Annals of the Former World

by John McPhee

  As it happened, Fred Vine was also working at Princeton at the time—Vine, who had co-authored with his Cambridge University colleague Drummond Matthews the paper that contributed the manifesting insight into the movement of the ocean crust and placed Vine and Matthews among the handful of people in various parts of the world who collectively brought about the plate-tectonics revolution. When Moores’ packet arrived from Cyprus, Moores already had behind him three years’ experience in Macedonia among the lower components of ophiolitic rock, but he had not sensed their origins. He had not imagined that they had formed in one milieu and been transported to another. Like most geologists, he thought of them as rocks formed from magmas that had welled up under Greece. Opening a geologic map of Cyprus, he saw diabase dikes all running in the same direction; he saw mantle-derived serpentine at the bottom of the section and basaltic pillows at the top. This time, his imagination made the jump. Unfolding the map in front of Fred Vine, he said, “How does this look for oceanic crust formed at a spreading center?”

  In the science, the ancestral glimmerings of that intuition were nearly a century old. By the eighteen-eighties, geologists had begun to reflect on a common association of serpentine, gabbro, diabase, and basalt; and in The Face of the Earth, Eduard Suess, of Vienna, observed that all these “green rocks,” as he called them, could characteristically be found in folded-and-faulted mountains. He said that they had formed in the geosynclines from which the mountains derived.

  In 1892, when the German geologist Gustav Steinmann visited San Francisco, Berkeley’s Andrew Lawson took him to the north side of the bridgeless Golden Gate to see the rocks of Marin. (Moores came upon this remarkable account while he was reading Steinmann in German in 1967.) Steinmann had wandered the Apennines and Alps noticing serpentines, pillow lavas, and radiolarian chert—always in that order, upsection. Now, in Marin, he said to Lawson, “These rocks are the same.” There were solid cliffs of red chert, and pillow lavas as well. On the San Francisco side of the strait was a headland of serpentine. Steinmann commented that since the chert was stratigraphically at the top of the sequence the whole assembly must have come from deep sea. In 1905, Steinmann published a definitive study of the same three rock types threading through the Alps. They became known in the science as the Steinmann Trinity.

  After the German meteorologist Alfred Wegener presented his hypothesis of continental drift (1912), no one connected it to the Steinmann Trinity. For about four decades, the two ideas hung on to the trailing edge of the science, with no one suspecting that Wegener’s idea would effloresce as a scientific paradigm, or that the ophiolite story (the advanced edition of the Steinmann Trinity) would, thereafter, provide the chapter headings to the plate-tectonic history of the world.

  There were hints. In 1936, Harry Hess, of Princeton, whose “History of Ocean Basins” would—in 1960—introduce the spreading seafloor and begin the new tectonic story, gave a paper in Moscow in which he related Alpine peridotites to island arcs and called this “a contribution to the ophiolite problem.” The ophiolite problem was manifold. Most geologists did not accept the association of such disparate rocks. The few who did asked elemental and unanswerable questions: Are the rocks in their original place? If not, where have they come from and how did they move? Peridotite, which in alteration becomes serpentine, is now seen to be of the deepest origin of any rock found at the earth’s surface. It is now thought to be rock of the earth’s mantle, and few disagree. Hess had studied two belts of Alpine-type peridotites in the Appalachians. He asserted that they were not just garden-variety igneous rocks that had come up as magma under existing Appalachians but rock that had intruded much earlier, on the edges of a geosyncline. Hess went on to say that peridotites seemed to be introduced in the initial phases of mountain building—and not thereafter. He said they were cold, intact, and solid as they came up in the rising mountains—that they had been, in other words, tectonically emplaced. Hess was describing a collision between a continent and an oceanic subduction zone, but in 1936 he didn’t know it. In Moores’ words, “This is the clearest example I know of of a guy saying the right thing for the wrong reason. Hess said this was the most important event in the forming of the Appalachians, this ‘intrusion’ in the Ordovician. It was. But plate tectonics did it.”

