by John McPhee
Five minutes after five, and San Francisco’s Red Cross Volunteer Disaster Services Committee is in the middle of a disaster-preparedness meeting. The Red Cross Building is shivering. The committee has reconvened underneath its table.
In yards and parks in the Marina, sand boils are spitting muds from orifices that resemble the bell rims of bugles. In architectural terminology, the Marina at street level is full of soft stories. A soft story has at least one open wall and is not well supported. Numerous ground floors in the Marina are garages. As buildings collapse upon themselves, the soft stories vanish. In a fourth-floor apartment, a woman in her kitchen has been cooking Rice-A-Roni. She has put on long johns and a sweatshirt and turned on the television to watch the World Series. As the building shakes, she moves with experience into a doorway and grips the jamb. Nevertheless, the vibrations are so intense that she is thrown to the floor. When the shaking stops, she will notice a man’s legs, standing upright, outside her fourthstory window, as if he were floating in air. She will think that she is hallucinating. But the three floors below her no longer exist, and the collapsing building has carried her apartment to the sidewalk. Aqueducts are breaking, and water pressure is falling. Flames from broken gas mains will rise two hundred feet. As in 1906, water to fight fires will be scarce. There are numbers of deaths in the Marina, including a man and a woman later found hand in hand. A man feels the ground move under his bicycle. When he returns from his ride, he will find his wife severely injured and his infant son dead. An apartment building at Fillmore and Bay has pitched forward onto the street. Beds inside the building are standing on end.
The Marina in 1906 was a salt lagoon. After the Panama Canal opened, in 1914, San Francisco planned its Panama-Pacific International Exposition for the following year, not only to demonstrate that the city had recovered from the great earthquake to end all earthquakes but also to show itself off as a golden destination for shipping. The site chosen for the Exposition was the lagoon. To fill it up, fine sands were hydraulically pumped into it and mixed with miscellaneous debris, creating the hundred and sixty-five dry acres that flourished under the Exposition and are now the Marina. Nearly a minute has passed since the rock slipped at the hypocenter. In San Francisco, the tremors this time will last fifteen seconds. As the ground violently shakes and the sand boils of the Marina discharge material from the liquefying depths, the things they spit up include tarpaper and bits of redwood—the charred remains of houses from the earthquake of 1906.
An earthquake.
A small flex of mobility in a planetary shell so mobile that nothing on it resembles itself as it was some years before, when nothing on it resembled itself as it was some years before that, when nothing on it …
Not long ago, at Mussel Rock, a man named Araullo was fishing. He had a long pole that looked European. He seemed not so much to be casting his lure as sweeping it through the sea. His home was near the top of the cliff. He pointed proudly. The one nearest the view.
He had come down the trail and jumped over water to a wide, flat boulder. The seismic crack that came down the cliff ran into the water and under the boulder. He was fishing the San Andreas Fault, and he was having no luck.
I asked him, “What are you after?”
He said, “Sea perch. I also get salmon and striped bass here. Now I don’t know where they are. Someday, they come.”
He said that he felt very fortunate to have a house so close to the fish and the ocean, to have been able to afford it. He had bought it six months before. In this particular location, real estate was cheap. He had bought the house for a hundred and seventy thousand. I could barely hear him over the sound of the waves.
“If it going to go down, it going to go down,” he shouted, and he flailed the green sea. “You never know what going to happen. Only God knows. Hey, we got the whole view of the ocean. We got the Mussel Rock. What else we need for? This is life. If it go down, we go down with it.”
The cormorants were present, and the pelicans. The big fishing boulder was echeloned with shears. From somewhere near Araullo’s house, a hang glider had left the jumpy earth and now hovered safely above us.
Araullo ignored the hang glider and kept on swinging his pole.
“I don’t know where they are,” he said again. “But someday they come. They always come.”
