J. Tuzo Wilson

Magnetic Anomalies over a Young Oceanic Ridge off Vancouver Island

The recent speculation that the magnetic anomalies observed over oceanic ridges might be explained in terms of ocean-floor spreading and periodic reversals of the earths magnetic field may now be reexamined in the light of suggested reversals during the past 4 million years and the newly described Juan de Fuca Ridge.

Evidence from Ocean Islands Suggesting Movement in the Earth

Oceanic islands increase in age from the mid-ocean ridges towards continents and the andesite line reaching a maximum known age of Upper Jurassic. The Seychelles appear to be a continental fragment. Several pairs of lateral aseismic ridges extend from islands on the mid-ocean ridge to adjacent continents. Their continental junctions mark points on opposite coasts which would also fit if the continents were reassembled according to the criteria used by Wegener. As Holmes has shown each pair of ridges tends to have distinctive chemical characteristics. One possible explanation is that convection currents in the mantle rising along the mid-ocean ridges and sinking beneath trenches have carried the crust apart across the Atlantic, India and East Pacific Oceans. The lateral ridges may be approximately streamlines. Although Darwin showed that most volcanic islands sink, a few have been uplifted. Most of these lie a few hundred kilometres in front of deep trenches, suggesting that they may be on the crest of a standing wave in front of the trenches and that the crust is rigid. Of eleven straight chains of young islands in the Pacific ten get older away from the East Pacific Ridge. They could also be streamlines, fed by lava rising from deep within convection cells with stagnant cores. The regularity of ridges suggests non-turbulent flow.

Mantle plumes and plate motions

Abstract This paper elaborates the hypothesis that convection plumes may be rising from the lower mantle to spread out in the asthenosphere and drive lithospheric plates about and thus possibly provide the primary mechanism which governs the behaviour of the earths surface. The paper notes some characteristics of plumes and identifies more than thirty by the hot spots which overlie them. Most lie close to mid-ocean ridges and have produced aseismic ridges trending away from them on either plate. A few have been overridden by plates to produce single, isolated chains of seamounts and islands. One plume may have uplifted the Colorado Plateau. Such distinctions serve to identify five types of hot spots. Most plates are in motion over the lower mantle. They are considered to be driven by the plumes, but their paths are influenced by interactions with other plates. Some temporarily become more or less stationary relative to the lower mantle. It is held that stationary plates, of which Africa and Southeast Asia may be present examples, develop special characteristics among which much volcanism, epeirogenic uplift, rifting and the development of basins and swells are diagnostic. It is well-known that if two plates approach one another at a subduction zone that a continental plate generally overrides an oceanic one. It is here suggested that the question of which plate is more nearly stationary over the mantle is important and determines the character of the continental margin. It is held that, if a continental plate advances over an oceanic one which is fixed over the mantle, a migrating marginal trench and mountains of Andean type with huge batholiths will form on the leading edge of the continent. On the other hand, if a continental plate is fixed and one or more oceanic plates are advancing and sliding under it, island arcs (and, when a collision with another continent occurs, mountains of Appalachian type) will form along each coast towards which a plate is advancing.

