by John McPhee
The gap between the Americas was subsequently filled in two stages. Not long after the arrival of the Caribbean Plate, Honduras and Nicaragua drifted in from who knows where. Ultimately, Panama came in from the Pacific, about seven million years ago. The collision calls to mind a cork going into a bottle, but was not so neat a fit. Panama was a part of an island arc, and the ophiolite that preceded it runs from Costa Rica to the Colombian Cordillera Occidental and on down to the Gulf of Guayaquil, indicating that the Choco Terrane, as the arc is called, was nearly a thousand miles long.
In Colombia’s Cordillera Central, one range east of the Choco Terrane, are older ophiolites that report like a tree ring the continent’s growth in successive laminations. Moores has been in Brazil among Precambrian ophiolites that seem to describe the basement of the continent collecting. In southern Patagonia, he has traced a line of ophiolites, called the Rocas Verdes Complex, from the fiftieth parallel to Tierra del Fuego. The Atlantic island of South Georgia, two thousand miles east of Tierra del Fuego, is part ophiolite and is believed to be a travelled piece of the Rocas Verdes Complex.
“Travelled?”
“Probably on a transform fault,” Moores says.
In other words, South Georgia was once the easternmost part of Tierra del Fuego, and it took off.
Another part of Tierra del Fuego appears to have departed but returned. That, at any rate, is what Moores and others concluded in 1989 during a geologic voyage there. The Straits of Magellan lie across an ophiolite complex that they interpret as the back-arc basin behind a piece of South America that moved west before changing its mind. Perhaps South Georgia will come back, too.
Down the high Andes from Ecuador to fifty degrees south are no known ophiolites. This is a surprising interruption in a story that touches every other plate-boundary mountain range or former-plate-boundary mountain range in the world. After being tucked into the tectonics all the way from Alaska to Ecuador, ophiolites disappear. The gap in which they are missing is equal to the distance from the equator to Seattle. In ophiolite tectonics, perhaps the largest question at present is: What has happened in the central Andes?
In his office in Davis, with the Americas spread out on paper before him, Moores greets the question laconically. He says, “It makes you wonder.”
I remark, “It’s the one place on the western margin of the Americas where you don’t find ophiolites providing some sort of story about exotic terranes—about the Smartville Block joining California, about Caribbean crust coming several thousand miles to its present position. Your intuition must be telling you something.”
Moores says, “There are two possibilities: one, it is different from everywhere else that we know of; two, the evidence is hidden or is not preserved. My guess—this will make Andean geologists cringe—is that much of the Mesozoic volcanics one sees in the western Andes and the coastal ranges of Chile and Peru represents something that was at one time exotic to South America. My guess is that there’s a suture. We just haven’t found it. It may well be on the Chilean-Argentine frontier.”
“So you’re talking about an ophiolite announcing a terrane that came drifting in from wherever to fill the four thousand miles from the equator to fifty south, and you don’t even know where the ophiolite is?”
“That’s right.”
“So you’re …”
“Doing violence to the geology as it’s known.”
“The understanding of plate-tectonic history as it appears to be developing for the Pacific margin of the two Americas from Alaska to Tierra del Fuego includes lots of evidence for such terranes attaching themselves all the way, with the single exception of …”
“Zero to fifty south.”
“And you think that …”
“Chile is exotic terrane.”
“Chile and western Peru are from somewhere off in the South Pacific?”
“It’s not a credible position to defend. But that’s what I think.”
“Where are the ophiolites on the east side of the Andes?”
“They haven’t been found.”
We go out of Davis one morning past a sign that says “OPEN TRENCH,” and head for San Francisco on Interstate 80. In the median are dense effusions of oleander, in blossom pink and white, beguiling the westbound traffic with the pastel promise of California. A pickup in front of us is carrying an all-terrain vehicle studded with flashing sequins. As we move out to pass, the pickup abruptly moves out and blocks us. The pickup has California plates and a bumper sticker. “Don’t Like My Driving? Dial 1-800-EAT-SHIT.”
By the edge of the western hills, Mt. Diablo stands up like a hat on a table. All the way from Sacramento to the Coast Ranges you can see it from the highway. Moores calls it a piercement structure. It is a balloonlike mass of Franciscan melange that has been squeezed up through valley sediments as if from a pastry sleeve. It is nearly four thousand feet high. Its topographical base is at sea level, beside the common delta of the great conjoining rivers, where a canal with no locks takes oceanic merchant ships across the Great Central Valley to Sacramento.
To climb Mt. Diablo, you go up through Pittsburg out of Honker Bay. In the tranquillity of its oak woodland (the mountain is a park) you see the rhythmically bedded red cherts characteristic of the Franciscan. You see its featureless, unbedded sandstones. You see elements of the Coast Range Ophiolite. Moores calls the mountain “low-cost fruitcake, stirred up even more than most Franciscan.” Interlayered volcanics and cherts are folded and refolded there like the leaves of a croissant. From the summit, you can see more than a hundred miles to the High Sierra or look down into a quarry cut in sheeted diabase from some ocean’s spreading center who knows where.