Book 5
Crossing the Craton
Crossing the craton—the Stable Interior Craton, core of the continent: Illinois, Iowa, Nebraska, and friends—you don’t see a lot of rock. You see in the roadcuts emerald vetch moving under wind like wheat. You see the dark fine loess of airborne hills. You know, of course, that you are riding over subcrop, tens and hundreds and even thousands of feet down, but seldom does it outcrop, and, where it does, it is such an event that it is likely to have been named a state park. In Starved Rock State Park, in La Salle County, Illinois (Ordovician marine sand, exposed in a bluff of the Illinois River), you rub your hand against a massive quartz sandstone so lightly bound that white sand rains down the rockwall and onto your toes. Near Davenport, in Iowa, you stop to photograph a small farm on a drumlin, its barn like a biscuit on the summit of a loaf of bread. Under the hay and the windmills of Iowa is large-groundswell chocolate land—glaciated terrain and nothing flat about it, and a wind that sounds like white water as it moves through isolated stands of trees. A Devonian limestone outcrops at 1100 Dubuque Street in Iowa City and is three stories high. West of Skunk River comes a steady, eight-mile climb to the Des Moines lobe of the Wisconsinan ice sheet—younger country now and without relief. A bubble would center and rest on the horizon. In an overhanging streamcut at Pammel State Park is a limestone younger than the last. “Expect to see rattlesnakes,” you are told; but you see what you expect: brachiopods, nautiloids, and crinoids, of Carboniferous time. Ledges State Park, near Ames, is a streamcut lithic sandstone, meaning there’s a lot in it besides quartz and feldspar. The whole exposure is so weathered you can’t get a decent sample. It is punk rock. There are carbon patches in the rock. The bluffs of Council Bluffs are loess that came in on the wind, and rising toward their summits are the terracettes called catsteps. From a river bar of the braided Platte, in Nebraska, you collect varied pebbles from scattered sources hundreds of miles away. All this notwithstanding, in fifteen hundred miles of the midcontinent there is a great deal less rock to see on the surface than you would see in Wyoming if you opened one eye. In The Evolution of North America (Princeton, 1959)—the definitive volume of its time—Philip King, of the United States Geological Survey, encapsulated the Midwest in one memorable sentence: “The rather monotonous geologic features of the Interior Lowlands would seem of less interest than the complex rocks and structures of the Canadian Shield, the Appalachians, or the [western] Cordilleras, nor can we, in this book, devote space to them commensurate with their surface area.”
There would be more to tell if you could sense what you can’t see. All the ledges and bluffs have been cut in the limestones, shales, and sands that came over the midcontinent in the twelve periods known in geology as Phanerozoic time—the five hundred and forty-four million years since creatures with hard skeletal parts first came into the world and began to leave their fossils in rock. In the American Midwest, the rock of Phanerozoic time rests in thin veneer on basement rock from deeper time. Some of it is a southern extension of the vast Canadian Shield. Geologists long taught that the basement was a platform at the edges of which the continent grew and that the platform had been there forever, down through the eons of Precambrian time. The Precambrian—beginning, as it does, with the beginning of the earth—covers more than four billion years. This is seven-eighths of earth history. Geological textbooks, nevertheless, would typically give the Precambrian one short chapter. First there was the basement, and on that grew the world.
In the three hundred and forty-two pages of Our Mobile Earth (Scribner’s, 1926), Reginald Aldworth Daly, of Harvard University, composed one page on Precambrian rocks and one page on �
��Pre-Paleozoic eras.” Daly was a giant in the geology of his time. He summarized Precambrian lithology as “the crumpled Basement Complex.” Even Reverend H. N. Hutchinson, B.A., F.G.S., did a little better than that in The Autobiography of the Earth (Appleton, 1891). Referring to an undifferentiated mass of “fundamental gneiss,” Hutchinson devoted one of his sixteen chapters to the Precambrian—fifteen pages in two hundred and eighty-three—calling it “An Archaic Era.” Sixty years later, in 1951, A. J. Eardley, of the University of Utah, gave the first eighty-eight per cent of the history of the earth one chapter among the forty-three chapters of his Structural Geology of North America. He opened the chapter with this sentence: “The continent of North America is made up in a broad way of a stable interior and surrounding belts of deformed, intruded, and metamorphosed rocks.” Of its oldest component, the Canadian Shield, he went on to say, “The time has not yet arrived … when the vast region can be broken down into divisions with confidence.” Even as the twentieth century began to fade, three Canadian geologists—Colin W. Stearn, Robert L. Carroll, and Thomas H. Clark, of McGill University—reserved twenty pages in five hundred and forty-nine for Precambrian events in their Geological Evolution of North America (Wiley, 1979). W. Randall Van Schmus, of the University of Kansas, who did his doctoral dissertation on Canadian rock 2.4 billion years old, has remarked that to some extent geologists still tend to divide the history of the earth into two units, giving attention to the first in inverse proportion to its overwhelming eons. “There’s basement, and there’s Phanerozoic. There exists a longstanding prejudice that the Precambrian has to be different. The only difference is that it’s older. And there’s no bugs in it. No fossils that we can use for stratigraphic correlation. Isotopes are our fossils. There’s always something in the rocks that will give you the answer you are looking for. You need to wait for the development of techniques. We’re still dealing with a frontier in continental evolution. The information’s there; we just have to figure how to get it out.”
