridge crests are the culminations from which elevation falls off with a smooth square-root-of-age law and heatflow approximately with an inverse square-root-of-age law (Parsons and Sclater, 1977), as predicted from cooling of the lithosphere. Complications are details of volcanism, tectonics and hydrothermal circulation as well as systematic variations of bathymetry related to poorly known deeper mantle sources, or more locally, to junctions, e.g. with transform faults. The existence, or at least extent, of sub-crestal magma chambers is still debated. Very narrow crestal rifts occur on the fast East Pacific Rise (Kurras et al., 1998). Axial rift valley topography of slow ridges is not fully understood, probably caused by lagging melt rise through a cool thick lithosphere (Phipps-Morgan and Parmentier, 1987; Chen and Morgan, 1990); oblique slow spreading SW of Iceland with no rift valley but volcanic ridges (Jacoby, 1980a; White, 1997) is explained by ample supply of hot melt from the Iceland Plume. Questions also remain why the orientation of ridge axes is mostly normal to spreading. It is noteworthy that the prevailing symmetry of spreading as a consequence of symmetrical cooling and self-perpetuation of the weakest part helps self-organizing the plates. Asymmetry does, however, occur: is it generated by asymmetric heat supply, by ridge migration (Stein et al.,

W.R. Jacoby / Journal of Geodynamics 32 (2001) 3–27

11

1977) or by flow from plumes to ridge axes? There is a tendency of slower/faster spreading on the leading/trailing side (Jacoby, 1985): is it explained by thermal inertia, i.e. a migrating ridge leaving ‘‘behind’’ hotter material and slowing the cooling of the ‘‘hindside’’? Plate divergence can be studied in great detail in Iceland, but leaving questions open, too. There is no axial line or single fault, but two parallel active belts with a temporary ‘‘Hreppar’’ microplate between NAM and EUR. Is this related to westward migration of the axial rift versus the Iceland plume? Why is basalt produced in episodes from en echelon volcanic systems? Volcanic systems consist of central volcanoes (some of them with calderas) and fissure swarms which trend normal to the minimum compressive stress as the plates diverge. There is a clear change of fissure trend at 65 N. Does the Iceland plume generate the change in stress direction? There is generally no axial rift valley, but the gaping fissures are obvious (e.g. in the Northern Volcanic Zone, NVZ, Fig. 2). Graben formation occurs only rarely. Is magma supply the governing factor, e.g. in the famous [Thorn]ingvellir region (Fig. 3) and at the northern end of the Northern Volcanic Zone in Axarfjo¨rdur where the eastern slopes of the Tjo¨rnes peninsula belong to the most beautifull examples of stepping-down normal faults (Fig. 4). What is the mechanism producing very large basalt flood events every 104 a, or so, beautifully exposed in the deeply eroded fjords (Fig. 5). In other large igneous provices, e.g. the Deccan Traps, the eruption activity seems to have been similar. EW compressive focal mechanisms at the volcanic centre Ba´rdabunga appear to contradict plate divergence; do they represent roof collapse or cauldron subsidence in a deflating magma chamber? Are the abundent intermediate to acidic volcanic rocks and the large crustal thickness related to plume-ridge interaction? The nature of the Iceland crust is debated. The Tjo¨rnes Fracture Zone (TFZ) and the South Iceland Seismic Zone (SISZ) are not single transform faults. TFZ as a whole is in oblique transtension. In the SISZ, faulting occurs normal to the transform boundary, possibly representing transient bookshelf tectonics, but evidence for block rotation is debated. The above demonstrates that transform faults and fracture zones may strongly deviate from ideal single ‘‘straight’’ faults, even with anti-intuitive features. Transpression or transtension frequently occurs creating pressure ridges and pull-apart basins and volcanically active, ‘leaky’ transform faults. A recent gravity study by the author of the TFZ revealed that a pull-apart basin at the surface is accompanied by distinct magmatic ‘‘counterparts’’ from below. In plate convergence the trench axis bounds only the upper plate. Back-arc basins or Andeantype volcanic mountain ranges are formed above the descending plate with some asymmetry around the Pacific: basins over westward subduction and Andean mountains over eastward subduction. This difference is still a question, perhaps related to kinematics of the bounding plates (Chase, 1978). An explanation by rotational or tidal effects is physically not feasible (see below). Continental collision or orogeny is highly complex plate tectonics (Dewey and Bird, 1970; Brown, in press). It seems that arrivals of spreading ridges at trench have not received much attention although heating may induce much deformation and metamorphism (Pacific-type orogeny; Maruyama, 1997; Matsuda and Uyeda, 1971). It is not clear how slivers of oceanic crust and mantle are transported onto continental crust to form ophiolites (Maruyama, 1997). Obviously, however, spreading ridges cannot subduct as such; the normal spreading situation stops; rather the relative plate velocities must change. If a ridge axis arrives ‘‘obliquely’’ at a trench it will probably somewhat subduct (e.g. Chile Rise and South America trench). It seems that plate motion re-arrangement by trench-ridge interaction has not been systematically studied. When in western North America Kula plate subduction terminated and the predecessor East

12

W.R. Jacoby / Journal of Geodynamics 32 (2001) 3–27

Fig. 2. Fissures in Kelduhverfi, E of Tjo¨rnes after the Krafla rifting episode 1975–1984. The fissure shown widened a few decimeters in December 1975 and January 1978 (sketch by author, August 1989).

W.R. Jacoby / Journal of Geodynamics 32 (2001) 3–27

13

Fig. 3. Pingvellir, the old parliament site of Iceland. The gaping fissure in the foreground is Allmannagja´ following the western edge of the axial graben of the active Western Volcanic Zone; fissures contiue to the NNE and can be seen as gullies obliquely crossing the slope of the hyaloclastite mountain in the background (photograph by author, 1974).

Pacific Rise was ‘‘absorbed’’ by San Andreas Fault movement (Atwater, 1975) the Pacific plate may have ‘‘turned’’ more northerly. In other cases young lithosphere may be forced to subduct with complex consequences; the plate ‘‘conglomerate’’ between Asia and Australia holds several examples of such complexities including arc–arc collision. Orogens are different from one to the other: why has the thick crustal root of the Urals survived, but not in the Variscan belt? When does ‘‘delamination’’ (Bird, 1978) occur, when not? What is the role of continent or terrane collision? Why is, in oblique convergence or transpression, the relative motion partitioned into normal thrusting and strike slip earthquakes, as observed in the Sunda arc and in New Zealand and the Macquarie Ridge where the relative plate motion varies from normal to oblique subduction and to transform motion and divergence, involving oceanic subduction and continent–continent collision (Ruff et al., 1989)? It is becoming obvious that the simple model of normal convergence has to be generally replaced by transpression or time-varying phases of transpression resulting in increased complexity (Holdsworth et al, 1998). It emerges that continental crust or lithosphere is a complex ‘‘conglomerate’’ of terranes from past collisions and that the the general concept of lithosphere and asthenosphere is too simple in the process involving several ‘‘strong’’ and ‘‘weak’’ layers; evidence is growing for very low viscosities of the lower crust to 1017 Pa s (e.g. Klein et al., 1997). Usage of the terms ‘‘lithosphere’’ and ‘‘asthenosphere’’ has therefore become mixed. Finally, considering the time scale, it is not yet established how steady the motions are. Geophysical and geological data used to derive plate kinematics average 3–5 Ma periods (deMets et al., 1990). Space geodesy (GPS, SLR and VLBI) allow now relative velocities to be measured in intervals as short as years or less. This is observational proof of plate movements validating the earlier geophysical models and defining plate deformation. Plate interiors move at steady rates at

