by John McPhee
I have a question. Why all this Fourth of July geology as recently as three million years ago, when all we have in these latitudes now are run-of-the-mill earthquakes?
Because in the Pliocene a triple junction of lithospheric plates is just off San Francisco, Moores replies. A subduction zone is dying out as its trench turns into the San Andreas Fault. The volcanism relates to that. For tens of millions of years, a lithospheric plate of considerable size lay between North America and the Pacific Plate. It is known in geology as the Farallon Plate. By the late Pliocene, this great segment of crust and mantle, possibly at one time a tenth of the shell of the earth, had in large part been consumed. Fragments of it remain: in the north, the Juan de Fuca Plate, whose subduction under North America has produced Lassen Peak, Mt. Shasta, Mt. Rainier, Mt. St. Helens, Glacier Peak, and the rest of the volcanoes of the Cascades; in the south, the Cocos Plate and the Nazca Plate, whose subduction has created Central America and elevated the Andes. For tens of millions of years, the Farallon Plate went under the western margin of North America, while North America gradually scraped off the Franciscan melange of coast-range California. To the west, under the ocean, was the spreading center that divided the Farallon Plate from the Pacific Plate. As the Farallon Plate, moving eastward, was consumed, the spreading center came ever closer to California. At Los Angeles and Santa Barbara, the Pacific Plate first touched North America, twenty-nine million years before the present. Where it touched, the trench ceased to function, the spreading center ceased to function, and the plate boundary became a transform fault. It was only a few miles long at first, but steadily the great fault propagated from Los Angeles and Santa Barbara to the north and to the south, shutting the trench like a closing zipper. The triple junction of the Farallon Plate, the Pacific Plate, and the North American Plate migrated northward with the northern end of the fault. And so, in the Pliocene, three million years ago, the triple junction was off San Francisco. The volcanoes in the Sierra were the dying embers of Farallon subduction. The volcanoes in the Napa Valley and adjacent coastal ranges were a result of the new fault pulling the earth apart at kinks and bends. The eruption of the Sutter Buttes almost surely relates to the dying subduction or the new plate motions but is, as they say, not well understood. Now, in the Holocene, the triple junction is still moving north. For the moment, it is at Cape Mendocino, where the San Andreas ends and what is left of the Farallon Trench continues. That is how things appear, anyway, in present theory.
Six million years before the present, in the late Miocene, Moores and his apricot tree would be in or beside a saltwater bay that covers most of the Great Central Valley. It is full of tuna and other large fish, because an upwelling of cold water (like the upwelling in the Humboldt Current off modern Peru) has filled the bay with nutrients. There is no Golden Gate. The bay’s outlet is at Monterey. A terrane is moving along the west side of the San Andreas Fault. Carrying with it the sites of San Diego, Los Angeles, Santa Barbara, San Luis Obispo, Big Sur, Monterey, and Salinas, it will someday be known as Salinia.
In the Eocene, fifty million years ago, Moores’ backyard in Davis is mud at the bottom of the Farallon Ocean, some thirty miles offshore, on the continental shelf. As Eocene rivers pour into these waters—having advanced their gravels from Tibet-like altitudes and across the low country that will one day rise as the Sierra—they cut submarine canyons through the future Great Valley. The rock that preserves this story is a marine shale, loaded with shelf creatures of Eocene age. Below Davis, in the Great Valley Sequence of sediments, it lies about twenty-five hundred feet down.
In the Cretaceous, some eighty or ninety million years ago, Moores’ address is a precariously inclined deep-sea fan—a spilling of sediment down the continental slope toward the trench where the Farallon Plate is disappearing. About fifty miles wide, the trench lies in the space that will one day separate San Francisco and Fairfield. As the slab of the Farallon Plate melts beneath North America, it contributes to the magmas of the great batholith and the superjacent volcanoes of the ancestral Sierra Nevada.
At the end of the Jurassic, about twenty million years after the docking of the Smartville Block, another island arc comes in and docks against Smartville, more or less directly under Moores and his tree. Geology will call it the Coast Range Ophiolite, and it will lie under forty thousand feet of Great Valley sediments and be warped into the coastal mountains. One of its large fragments will end up in the Oakland hills.