The development of techniques in recent years has sent light into deep time, revealing structures that could not have been imagined before. The basement of the continent is no longer the undifferentiated mass it was through most of the history of the science. There have been inventions or advances in metamorphic petrology, in samarium/neodymium geochronology, in argon/argon thermochronology, in uranium/lead dating, in zircon dating, in aeromagnetic mapping, in filtered-gravity mapping, in trace-element geochemistry, and in the isotopic monitoring of the crustal history of rocks and their origins in the mantle. Not to mention—at the two ends of the technological spectrum—the novelty of the ion-probe mass spectrometer and the undiminished relevance of oil wells. Many advances date only from the early nineteen-eighties, brought on by the evolution of computers and the programming to process the data. The collective result has been a new and rapid sketching-in of whole Precambrian scenes.
As you cross Iowa and approach Des Moines, nothing on the surface—not a streamcourse, a fault line, an outcrop, a rise—so much as hints at what is now beneath you. Six hundred feet down is the eastern edge of a great tectonic rift—a rupture of the lithosphere—that reposes there like a sunken boat in the waters of a lake. Filled in during the Precambrian and covered over by sediments of Paleozoic time, the central rift is about thirty miles wide, and trends southwest. If you are on Interstate 80, you angle across it. At Lincoln, Nebraska, you reach the far side. In many places, the walls of the rift are three thousand feet sheer. In the other direction, the buried rift valley runs far to the north under Iowa and under Wisconsin and under Lake Superior, where it forms a triple junction with a rift that trends off through Michigan to the southeast and a third but incomplete rift (a mere crack known as a failed arm) that goes north-northwest into Canada. This great rifting of the “stable” craton—basement of the continent—began eleven hundred and eight million years before the present and ended a thousand and eighty-six million years before the present. Continental in scale, it split North America right up the middle and down one side, threatening to scatter it to who knows what distant corners of the globe. On modern gravity maps and magnetic maps of North America, it is the single most prominent feature that you see. Rifts meeting at a triple junction are a signature of plate tectonics and can be seen all over the modern world—in the far-south Atlantic, where the African, Antarctic, and South American plates conjoin; in the Azores, where the African, Eurasian, and North American plates conjoin; in the Indian Ocean; at the Galapagos Islands; at Cape Mendocino in California. To sense most clearly, though, the Precambrian rift system that lies under the middle of North America, look at a map of Arabia and Africa.
The Red Sea, the Gulf of Aden, and the East African Rift Valley—with its rift-provoked volcanoes (Kilimanjaro) and riftdepression lakes (Tanganyika, Victoria)—meet in a triple junction of plates off the southern tip of Arabia. The splitting is young and has been going on only about twenty million years, but the relic of something similar lies under the Middle West and was not seen to be what it is until the third quarter of the twentieth century. In the context of this discussion, the eleven hundred million years since the North American rifting occurred is not, in its own way, a particularly large number, for its date is much closer to the end than to the beginning of Precambrian time, with fully three-quarters of the earth’s history stretching back before it. The fresh insights and technological advances mentioned above have gone far deeper into time and wider in geography than the relatively modern Midcontinent Rift. They have reached back to the beginning of the Proterozoic Eon (twenty-five hundred million years before the present) and beyond that to the early Archean Eon, when the North American craton, once assumed to have been in place forever, evidently did not exist. By some geologists, the first six hundred million years of the earth’s history have recently been given status of their own as the Hadean Eon. Why geologists have decided that the earth’s beginnings were in Hades is for them to say, but their technologically informed guesswork in describing former scenes does extend, in a general way, into seascapes older than four billion years.