14

W.R. Jacoby / Journal of Geodynamics 32 (2001) 3–27

Fig. 4. Tjo¨rnes peninsula viewed across low-elevation Kelduhverfi. It represents the western shoulder of the Axarfjo¨rdur graben formed by recent, beautifully stepping down, normal faulting with little erosional effects; the scarps have been emphasized in this sketch (by author, 1986). The graben is the northern extension of the Northern Volcanic Zone (NVZ) of Iceland.

these scales, but the differences between geophysical and space–geodetic models indicate possible variations of unknown time behaviour. At plate boundaries movements are intermittent and episodic. Plate margin and plate interior strains will replace the assumption of rigidity. 2.6. The reference frame Until recently information on relative plate movements came from plate margins, where measurements suffer from margin deformation. VLBI, SLR and GPS measurements at plate interior points render the ensemble of observation points as frame, and the relative individual point movements in that frame. ‘‘Absolute’’ motions versus the mantle are not definable since the mantle is flowing itself. The hotspot frame of reference has been proposed on the assumption that plumes (or the ensemble of all of them) are fixed in the deeper mantle (Wilson, 1963), and their mutual relative movements seem slow, an order of magnitude slower than fast plates (Molnar and Atwater, 1973), consequently hotspot paths on slow plates will be erratic. Other definitions of ‘‘absolute’’ reference

W.R. Jacoby / Journal of Geodynamics 32 (2001) 3–27

15

Fig. 5. Pile of flood basalts exposed in the glacially excavated Berufjo¨rdur. The age range can be estimated by counting the single flows and multiplying by about 10,000 a (sketch by author, 1992).

frames have been based on ‘‘no net rotation’’, balancing torques between the mantle and the lithosphere, or some other minimizing principle, leading to similar results (e.g. Kaula, 1975). However, a remaining unsolved problem is whether the lithosphere as a whole has ‘‘net rotation’’, i.e. a residual component to the west (degree one component). Two recent estimates are 0.15 /Ma about a pole at 56 S, 84 E and 0.33 /Ma about 49 S, 65 E (maximum velocity: 1.7, Ricard et al., 1991; or 3.7 cm/a, Gordon, 1995). It has been argued that interaction with Earth rotation, e.g. by tidal drag, may be important (Bostrom, 1971), since e.g. present plate boundaries are preferentially oriented north–south. It can however, be shown (Gordon, 1995; Ranalli, 2000) that the torques required by the above magnitude of net rotation are 10 orders of magnitude greater than tidal torques or, alternatively, that the inferred asthenosphere viscosity would have to be that much lower than otherwise indicated. From paleomagnetic data through most of the Phanerozoic true polar wander (mantle moving relative to spin axis) was slow; Grohmann (1985) suggested that it occurred sub-normal to trenches and sub-parallel to the ridges inferring a dynamic dominance of subduction. During ‘‘short’’ periods large-scale polar wander is suggested (Marcano et al., 1999), perhaps by small changes in the inertia tensor, including a switch of axes of greatest moment, generated, e.g., by time dependent

16

W.R. Jacoby / Journal of Geodynamics 32 (2001) 3–27

convection currents or major meteorite impacts; however, one of the largest post-accretion impacts at the Cretaceous–Tertiary boundary (Alvarez et al., 1977) did not generate large polar wander; the relations between the Chicxolub impact, volcanism (Deccan traps) and biological extinction and evolution are debated, e.g. extensively by Courtillot (e.g. 1999). Finally, spatial relationships between deep seismic structures and surface features (continents or shields and oceans) have been suggested to contradict large relative surface motions over the mantle (Pavlenkova, 1995), favouring some form of moderate ‘‘fixism’’. Since direct measurements now demonstrate the motions, surface-depth relationships may reflect transient features in a dynamic system, not yet fully understood. 2.7. Plate dynamics The question why the plates move is still an open problem. In principle, plate movements are an integral part of thermal mantle convection (McKenzie, 1969), but how the plate movements and the deeper flow interact is unknown. It is not a real failure, but a weakness of the plate model. The gravity field will not tell, although long speculated (Runcorn, 1967) due to its ambiguity, especially due to competing effects of internal temperature and surface deformation (McKenzie, 1977). A few aspects of the mechanism are less uncertain than others, and of these we consider the forces ridge push (RP) and slab pull (SP), inherent in the density and rheological structure of divergent and convergent plate margins, supported by global force or torque balance models and increasingly by mantle tomography. However, as long as the interactions between mantle flow and surface plates are not understood, ideas on plate dynamics remain speculative. 2.7.1. Plate driving and resisting forces Ridge push (RP) is best described by considering a quasi-lithostatic pressure acting on the vertical sides of a block of lithosphere and asthenosphere between the ridge axis and downslope (Hales, 1969; Jacoby, 1970; 1980b; Frank, 1972). From the different pressure increments rgdz on both sides, RP is the difference between pressure-depth integrals as a positive force balance downslope from crest at 2.5 km to 5 or 6 km depth in the order of 1012 N/m. This can be considered active or passive and does not tell much about initiation of rifting. Plate divergence is accompanied by sub-axial melt ascent, and both lead to tensile stress concentration at the axis (Jacoby and Higgs, 1995). Lliboutry (1971) proposed mantle melt to rise along steep dykes. Others prefer pull from the distance, at least as the trigger. Quantitative RP estimates are not very sensitive to detailed assumptions on deep ridge structure (Jacoby, 1980b). Ridges are ‘‘passive’’ by being not fixed by deep upwelling and requiring ‘‘external’’ action for initiation, but generate ‘‘active’’ RP and move in a self-organizing plate system. A variant of RP may be plume push (PP) by (see below; Morgan, 1971). Driving through horizontal ‘‘radial’’ drag from plume outflow has not been demonstrated to be significant. But a super plume or a row of plumes may trigger continental break-up and generate RP and seafloor spreading. PP is non-negligible if a plume feeds strongly into a nearby ridge as in the case of Iceland. Slab pull (SP) seems an obvious force, since cool heavy slabs of lithosphere should sink (Elsasser, 1967; Isacks and Molnar, 1971). However, how is the vertical negative buoyancy transmitted to the horizontal lithosphere? Pieces of plate would sink vertically transmitting no horizontal force but generating downwelling and flow towards it possibly driving the adjacent plates. Alternatively, if a slab sinks vertically intact, it would ‘‘drive’’ trench and subjacent asthenosphere oceanward but