When Smartville docks, in the Jurassic, its individual islands possibly resemble Hokkaido, Kyushu, and Honshu. The trench closes east of Sacramento and a new one opens west of Davis and begins to consume the Farallon Plate. The downgoing slab of the Farallon Plate depresses the region and creates the structural basin that will fill up with sediments and become the Great Central Valley. Since there will be no Sierra Nevada and no Coast Ranges for nearly a hundred and fifty million years to follow, the result will be a valley that is not a conventional river valley but a structural basin filled to the brim with sediments that (almost wholly) do not derive from the mountains around it.
Before Smartville, blue ocean—extracontinental, abyssal ocean. In the earliest Triassic, the site of Davis is far out to sea. The continental shelf is back in Idaho and Nevada. North America in these latitudes has been growing. Two terranes have already come in. But here at the dawn of the Mesozoic the continent has not yet received so much as a hint of California.
The Napa Valley is thirty-five miles due west of Davis—an easy run for a field trip, a third of it flat and straight. The occasions have been several, not to mention spontaneous, when Moores and I have made westering traverses, collecting roadside samples of rock and wine.
After the level miles of field crops and fruit trees and almond groves, the ground suddenly and steeply rises in oak-woodland hills, so brown and dry for much of the year that geologists working among them can accidentally start fires with sparks from their hammers. Putah Creek, the stream that has spread its fine silts to Davis, is here a kind of door to the Coast Ranges, spilling forth their contents, coarsely bedded. Among the stream’s cutbank gravels are layers of air-fall tuff that descended from the coast-range volcanoes of the Pliocene, and conglomerates that contain serpentine pebbles, peridotite pebbles, chert pebbles, graywacke pebbles, volcanic pebbles—the amassed detritus of several geologies, suggesting the commotion in the rock to come. Also present are fine-grained remnants of extremely fluid basalts that burst out in the northwest in middle Miocene time, covered areas the size of Iceland in a single day, and are thought to have been the beginnings of the geophysical hot spot that has since migrated to Yellowstone. The Columbia River flood basalts reached their southern extremity here.
As we go up the stream valley and arrive at the shore of Lake Berryessa, we pass through huge roadcuts of sedimentary rock whose bedding planes, originally horizontal, have been bent almost ninety degrees and are nearly vertical. Reaching for the sky in distinct unrumpled stripes, the rock ends in hogbacks, jagged ridges. Cretaceous in age, these are the bottom layers of the Great Valley Sequence, bent high enough to resemble the bleaching ribs of a shipwreck. They are some of the strata that were folded against the Franciscan melange when it rose (or was pushed) to the surface as the latest addition to the western end of the continent. In the heat and pressure of the Farallon Trench, the Franciscan sediments had been metamorphosed to varying extents, with the result that when they ultimately appeared on the surface they were miscellaneous and heterogeneous well beyond the brink of chaos. This lithic compote is the essence of the Coast Ranges. You leave the precise bedding planes and jagged ridgelines of the Great Valley Sequence and enter a country of precipitous nobs and rootless outcrops resting in scaly clay. In its lumpiness it resembles a glacial topography magnified many times. If the Great Valley Sequence can be compared to regimental stripes, the Franciscan is paisley.
“Look at this munged-up Franciscan glop!” Moores exclaims.
Narrow thoroughfares twist amo
ng the giddy hills. Ink Grade Road. Dollarhide Road.
“Look at that melange! Holy moly, look at the lumps!”
Between the grinding lithospheric plates, the rock of this terrain was so pervasively sheared that a roadcut in metabasalt looks like green hamburger. We clearly see its contact with the scaly clay.
“That clay is the matrix of the Franciscan, in which blobs of various material are everywhere contained, and that is the guts of the Coast Range story. The metabasalt is a tectonic block in the matrix. You can see why people who tried to map stratigraphy went crazy. Imagine—before plate tectonics—the aching problems that this fruitcake, this raisins-in-a-pudding kind of stuff, produced. It doesn’t fit the stratigraphic rules we all grew up on. It was assumed that you had a stratigraphic sequence here, and for years people tried unsuccessfully to explain these places in terms of eroded and deformed stratigraphies. In 1965, Ken Hsü proposed the melange idea. But he suggested that the melange had come here by gravity —that it had slid off the Sierra. No one had the idea of underthrusting—what we now see as the subduction of one plate beneath another, with all this miscellaneous material being scraped together and otherwise accumulating at the edge of the overriding plate. In 1969, Warren Hamilton, of the U.S.G.S., published a paper on the underflow of the Pacific crust beneath North America in Cretaceous and Cenozoic time. He presented the paper at the Penrose conference on the new global tectonics. Suddenly, people had a new view of the Franciscan. They said, ‘Oh, that must be a berm resulting from subduction.’ And the whole story broke open.”