That, as it happens, is the rounded age of the oldest rock ever found on earth (actually 3.96). The oldest rock ever found on earth is found every couple of years, it seems, as still another crustal fragment, radiometrically dated, comes nearer to 4.0. The oldest rock in the United States is in the Minnesota River Valley—about 3.5 billion years. There is rock in West Greenland that is 3.8, in Australia about 3.5. Some rocks in Africa are as old as 3.6. No contemporary continent is anywhere near as old as these Archean dates, but it is interesting that the oldest known rock comes from North America. Discovered and dated by Samuel A. Bowring, who is now at M.I.T., the incumbent oldest rock ever found on earth is east of Great Bear Lake, in the Canadian Northwest Territories, almost exactly on the Arctic Circle. The University of Kansas geochronologist Randy Van Schmus, who has vetted and guided this essay, continues the description: “It’s a very strongly deformed foliated gneiss. None of the primary rock structure or texture is preserved. It’s in a basement block of [the mountain-building events known in geology as] the Wopmay Orogen. Chemically, it’s an evolved rock, in the sense that it was derived from the partial melting of something that predated it. So it’s clearly not the oldest. And we know from zircons in sandstones in Australia that there were igneous rocks crystallizing 4.2 billion years ago. So there were older rocks, but they’ve either been destroyed or not found yet.”
The scenes that lie before the oldest rocks and go back to the beginnings of Hadean time rest on isotopic and chemical signatures, cosmological data, and conjecture. From an interstellar gas cloud, evidently, the solar system began to form about 4.56 billion years ago. The first eleven verses of Genesis cover more than four billion of those years—the entire Precambrian and the first hundred and fifty million years of Phanerozoic time. Meanwhile, gravity, a shock wave from a supernova—or something—caused the gas cloud to collapse, becoming incandescent vapor, in which mine
rals formed dust. According to present theory, planetesimals formed from the dust and were swept up into planets. The collecting and compacting of the earth happened quickly—in a few tens of millions of years—and included cometary material that had water in it. From the beginning, water would have been expelled into the earth’s atmosphere or onto its surface. Meteors kept on showering, accreting, increasing the size of the earth. Around 3.9 billion years before the present, meteor impacts were particularly intense. Many of the meteors were large objects hundreds of kilometres across. Stable continental crust could not develop until the earth to some extent cooled and meteor bombardment stopped. It stopped because so much debris had been gathered into planets. The present asteroid belt seems to be a planet that never formed. An object the size of Mars is thought to have collided with the very early earth, sending vaporized material into orbit. It cooled and coalesced as the moon.
On the early Archean earth, was the face of the waters an unfeatured globe-girdling sea? Theoreticians consider that possible, but highly improbable. Most likely, something would have been sticking up, such as a global mob of islands, collectively representing about twenty-five per cent of the surface of the earth. Geophysicists have calculated the production of heat from decaying uranium, potassium, and thorium in the early Archean, and the heat seems to have been three or four times the amount that the earth produces today. For the most part, the modern earth vents heat through geophysical hot spots like Etna, Yellowstone, and Hawaii, through tectonic spreading centers like the Mid-Atlantic Ridge and the East Pacific Rise, or through the volcanism of subduction zones, where one lithospheric plate slides beneath another and to some extent melts, while the resulting magma breaks the surface as a Mt. Rainier, a Mt. St. Helens, an Aconcagua, a Fujiyama. The partial melting of ocean crust would draw off a magma chemically different from the ocean crust, and it would harden as less dense, lighter, continental-style rock, examples of which are andesite and granite. To rid itself of four times as much heat, the early Archean earth must have had many more places where the heat could get out, and at this point the conversation reaches the center of an unresolved question about the look of the Archean world. Some think that lithospheric plates, similar in size to the modern plates, moved a good deal faster, with much more volcanism at the plate boundaries. A majority leans toward a picture of the earth with its eggshell more shattered, with several times as many plates as exist at the moment—smaller ones, of course, and a vastly greater linear aggregate of plate boundaries, all venting heat. The role of hot spots—plumes of heat rising to the surface from deep in the mantle—is a third and considerable factor. They could have been all over the earth in great numbers, accounting for a very large percentage of the vented heat, while early plate motions were less significant. Some as-yet-unknown proportion of these three views composes the picture that geophysical science is trying to see.