Static or mobile earth: The current scientific revolution

THE FIRST STUDENTS of the solid earth were miners. Early they founded the sciences of mineralogy and metallurgy, but their studies were local. In spite of the broader insights of such men as Pliny the Elder, Leonardo, Steno, Agricola, Werner, Cuvier, and Lamarck, it was not until the close of the eighteenth and the early nineteenth centuries that Hutton, Smith, and Lyell demolished semi-religious beliefs in a cataclysmic origin of the earth and established the principles of geology (Adams, 1938). As a result, since 1830, the earth has been regarded as a rigid, stable body whose surface features have evolved slowly by processes which we see in action today. In this belief and with steadily improving techniques geologists have spent the past 150 years mapping the rocks exposed on land. Preoccupation with the beauty and intricacy of these discoveries and confinement to the land surface has prevented mankind from appreciating how little of the whole earth is visible. The inaccessibility of most of the earth is a limitation which has prevented geologists from developing general theories. One cannot discover everything about an egg by examining only one-third of its shell. The first precise and general theories of the earth were formulated in the seventeenth century by William Gilbert, Newton and Halley from their investigations of the earths magnetic and gravitational fields. Unfortunately their elegant theories were not matched by the development of good instruments until a few years ago. Whereas early geologists rapidly accumulated good data, but lacked theory, early geophysicists had precise theories, but could not make enough observations; the two groups could not correlate their ideas. Both long remained in ignorance of those greater parts of the earth, its interior and its ocean floors. Only during this century have seismologists discovered how to use earthquake waves, like giant X-rays, to illuminate the dark interior; only since World War II have adequate ships, instruments and expenditures been available to explore the ocean basins. Suddenly within the past two or three years, the wraps are off and we glimpse for the first time the full beauty of the naked earth. The vision is not what any of us had expected from our limited peeps at the constituent parts. The earth, instead of appearing as an inert statue, is a living, mobile thing. The vision is exciting. It is a major scientific revolution in our own time, but before expatiating upon its nature, let us examine the evidence.

New insights into old shields

Abstract The first part of this paper briefly reviews some major developments in Precambrian geology during the past decade since the Upper Mantle Project was proposed. These include widespread use of radiometric age determinations which have completely revised ideas about the Australian shield, shown an excellent match in provinces of the same age between Africa and South America and discovered rocks more than 3.5 billion years old in South Africa and Greenland. Deep holes have been drilled into ancient rocks in the Soviet Union and the top of the buried basement contoured and studied in the United States and Canada. The second part discusses division of old shields into cratons, which are the oldest parts formed during the Archean Era, folded belts and mobile belts which are younger than cratons in age and overlap one another. The rocks of the mobile belts are more highly metamorphosed. It is suggested that the best time scale possible for the Precambrian consists of three apparently synchronous and nearly equal divisions of Proterozoic time, an Archean Era when the cratons formed and a pre-Archean Era, from which no rocks have been preserved on earth. The final section outlines briefly a possible mechanism causing continental drift and the renewal of shields. It is that plumes may flow up like great pipes from deep in the mantle to uplift domes in some shields, notably Africa, or to be overridden by other shields as may be the case beneath the Colorado Plateau.

Aspects of the different mechanics of ocean floors and continents

Abstract Taking the concept of ocean-floor spreading as accepted, this implies that the mechanics of ocean floors and of continents are different in character. The direction of motion along transform faults is the reverse of that expected for ordinary transcurrent faults. A Proto-Atlantic Ocean is suggested as the explanation for the separation of the contemporaneous realms of Olenellus and Paradoxides in North America and Europe, which indicates that continental drift has been recurrent and ocean basins have repeatedly opened and closed. It seems likely that it is not a pattern of convection currents within the mantle which accounts for this behaviour, but that instead the rigid lithospheric plates themselves form the outward current away from mid-oceanic ridges and that the return flow is the only flow occurring in theasthenosphere. Mid-oceanic ridges should therefore be regarded as diapiric structures rather than as parts of a convective system.

Awards presented at Fifty‐Third Annual Meeting

Carl Eckart entered geophysics through an auspicious incident at the outbreak of World War II. He was then, at age 40, Professor of Theoretical Physics at the University of Chicago, having made very major contributions to the development of quantum mechanics. He found himself involved in the war effort as an administrator and researcher in the work on underwater sound, fundamental for submarine detection, which was then starting at San Diego. Eventually Eckart settled in La Jolla, a suburb of San Diego, as Professor of Marine Geophysics at the Scripps Institution of Oceanography. He has been in La Jolla ever since. Although by inclination he is more of a scholar than an administrator, Eckart has served his country and the University of California in a succession of administrative positions: as Director of the Marine Physical Laboratory, Director of the Scripps Institution of Oceanography, and as Vice Chancellor of the San Diego campus, not to mention many national committees.