W.R. Jacoby / Journal of Geodynamics 32 (2001) 3–27

17

not the plate. A ‘‘stagnant’’ asthenosphere acts like a ‘‘roller‘‘ about which the plate moves down. In this case and if the slab is a stress guide, SP (=d
D

r
g; slab thickness d, depth of descent D; density excess
r; gravity g) may become larger than 10
RP, with no explicit influence of dip angle (because total buoyancy
longitudinal force component=constant). A transverse (vertical) component of motion, with oceanward trench migration, reduces RP (as inferred around the Pacific in the hotspot frame of reference; Hyndman, 1972), and a ‘‘suction force’’ SF on the ‘‘upper’’ plate results. The mutual importance of SP and SF depend on unknown mantle flow (Jacoby, 1973; 1976; Hager and O’Connell, 1978, 1981; Uyeda and Kanamori, 1979). Another unsolved problem is how subduction is initiated. Candidates are aging and local gravitational instability through phase transformations (basalt to eclogite), increased compresssive stress, lateral propagation of convergent movement or deep mantle flow. As the plates AFR and ANT grow by seafloor spreading, and the Pacific region is shrinking, the Atlantic will eventually have to take over the role of the present Pacific with activated margins (‘‘active’’: with subduction). ‘‘Rate-limiting’’ resistive forces, as bottom drag, opposite to plates being dragged by convection, prevent plates to be accelerated. The debate has not ended and recent data on seismic anisotropy in the mantle lithosphere, e.g. below the Canadian shield (G. Bokelmann, pers. commun., 2001), may suggest a driving drag from below. For the Pacific plate it seems small (Jacoby, 1970; Harper, 1975; Parmentier and Oliver, 1979). Other velocity limiting forces are friction at transform faults and trenches. Resistance against slabs penetrating the lower mantle is indicated by compressive focal mechanisms and a seismicity maximum above 660 km depth (Das et al., 2000), possibly generated by increase in effective viscosity and/or endothermic phase transitions depressing the phase boundary to greater depth generating buoyancy. Tomography suggests slab penetration into the lower mantle, possibly reaching the core-mantle boundary (Albere`de and Van der Hist, 1999, Van der Hilst and Ka´rason, 1999), but the evidence for penetration or accumulation of slab material is at least regionally variable (see below). Global force balance models have been studied extensively in the 1970s (e.g. Forsyth and Uyeda, 1975; Harper, 1975; Solomon et al., 1975; Richardson et al., 1976, 1979; Chapple and Tullis, 1977) with parameters fitted to observed plate kinematics and stress. Generally RP, SP and resistance at 660 km were found to be most important with no significant influence from plumes. Bottom drag seems to be resistant, but small. The 660 km resistance leaves only a fraction of the negative buoyancy for horizontal SP, leading to compressive stress prevailing in the plates (Mendiguren, 1971; Sykes and Sbar, 1973; Wiens and Stein, 1985; Richardson et al., 1976; Zoback et al., 1989). For a review of force balance models see Jacoby (1985). The picture is one of self organized plates driven by boundary processes. The role of continents remains ambiguous, self-propelling or drag increasing. Experimental heat sources floating on a fluid propell themselves by leaving hot, slightly elevated material behind them (similar to RP), but continental ‘‘heaters’’ or ‘‘heat blankets’’ within rigid plates are not ‘‘free’’ to move, though perhaps leaving a ‘‘hot trail’’ behind. On the other hand, seismology, e.g. tomography, indicates cool thick lithospheric ‘‘keels’’ of cratons, and presently the slow continental plates seem to experience a large ‘‘continental bottom resistance’’; during Cretatious, however, continental plates were fast, probably subject to SP, suggesting that it is most important. 2.7.2. The convection aspect That plate dynamics is a form of thermal mantle convection does not say in which form it takes place (although the force balance models suggest that deep flow does not fully govern the kinematics

18

W.R. Jacoby / Journal of Geodynamics 32 (2001) 3–27

of plates); early computers permitted only homogeneous steady-state models which influenced the ideas. Such cells organize themselves by space limitations, and spherical harmonics of degree 4 may dominate (Busse, 1983; Busse and Riahi, 1982). Lateral walls affect the flow and a large deep subducting plate in the western Pacific could dominate the whole degree 4 flow pattern. Degree 2 patterns have been inferred from plate boundaries (Pavoni and Mu¨ller, 2000), mantle tomography (Laske and Masters, 1996), the geoid and hot spot distributions (Seidler et al., 1999), but their significance is uncertain. Indeed, the mutual influence of deeper mantle flow and the plates remains an open problem; the recent history of ideas should make us cautious. Whether plate boundary forces or mantle convection drive the plates, hinges on where the gravitational potential is concentrated, all over the mantle or in the plates? If plates are simply the top of cells, why should plate boundaries migrate? If the plates drive the mantle flow, e.g. in the asthenosphere (Parmentier and Oliver, 1979; Harper, 1975, 1978) or mantle-wide return flow (Hager and O’Connell, 1978, 1981), it is governed by moving surface boundary conditions and internal phase changes; there may be no simple convection cells at all. Mantle convection and plate force models may be reconciled in high Rayleigh number convection modelling with non-linear rheology (including strength of plates) and increased spatial resolution (Trompert and Hansen, 1998; Tackley, 2000); chaotic, boundary layer dominated flow results. Partition of poloidal and torroidal occurs if plates are ‘‘rigid’’; self-propelling RP and SP as well as self-organization are allowed and the boundary layers penetrate the mantle as cold lithospheric slabs (replenishing the lower reservoir: D00 or ‘‘lava lamp model’’), while plumes advect heat into the asthenosphere which, in turn, is drained by lithosphere formation and subduction. This drainage alone would ‘‘consume’’ the MORB reservoir in 100 Ma, or so. How much chemical exchange occurs between the penetrating plumes and slabs and the middle mantle (till the system is interrupted by a megaplume; see below)? Indeed, mixing and chemical heterogeneity remain big problems. This scenario creates also temporal differences, e.g. between the Pacific and African hemispheres. In 3-D convection models (Trubitsyn et al., 1999) continents move toward downwelling, come to rest and reverse the flow to upwelling causing continental break-up. This mimics the Wilson cycle. Continents influence convection because they obstruct heat escape and resist deep subduction due to negative buoyancy. In spite of the above positive model arguments, plumes (proposed originally to explain ‘‘hotspots’’ or places of ‘‘anomalous’’ volcanism, but some volcano chains have no clear age succession, e.g. Easter Islands) remain enigmatic, and their very existence is debated. This is mostly so because plumes have evaded clear seismological evidence till recently, and even now it is weak (Bijward and Spakman, 1999). Model plumes have a ‘‘head’’ and stem structure; if a head hits the lithosphere a large igneous province (LIP) of flood basalts may be created in a very short time span (e.g. Deccan traps of India; White and McKenzie, 1995) and trigger continental break-up (Cox, 1978). Deep megaplumes may generate mega-swells and clusters of secondary plumes in the upper mantle (Davaille, 1999). Plume-generated basalts (e.g. ocean island basalts OIB) are geochemically distinct from MORB. Recent geochemical and tomographic results have led to the so called ‘‘lava lamp model’’ (Kellog et al., 1999; van der Hilst et al., 1997; van der Hilst and Ka´rason, 1999) where a deep mantle reservoir is geochemically slightly different from the rest. Recent convection modelling also demonstrates a complex, strongly time-dependent behavior which cannot be literally transferred to Earth history. Phases of slow, quasi-steady state evolution may be followed by drastic ‘‘unpredictable’’ revolutions. Plumes may be triggered by ‘‘downwelling’’ slabs (Schaeffer and Manga, 2001). The time-variability is driven by boundary layer

