This piece is part of a special project on deep time examining what the Western U.S. was like thousands, millions and even billions of years ago, and how that history is still visible and consequential today. Read more stories from the series.
The Scripps Institution of Oceanography in La Jolla, California, was in chaos when 24-year-old Tanya Atwater arrived to pursue her graduate studies. It was January 1967, and a buzz of frenetic energy filled the air. Long rolls of paper printed with squiggles of magnetic data spooled down the hallways, retrieved from odd corners where they had lain covered in dust.
A few weeks earlier, a Cambridge geophysicist named Fred Vine had visited Scripps to explain a theory so new it didn’t yet have a name. Some called it the Vine-Matthews Hypothesis; others referred to it as seafloor spreading. It was a critical advancement on the old, long-discredited notion of continental drift. The new theory would come to be known as plate tectonics.
Atwater stood out among the other scientists. For one thing, she was a woman in a field dominated by men. She was also a self-described “full-on Berkeley hippie … barefoot, with beads and flowers.” She had an undergraduate degree in geophysics and a knack for seeing the big picture.
Her first class was marine geology. The professor didn’t bother with the syllabus. Chalk flew over the blackboard as he dove right into this “wonderful new idea,” which had transformed the study of the oceans and would, in time, upend the story of the continents and how they came to be. No point in opening the textbooks; they were suddenly out of date.
A revolution was going on, and Atwater had stepped into the middle of it.
THE IDEA THAT CONTINENTS ONCE FITTED TOGETHER like puzzle pieces had been around at least as long as accurate maps of the world had existed. But it was easily dismissed; until the mid-20th century, most geologists were “fixists” who believed the land masses hadn’t budged since the beginning of time.
Yet all over the planet, paleontologists were finding similar rock layers cradling similar types of fossils. A reptilian creature from the Permian period showed up in both Brazil and southwestern Africa. One particular fern was scattered throughout the Southern Hemisphere. How had plants and land-bound animals crossed vast oceans to reach other continents?
In a 1915 book, German scientist Alfred Wegener offered a solution. Maybe the continents had once been joined and subsequently drifted apart. World War I was raging, so few people paid attention until the third edition of his book was translated into English, French, Spanish, Russian and Swedish. Scientists could now ridicule Wegener’s theory of continental drift in six languages. According to Atwater, American geologists especially “pooh-poohed” the idea.
Still, the notion that continents drifted didn’t go away.
ATWATER HADN’T SET OUT TO BE A PART OF A SCIENTIFIC REVOLUTION. “It was just a lucky chance of having the right background at the right moment in the right place,” she said.
She was born in California on Aug. 27, 1942. As a girl, she liked to draw, but her dreams of becoming an artist took a turn when the Soviet Union launched Sputnik in 1957. “It was just so astonishing that science could put something into outer space,” she said.
Her engineer father and botanist mother encouraged her newfound interest. When a recruiter from the California Institute of Technology visited Atwater’s high school, she asked about science degrees. But Caltech didn’t accept women; they would just get married, quit and waste their educations, Atwater remembered being told. She visited Harvard next, which pointed her toward the neighboring women’s college, Radcliffe — a nonstarter, as she lacked the Greek or Latin prerequisites.
Never mind: The Massachusetts Institute of Technology would do.
MIT welcomed women into its science programs. When Atwater asked a professor why, he replied MIT-educated women would raise great children, which she took to mean boys. “I didn’t even blink,” she wrote in Plate Tectonics: An Insider’s History of the Modern Theory of Earth, to which she contributed a chapter. “I guess I was used to it by then.”
Atwater wandered through five majors in the next three years, churning through organic chemistry (“just horrible”) and electrical engineering (“I kept electrocuting myself”). Then she enrolled in a field course in Montana. It was her first introduction to practicing geology in the field. She was in heaven; she hiked and marveled at the mountains, learning how their geometry fit together and translating that knowledge onto maps. Could you actually get paid to do this? she wondered.
“Anything that you study, you see much better than the rest of the world,” she said. “Suddenly, the landscapes and the rocks, they were talking to me.”
Still — geology. It was, in Atwater’s words, a grubby and obscure field, totally removed from the slick, high-tech satellite dreams that propelled her into science. She had no memory for facts, and geology was allfacts. What forces spit out volcanoes and crumpled mountains? At the time, professors had no explanation, and students drew arrows on their maps like “the hands of a capricious god shortening or extending our landscapes, willy-nilly.” Was this to be her career?
She dropped out of MIT. She traveled and deliberated. All the while, the mountains kept talking.
“Anything that you study, you see much better than the rest of the world. Suddenly, the landscapes and the rocks, they were talking to me.”
IF ATWATER HAD BEEN BORN IN THE VERDANT EAST, her career might have turned out differently. Much of California’s landscape is stark and exposed. “In the West,” she said, “the rocks are all standing up and yelling at you.” She realized that if she was going to study geology, she needed to go back to her home state.