The Franciscan melange contains rock of such widespread provenance that it is quite literally a collection from the entire Pacific basin, or even half of the surface of the planet. As fossils and paleomagnetism indicate, there are sediments from continents (sandstones and so forth) and rocks from scattered marine sources (cherts, graywackes, serpentines, gabbros, pillow lavas, and other volcanics) assembled at random in the matrix clay. Caught between the plates in the subduction, many of these things were taken down sixty-five thousand to a hundred thousand feet and spit back up as blue schist. This dense, heavy blue-gray rock, characteristic of subduction zones wherever found, is raspberried with garnets.
In a 1973 paper by Kenneth Jingwha Hsü appears a sentence describing the Franciscan melange—this five-hundred-mile formation, the structural nature of which he was the first to recognize—in terms that could be applied to almost any extended family sitting down to a Thanksgiving dinner:
These Mesozoic rocks are characterized by a general destruction of original junctions, whether igneous structures or sedimentary bedding, and by the shearing down of the more ductile material until it functions as a matrix in which fragments of the more brittle rocks float as isolated lenticles or boudins.
Hsü was born in China and began to use his umlaut as a tenured professor at the Swiss Federal Institute of Technology.
The melange above Auburn, which collected against North America before the arrival of the Smartville Block, tells the same sort of story as the Franciscan, with the difference that the rock in the Sierra melange has been almost wholly recrystallized, as a result of the collisions that completed California. Kodiak Island and the Shumagin Islands are accretionary wedges, too—shoved against Alaska by the north-bound Pacific Plate. The Oregon coast is an accretionary wedge (the Juan de Fuca Plate versus the North American Plate), complicated by a chain of seamounts that have come drifting in, making, among other things, Oregon’s spectacular sea stacks. The outer islands of Indonesia are accretionary berms like the California Coast Ranges (the Indo-Australian Plate versus the Eurasian Plate), not to mention the Apennines of Italy, the north coast of the Gulf of Oman, and the Arakan ranges of Burma.
Now and again in the Coast Ranges you see ophiolite pillows on top of the melange—a typical relationship, since the melange forms at the edge of the overriding plate and the ophiolite is already on the overriding plate, having been previously emplaced there. Ocean-crustal detritus is widespread and prominent among the rocks of the Franciscan, but the Coast Range Ophiolite, in more concentrated form, is in the eastern part of the mountains, where it has been bent upward with the overlying Great Valley sediments, and pretty much shattered. Between Davis and Rutherford is a block of serpentine—disjunct. floating in the Franciscan-that underlies the bowl of a small mountain valley. The serpentine has weathered into soil, now planted to vines. These are some of the few grapes in California that are grown in the soil of the state rock. Moores is predisposed toward the wine. To him, its bouquet is ophiolitic, its aftertaste slow to part with serpentine’s lingering mystery. To me, it tastes less of the deep ocean than of low tide. The stuff is fermented peridotite—a Mohorivicic red with the lustre of chromium.
The winery is in the deep shade of redwoods on a tertiary road. It makes only ten thousand gallons and has been in one family for a hundred years. The cave is in Franciscan sandstone. The kegs, tanks, and barrels are wood. Outside the cave, we stand on a wooden deck looking into a steep valley through the trunks of the big trees. Passing a glass under his nose, Moores remarks that the aroma is profound and reminds him of the wines of Cyprus. There is an intact ophiolite on the side of Mt. St. Helena, at the northwest end of the Napa Valley, he tells me—an almost complete sequence, capped with sediments but lacking pillows. There’s a complete sequence on the east side of Mt. Diablo. “If you mapped the Coast Range Ophiolite, it would go from Oregon all the way down, in discontinuous blobs, plus the shards you see around San Francisco and elsewhere—rocks of the ophiolitic suite that just lie around as broken pieces, like the block that is under these grapes, and cannot be read in sequence.” When Moores was first in California, he happened upon a report about mercury deposits at the north end of the Napa Valley. It mentioned “gabbro … along the contact between serpentine and volcanics.” Moores got into his van, went to the Napa Valley, and looked. He then interested Steven Bezore, a graduate student, in working there. Bezore’s master’s thesis was the first demonstration of an ophiolitic complex in California, and led to the recognition of the Coast Range Ophiolite. After the winery, we stop at a crossroads store, Moores explaining that he requires coffee “to back-titrate the wine.”
