Earthquake Storms

by John Dvorak

  When Pangaea existed and while it was being broken up, the other side of the planet was covered by a vast ocean—the Panthalassa Ocean, which is Greek for “all seas”—which was a much larger version of today’s Pacific Ocean. One of the things known for certain about the Panthalassa Ocean is that its floor was crossed by at least one mid-ocean ridge because segments of this prehistoric ridge still exist—but that is getting ahead of the story.

  As the north-south split of Pangaea grew wider—forming a new ocean, the Atlantic Ocean, between the Americas and Eurasia and Africa—the two American continents drifted west, causing the Panthalassa Ocean to get smaller, its seafloor plunging beneath the American continents.

  It is a process still seen today. Rocks on the seafloor—mainly basalt—are denser than continental rocks. Ocean basins are low and continental mountains are high because when a continent and an ocean basin collide, the denser rocks on the floor of the ocean plunge beneath the lighter rocks of the continent. The resulting plate boundary is a subduction zone with a deep trench. This is happening today along the entire west coast of South America, and for a long time it happened along the entire west coast of North America. But the situation changed when North America encountered the mid-ocean ridge that was already running along the bottom of the Panthalassa Ocean.

  That mid-ocean ridge was, of course, the boundary between two plates. The plate moving away from the ridge to the west by northwest was and is still known as the Pacific plate. The other plate, the Farallon plate, named for the Farallon Islands west of the Golden Gate, was and still is moving to the southeast. North America got a head start over South America on its westerly movement—the north Atlantic opened before the south Atlantic—and that continent has drifted far enough west so that it has overridden a segment of the mid-ocean spreading ridge of the Panthalassa Ocean. At the moment the North American plate touched the mid-ocean spreading ridge, the San Andreas Fault started to form.

  To see it another way, the Juan de Fuca Ridge and the East Pacific Rise are segments of a once-continuous mid-ocean ridge of the Panthalassa Ocean—now the Pacific Ocean. What was once the Farallon plate is now, in the north, the Juan de Fuca plate, which moves eastward from the Juan de Fuca Ridge and plunges under Washington and Oregon. To the south, what still exists of the Farallon plate is now the Cocos plate off Central America and the Nazca plate off South America; both move eastward away from the East Pacific Ridge. And connecting the two segments of mid-ocean ridge—that is, the Juan de Fuca Ridge and the East Pacific Rise—according to Wilson, is the San Andreas transform fault.

  Much detail has been added since 1965, when Wilson made his original proposal. The Juan de Fuca Ridge has been divided into two pieces, the northern one still known as the Juan de Fuca plate, and the southern one, off the coast of southern Oregon and northern California, is called the Gorda plate. Connecting the Gorda plate to the California coast is a long straight feature called the Mendocino escarpment. And where the escarpment touches the California coastline, near Cape Mendocino, that point is regarded as the northern end of the San Andreas Fault.

  Features at the southern end of the fault have also been greatly refined. The East Pacific Rise runs north continuously as far as the opening to the Gulf of California where, from that point north, running along the axis of the gulf, there is a series of short spreading centers connected by short transform faults that continue north until they run across land as far as the southern part of the Salton Sea. Formally, the northernmost spreading center ends on the east coast of the Salton Sea at Bombay Beach, which is regarded as the southern end of the San Andreas Fault.

  All this raises the question: When did the transition occur from subduction zone to transform fault? When did the earliest vestige of something like the San Andreas Fault form in California? To answer that question, one has to look to the sea.

  Tanya Atwater was still in high school when Wilson was standing on the slope of Mauna Loa, thinking the Earth’s surface was immobile. She was in college when he proposed the idea of mobile rigid plates, and had completed her degree in geophysics the year Wilson proposed the San Andreas was a transform fault—an indication of how quickly ideas were changing. Anxious to be part of the revolution in understanding how the Earth worked, Atwater attended the Scripps Institute of Oceanography near San Diego, where she hoped to do her own research that would reveal even more tantalizing details of the Earth’s mobility. But initially she was stymied in her attempt—because she was a woman.