W.R. Jacoby / Journal of Geodynamics 32 (2001) 3–27

19

instabilities. Maruyama (1994), based on a mantle tomographic solution of Fukao et al. (1994), proposed a model of ‘‘plate-plume tectonics’’ where metastable mass accumulations (e.g. at 660 km depth: ‘‘megaliths’’, Ringwood and Irifune, 1988) resulting from massive subduction (e.g. Pangea–Panthalassa stage) may plunge to the core-mantle boundary in a ‘‘big flush‘‘ (Brunet and Machetel, 1989; Yuen et al., 1994), upsetting the system and triggering subsequent megaplumes from instabilities accumulated in the deep mantle. Reorganization of the plate movements would follow. On a shorter time scale, plumes vary in discharge through time (Iceland plume: White, 1997; Vogt, 1971). These phenomen are not yet clearly understood. Recent good overviews of mantle processes, convection, plates and plumes can be found in Davies (1999) and Jackson (1998). By no means do we fully understand mantle dynamics. Advances in modelling and mantle tomography will narrow the range of possible models, though poor seismic illumination in large mantle regions restricts resolution of structure, as demonstrated by H.P. Bunge (pers. commun., 2000; see also Bunge et al., 1998), calling for ocean bottom seismometry. 2.7.3. Further problems This section closes with a reminder of some additional points of persistent uncertainty or ignorance without detailed discussion. The role of life in the Earth’s dynamic evolution attracts increasing interest, e.g. in the Gaia concept; life has influenced Earth, but how much? Life has adapted to Earth to an amazing extent and exists deep in the oceans and the lithosphere. The greenhouse effect has buffered the surface temperature, as solar radiation increased by 20%. Is this true co-evolution of life and Earth? Life affects the atmosphere, hydrosphere, lithosphere and, through subduction, also the mantle. How sensitive is the mantle to small changes of the input e.g. of H2O, CO2, SO2, biogenic sediments? The role of water is illustrated by Venus, which has a hot dry atmosphere and no plate tectonics. Biological evolution itself is an evolving science, feeding back on the question of coevolution. Biogeology dealing with the system of Earth and life is critical for future sustaining the human habitat. We are now a successful product of coevolution, but how long? Species not prepared by co-evolution with man were wiped out when man arrived, e.g. in North America or the Pacific islands. Incidentally, Earth l evolution models have also been influenced by biological evolution ideas, especially when Wegener proposed continental drift to replace the land bridges called for by paleontologists. Large extinctions affected not only the atmosphere, hydrosphere and life, but also geodynamic processes (Glikson, in press). Nicolaysen (1985) proposed a ‘‘bistable’’ Earth model consisting of poroelastic shells permitting fluid transfer between depth and surface connected with inflations and deflations of the mantle, modulating the crystallisation of the inner core through stress variations. Storetvedt (1997, 1999) advocates a global view called ‘‘wrench tectonics’’ which accepts minor continental or plate movements, but not large ones. This view seems similar to Belousov’s (1967, 1970) hypothesis of oceanization and considers also changes in moment of inertia and polar wander. One should definitely look beyond the mainstream, but the consistency and wealth of current geodetic and geological–geophysical observations on the large-scale plate movements cannot be debated away, and any new theory of the Earth will have to incorporate them.

20

W.R. Jacoby / Journal of Geodynamics 32 (2001) 3–27

3. Summary and conclusions The concluding discussion concentrates on general aspects. Summarizing we note: . Progress of ‘‘geodetic’’ and ‘‘geological’’ Earth models differed, as none of the latter explains ‘‘all geological observations‘‘, while space and time are fundamental. Geological models had no straightforward evolution, often competing with eachother, due to the confusingly complex observations; in contrast, physics tries to idealize and tends to ‘‘unify’’. Models have the paradox aim of explaining complexity and remaining simple. . Earth models have evolved from mythical to scientific, from the belief in divine creation, and up to the 18th century, from catastrophic to more ‘‘civilized’’ processes. The science of Earth began with actualism. Geological models evolved from static to dynamic, via the hypothesis of continental drift to plate tectonics which triggered today’s ‘‘turbulent’’ growth of the earth sciences, but does not explain ‘‘everything’’. . No doubt, plate tectonics has passed many critical tests, so what kind of failures are conceivable? A total failure, a direct contradiction to observations, requiring its dismissal, a return to fixist models are out of the question. . The plate boundary force model is a reasonable, but incomplete approach to plate dynamics. Thermal mantle convection in all its aspects is not yet fully understood. Ever improving computer power is beginning to merge plates with mantle flow. . Future modifications and additions will involve the non-rigidity of plates, continental tectonics, the driving mechanism, mixing and evolutionary processes, co-evolution of Earth and life, plumes and impacts. . Mantle tomography is beginning to resolve slabs and plumes, hampered however by illumination problems. Modelling, tomography and new assessements of geological history will clarify the nature of different kinds of plumes. The deep mantle including the ‘‘lava lamp model’’ and the D00 layer are being assessed. . New observations will answer some questions and pose new ones. Future satellite gravity missions will constrain stresses and driving mechanisms; geochemistry and geobiology will open windows into the transport and mixing processes, reservoirs of mantle convection and evolutionary trends. One of the great challenges is to bring all the different fields of Earth sciences together, especially geophysics and geochemistry. . Theory will progressively unify mantle physics and chemistry, including life, and external processes. The enormous complexity of Earth ‘‘reality’’ is a serious problem, but as models must be ‘‘simple’’ in principle, it is challenging to reconcile complexity and simplicity. Models are tools for deeper understanding, though not proving, theories. The recent change of paradigms had the characteristics of an unstable system with metastable states; the change began gradually and grew ‘‘exponentially’’, but then slowed down approaching a new level (time behavior as arc tan t/t where t was a matter of only few years) At this new level, we must again search for failures and shortcomings leading to improvements and new breakthroughs. Another revolution may not be in sight, but revolutions are never in sight a long way in advance. Yet mantle plumes may lead to merging views on ‘‘vertical’’ and ‘‘horizontal’’ tectonics, and plate–plume interaction comes into view. ‘‘Self-consistent’’ boundary layer convection with