Atwater arrived at the University of California-Berkeley at the height of the Free Speech Movement. Berkeley was a hub of counterculture, civil rights and protests against the Vietnam War — “not a war that my generation could believe in,” Atwater reflected. Students vanished from the hallways, taken by the draft, or dropped out to enlist in the Coast Guard and avoid being shipped overseas.
Atwater enrolled in geophysics, the study of Earth’s processes. She transferred her math and physics credits from MIT and crammed as many geology classes into her schedule as she could. It wasn’t an obvious time to dwell on slow forces and distant pasts. Enrollment in undergraduate geology programs was dropping, and students mostly ended up working in the petroleum industry. Amid a cultural revolution, it seemed to be a field as stolid and immovable as the mountains themselves.
She graduated with a B.A. in geophysics in 1965, and once again headed east, to Massachusetts, this time for a summer internship at the Woods Hole Oceanographic Institute. At the time, oceanography and geology were separate realms, with little communication between them. Atwater didn’t know there was anything geologically interesting happening underwater; she was simply “drawn to the romance of going to sea.”
When her colleagues began preparing for a meeting in Ottawa, Canada, Atwater asked if she could go. There, she heard geophysicist J. Tuzo Wilson give a lecture about a new kind of geologic feature. He called it a “transform fault.”
A fault is a fracture in the Earth’s crust. The simplest kind is when one block of earth rises or drops vertically in relation to another. But a transform fault, Wilson explained, is defined by horizontal movement rather than vertical. Along the seafloor, many such faults intersected “mid-ocean ridges,” long chains of undersea mountains cloven by lava-spitting rifts. Mid-ocean ridges delineate the Earth’s crust into large, rigid plates — plates that can move apart slowly as magma wells up between them. The edges aren’t smooth lines, however; they’re more jagged, and follow a sort of stairstep pattern, with transform faults linking separate ridges together like messy stitches in a seam.
Wilson had been a fixist until recently. Now he was declaring himself a “drifter” who believed the continents rode atop moving plates. He passed out paper diagrams to explain his hypothesis. “Cut here, fold here, pull here,” read the instructions. The crowd laughed. Atwater felt embarrassed. A children’s game, at a serious scientific meeting? But something about the diagrams hooked her imagination. In the privacy of her hotel room, she cut and folded and pulled.
AMID ALL THE TALK OF THE OCEAN, one thing stuck with her. Wilson had mentioned an unusual transform fault that occurred on land — in Atwater’s home state, in fact. It was the San Andreas.
Stretching 800 miles from Mendocino to the U.S.-Mexico border and reaching 10 miles deep, the San Andreas Fault is not a single crack in the Earth’s surface but a network of fractures. It’s most visible on the Carrizo Plain, a rumpled piece of grassland bordering the Temblor Range in central California. There, dry creek beds cut downhill toward the fault, reach it, and jump hundreds of feet sideways before continuing on.
Catastrophe prompted the first study of the San Andreas — the San Francisco earthquake of 1906. Thousands died; tens of thousands lost their homes to the quake and the fires that burned for days afterward. Smoke was still rising from the ruins when a geologist named Andrew Lawson assembled an Earthquake Investigation Commission. A decade earlier, Lawson had identified a fault in the San Andreas Valley. Now, his team painstakingly traced this fault on foot and horseback for hundreds of miles, documenting recently toppled statues and disjointed fences, roads and bridges. The commission concluded that the land on one side of the San Andreas had slipped northwesterly while the other side lurched to the southeast, causing the quake. In some places, the land shifted as much as 21 feet.
Credit: Courtesy of Tanya Atwater
THE IDEA OF CONTINENTAL DRIFT LANGUISHED FOR DECADES, largely ignored by the scientific community. But in 1963, the first real advancement on the concept came from an unexpected location: under the ocean. That year, Cambridge geophysicist Fred Vine and his colleague, Drummond Matthews, published an obscure paper in Nature. It pointed out magnetic anomalies on the ocean floor that, they claimed, could be explained by the signature of the Earth’s magnetic field on freshly cooled lava. The patterns appeared in stripes, some with normal polarity — minerals within the lava pointed north, like tiny compass needles — alternating with stripes that had reversed polarity, in which the minerals pointed south instead.
Vine and Matthews inferred two things from the anomalies: Earth’s magnetic field had, at various times in the deep past, reversed, and more importantly, the seafloor was spreading. Magma welled up at a mid-ocean ridge and bubbled through the long rift in the center. As it cooled, it took on the imprint of the Earth’s magnetic field. More magma surfaced and shouldered the new rock aside. Over millions of years, a zebra-pattern formed on either side of the ridge, with stripes running parallel to it — bands of seafloor marked by alternating magnetic signatures.