The geology along the interstate west-southwest of Davis recapitulates the traverse to the Napa Valley, but in less obvious fashion, for the hills are lower; the delta and the bays are in a structural depression. In the swells of the oak woodland, the bent-upward sediments of the Great Valley are under the straw-brown grass. Where the grass is thin, you can see the bedding. Near Cordelia, Green Valley Creek comes in from the north, following the Green Valley Fault, which is actively slipping. Here in the hill country between Fairfield and Vallejo, the lovely hummocky topography is the result of creeping landslides, earthflow, solifluction. The solifluction scars are like stretch marks, waves advancing downhill. This is not the sort of place to site a house, but it is the sort of place where houses are sited. Here geology in motion is just another factor in daily life. When a classified ad in a local paper says “Owner Suddenly Called East Must Sell,” the possibility is not inconceivable that the house is a sled.
On the top of Sulphur Springs Mountain, before Vallejo, the highway is in a benched throughcut a hundred feet high. On I-80, this is the beginning of the Franciscan. The Coast Range Ophiolite is here as well. Some of the rock is serpentine. We pull over, and walk the cut from one item to another in the melange. After the serpentine comes a black wall that reflects light as if it were made of obsidian. Moores opens a pocketknife and easily carves the rock. He studies it in his hand lens. He says it is “the decrepitated matrix of the Franciscan”—the scaly clay in which are embedded all the continental shards and abyssal sediments, the bits of seamounts and ocean crust, the litter of half the world.
The interstate descends toward Vallejo, toward Benicia, toward Suisun and San Pablo bays. When Robert Louis Stevenson first saw Vallejo, he described the community as “a blunder.” Vallejo was twice, briefly, the capital of California. Benicia was Mrs. Vallejo. Between the two bays, we cross Carquinez Strait and immediately pass through the soft marine sediments of the largest roadcut on Interstate 80 between the Atlantic and Pacific oceans. Its vertical dimension is three hundred and six feet, but the sediment is so weakly cemented that the two sides lie open like butterfly wings and are thus immense. After the Farallon Trench quit, these marine deposits settled upon the Franciscan melange. Throughout the Coast Ranges, the melange has an icing of sediment, acquired while it was s
till underwater.
Running through Richmond over landfilled marshes beside San Pablo Bay, we cross the Wildcat Fault and, moments later, the close and parallel Hayward Fault. Off to our left, southeast, runs the long escarpment of the Berkeley Hills. Their steepness breaks at the Hayward Fault, below which is gently sloping ground. The obeliscal Berkeley Campanile, of the University of California, stands like a marker close by the fault, whose itinerary on the campus goes right through Memorial Stadium. The Hayward Fault is like one of the yard lines.
The road veers west, and we are suddenly high above water, on the upper deck of the San Francisco—Oakland Bay Bridge. Left and right of us are fifty miles of safe anchorage, waters that are in places twelve miles across. It strains credulity that in two centuries of nautical exploration this most prodigious harbor in North America did not reveal itself to a single ship. So far as is known, no ship passed through the Golden Gate until 1775—a modern date, even in California time.
As a geologic feature, the bay is the youngest thing in sight—younger than the rivers that feed it. During the glacial pulses of the Pleistocene, when so much water rested on the continents as ice and sea level was lower than it is now by a couple of hundred feet and shorelines worldwide were far outboard of present beaches, the Sacramento-San Joaquin flowed through the Golden Gate and forty or fifty miles west before it emptied into the sea. When the ice melted, the sea came up, and drowned innumerable river valleys—drowned the Susquehanna and made the Chesapeake, drowned the Delaware to Trenton, drowned the Hudson to Albany, drowned the Sacramento-San Joaquin from the Golden Gate through the Coast Ranges and into the Great Central Valley, filling the Bay Area’s bays.
If Alcatraz, Angel Island, Yerba Buena, and so forth were elsewhere in the Coast Ranges, they would be the summits of mountains, and not islands in a bay. Something has depressed the bay region, incidentally allowing the rivers to get out of the Great Valley. In breadth and depth, the depression is unlike any other among the mountains of the California coast. Asked to explain, Moores can only speculate. Possibly the depression is just a system of erosional valleys, at the business end of the state’s great watershed. Maybe it’s a synclinal fold—a compressional trough, made as a side effect of San Andreas motion. Maybe it’s a huge pull-apart basin. Maybe it’s all three. Of his guesses, he likes best the pull-apart basin.
“What’s under the bay?”
“Volcanics, conglomerates, glaucophane schist, sandstone, serpentine, chert—Franciscan. This is the place it was named for, and this is the tectonic product. The Franciscan is under the city of San Francisco and is the basement of the bay. Franciscan rocks in the Bay Area are from more parts of the subduction complex than you are likely to find in such concentration anywhere else on the coast.”
The bridge arrives at Yerba Buena Island, and a tunnel runs through it: through sandstone of the Franciscan, outcropping above the entrance and derived from some continent somewhere, perhaps from our own—indurated American sediment that slid down the slope to the trench and there became incorporated into the Franciscan. In the light again and on the western span of the bridge, the run is short to the white city.