W.R. Jacoby / Journal of Geodynamics 32 (2001) 3–27

21

chaotic behavior may be better understood by simple physical laws. Material properties become more comprehensible by quantum physics. Better predictions, e.g. of earthquakes, will result. By no means have we firm final knowledge of the mantle processes yet. Irrespective of more optimistic views on some ‘‘very convincing models’’, dynamics is more complex than e.g. potential fields, where fitting data is only a necessary, not a sufficient condition for a ‘‘true’’ model. The ‘‘ultimate theory’’ is an aim approached by a tedious iterative process. The aim is not the ‘‘perfect’’ formal description of all the complexities, but a comprehensive view open to new observations, necessary because we are selective and never see ‘‘all’’. Time has a human dimension; biological evolution takes many generations and scientific evolution takes time to think. Exaggerated competitiveness may be good for science, but the best? Science is a deeply human endeavor for truth and it evolves with ideas and their ultimate falsification. The speed of today’s technical evolution by far exceeds the human biological evolution, endangering homo sapiens and the human episode on Earth (though co-evolution of life and Earth will continue). Science moves between the ‘‘poles’’ of free uninhibited play of thought and the most rigorous control by measurement and calculation: play and selection—evolution. In spite of what politicians may think, one cannot plan ideas. Main stream thinking and fashions in science exist, as they make life simpler, but scientists must be open for new data and new ideas. If new ideas are against current models, all evidence must be weighed: new evidence may or may not be an unexpected flash of light. To take it seriously means to think it to the end. It requires courage to think against current wisdom and to question it in public. And it requires belief in thriving for truth although knowledge of it remains preliminary. Alfred Wegener was a good example of a scientist under pressure from opposing current wisdom. He had good evidence and clear model and scaling ideas. This made him steadfast against massive opposition. He was more right than most of his critics. Later he became perhaps too defensive in his arguments as published in the four editions of his book ‘‘Die Entstehung der Kontinente und Ozeane’’ (1915–1930) after he had first (1912) confidently presented his ideas and arguments (see translation, this volume). The ‘‘human’’ conflict between necessary specialization and vision of the whole has no standard solution. Progress requires teams of scientists where individuals must be specialists and cooperating generalists with perspective of the geological problems and feeling for physics, with ‘‘physical thinking’’ and ‘‘geological thinking’’, with a ‘‘feeling’’ for the general hierarchy or weight of evidence. If this is ignored the picture gets distorted. Science is a community effort requiring communication and modesty, not the attitude to be the only person in the world who knows the truth. Of all these qualities Alfred Wegener gave clear evidence. Finally, what Karl Popper (1994) said about ‘‘Darwinism’’ is also true of plate tectonics Translated from German, ‘‘Darwinism’’ replaced by ‘‘plate tectonics’’): ‘‘Although we do our best in science to find out the truth, we are conscious of the fact that we can never be sure to have found the truth. ... We have learnt not to be disappointed if our scientific theories are disproven. ... We know that our scientific theories always remain hypotheses, but that in many cases, we can find out whether or not a new hypothesis is superior to an old one. ... It can be useful to speculate about the possible limitations of plate tectonics, for we should always search for possible alternatives to the dominating theory. ... I suspect that plate tectonics is correct, even on the level of new scientific discoveries, I suspect, but I do not know. ...’’

22

W.R. Jacoby / Journal of Geodynamics 32 (2001) 3–27

Acknowledgements I wish to thank Helmut Gebrande who originally pointed out to me Wegener’s 1912 paper. This set me on the path of the history of geodynamics. Alik Izmail Zadeh suggested to speak about ‘‘The plate tectonic paradigm: Successes and failures’’ at the Zonenshain Conference in Moscow 1998. It was a challenge and invited continuation along these lines on evolution of Earth and the image of Earth. Many discussions ensued which formed my views. Giorgio Ranalli read a draft of the paper and gave me constructive criticism (but the responsibility for this paper is mine). References Albarede, F., Van der Hilst, R.D., 1999. New mantle convection model may reconcile conflicting evidence. Eos Trans. Am. Geophys. Un. 45, 535–539. Alvares, W., Arthur, M.A., Fischer, A.G., Lowrie, W., Napoleone, G., Premoli Silva, I., Roggenthen, W.M., 1977. Upper Cretaceous–Paleocene magnetic stratigraphy at Gubbio, Italy. V. Type section for the late Cretaceous– Paleocene geomagnetic reversal time scale. Geol. Soc. Am. Bull. 88, 383–389. Ampferer, O., 1925. U¨ber die Kontinentalverschiebungen. Die Naturwissenschaften 13, 669–675. Argand, E., 1924. La tectonique de l’Asie. 13th Int. Geol. Congr. 1922, Compt. Rend., 171–372, Brussels. Atwater, T., 1975. Implications of plate tectonocs for the Cenozoic tectonic evolution of western North America. Geol. Soc. Am. Bull. 81, 3513–3535. Barazangi, N., Dorman, J., 1989. World seismicity map compiled from ESSA Coast and Geodetic Survey epicenter data 1961–1967. Bull. Seis. Soc. Am. 59, 380–389. Belousov, V.V., 1967. Some problems concerning the oceanic earth’s crust and upper mantle evolution. Geotectonics 1, 1–6. Belousov, V.V., 1970. Against the hypothesis of ocean-floor spreading. Tectonophysics 9, 489–511. Benioff, H., 1954. Orogenesis and deep crustal structure—additional evidence from seismology. Bull. Geol. Soc. Am. 66, 385–400. Bijwaard, H., Spakman, W., 1999. Tomographic evidence for a narrow whole mantle plume below Iceland. Earth Planet. Sci. Lett. 166, 121–126. Bird, J.M., Isacks, B., 1972. Plate Tectonics. Selected Papers from the Journal of Geophysical Research, AGU, Washington (republished enlarged: 1980). Bird, P., 1978. Initiation of intracontinental subduction in the Himalaya. J. Geophys. Res. 83, 4975–4987. Bostrom, R.C., 1971. Westward displacement of the lithosphere. Nature 234, 536–538. Brinkmann, R., 1940. Emanuel Kayser’s Abriss der Geologie, 1. Bd. Enke, Stuttgart. Brinkmann, R., 1964. Lehrbuch der Allgemeinen Geologie. Enke, Stuttgart. Brown, M. (2001). From microscope to mountain belt: 150 years of petrology and its contribution to understanding the tectonics of orogens. J. Geod. 32, 115–164 Brunet, D., Machetel, P., 1989. Large-scale tectonic features induced by mantle avalanches with phase, temperature and pressure lateral variations of viscosity. J. Geophys. Res. 103, 4929–4945. Buber, M., 1950. Israel und Palestina. Artemis Verl, Zurich. Bullard, E.C., Everett, J.E., Smith, A.G., 1965. The fit of the continents around the Atlantic. In: Blackett, P.M.S., Bullard, E.C., Runcorn, S.K. (Eds.), A Symposium on Continental Drift. Phil. Trans. Soc. London, A, 1088, 41–51. Bunge, H.P., Richards, M.A., Lithgow-Bertelloni, C., Baumgardner, J.R., Grand, S.P., Romanowics, B.A., 1998. Time scales and heterogeneous structure in geodynamic models. Science 280, 91–95. Busse, F.H., 1983. Quadropole convection in the lower mantle? Geophys. Res. Lett. 10, 285–288. Busse, F.H., Riahi, N., 1982. Patterns of convection in spherical shells. Part 2. J. Fluid Mech. 123, 283–301. Carey, S.W., 1958. A tectonic approach to continental drift. In Continental Drift: A Symposium, U. Tasmania, 117– 355. Chapple, W.M., Tullis, T.E., 1977. Evaluation of the forces that drive the plates. J. Geophys. Res. 83, 1969–1984. Chase, C.G., 1978. Extension behind island arcs and motion relative to the hot spots. J. Geophys. Res. 82, 5385–5387.