  To make the crucial measurements that would change our view of the solid Earth and lead to the concept of plate tectonics—the phrase “plate tectonics” would not be used until 1969—one had to go to sea, but in the 1960s, to go to sea on an oceanographic ship, such as those operated by Scripps, one had to be a man.

  “She would have no privacy,” Atwater remembered was a common reason to exclude her. Another was that a woman would have no convenient or comfortable place to bathe or to urinate. And there was the possibility that just the presence of a woman working in the deep confines of a ship might be enough to tempt even the most self-disciplined male crew member.

  Atwater also endured personal slights. She would remember that, when visiting other institutions, anxious to discuss her own work, she was often not introduced, others assuming she must be some guy’s tag-along girlfriend.

  But Atwater was fortunate in that her male mentors—at the time all senior scientists at Scripps were male—gave her the respect she deserved and insisted that she have the opportunity to work on a ship and go to sea. And when she went to sea, one of the things she noticed on those oceanographic cruises was that the pattern of seafloor magnetic anomalies off the California coast was different from those elsewhere in the ocean.

  Here it is instructive to step back a bit. During the informal meeting held in Wilson’s office when Vine unrolled the map of seafloor anomalies in the northeast Pacific, Wilson, Vine, and Hess all instantly recognized a pronounced linear anomaly that indicated the crest of the subsequently named Juan de Fuca Ridge. They also noticed something else. As Vine would recall the scene, the three men stared in amazement at the map because not only were there other linear anomalies parallel to Wilson’s proposed ridge, but a symmetry to the pattern of anomalies about the ridge crest. And Vine soon had an explanation.

  As Hess had suggested years earlier, hot material from the mantle rose along a ridge crest and formed new seafloor. Then, as Vine now explained, as the material cooled its temperature passed below the Curie point, the temperature below which a material with suitable minerals, known as ferromagnetic minerals, would become magnetized in the Earth’s field. Mantle material had such minerals, so after the material cooled, it left a pronounced magnetic anomaly along the ridge crest. Then as the ridge continued to spread open, the now-cooled material moved off to the sides of the ridge and new, hot mantle material rose to replace it.

  However, the direction of the Earth’s magnetic field flips every hundred thousand or few million years, so that the north magnetic pole becomes the south magnetic pole and the south magnetic pole becomes the north magnetic pole. Why the magnetic poles flip is still not known, but the fact that they do produces an interesting pattern of linear magnetic anomalies on the seafloor—known as magnetic remanence—one that is often compared to zebra stripes.

  All seafloor material that was at a ridge crest and cooled when the Earth’s magnetic field was in its current alignment has the same direction of magnetization and is known as “normal.” Material that cooled when the Earth’s field was in the opposite alignment is “reverse.” And so, as a continuous supply of hot material rises from the mantle, cools, and moves off and away from a ridge crest—as Hess described it, in the manner of “a conveyor belt”—an alternating-line pattern of normal and reverse magnetic anomalies is produced. Therefore—and this is the important part for what Atwater would discover—if one finds the same pattern of normal and reverse ma
gnetic anomalies on different sections of the seafloor, then those two sections of seafloor formed over the same age range. Furthermore, one can match individual anomalies on different sections of seafloor and know they were created at the same time.

  The basic idea is straightforward, but is a challenge to put into practice. For example, the age of seafloor magnetic anomalies had to be determined from both radiometric age dating of lava flows on land and from the recovery of sedimentary samples taken from the seafloor that contained microscopic fossils. Also, it had to be determined whether seafloor-spreading rates were constant or ebbed and flowed.

  It might seem hopelessly complicated, but Atwater found a way to do it by going on oceanographic cruises and collecting and compiling an immense amount of data, and from that making a detailed map of magnetic stripes across much of the northeast Pacific.

  Her map, published in 1970, showed 32 stripes, each one identified with a “magnetic isochron number,” or “chron.” Chron 1 was the magnetic stripe currently forming over the crest of the Juan de Fuca Ridge. Chron 32 was found out in the Pacific almost as far as the Hawaiian Islands and she identified it as having formed along the crest of the mid-ocean ridge 75 million years ago.