The descent is deep to the floor of the Napa Valley, which is flat. For a Coast Range valley, it is also spacious—as much as three miles wide. Vines cover it. Up the axis runs the two-lane St. Helena Highway, which seems to be lined with movie sets. This road is the vague but startling equivalent of the Route des Vins from Gevrey-Chambertin to Meursault through Beaune. The apparent stage sets are agricultural Disneylands: Beringer’s Gothic half-timber Rhine House, Christian Brothers’ Laotian Buddhist monastic chateau, Robert Mondavi’s Spanish mission. Most offer tours, and wines to sip. As a day progresses, tongues thicken on the St. Helena Highway, where the traffic begins to weave in the late morning and is a war zone by midafternoon. The safest sippers are in stretch limos, which seem to outnumber Chevrolets.
Most valleys in the Coast Ranges are smaller and higher than this one, their typical altitude at least a thousand feet. The southern end of the Napa Valley, being close to the San Francisco bays, is essentially at sea level. The valley floor rises with distance from the water, but not much. St. Helena, in the north-central part of the Napa Valley, has an elevation of two hundred and fifty-five feet. It is surrounded by mountains that are comparable in height to the Green Mountains of Vermont or the White Mountains of New Hampshire. Why this deep hole in such a setting?
The San Andreas family of faults is spread through the Coast Ranges, and outlying members are beneath the Great Valley. Where a transform fault develops a releasing bend—which is not uncommon—the bend will pull apart as the two sides move, opening a sort of parallelogram, which, among soft mountains, will soon be vastly deeper than an ordinary water-sculpted valley. In the Coast Ranges, most depressions are high and erosional. Some are deep tectonic valleys that are known in geology as pull-apart basins. In the Napa region, Sonoma Valley
, Ukiah Valley, Willits Valley, and Round Valley are also pull-apart basins. Lake Berryessa lies in a pull-apart basin, and so does Clear Lake.
Where pull-apart basins develop—stretching and thinning the local crust, drawing the mantle closer to the surface—volcanic eruptions cannot be far behind. In the Pliocene, after the Farallon Trench at this latitude ceased to operate and the San Andreas family appeared, the Napa basin had scarcely pulled itself apart before the fresh red rhyolite lavas came up and air-fall tuffs poured in. The Coast Ranges were aglow with sulphurous volcanism, its products hardening upon the Franciscan. The nutritive soils derived from these rocks prepared the geography of wine.
The rocks are known in geology as the Sonoma Volcanics. Napa and Sonoma are Patwin Indian names: “Napa” means house; “Sonoma” means nose or the Land of Chief Nose. The rocks are the Land of Chief Nose Volcanics. Chief Nose was a Tastevin before his time. The heat of the volcanics lingers in the mud baths and hot springs of Calistoga. The heat lingers under cleared woods near Mt. St. Helena, where small power stations dot the high ground like isolated geothermal farms.
As the new fault system wrenched the country, fissures opened, and hot groundwater burst out in the form of geysers and springs. They precipitated cryptocrystalline quartz and—in this matrix—various metals. Some gold. More silver. Near the surface, easiest to mine, were brilliant red crystals of cinnabar (mercuric sulphide). Mercury will effectively pluck up gold from crushed ores. In the nineteenth century, the Coast Ranges were tunnelled for mercury. It was carried across the Great Central Valley and used in the Sierra. The gold of the Coast Ranges was in those days insignificant but is more than significant now. In the nineteen-eighties, the Homestake Mining Company dug two open pits in ridges north of the Napa Valley. In surface area, they aggregate roughly a square mile. The gold is too fine to be seen through a microscope but is nonetheless there in sufficient concentration to be dissolved economically with cyanide. Discoveries of submicroscopic gold in California, Nevada, and elsewhere put the United States in a position to surpass South Africa in gold production by the turn of the twenty-first century—news that geologists regard as only slightly less astounding than the landings on the moon. Homestake’s underground mine in the Black Hills of South Dakota is about a century old, and at latest count was eight thousand and fifty feet deep—the deepest mine in the Western Hemisphere. Homestake has produced more gold than any corporation in North America. With these new claims in the Coast Ranges, the company announced that it had more than doubled its reserves.