W.R. Jacoby / Journal of Geodynamics 32 (2001) 3–27

23

Chen, Y., Morgan, W.J., 1990. A nonlinear rheology model for the mid-ocean ridge axis topography. J. Geophys. Res. 95, 17583–17604. Courtillot, V., 1999. Evolutionary Catastrophes: The Science of Mass Extinction. Cambridge University Press, Cambridge. Cox, A. (Ed.), 1973. Plate Tectonics and Geomagnetic reversals. Freeman, San Francisco. Cox, K.G., 1978. Flood basalts, subduction and the break-up of Gondwanaland. Nature 274, 47–49. Darwin, C., 1842. The Structure and Distribution of Coral Reefs. Smith Elder & Co, London. Das, Sh., Scho¨ffel, H.J., Gelbert, F., 2000. Mechanism of slab thickening near 670 km under Indonesia. Geophys. Res. Lett. 27, 831–834. Davaille, A., 1999. Simultaneous generation of hotspots and superswells by convection in a heterogeneous planetary mantle. Nature 402, 756–760. Davies, G.F., 1999. Dynamic Earth: Plates, Plumes and Mantle Convection. Cambridge University Press, Cambridge. DeMets, C., Gordon, R.G., Argus, D.F., Stein, S., 1990. Current plate motions. Geophys. J. Int. 101, 425–478. Dewey, J.F., Bird, J.M., 1970. Mountain belts and the New Global Tectonics. J. Geophys. Res. 75, 2625–2647. Dietz, R.S., 1961. Continent and ocean evolution by spreading of the sea floor. Nature 190, 854–857. Dietz, R.S., Holden, J.C., 1970. Reconstruction of Pangea: Breakup and dispersion of continents, Permian to Present. J. Geophys. Res. 75, 4939–4956. Dirac, P.A.M., 1938. A new basis for cosmology. Proc. Roy. Soc. London 165, 199–208. Du Toit, A.L., 1937. Our Wandering Continents. Oliver & Boyd, London. Egyed, L., 1956. The change of the earth’s dimensions determined from paleogeographical data. Geofis. Pur. Appl. 33, 42–48. Egyed, L., 1957. A new dynamic conception of the internal constitution of the earth. Geol. Rundschau 46, 101–121. Elsasser, W.M., 1967. Convection and stress propagation in the upper mantle. Princeton University Tech. Rep. 5, 15 June. Fisher, O., 1881. Physics of the earth’s crust. MacMillan, London. Forsyth, D., Uyeda, S., 1975. On the relative importance of the driving forces of plate motion. Geophys. J. 43, 163–200. Franck, S., Bounama, C. (2001). Global water cycle and earth’s thermal evolution. J .Geod. 32, 231–246. Frank, F.C., 1972. Plate tectonics, the analogy with glacier flow, and isostasy. In: Carter, N.L., Rayleigh, C.B. (Eds.) Flow and Fracture of Rocks. AGU Monograph 18, 285–292. Fukao, Y., Maruyama, S., Obayashi, M., Inonue, H., 1994. Geologic implications of the whole mantle P- wave tomography. J. Geol. Soc. Japan 100, 4–23. Glikson, A.Y. (2001). The astronomocal connection of crustal evolution: Post-LHB mega-impact rates, origin of sialic nuclei and episodic perturbation of plate tectonic patterns. J. Geod. 32, 205–229. Goldreich, P., Toomre, A., 1969. Some remarks on polar wandering. J. Geophys. Res. 74, 2555–2569. Gordon, R.G., 1995. Present plate motions and plate boundaries. In: Ahrens, T.J. (Ed.), Global Earth Physics, Reference Shelf 1. Am. Geophys. Un, Washington, pp. 66–87. Griggs, D.T., 1939. A theory of mountain building. Am. J. Sci. 237, 611–650. Grohmann, N., 1985. Drift tectonics—the fundamental rythm of crustal drift and deformation. Geol. Rundschau 74, 267–310. Gutenberg, B., 1930. Hypotheses on the development of the earth. J. Wash. Acad. Sci. 20, 17–25. Haarmann, E., 1930. Die Oszillationstheorie. Enke, Stuttgart. Hager, B.H., O’Connell, R.J., 1978. Subduction zone dip angles and flow driven by plate motion. Tectonophysics ?, 111–133. Hager, B.H., O’Connell, R.J., 1981. A simple global model of plate dynamics and mantle convection. J. Geophys. Res. 86, 4843–4867. Hales, A.L., 1969. Gravitational sliding and continental drift. Earth Planet. Sci. Lett. 6, 31–34. Hamilton, E.L., 1959. Thickness and consolidation of deep-sea sediments. Bull. Geol. Soc. Am. 70, 1399–1424. Harper, J.F., 1975. On the driving forces of plate tectonics. Geophys. J. R. Astron. Soc. 40, 465–474. Harper, J.F., 1978. Athenosphere flow and plate motions. Geophys. J. R. Astron. Soc. 55, 87–110. Hess, H.H., 1946. Drowned ancient islands of the Pacific basin. Am. J. Sci. 244, 722–791. Holdsworth, R.E., Strachan, R.A., Dewey, J.F. (eds.), 1998. Continental Transpressional and Transtensional Tectonics. Geol. Soc. Spec. Publ. No. 135, Geol. Soc. London.