  All mid-ocean ridges spread symmetrically. But Atwater realized that the pattern of magnetic stripes off the coast of California has no symmetrical counterpart because the mid-ocean ridge that formed those stripes has disappeared—that is, it has slid under the North American plate. And that was the key: The age of the magnetic anomaly next to the coast of California would be the age of the first San Andreas–type fault, and thus how old the fault was would finally be revealed. After considerable work, Atwater identified the magnetic anomaly, Chron 8, and determined the age of the San Andreas Fault: 25 million years old.

  The fault may be 25 million years old, but as far as human understanding of the fault and geophysics in general, it is astounding how quickly ideas progressed from Hess’s geopoetry to Wilson’s mobile plates and to understanding the origin of the San Andreas Fault, then to Atwater’s success in answering a question that, a decade earlier, no one could have imagined to ask or, if someone then had such a fanciful notion, no one could have conceived how the question might be answered. And even more astounding considering how little men like Richter, Lawson, and others understood about the geological anomalies they were seeing and earthquakes they were feeling and measuring just a generation before. And it was just the beginning. Now a whole range of geologic puzzles could be solved.

  One that had been perplexing geologists since the time of Lawson and his discovery of the San Andreas Fault was the origin of a chaotic mixture of mangled sandstones, shales, cherts, and basalts that comprise most of the Coast Ranges of California. That includes every major hill of San Francisco, including Telegraph Hill, Russian Hill, and Nob Hill, as well as Alcatraz Island in San Francisco Bay, and Twin Peaks and San Bruno Mountain south of the city, a complex assemblage that Lawson had named in 1892 the Franciscan Series.

  What was puzzling and intriguing about the Franciscan Series was that it was impossible to put the various rock units in stratigraphic order. For decades, Lawson and, later, other geologists strained to use the tried-and-true methods of superposition—whereby rock layers are deposited in a time sequence with the oldest at the bottom and the youngest at top—to decide in what order the various sandstones, shales, cherts, and so forth had been deposited. But this conflicted with what fossils recovered from those same layers indicated about the relative ages. Fossils indicated that some layers higher in the sequence were actually older than layers underneath. The solution became clear after Wilson’s work and the development of the theory of plate tectonics, after it was realized that ocean crust could subduct beneath continental crust.

  To see the evidence for oneself, I suggest going to the rocky Marin Headlands immediately north of the Golden Gate Bridge. After gazing at the fantastic view of San Francisco’s cityscape, turn around and look at the rocks exposed in the roadcut.

  What catches the eye are the parallel ribbons of red chert folded into giant kinks and the occasional chevron. If the chert is examined closely—a glass hand lens is of great utility—one sees countless white dots. Those dots are skeletons of a tiny sea creature, a radiolarian, that built its tiny shell out of the mineral silica. Radiolarians thrive only in warm equatorial waters, and when they die their skeletons sink slowly to the ocean floor to form what is inelegantly though accurately known as radiolarian ooze. But how did the skeletal fossils of these tiny equatorial creatures get to California, and why can they be found interspersed among the various sandstones, shales, and basalts of the Franciscan? The answer: because they rode in, conveyor-belt style, on the Farallon oceanic plate, then were smashed against the continent of North America.

  To elaborate, as the North American plate drifted west, the Farallon plate moved southeast. The collision formed a subduction zone as the denser Farallon plate slid under the lighter continental rocks of North America. As it did so, the brow of North America acted like a bulldozer and scraped off some of the rocks of the seafloor, forming what plate tectonophysicists call “an accretionary wedge.” The rocks of this wedge are the crushed and mangled and highly deformed rocks of the Coast Ranges—which includes the red ribbon chert north of the Golden Gate—as well as the hills of San Francisco, most of the mountain mass of Big Sur and its spectacular sea cliffs, and much more. In short, the Franciscan contains rocks that, quite literally, came from a wide stretch of the seafloor of the ancient Pacific Ocean, then were piled up against the northern and central coasts of California.