Not just any city is the topic of a serious tourist guidebook called A Streetcar to Subduction, a copy of which Moores and I have with us. Written by the geomorphologist Clyde Wahrhaftig, of Berkeley, it is a shining little book of geological field trips on public transportation.
Not just any city can claim to have formed in a trench where the slab of a great ocean dived toward the center of the earth, where large pieces of varicolored country came together, and where competent rock was crushed to scaly clay. After the churning stopped and the whole mixture was lifted into the weather, the more solid chunks very soon stood high and the softer stuff washed down. In Ina Coolbrith Park, we climb to the top of Russian Hill. Lining the path are large sharp pieces of red Franciscan chert, but Russian Hill is sandstone and the chert is decorative. It has come in trucks. Moores trains his binoculars on Alcatraz. He has read the geology of Alcatraz. He says it is a sequence of quartzofeldspathic sandstone and shale. In the compote of the Franciscan, Alcatraz is lying on its side. From Russian Hill he can see that—in the bedding planes. The rock of Alcatraz is continental in origin. There is no telling what continent, or when it escaped. The hill we stand on is also quartzofeldspathic sandstone and is believed to be of the same provenance as Alcatraz. Behind us is the summit of Nob Hill. Russian Hill and Nob Hill are actually the mammary climax of the same small mountain. Some tectonicists look upon this sandstone as being so discretely integral that it deserves nomenclatural distinction. They see it as a picocontinent that drifted into the Franciscan. In their terminology, Nob Hill is part and parcel of the Alcatraz Terrane.
Because the melange contains rocks from all over the Farallon rim, it lends itself to reclassification by those who prefer to describe it not as a tectonic unit but as a boneyard of exotica. To them the Bay Area is not so much the type locality of the Franciscan as a tessellation of six miniterranes. Over the water past Alcatraz we see Angel Island and the southern tip of the Tiburon Peninsula, which, with scattered bits along the Hayward Fault in Richmond, Berkeley, and Oakland, have been called the Yolla Bolly Terrane. The Marin Headlands Terrane includes not only southern Marin County but much of the city of San Francisco. There is a San Bruno Mountain Terrane, a Permanente Terrane, and, where doubt retains a foothold, an Unnamed Terrane.
We cross the Golden Gate, take the first exit, and curl downhill in overshadowing roadcuts of radiolarian chert. Radiolaria are creatures that live near the tops of warm oceans and look like microscopic sea urchins. After they die, their external skeletons go to the bottom to be radiolarian ooze, which lithifies as a very hard, very beautiful rock—a wine-red cryptocrystalline quartz of arrowhead quality. In fifty per cent of the world ocean, radiolarian chert lies like an enamel on the ophiolitic sequence. Where the sequence goes, so goes the chert.
Where you find chert from the open ocean, if you keep going downsection, basalt will soon follow, Moores remarks. Walking downsection, he finds the contact of red upon black. He says that the basalt was a seamount, one of the countless submarine volcanoes that ride the ocean floor.
In the early nineteen-thirties, the north pier of the Golden Gate Bridge was sunk two hundred feet through the red chert and into the basalt below it. At the north pier, the competence of the rock was never a problem. Support for the south pier, in San Francisco, was a good deal less promising. The south pier had to stand, in water, on sinuous, slippery serpentine, the state rock. Also under the ocean, two miles away, was the San Andreas Fault. The serpentine was thought to be potentially unstable, so it was hollowed out, like a rotten molar. The hollow was a little more than an acre, and ten stories deep. It was to be filled with concrete to anchor the bridge. While it still lay open and dry, within coffering walls thirty feet thick, the structural geologist Andrew Lawson, of Berkeley, was lowered in a bucket to inspect the surface of the bedrock. With his purewhite hair, his large frame, his tetragrammatonic mustache, Lawson personified Higher Authority. The stability of the serpentine had been called into question and made a public issue not only by a mining engineer but by the world-renowned structural geologist Bailey Willis, of Stanford, who predicted disaster. Lawson regarded Willis’ assessment as “pure buncomb.” Getting out of his bucket a hundred and seven feet below the strait, Lawson found that “the rock of the entire area is compact, strong serpentine remarkably free from seams of any kind.” He wrote in his report, “When struck with a hammer, it rings like steel.”
About the proximity of the great fault, Lawson had realistically observed (during the design phase) that an earthquake strong enough to knock down the bridge would also raze the city. He went on to say, “Though it faces possible destruction, San Francisco does not stop growing and that growth necessarily involves the erection of large and expensive structures.”
In June, 1935, when the south tower stood nearly complete—seven hundr
ed and forty-seven feet high, with no cables attached—it began to sway in a middle-energy earthquake. A construction worker named Frenchy Gales—as quoted in John van der Zee’s The Gate—continues the story:
It was so limber the tower swayed sixteen feet each way … . There were twelve or thirteen guys on top, with no way to get down. The elevator wouldn’t run. The whole thing would sway toward the ocean, guys would say, “Here we go!” Then it would sway back, toward the Bay. Guys were laying on the deck, throwing up and everything. I figured if we go in, the iron would hit the water.