24

W.R. Jacoby / Journal of Geodynamics 32 (2001) 3–27

Holmes, A., 1928. Radioactivity and earth movements. Trans. Geol. Soc. Glasgow 18, 559–606. Holmes, A., 1965. Principles of Physical Geology, 2nd ed.. Nelson, London. Hopkins, W., 1839. Researches in physical geology. Roy. Soc. London Philos. Trans. 129, 381–385. Horn, S., Kreher-Hartmann, B., Heide, K. (2001). Melting experiments and field work on Kormı´ Hu`rka volcano, Bohemia, by Johann Wolfgang Goethe. J. Geod. 32, 77–97. Hyndman, R., 1972. Plate motions relative to the deep mantle and the deveolpment of subduction zones. Nature 238 (5362), 263–265. Irving, E., Wynne, P.J., Thorkelson, D.J., Schiarizza, P., 1996. Large (1000–4000 km) northward movements of tectonic domains in the northern Cordillera, 83–45 Ma. J. Geophys. Res. 101, 17901–17916. Irwin, W.P., 1972. Terranes of the western Paleozoic and Triassic belt in the southern Klamath Mountains, California. US Geol. Surv. Prof. Paper 800-C, 103–111. Isacks, B., Molnar, P., 1971. Distribution of stresses in the descending lithosphere from a global survey of focal mechanism solutions of mantle earthquakes. Rev. Geophys., Space Phys. 9, 103–174. Isacks, B., Oliver, J., Sykes, L.R., 1968. Seismology and the New Global tectonics. J. Geophys. Res. 73, 5855–5899. Jackson, I.N.S. (Ed.), 1998. The Earth’s Mantle: Composition, Structure and Evolution566. Cambridge University Press, Cambridge. Jacoby, W.R., 1970. Instability in the upper mantle and global plate movements. J. Geophys. Res. 75, 5671–5680. Jacoby, W.R., 1973. Model experiment of plate movements. Nature Phys. Sci. 242, 130–134. Jacoby, W.R., 1976. Paraffin model experiment of plate tectonics. Tectonophysics 35, 103–113. Jacoby, W.R., 1980. Plate sliding and sinking in mantle convection and the driving mechanism. In: Davies, P.A., Runcorn, S.K. (Eds.), Mechanisms of Continental Drift and Plate Tectonics. Academic Press, London, pp. 159–172. Jacoby, W.R., 1980a. Morphology of the Reykjanes Ridge crest near 62 N. J. Geophys. 47, 81–85. Jacoby, W.R., 1981. Modern concepts of earth dynamics anticipated by Alfred Wegener in 1912. Geology 9, 25–27. Jacoby, W.R., 1985. Theories and hypotheses of global tectonics. In: Fuchs, K., Soffel, H. (Eds.), Landolt-Bo¨rnstein. Numerical Data and Functional Relationships in Science and Technology V/2b. Springer, Heidelberg, pp. 298–369. Jacoby, W.R., Higgs, B., 1995. On the rifting dynamics of plate divergence and magma accumulation at oceanic ridge axes. Pure Appl. Geophys. 145, 505–521. Jeffreys, H., 1970. The Earth, 5th Edition. Cambridge University Press, London. Kaula, W.M., 1975. Absolute plate motions by boundary velocity minimization. J. Geophys. Res. 80, 244–248. Kellog, L.H., Hager, B., van der Hilst, R.D., 1999. Compositional stratification in the deep mantle. Science 283, 1881– 1884. Klein, A., Jacoby, W.R., Smilde, P., 1997. Mining-induced crustal deformation in northwest Germany: modeling the rheological structure of the lithosphere. Earth Planet. Sci. Lett. 147, 107–123. Kraus, E.C., 1936. Der Abbau der Gebirge, I. Der alpine Bauplan. Borntraeger, Berlin. Kurras, G.J., Edwards, M.H., Fornari, D.J., 1976. High-resolution bathymetry of the East Pacific Rise axial summit trough 9 4900 510 N: a compilation of Alvin scanning sonar and altimetry data from 1991–1995. Geophys. Res. Lett. 25, 1209–1212. Laske, G., Masters, G., 1996. Constraints on global phase velocity maps by long-period polarization data. J. Geophys. Res. 101, 16059–16075. LePichon, X., 1968. Sea-floor spreading and continental drift. J. Geophys. Res. 73, 3661–3697. LePichon, X., Francheteau, J., Bonnin, J., 1976. Plate Tectonics. Devel. Geotecton., 6. Elsevier, Amsterdam. Lliboutry, L.A., 1971. Sea floor spreading, continental drift, and lithosphere sinking with an asthenosphere at melting point. J. Geophys. Res. 76, 1433–1446. Lovelock, J., 1991. Gaia—The Practical Science of Planetary Medicine. Gaia Books Ltd, London. Maruyama, S., 1994. Plume tectonics. J. Geol. Soc. Japan 100, 24–49. Maruyama, S., 1997. Pacific-type orogeny revisited: Miyashiro-type orogeny proposed. The Island Arc 6, 91–120. Matsuda, T., Uyeda, S., 1971. On the Pacific-type orogeny and its extension of the paired belts concept and possible origin of marginal seas. Tectonophysics 11, 5–27. McKenzie, D.P., 1967. Some remarks on heat flow over ridges. J. Geophys. Res. 72, 6261–6273. McKenzie, D.P., 1969. Speculations on the consequences and causes of plate motion. Geophys. J.R. Astron. Soc. 18, 1–32.

W.R. Jacoby / Journal of Geodynamics 32 (2001) 3–27

25

McKenzie, D.P., 1984. The generation and compaction of partially molten rock. J. Petrol. 21, 713–765. McKenzie, D.P., 1977. Surface deformation, gravity anomalies, and convection. Geophys. J. Roy. Astr. Soc. 48, 211– 238. McKenzie, D., Bickle, M.J., 1988. The volume and composition of melt generated by extension of the lithosphere. J. Petrol. 29, 625–679. McKenzie, D., Morgan, W.J., 1969. Evolution of triple junctions. Nature 224, 125–133. McKenzie, D.P., Parker, D.C., 1967. The North Pacific: an example of tectonics on a sphere. Nature 216, 1276–1280. Marcano, M.C., van der Voo, R., Niocaill, C.M., 1999. True polar wander during the Permo-Triassic. J.. Geodynamics 28, 75–95. Mendiguren, J.A., 1971. Focal mechanism of a shock in the middle of the Nazca plate. J. Geophys. Res. 76, 3861–3879. Minster, J.B., Jordan, T.H., Molnar, P., Haines, E., 1974. Numerical modelling of instantaneous plate tectonics. Geophys. J. R. Astron. Soc. 36, 541–576. Minster, J.B., Jordan, T.H., 1978. Present day plate motion. J. Geophys. Res. 83, 5331–5354. Molnar, P., Atwater, T., 1973. Relative motions of hot spots in the mantle. Nature 246, 288–291. Morgan, W.J., 1971. Convection plumes in the lower mantle. Nature 230, 42–43. Nicolaysen, L.O., 1985. Renewed ferment in the earth sciences—especially about power supplies for the core, for mantle and for crises in the faunal record. South Afr. J. Sci. 81, 120–132. Parmentier, E.M., Oliver, J.E., 1979. A study of shallow global mantle flow due to the accretion and subduction of lithospheric plates. Geophys. J. R. Astron. Soc. 57, 1–21. Parsons, B., Sclater, J.G., 1977. An analysis of the variation of ocean floor bathymetry and heat flow with age. J. Geophys. Res. 82, 803–827. Pavlenkova, N., 1995. Structural regularities in the lithosphere of continents and plate tectonics. Tectonophysics 243, 223–239. Pavoni, N., Mu¨ller, M.V., 2000. Geotectonic bipolarity, evidence from the pattern of active oceanic ridges bordering the Pacific and African plates. J. Geod. 30, 593–601. Phipps-Morgan, J., Parmentier, E.M., 1987. Mechanisms for the origin od mid-ocean ridge axial topography: implication for the thermal and mechanics structure of accreting plate boundaries. J. Geophys. Res. 92, 12823– 12836. Popper, K., 1994. Alles Leben is Problem-Lo¨sen. Wiss Buchges, Darmstadt. Ranalli, G., 2000. Westward drift of the lithosphere: not a result of rotational drag. Geophys. J. Int. 141, 535–537. Ricard, Y., Doglioni, C., Sabadini, R., 1991. Differential motion between lithosphere and mantle: a conseqeunce of lateral mantle viscosity variations. J. Geophys. Res. 96, 8407–8415. Richardson, R.M., Solomon, S., Sleep, N., 1976. Intraplate stress as an indicator of plate tectonic driving forces. J. Geophys. Res. 81, 1847–1856. Richardson, R.M., Solomon, S., Sleep, N., 1979. Tectonic stress in the plates. Rev. Geophys. Space Phys. 17, 981– 1019. Ringwood, A.E., Irifune, T., 1988. Nature of the 650-km seismic discontinuity: implications for mantle dynamics and differentiation. Nature 331, 397–403. Ruff, L.J., Given, J.W., Sanders, C.O., Sperber, C.M., 1989. Large earthquakes in the Macquarie Ridge complex: Transitional tectonics and subduction initiation. Pure Appl. Geophys. 129, 71–129. Runcorn, S.K., 1962. Palaeomagnetic evidence for continental drift and its geophysical cause. In Continental Drift. Int. Geophys. Ser., 3, Acad. Press, New York, pp. 1–40. Runcorn, S.K., 1967. Flow in the mantle inferred from the low degree harmonics of the geopotential. Geophys. J. Roy. Astr. Soc. 14, 375–384. Schaeffer, N., Manga, M., 2001. Interaction of rising and sinking mantle plumes. Geophys. Res. Lett. 28, 455–458. Schwinner, R., 1920. Vulkanismus und Gebirgsbildung. Ein Versuch. Z. Vulkanologie 5, 175–230. Sclater, J.G., Francheteau, J., 1970. The implication of terrestrial hear flow observations on current tectonic and geochemical models of the crust and upper mantle. Geophys. J. Roy. Astr. Soc. 20, 509–542. Seidler, E., Jacoby, W.R., Cavsak, H., 1999. Hotspot distribution, gravity, mantle tomography: evidence for plumes. J. Geodynamics 27, 585–608. Smith, R.B., 1977. Intra-plate tectonics of the western North American plate. Tectonophysics 37, 323–336.