The stripes had been documented by magnetometers that were towed behind research ships; oceanographers had amassed stacks of these measurements without realizing their importance. Vine and Matthews’s work suggested that you could read them like a history book, albeit one missing any dates — assuming the seafloor really did spread, and the magnetic field really did flip. Using one radical unproven theory to justify another radical unproven theory was like trying to net wind or squeeze water. According to Vine, the idea “went over like a lead balloon.” Other scientists called the work unconvincing, even heretical.
James Heirtzler, who led a team at Lamont Geological Observatory in New York that specialized in magnetic profiles, was among the critics. But then, in early 1966, two of his graduate students noticed a startling pattern in the magnetic data collected the year before on the Eltanin, a floating science lab devoted to exploring Antarctic waters. When they first brought Heirtzler the results, he refused to believe his eyes. It took him a month to accept the implications. Later that year, while passing through Santiago, Chile, he elbowed his way onto the agenda of an international scientific meeting about Antarctica to share the data.
Atwater was in Santiago, too, having moved there for a geophysics job. She’d been spending her days reading recordings of earthquakes in dramatic landscapes like the Atacama Desert, where, she said, “it was so quiet we could hear the roots of the tree groaning in the ground when the wind blew.”
At the Antarctica meeting, Atwater was dozing through Latin-riddled presentations about plankton and other minuscule bits of marine life. When it was time for Heirtzler’s presentation, Atwater recalled, “Everybody … snuck out to go to lunch. I’m sure there were other people in the room, but I remember being all alone at this talk.”
Heirtzler showed a slide of the Eltanin data.
“It was just a line, a wiggly line,” Atwater said. “But it was symmetrical.” The data fit the ridiculed Vine-Matthews Hypothesis precisely. If volcanic rock really squeezed up out of a mid-ocean ridge and spread in either direction, and if it really were imprinted by the Earth’s magnetic field, then the pattern would be symmetrical on either side of the ridge. “There may be no other natural phenomena where nature shows such order,” Heirtzler wrote.
This animation shows seafloor spreading from a mid-oceanic ridge connected by a series of transform faults with stripes showing the polarity of seafloor magnetization. Made by Tanya Atwater with helpful comments from Ken MacDonald and Doug Burbank.
Even better, the profile had timestamps, of a sort. A U.S. Geological Survey research team in Menlo Park had recently measured the age and polarity of volcanic rock samples to work out a timeline for when the magnetic field randomly switched direction in the past 4 million years. It was meticulous research that many geologists dismissed as meaningless and fringe. Now, that calendar made it possible to interpret the groundbreaking Eltanin profile. Heirtzler showed the two datasets side-by-side: the zebra-stripe pattern and magnetic reversal timeline. “It matched perfectly,” Atwater marveled. “It was ironclad.” Seafloor spreading was real.
The revelation hit like a lightning bolt for the 24-year-old scientist who had remained in the emptying room. California was the epicenter of a radical new understanding of the Earth. She had to get back there.
ARRIVING AT SCRIPPS IN JANUARY, Atwater plunged straight into the pandemonium caused by Fred Vine’s recent visit. Students and professors pored over old magnetic profiles in search of symmetry and found it everywhere. “They’d been collecting magnetic data from the ships just because it was an easy measurement to make,” Atwater said. “They dug them all out and looked at them again: ‘Oh my God, there it is, the pattern!’” Longtime fixists transformed, seemingly overnight, into drifters. The paradigm shift was in full swing — at least for those who studied the ocean floor.
“The people on land knew there was some revolution going on in the ocean,” Atwater said. She put an inflection of contempt into her voice: “But, you know, the ocean.”
Geologists didn’t understand how seafloor spreading was relevant to their work. They needed a translator, and Atwater, with one foot on land and another at sea, was destined for that role. It didn’t matter that she was still a student. Everything had been upended. As one of Atwater’s contemporaries wrote, “The plate tectonic revolution was a great leveller: everyone found themselves in the same boat, regardless of age or experience.”
Well, not quite the same boat. Women weren’t allowed on boats.
It wasn’t a rule, really, just a lingering superstition about bad luck and concerns over the propriety of shared bathrooms and cramped quarters. This was inconvenient, since Atwater belonged to the Deep Tow research group, which would soon sail over the Gorda Rift offshore of Northern California, towing instruments to collect new data. Previous research vessels had collected data close to the surface, but Deep Tow would drop a cable down to the bottom, outfitted with sonar, sounders, underwater cameras, a magnetometer and other instruments. The expedition would provide the closest look yet at a seafloor-spreading center. In closed-door meetings, Scripps scientists argued about what to do with “the girl.” Atwater’s advisor, John Mudie, cut to the chase: “She could sue you, you know.”
“That’s all it took,” she said. She was going to the Gorda Rift.