  By 1955, the burst of data that had come with the ocean-exploration programs of the Cold War had yielded—in the work of Russell Raitt, of the Scripps Institution of Oceanography, and Maurice Ewing, of Columbia University—the first descriptions of ocean crust to be derived from seismic refraction. Everywhere, the ocean crust seemed to be some sort of package—a once molten but nonetheless zoned assembly, in three general bands.

  The German geologist W. P. de Roever was the first to imagine rock of the earth’s mantle in the thin air of the Alps. In a 1957 paper, he said that the Alpine peridotites appear to be solid intrusions; they have deformed fabrics; they appear to be coming up from the mantle. For “solid intrusions” you could read “emplacements from elsewhere.” In their deformed fabrics you could see that they had been moved. How they had been moved remained unclear.

  In 1959, Jan Brunn, who had mapped the Vourinos ophiolitic complex in Macedonia, published an abstract in French in which he compared ophiolites to the Mid-Atlantic Ridge. He was the first person ever to suggest that the ophiolites of the aerial world were similar to crust found at mid-ocean ridges. He compared ophiolitic rocks to dredged rocks. All this was before seafloor spreading was recognized, and no one noticed the work of Brunn. But it was Brunn who took the Steinmann Trinity and the ophiolitic sequence out of the Oz of geosynclines and placed them in the center of the widening oceans.

  Over the next nine years (1960-68) appeared the twenty-odd scientific papers reporting the plate-tectonics story: that plates are essentially rigid, and deform at their boundaries; that all plates include ocean crust, and generally a very large amount of it (the continents are passengers on the plates); that new seafloor moves away from a spreading center until it goes down into a trench to be consumed; that plates sliding past each other (as at San Francisco) do so in strike-slip sporadic jumps; that ocean crust colliding with continental crust can pry up something like the Andes; that continental crust hitting continental crust will build Himalayas, Urals, Appalachians, and Alps.

  While these novel facts were still for the most part unknown, Moores, in Macedonia in the early nineteen-sixties, was becoming thoroughly at home with the petrology and structure of the Vourinos Complex, about thirty miles west of Mt. Olympus. Brunn’s ideas notwithstanding, Moores thought of the Vourinos rocks as homegrown—in his words, “a partially molten diapiric blob.” Diapirs are the bodies of rock that, balloonlike, rise, crashing their way into the country rock above them. Harry Hess, one of Moores’ supervisors, went to Greece to inspect his work. Hess was already in the process of abandoning the geosyncline and shedding the Old Geology like old skin. He decided that the Vourinos rock had formed in an oceanic environment, and probably at a spreading center. Moores, clinging to what he had been taught (by, among others, Hess), decided that Hess was crazy.

  Moores’ conservatism can be understood in the light of the disregard in which ophiolites were generally held in the United States at that time. When a graduate student at Stanford, doing field work not far from the university, suggested that the local sediments had been deposited on an ophiolitic complex, the Geology Department specifically forbade him to use the word “ophiolite” in a Ph.D. thesis. The professors explained that ophiolites were a wild European idea, clearly wrong—and, in any case, not applicable to California.

  The concept of the spreading seafloor had gained acceptance by 1966, when Moores first saw the geologic map of Cyprus. Moores and Vine prepared to go there, but had to wait, because of the political tension in the six-year-old country. They worked there in 1968 and 1969. The evidence was convincing that Cyprus was essentially a piece of ocean crust, thrust up in some way and now aerially exposed. Moores and Vine’s paper, whic
h has influenced all subsequent understanding of ocean crust, was the first to establish ophiolites as ocean-floor remnants, for the most part formed by spreading processes. This odd collection of rocks ranging from water-cooled lavas to mantle blocks, so difficult to explain in continental settings, could now be seen not as ordinary igneous formations but as tectonic features that had moved from one place to another in the course of epic alterations of landscape.