26

W.R. Jacoby / Journal of Geodynamics 32 (2001) 3–27

Solomon, S., Sleep, N., Richardson, R.M., 1975. On the forces driving plate tectonics: inferences from absolute plate velocities and intraplate stress. Geophys. J. R. Astron. Soc. 421, 769–801. Stein, S., Melosh, H.J., Minster, J.B., 1977. Ridge migration and asymmetric sea-floor spreading. Earth. Planet. Sci. Lett. 36, 51–62. Storetvedt, K.M., 1997. Our Evolving Planet. Earth History in New Perspective. Alma Mater., 456 pp. Storetvedt, K.M., 1999. Global wrench tectonics. Replacement model for plate tectonics. Mem. Geol. Soc. India 43, 521–547. Suess, E., 1885. Das Antlitz der Erde, I, II, III. Prag, Wien, Tampsley. Sykes, L.R., 1969. Seismicity of the mid-ocean ridge system. Geophys. Monogr. 13, 148–153. Sykes, L.R., Sbar, M.L., 1973. Intra-plate earthquakes, lithosphere stresses and the driving machanism of plate tectonics. Nature 245, 298–302. Tackley, P.J., 2000. Mantle convection and plate tectonics: towards an integrated physical and chemical theory. Science 288, 2002–2007. Tapponnier, P., Molnar, P., 1979. Active faulting and Cenozoic tectonics of the Tien Shan, Mongolia, and Baykal region. J. Geophys. Res. 84, 3425–3459. Taylor, F.B., 1910. Bearing of the Tertiary mountain belt in the origin of the earth’s plan. Geol. Soc. Am. Bull. 21, 179–226. Trompert, R., Hansen, U., 1998. Mantle convection simulations with rheologies that generate plate-like behaviour. Nature 395, 686–689. Trubitsyn, V.P., Rykov, V.V., Jacoby, W., 1999. A self-consistent 2-D model for the dip angle of mantle downflow beneath an overriding continent. J. Geod. 28, 215–224. Uyeda, S., Kanamori, H., 1979. Back-arc opening and the mode of subduction. J. Geophys. Res. 84, 1049–1061. van Bemmelen, R.W., Berlage, H.P., 1954. Versuch einer mathematischen Behandlung geotektonischer Bewegung unter besonderer Beru¨cksichtigung der Undationstheorie. Gerlands Beitr. Geophys. 75, 3941–3954. Van der Hilst, R.D., Ka´rason, H., 1999. Compositional heterogeneity in the bottom 1000 kilometers of Earth’s mantle: toward a hybrid convection model. Science 283, 1885–1888. Van der Hilst, R.D., Widyantoro, S., Engdahl, E.R., 1997. Evidence for deep mantle circulation from global tomography. Nature 386, 578–584. Vening-Meinesz, F.A., 1962. Pattern of convection currents in the earth’s mantle. Proc. Konink. Ned. Akad. Wetensch., B 65, 131–143. Vine, F.J., Matthews, D.H., 1962. Magnetic anomalies over oceanic ridges. Nature 199, 947. Vogt, P.R., 1971. Asthenosphere motion recorded by the ocean floor south of Iceland. Earth Planet. Sci. Lett. 13, 153– 160. Wadati, K., 1935. On the activity of deep-focus earthquakes in the Japan island and neighbourhoods. Geophys. Mag. 8, 305–325. Walcott, R.I., 1970. Flexure, rigidity, thickness, and viscosity of the lithosphere. J. Geophys. Res. 75, 3941–3954. Wegener, A., 1912. Die Entstehung der Kontinente. Petermanns Geogr. Mitt., 58, 185–195, 253–256, 305–309. Wegener, A., 1922. Die Entstehung der Kontinente und Ozeane. Vieweg, Braunschweig, 1915, 1920, 1922, 1930 (first and fourth edition reprinted, Vieweg, 1980; The Origin of Continents and Oceans. Methuen, London, 1924; Dover, New York, 1966) White, R.S., 1997. Rift-plume interaction in the North Atlantic. Phil. Trans. R. Soc. Lond. A 355, 319–339. White, R.S., McKenzie, D., 1995. Mantle plumes and flood basalts. J. Geophys. Res. 100, 17543–17585. Wiens, D.A., Stein, S., 1985. Implications of oceanic intraplate seismicity for plate stresses, driving forces and rheology. Tectonophysics 116, 143–162. Wilson, J.T., 1963. Evidence from islands on the spreading ocean floors. Nature 197, 536–538. Wilson, J.T., 1965. A new class of faults and their bearing on continental drift. Nature 207, 343–347. Wilson, J.T., 1966. Did the Atlanic close and re-open? Nature 211, 676–681. Wilson, J.T., 1968. Static or mobile earth: the current scientific revolution. Proc. Am. Phil. Soc. 112, 309–320. Wilson, J.T., 1973. Mantle plumes and plate motions. Tectonophysics 19, 149–164. Yuen, D.A., Cadek, O., Boehler, R., Matyska, C., 1994. Large cold anomalies in the deep mantle and past mantle instability. Terra Nova 6, 238–245.

W.R. Jacoby / Journal of Geodynamics 32 (2001) 3–27

27

Zoback, M.L., Zoback, M.D., Adams, J., Assumpcao, M., Bell, S., Bergman, E.A., Bluemling, P., Brereton, N.R., Denham, D., Ding, J., Fuchs, K., Gay, N., Gregersen, S., Gupta, K.H., Gvishiani, A., Jacob, K., Klein, R., Knoll, P., Magee, M., Mercier, J.L., Mueller, B.C., Pauin, C., Rajendran, K., Stephansson, O., Saurez, G., Suter, M., Uedias, A., Xu, Z.H., Zhizhin, M., 1989. Global patterns of intraplate stress: a status report on the World Stress Map Project of the International Lithosphere Program. Nature 341, 291–298.