  To see the full suite of rocks of the Franciscan, one could drive hundreds of miles along back roads that wind through the Coast Ranges. But there is a simpler way.

  At a few places along the coast, hillside streams and ocean currents have combined to concentrate rocks washed down from the Franciscan at several beaches. The most famous is Moonstone Beach near San Simeon.

  Here the beach is covered with pebbles of a wide variety of color, texture, and luster, making it a favorite of rock collectors—which is why one sees, at low tide, people walking slowly up and down the beach, their eyes fixed to the ground, their bodies occasionally bent down to examine a particular pebble. Among the most common of the pebbles are those of the red chert. Another is a drab gray sample of seafloor basalt. Pebbles of green serpentinite struck through with tiny veins of white chrysotile—that is, asbestos—are not uncommon. But the one that most collectors seek—and is the mistaken namesake of the beach—is known to locals as moonstone.

  True moonstone is a low-quality gem that displays an iridescent, silvery moonlight-like quality. But there are no moonstone gems at Moonstone Beach. What people find is a white translucent stone, often with gray or brown bands, composed of the mineral chalcedony, a form of silica. Ironically, considering how some geologists flock to Moonstone Beach to examine pebbles of the Franciscan—it is probably the best place to hold samples of California’s diverse geology in one’s hand—the pebbles of chalcedony that most rock collectors seek are not products of the collision of the Farallon and North American plates. Instead, they formed long after the seafloor was plowed up and the Franciscan created, and thus are the result of the slow dripping of silicate-rich water through cracks and crevices in caves on dry land. The mistaken moonstones at Moonstone Beach are not part of the plate tectonics story of California, but of a geologic process that can be found in caves around the world.

  Nevertheless, almost every other pebble on Moonstone Beach is. And if one wants more evidence of the plate-against-plate collision and proof that it changed from subduction to a San Andreas–type transform fault, one has only to ask: Why is there oil in California?

  Almost all of the oil in the Golden State is derived from the organics of siliceous diatom frustules. Diatoms are microscopic bivalves; frustules are the hard external cell walls covering diatoms, composed almost entirely of si
lica. When diatoms die, their frustules, now containing decaying organic matter, sink. Unlike radiolarians, which live far out in the ocean, diatoms thrive close to shore in shallow basins. During the middle Miocene, about 16 million years ago, there was a great proliferation of diatoms along the California coast. In part, this proliferation, which produced a thick section of organic-rich sediment, was caused by a change in the pattern of ocean currents in the Pacific Ocean—a change in pattern caused by a closing of the Indonesian Seaway as the plate carrying Australia pushed up against southern Asia. There was also a proliferation because there were the exact geologic conditions for the organic bodies of diatoms and their attendant frustules to accumulate and, later, to be buried.

  Sixteen million years ago, as the transition was still under way from subduction to transform fault, a series of shallow undersea basins formed along the California coast from Newport Beach south of Los Angeles to Eel River Basin near Cape Mendocino. The geologic conditions were such that, at first, little sediment washed from the continent and over the thick accumulation of frustules; but after a few million years that changed, and those carbon-rich beds were buried. Those beds now comprise the most economically important geologic layer in California—the Monterey Formation.

  The frustules in the Monterey Formation were buried at the right depth and for the right amount of time to be changed into a waxy material known as kerogen, and if the depth and time were exactly right, some of the kerogen was changed into oil, which flowed and accumulated in reservoirs.

  The Monterey Formation can be found exposed at many places. One of the more spectacular is at Montaña de Oro State Park near Morro Bay, a few miles south of Moonstone Beach. Here the organic-rich rocks are a white shale. If one drives about 100 miles farther south to the seaside community of Carpinteria, one can actually see oil seeping out of the ground and onto the sandy beach—and offshore derricks where oil is being pumped out of the ground. And, of course, it is oil from the Monterey Formation that feeds the black tar to the famous La Brea Tar Pits along the 5800 block of Wilshire Boulevard in Los Angeles.