  In Pacific Grove, California, at the end of 1969, a Penrose conference on “The Meaning of the New Global Tectonics” drew structural geologists from all over the world. William Dickinson, of Stanford, dismantled the geosyncline and assigned its parts to various aspects of plate tectonics—collisions, island arcs, abyssal plains, mélanges, trenches, transform faults. Moores describes the conference as “a watershed of geology—that was when people really began to realize how important plate tectonics was.” Listening to Dickinson, he thought of all the ophiolitic and volcanic-island rock that he was seeing in the Sierra Nevada and the Coast Ranges, and it occurred to him that these mountain systems could be understood in terms of island arcs accreting. The arrival times of ophiolites could date successive mountain-building events. In his words, “Recognition of the fact that ophiolite emplacement had to be by the collision process meant that you could explain the western part of the United States by that kind of sequence. That idea came to me on the last morning of the Penrose conference in 1969, and I wrote it down: the idea that you could explain the progressive eugeosynclinemiogeosyncline development and the progressive orogenies that you see in western North America as a series of island-arc complexes—the things we call terranes—that have collided with the continent. I was so excited I could hardly sit still for several days—in fact, for a couple of weeks after that. I came back to Davis all bubbling over.” Before long, he had sent a paper to Nature. It appeared in 1970 and was the first to suggest the collisional assembling of California and of vast related portions of the North American Plate.

  The idea that California is in large part a collection and compaction of oceanic islands was a reverberation of an ancient myth as well as a development in a science. For at least two thousand years, people described certain undiscovered islands with a force of imagination that became belief. In the Middle Ages and the Renaissance, such islands appeared on global maps, and when navigation revealed that they were not where they were said to be cartographers moved the islands to new locations in unexplored seas—the Fortunate Isles, for example, and the Seven Cities of Cibola, and the Lost Atlantis. One such island was California, a utopia of the western Atlantic Ocean. It is described as follows in Las Sergas de Esplandián, a Spanish romance published in 1508:

  Know, then, that, on the right hand of the Indies, there is an island called California, very close to the side of the Terrestrial Paradise, and it was peopled by black women, without any man among them, for they lived in the fashion of Amazons. They were of strong and hardy bodies, of ardent courage and great force. Their island was the strongest in all the world, with its steep cliffs and rocky shores. Their arms were all of gold, and so was the harness of the wild beasts which they tamed and rode. For, in the whole island, there was no metal but gold.

  The wild beasts were griffins—half lion, half eagle—which the women rode through the air into battle, and which they trapped as fledglings. To the griffins they fed the voyaging men who came their way, and their own male infants. Ruling California was the “mighty Queen, Calafia … the most beautiful of all of them, of blooming years.” (Translation by Edward Everett Hale, 1864.)

  With an eye on the new tectonics, the paleontologist James W. Valentine, of the University of California, Davis, worked out a curve of the distribution of marine-invertebrate families across Phanerozoic time (the past five hundred and forty-four million years). He saw the diversity of creatures expand, decline, rise again. The thought occurred to him that if numerous small continents were spread around the world in fair proximity to the equator one would expect the diversity of life to be very high, whereas if continental masses should happen to be clustered (and especially if they were clustered around a pole) diversity should be very low. A typical Valentine graph showed creatures from continental shelves starting out at a very low level of diversity in late Precambrian and early Cambrian time, coming up to a high level in mid-Paleozoic time, then crashing at the end of the Permian, and then rising again. Valentine showed his graphs to Moores, and said, “I wonder if you can explain these patterns of diversity in terms of continental drift and redistribution of land.”

  Among the results of this dialogue were papers by Valentine and Moores in Nature (1970) and the Journal of Geology (1972). The title of the second one was “Global Tectonics and the Fossil Record.” The concept of continental drift had always implied the preexistence of a supercontinent, which Wegener had called Pangaea. After all, if Australia and Africa and the Americas and Eurasia spread apart from their obvious fit, they had to have been together in the first place. The first place—according to the newly determined vectors of the lithospheric plates—was two hundred million years ago, when Pangaea began to split into Laurasia and Gondwanaland. In the second place, they split farther, to sketch the present globe. Valentine’s diversity patterns were in harmony with this story: the greater the breakup of landmasses, the more diverse the fossil families. Moores looked at mountain belts that had come into existence hundreds of millions of years before the dispersal of Pangaea. If the new theory worked, it would work not only forward in time but backward. Gradually, he reassembled Paleozoic and Precambrian continents—not only the continents that came together to create Pangaea but also the continents that came together to create an earlier supercontinent, which no one had thought of before. Moores and Valentine called it Protopangaea, or Pangaea Minus One. More widely, it has become known in the science as Rodinia (from a Russian word meaning “motherland”). Since agglomerations and dispersals of terrane seemed to be cyclical in nature, there may have been who knows how many supercontinents before Rodinia. The new theory was like a stable and inventive structure built in the mind of a composer in advance of a composition, but there were those who didn’t like what they heard. “Global Tectonics and the Fossil Record” was attempting to demonstrate how continental drift affects evolution, and—heaven knows—it succeeded in enraging no small number of geologists and paleontologists, who felt that Moores, especially, and other “plate-tectonics boys” were ignoring phenomena like ocean currents in their headlong lust to write all aspects of the geologic pageant into the plate-tectonic model. But in 1978 Patrick Morel and Ted Irving, of Canada’s Department of Energy, Mines, and Resources, presented paleomagnetic evidence for Pangaea Minus One.

  Moores and Valentine also sensed a relationship between plate-tectonic history and the history of the level of the sea. Moores explained, “If you look at the stratigraphic record on platforms such as the midcontinent of the United States, you see times of high stands of the seas, when the continent was nearly submerged, and times of really low stands of the seas. Could that be related to continental drift? Take an average ocean basin and add a hot and voluminous spreading ridge. You will diminish the volume of the ocean basin and force water up over the continents. Conversely, if ridges die for some reason—lose their heat and collapse, or otherwise disappear—that will increase the volume of ocean basins and allow seawater to drain off the continents. The ocean’s transgressions and regressions seem to represent seafloor spreading going on or not going on. Others worked on this before we did, but we extended it back to the Cambrian-Precambrian boundary. In the geologic record, you see a great regression of seas in late Precambrian time, then transgression in Cambrian and Ordovician time, then regression in Permo-Triassic time, and then the transgression of Cretaceous time. The late-Precambrian regression coincides with Pangaea Minus One. As it split apart, with spreading centers forming all over the place, the Cambro-Ordovician transgression occurred, and when the smaller continents came back together there wa
s the Permo-Triassic regression, and when they split apart again came the famous Cretaceous transgression, when Colorado was underwater.”

  Hugh Davies, of the Papuan Geologic Survey, published cross sections of New Guinea that showed a huge ophiolite dipping northward. The Indo-Australian Plate, like a shovel, had lifted this piece of Pacific mantle and crust. The ophiolitologists Robert Stevens, John Malpas, and Harold Williams, of Memorial University of Newfoundland, described a series of ophiolites in Newfoundland as remnants of an arc that collided with North America in Ordovician time, signing in the Taconic Orogeny. Eli Silver, of the University of California, Santa Cruz, working in Indonesia, traced a large ophiolite there from its emplaced position on a submerged microcontinent northward into an ocean basin.

  As comparable research went on around the world, a question that attended all of it was “What ancient geography can be deduced from ophiolites?” and spectacular deductions continued to be forthcoming. The ophiolite suite seemed not only to spell out in detail the process of formation of oceanic lithosphere but to record plate collisions of which all other evidence was long since gone. Vanished oceans were recalled, and vanished plates inferred, as continents were deconstructed and continents were reconstructed. An ocean (or oceans) that once existed where the Atlantic is now had closed from north to south in Cambro-Devonian time, and the Permian disappearance of the ocean where the Urals are now had completed Pangaea. The Pacific Plate, now the largest in the world, did not exist then, but a spreading ridge, propagating westward, split Pangaea into Laurasia and Gondwanaland in late Triassic and early Jurassic time, and opened the Tethys Ocean. The early central Atlantic was a part of Tethys. As oceans came and went and continents evolved, island arc after island arc had been swept into larger masses—a story that could suggest that the first dry lands of Genesis were arcs accreting in a globe-girdling sea.