Cover Image


View/Hide Left Panel

and looked and looked. Next, Lynn Sykes delivered the one-two punch by showing that earthquake focal mechanisms on transform faults were consistent with J. Tuzo Wilson's theory of ridge offset. Menard returned to Scripps a complete convert.

Although the battle for acceptance of plate tectonics was quickly waged and won in the mid-1960s, there were still a number of details to be filled in, much of which was done under the sponsorship of NSF. The present-day plate kinematics were to be sorted out using the azimuths of transforms and the width of the near-ridge magnetic anomalies. The history of plate motions and reorganizations needed to be worked out, a problem often requiring targeted expeditions funded by NSF to key areas where there were gaps or complexities in the magnetic records. Second-order effects, such as the existence of propagating ridges and microplates, were observed from detailed surveys and found to be important mechanisms for accommodating changes in the direction of relative plate motion.

The vertical motion of the seafloor was predicted from conductive cooling relations and compared with the depth data. The archives of heat flow observations were compared with what was predicted based on the thermal cooling model that fit the subsidence of the seafloor away from the ridges, but were found lacking. The conductive heat flow was less than predicted near the ridges and on the flanks, leading to the proposal that hydrothermal circulation was appreciable in young crust. Later expeditions funded by NSF, notably the RISE (Rivera Submersible Experiments) Expedition to the East Pacific Rise in 1979, found the "smoking gun" for hydrothermal circulation near mid-ocean ridges in the form of hot vents and the completely unexpected chemosynthetic food chain associated with them. Thus, even the crowning achievement in the field of marine biology can be claimed by MG&G.

Hotspots, although not a natural component of the plate tectonic paradigm, proved to be a useful indicator of the direction and speed of absolute plate motion. Observations of the flexure of the lithosphere beneath the weight of the hotspot islands and seamounts, and seaward of subduction zones, were used to calibrate the strength of the oceanic plates. These studies, funded mostly by NSF, led to unprecedented abilities to predict the horizontal and vertical history of seafloor in all of the world's oceans.

I recall the first time I heard about the theory in 1972. I was an undergraduate at Colorado College majoring in physics, soon to graduate. One of my physics professors gave me an article from Scientific American written by John Dewey describing the new theory. After the geology courses I had taken that spoke of geosynclines deformed under unknown forces, plate tectonics seemed so simple and elegant. Soon after, J. Tuzo Wilson came to speak at the college. I was hooked. I had already applied to graduate school in physical oceanography, but quickly decided that geophysics was what I really wanted to study—nothing like getting in on the first decade of a major paradigm shift. On my first oceanographic expedition, there was no one more senior than the graduate students, including the two co-chief scientists, Peter Lonsdale and Kim Klitgord. Everything had to be discovered anew and reinterpreted in terms of the new model, and who better to do it than the graduate students who had no stake in any previous ideas?

It is impossible to understate the importance of plate tectonics. It grandly explained the distribution of earthquakes and volcanic eruptions. It exactly predicted evolutionary patterns and distributions of related species. It predicted the history of possible pathways for ocean circulation, trends in ocean volume that controls sea level, and alteration of seawater chemistry via fluid circulation at ridges and trenches. In the chemosynthetic colonies in the hot vents, it might even explain the origin of life.

Reconstruction of Earth's Paleoclimates

The impact on society of the use of MG&G observations to reconstruct paleoclimates has been no less important and followed fast on the heels of the plate tectonic revolution. Whereas the time scales for plate tectonics are measured in millions of years, the deep sea record from sediment cores has taught us that Earth's climate vascillates on thou-sand-year time scales, and possibly much less. No great revolution sparked the acceptance of the climate proxies from the deep sea, as was the case in plate tectonics, but the impact on mankind could be much greater. We doubt that plate tectonics will render Earth uninhabitable for mankind on a human time scale, but there is every reason to believe that natural climate cycles enhanced by man's degradation of air, water, and land could result in an Earth unable to support the present population in a matter of centuries or less.

The climate story is also one of fortuitous gathering of samples, specifically the deep-sea cores, before their significance was established. A large number of researchers labored long and hard to work out the biostratigraphy of the cores using the carbonate and siliceous shells of microscopic marine animals. These cores demonstrated that the carbonate compensation depth in the oceans had varied over time, for not completely understood reasons, as had sea level. Furthermore, the microfossils indicated that there had been sudden swings of climate from warm-loving to cold-loving marine planktonic microfossils and back again at rates too fast to have been caused by plates drifting into different climate zones. But the resolution in the biostratigraphy was too poor to work out the rates of climate shift and to establish absolute global synchronicity. Here again the pioneering work of Cox et al. (1964) proved useful, in that the reversal of Earth's magnetic field at the beginning of the Bruhnes epoch, about 700,000 years ago, was often faithfully preserved in the paleomagnetic field of the core, such that it

The National Academies of Sciences, Engineering, and Medicine
500 Fifth St. N.W. | Washington, D.C. 20001

Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement