by John McPhee
When continents collided, Africa docked with, among other places, the Old South. About a hundred and fifty million years later, when Africa departed, it apparently left a large piece of Florida, which is now covered with what Moores calls “a lot of modern limestones that developed on top of the Appalachian suture, which can be traced seismologically under northern Florida and off into the continental shelf.”
Hesitating, I say to him, “Florida is covered with marine sand on top of limestone on top of Paleozoic rocks. The Paleozoic rocks derive from Africa. That is what you are saying?”
“That’s right. Southern Florida is a piece of Africa which was left behind when the Atlantic opened up.”
“People in Florida say that southern Florida is Northern and northern Florida is Southern.”
“Civilization reflects geology.”
Southeastern Staten Island is a piece of Europe glued to an ophiolite from the northwest Iapetus floor. Nova Scotia is European, and so is southeastern Newfoundland. Boston is African. The north of Ireland is American. The northwest Highlands of Scotland are American. So is much of Norway.
While the Atlantic continues to open, the western Tethys, or Mediterranean Sea, is closing like a pair of tongs, hinged in the Atlantic crust roughly a thousand miles west of Casablanca. During the initial spreading of Tethys and later in the narrowing and even the lengthening of Tethys as effects of the forces that opened the Atlantic, the Mediterranean seafloor has been such a battleground that ophiolitic pieces of it are scattered around the basin like shell cases. Introducing chapters of the Mediterranean story, they are all through the Alps, Corsica, the Apennines, the Carpathians, the Dinarides, the Balkans, the Hellenides, Crete, the Cyclades, and the western part of Turkey.
The Mediterranean is full of tectonic rubble, no other single example being as large or as destructive as the Italian microcontinent, also known as the Adriatic Plate. Its western boundary is a subduction zone off the Campanian coast whose melt has become Vesuvius, and whose compressional distortions have become the Apennines. The boundaries of the Italian microcontinent run north into Switzerland, northeast down the Rhine to Liechtenstein, east to include the Austrian Alps and Vienna, then south through Zagreb and Sarajevo and past the Vourinos Ophiolite in Macedonia to the central Peloponnesus, and back to the boot of Italy. In the Jurassic, the Italian microcontinent made its attempt to become a permanent part of Africa. As Africa moved northeast with the opening of the Atlantic, it returned Italy to sender, picking up the Tethyan ocean crust that became the ophiolites of the Alps. The Alpine collision began in the Eocene, about fifty million years ago, and has not completely stopped.
Muscat, in Oman, sits at the base of peridotite cliffs and is the only capital city in the world hewn into rock of the earth’s mantle. Almost all of northern Oman is ophiolite, lifted by the shelf of Arabia in the closing of Tethys.
Four large parts of Africa, dating from the Archean Eon, are more than three thousand million years old: the West African Craton, the Congo Craton, the Zimbabwe Craton, and the Kalahari Craton. Long defined in geology as continental basements, continental shields, or continental cores, cratons are the ancient fundament to which younger and more legible rocks adhere. Plate tectonics now suggests (not to everybody) that the older parts of continents were themselves assembled much as the younger parts have been. For example, the African cratons are separated by belts of deformation that occurred after the Archean Eon but still in deep Precambrian time. Perhaps the deformed rocks are suture belts where preexisting oceans disappeared. If plate tectonics was functioning then as it functions now, the crusts of those vanished African oceans consisted of rocks of the ophiolitic sequence. There are late Precambrian ophiolites in the Kalahari Desert of southwest Africa, in Sudan, in Egypt, in Arabia, and at Bou Azzer, in the western Sahara. Moores says that Sudan, Egypt, and other parts of Africa are full of exotic terranes—island arcs that were, as he puts it, “crunched in there in the late Precambrian.” He continues, “Geologists have long seen Africa as having developed in place, but the story must be wrong. Little is known about the mobile tectonics there in Precambrian time. I think it’s kind of an unknown frontier of the ophiolite story.”
The mobile tectonics of more recent years are a good deal easier to see. If you look on a world map at Antarctica, South America, Africa, and Australia, you virtually see them exploding away from one another. You can reassemble Gondwana in your mind and then watch it come apart. In the Cretaceous, Africa and South America start to separate from Antarctica, and from each other. India is still part of southern Africa, but soon it breaks away. Australia remains attached to Antarctica until the Eocene, when it breaks away, forms its own plate, and heads north. India and Australia move separately for a time but then weld themselves together to become a single plate.
Madagascar begins to separate from Africa soon after India does, but India leaves Madagascar far behind. The Seychelles move away from Africa in the same manner—rifting obliquely, opening a small ocean basin with an active spreading center whose stairlike geometry (as in the Gulf of California) consists of short ridges connected by long transform faults. When the spreading stops, Madagascar and the Seychelles rejoin the African Plate.
I ask Moores why transform faults, like the San Andreas Fault, are so few and far between on land, whereas ocean floors are full of them.
He says, “They are rare on land because when they do appear in such a setting they rapidly take the land away and turn into marine transform faults. A transform fault carried India away from Africa. Look at the east coast of Madagascar. It’s a long straight line, where India departed. Look at the corresponding part of India, the Malabar Coast below the Western Ghats. It’s a long straight line. The Salinian Block, in California—with San Diego, Los Angeles, and so forth—will go on out to sea to the northwest, away from North America. The fault-divided halves of New Zealand’s South Island, which were once apart, will come apart again. Equatorial Africa and northeast Brazil slid away from each other and developed an intervening sea.”
I ask him why spreading centers and subduction zones take up most of the length of the world’s plate boundaries and transform faults take up so little.
“Because the earth is a globe,” he answers. “The curves of the spreading centers and the curves of the subduction zones will meet, or nearly meet. Where they don’t meet, as in California, you find a transform fault.”
When India separated from Africa, India’s geographical center was more than a thousand miles farther south than the present position of the Cape of Good Hope. Three thousand miles of Tethys Ocean separated India from Tibet. India moved northeast as rapidly as any drifting continent in the calculable history of plate motions. At least one island arc lay in its path, and maybe several. There were microcontinents, too.
In and around the Himalaya are well-preserved ophiolitic sequences that describe the disappearance of that part of Tethys. These include ophiolites of Pakistan. Until the 1971 war, they were known in geology as the Hindubagh Complex. They are now called the Muslimbagh Complex. They run from the Indus Gorge east along the Indus River and the Tsangpo and Brahmaputra rivers for two thousand kilometres, acquiring other local names along the way. This continuous belt of ophiolites consists of ocean crust that formed at a spreading center late in the Cretaceous and was emplaced on the northern margin of India in Paleocene time, when India had completed about half of its journey north. India evidently reached a trench with an island arc behind it, choked the trench, and picked up the ocean crust from the peripheries of the arc. That, in any case, is how Moores tells the story. He likens the ophiolite to a cow on a cowcatcher in front of an old western train. When those ophiolites were emplaced and India swept up the island arc (about sixty million years before the present), Australia was still on its own plate. After the two plates joined, in the Eocene, the whole enterprise that geologists now refer to as the Indo-Australian Plate continued to move northward, gathering islands, for a few tens of million
s of years. Then, with India as its hammerhead, it struck the Asian mainland. Moores thinks that the collision has scarcely begun.
This most emphatic of all contemporary continent-to-continent collisions is often described as a head-on crash, as if it had occurred within the ticks of human time. As India moved north, its highest rate of speed was a hundred and forty-two miles per million years. The present rate of compression is about a quarter of that, or two inches a year. If this could be recorded in stop-action photography, like the boiling swirls of cumulus clouds or the unfolding of a rose, it could indeed express itself kinetically. But two inches a year is an encounter so slow that a word like “collision” distorts its scale.
While India was closing with Tibet, it buckled the intervening shelf, raising from the sea a slab of rock more than a mile thick, a part of which is now the top of Mt. Everest. From the depths of lithification to the rock’s present loft, it has been driven upward at least fifty thousand feet. In the tectonic history of the globe, we have no idea how many times something of this proportion has happened. The probability is that it has happened often.
The boundary between the Eurasian Plate and the plate that carries India and Australia seems pretty obvious, but actually it cannot be narrowly defined. It is not as simple and precise as the Indus suture, where ophiolites are embedded along the northern slopes of the higher mountains, and it cannot be limited to the Great Himalaya Range itself, though that appears to be a clear partition between the hitter and the hit. Across two thousand miles, from the Ganges River north to Lake Baikal, the boundary between the Indo-Australian Plate and the Eurasian Plate is indistinct. It was once described as a separate plate—the unfortunately named China Plate. The whole zone is seismically active. It contains the highest large plateau in the world. The Indian collision has produced additional mountain ranges north of the Himalaya that are comparable in altitude to the Andes. Included also is the Sinkiang Depression, where collisional downbending has put the ground below sea level. Essentially, all of China is a part of the plate boundary, and in all of China there are very few rocks that are undeformed. Chinese geologists travelling in America incessantly snap pictures of simple flat-lying sediments—a geological basic that they have seldom seen. The crust under the Tibetan Plateau is twice as thick as most continental crust. The Indo-Australian Plate, pressing northward, seems to have caused this. To accommodate two inches of relentless annual advance, various things have to bulge or give way. The mountains have risen. The plateau has thickened. But these two changes have not been enough to account for the total compression. A growing number of geologists, following work that is being done by a group of French tectonicists, are beginning to agree that a large part of Southeast Asia has also been forced to one side. Where Burma meets India, the high ranges bend almost at a right angle and go off to the southeast. This is the Burma Syntaxis (the term refers to a bend in a mountain chain), and near it are the beginnings of a whole series of great rivers—the Brahmaputra, the Mekong, the Irrawaddy, the Salween—initially in parallel valleys, veining Southeast Asia from the Bay of Bengal to the South China Sea. Controlling these valleys are long strike-slip faults, in motion like the San Andreas. The French tectonicists are proposing that Vietnam, Laos, Thailand, Cambodia, Burma—the whole of Indochina—slid southeastward among those strike-slip faults, like a great terrestrial hernia. On a relief map you can see India ramming Asia and squeezing all that country out to the southeast. As the mechanism has gained acceptance among tectonic theorists, it has become known as continental escape.
I ask Moores if he thinks there’s a chance that plate tectonics may someday seem to have been a rational fiction, as the geosynclinal cycle does now.
“For parts of the world, maybe so,” he says. “Whatever is going on in central Asia is no one’s idea of plate tectonics. But as an explanation for eighty per cent of the surface processes of the earth, plate tectonics is in, firm.” Repeating the words of the volcanologist Alex McBirney, he says, “Remember, ‘In the next ten years, our confusion will reach new heights of sophistication.’”
(Or, in words dubiously attributed to Mark Twain: “Researchers have already cast much darkness on the subject, and if they continue their investigations we shall soon know nothing at all about it.”)
The thought occurs to me, not for the first time, that I am following a science as it lurches forward from error to discovery and back to error. In my effort to describe some of the early discoveries of plate tectonics, I must also be preserving some of the early misconstructions.
“Inevitably,” Moores agrees. “That is the nature of science, and geology is surely no exception.” His mentor Harry Hess, a combat veteran with the rank of rear admiral in the Naval Reserve, once told him, “Geologists make better intelligence officers than physicists or chemists, because they are used to making decisions on faulty data.”
The data that sketch departed geographies are actually numerous. Where pictures are clearest, the data cross-check with confirming frequency. For example, the ophiolitic narrative will conform with ancient latitudes preserved in the remanent magnetism of rock. The fossil record must not disagree. Where strike-slip faults have sliced a landscape and carried two sides apart, matchups can be traced in time and space. Sedimentary sequences, blue-schist belts, batholithic belts, thrust belts, and melanges will orchestrally tell what happened. If they are not synchronous, it didn’t happen.
That Asian plate boundary two thousand miles wide untidies the theory of plate tectonics more than any other place in the world with the probable exception of the American West. Moores is among the growing number of geologists who believe that Salt Lake City is on the eastern side of a muddled and dishevelled boundary between the North American and Pacific plates. Not long ago, when the boundary was utterly different—when an ocean trench off North America was consuming the Farallon Plate—the Rocky Mountains appeared, from Alaska to Mexico. In a major way, they defy explanation. As you look at the world map and see India hitting Asia, with the Himalaya and all the additional deformation to the north to show for it, you might begin to wonder why there is no India against the west coast of the United States. Obviously, the Smartville Block and the other accreted arcs added something to the mountains’ compression, but the impact of those terranes could not have been sufficient to deform a third of a continent. Force from the west evidently made the mountains. But what force? If it was a colliding landmass, where has it gone?
If you run your eye up the coast a couple of thousand miles, you see protruding from North America a body of land at least as conspicuous as India. Staring at the map one day, I ask Moores if he is ready for some academic arm waving on a windmill scale.
“Shoot,” he says.
“Why isn’t Alaska the missing India of North America?”
“It is.”
“Why didn’t Alaska—not all at once but in parts and in successive collisions—strike North America at the California latitudes and then take off on transform faults and slide north to where it is now?”
“That’s exactly what Alaska has done. When the Brooks Range swung into place, none of the rest of Alaska was there. Below the Brooks Range, all of Alaska consists of exotic terranes. They seem to have come from the Southern Hemisphere, and even from the western Pacific. Where each and every part originated we have no idea, but a lot of it collided down here in California and then went north on transform faults. Sonomia, Smartville, and so forth are probably fragments of terranes that in large part broke away and went north.”
From radiometric dating, paleomagnetism, matching fossils, and connectable orogenies, George Gehrels, of the University of Arizona, and Jason Saleeby, of the California Institute of Technology, have proposed that the Alexander Terrane of southeastern Alaska, which includes Juneau and Sitka, drifted ten thousand miles from eastern Australia to Peru and then north to its present position. Vancouver Island seems to have followed; its paleomagnetism indicates that it came from the latitude of Bolivia and arrive
d in the Eocene.
Such travels seem modest in comparison with a vision that Moores has of the world half a billion years earlier. In 1991, he published a paper claiming that Antarctica and western North America were once conjunct. This was during the existence of Rodinia. Tectonicists have for some time agreed that something must have rifted away from western North America in latest Precambrian time—that the craton cracked and separated, much as the Nubian-Arabian Craton is rifting now and making the Red Sea. In proposing Antarctica as the other side of the rift, Moores traces Precambrian lithologies of eastern Canada down through Alabama, Texas, and Arizona into Queen Maud Land, East Antarctica. Ian Dalziel, of the University of Texas, who had made a field trip with Moores to Antarctica, took Moores’ proposal and extended its Precambrian juxtapositions, reconstructing the whole of Rodinia. According to Dalziel and Moores, if you had journeyed due north from Morocco you would have crossed western Africa and gone into Venezuela and through Brazil and Chile to West Virginia and on through Arizona into Antarctica, beyond which lay Australia. When they published their conclusions, Time, Science News, the New York Times, the Los Angeles Times, the San Francisco Chronicle, the Washington Post, and countless other publications adorned the story with maps beside which the cartographic efforts of the fifteenth century seem to be precision documents.
Running a finger down through Mexico and into Guatemala almost to Honduras, Moores says that an east-west fault zone there is laced with ophiolitic rock and seems to have been the southern extreme of Paleozoic North America. After Rodinia dispersed, what lay beyond Guatemala was open ocean. When Pangaea later coalesced, north and south touched there.
One of the places where the breaking up of Pangaea may have begun, in the Mesozoic, was not far away. If you reassemble the Triassic terranes around the Atlantic, the dikes radiate from a point in the Bahamas—suggesting that a hot spot that was centered there broke open a large part of Pangaea. As the supercontinent rifted, a large oceanic gap reopened between North and South America—in effect, a western extension of Tethys. It contained no Antilles, Lesser or Greater—none of the present island arcs or subduction zones of the Caribbean. It was blue, abyssal ocean. Its bottom was ocean crust-and-mantle—the ophiolitic sequence. Evidently, the Caribbean Plate, bringing arc sequences with it, came drifting in from far to the west to take up the position it holds today. It appears to have collided with the Bahama platform, and perhaps with the shelf of North America, in latest Cretaceous time. Ocean crust chipped off its edges as it fitted into place. All around the basin, island arcs came up above sea level, and fragments of ocean crust came up with them, as the ophiolites of northern Venezuela, of Cuba, of Hispaniola, of western Puerto Rico. The island of Margarita, near the Venezuelan coast, appears to be one large ophiolite. Off the Cayman Trench, in mid-Caribbean, the ocean crust is thicker than ocean crust commonly is. Thick basalts seem to have poured out over the original ophiolitic sequence in some sort of mid-plate volcanic event. “The only other place we know about where that sort of thing occurred in that time is out in the western Pacific, in the Nauru Basin,” Moores continues. “Some people have suggested that these two things may be related. If you do a reconstruction of the East Pacific Rise and the plates in the Pacific, you can bring the Caribbean back into contact with the Nauru Basin in early Cretaceous time.” If that is where the Caribbean Plate came from, the distance that it travelled eastward is eight thousand miles.