To record data from the research ship, the students had to keep the scientific instruments just above the seafloor without bashing them into rocks or cliffs. It was a hair-raising task: “We all had to take turns pulling in and letting out the cable,” she said, while others watched the readout of sonar images showing the undersea topography. To see real-time images of a mid-ocean rift, “the center fallen in and fresh lava pouring over the seafloor,” brought clarity to everything Atwater had learned so far.
By this time Vine and Matthews had refined their hypothesis, describing how, thousands of miles from its origin in a mid-ocean ridge, the seafloor was swallowed by trenches, chewed up and recycled in the ever-moving mantle. They called this process, unromantically, a “conveyor belt.” The seafloor was not static, but churning, seething and eternally young, at least by geologic standards. Atwater was seeing it for herself.
THE NEXT TURN IN ATWATER’S CAREER revolved around a sketch on a napkin.
One autumn evening in 1967, she was talking with a visiting scientist at a dance hall not far from Scripps. Between beers and blaring accordions, Dan McKenzie scrawled a diagram of the San Andreas Fault — the boundary between the Pacific and North American plates. McKenzie shared a secret: A third plate bordered the other two just off Cape Mendocino, California, one of the most earthquake-prone spots in the Lower 48.
McKenzie hadn’t yet published his paper on what he called “triple junctions.” For Atwater, the pieces clicked. Triple junctions, seafloor-spreading centers, transform faults: Altogether, “they were the key to the whole geometry of the ocean,” she realized. Nobody had figured out how these newfangled concepts applied to geologic features on land; at this point, plate tectonics was an oceanic revolution. But there was a plate boundary on land: the San Andreas Fault. If she could explain how the San Andreas formed, she could bring the entire theory onto dry land.
“The plate tectonic revolution was a great leveller: everyone found themselves in the same boat, regardless of age or experience.”
ONE SMALL PROBLEM. The timescale of magnetic reversals went back a paltry 4 million years, barely into the Pliocene. The San Andreas was older than that — some scientists speculated much older, perhaps 100 million years, though nobody knew for sure. Was it a fast-moving young fault, or a slow-moving old one? Atwater knew she could answer that question using the zebra-stripe patterns just offshore — if only she had reliable dates.
That work was underway at Lamont Geological Observatory under the guidance of James Heirtzler. In 1968, Heirtzler published a wildly speculative timeline for magnetic reversals for the last 85 million years, back to the Late Cretaceous, the age of dinosaurs. “Their best guess was very good,” Atwater said. “But nobody knew if it was going to hold up or not. You have to put in a lot of weasel words, ‘maybe this is so,’ blah blah blah, ‘but maybe that’s so.’” She didn’t want to write a paper chock-full of uncertainty.
Atwater went to Washington, D.C., that spring to talk about the Gorda Rift at an American Geophysical Union meeting. It was her first conference lecture, leading up to her first paper, which would appear in the prestigious journal Science. She was a rising star, flush with victory. Afterward, she joined a group of students who traveled from D.C. to New York to tour Lamont.
The glow of success winked out. “I was invisible,” she wrote. “At every lab we visited, they introduced all the young men and skipped me, every time.”
THAT AUTUMN, A RESEARCH VESSEL called the Glomar Challenger sailed over the Mid-Atlantic Ridge to drill holes and sample sediment. Fossils within the sediment samples they retrieved gave scientists a way to put dates to the bottommost, oldest layers. To everyone’s surprise, Heirtzler’s best guess for the past 85 million years was spot-on.
It was the missing piece. Within months, Atwater had abandoned her official work with Deep Tow and was immersed in the study of the San Andreas. It was the most intense period of her career. “There were nights I couldn’t sleep,” she said. “I’d call up my mentor at 2 in the morning.” Her focus was a zebra-stripe pattern off the Pacific coastline that was an oddity. It was entirely one-sided: The pattern on the eastern side of the ridge was missing. The aberration bothered scientists; it looked like a flaw in the case for seafloor spreading.
McKenzie, the napkin-sketching scientist, proposed an explanation. The third plate — the one he revealed to Atwater in the dance hall — was the remnant of an older plate, called the Farallon, which once stood between the Pacific and North American plates. Over time, the Farallon had been pulled, or subducted, beneath the North American plate, carrying with it the missing half of the zebra pattern.
Atwater took up the story from there. Only after the Farallon had vanished, leaving a few fractured pieces behind, could the Pacific plate rub up against the North American, forming the San Andreas. Thanks to the Glomar Challenger research, she could trace every step of this speculative history as far back as 85 million years.
She didn’t need to go that far back, however. Using magnetic data as the key, she unlocked the answer to a question that had dogged everybody. The San Andreas was younger than many had speculated, she determined; it couldn’t be older than 23 million years. And with that piece of information, she showed how the unseen movements of plates shaped the geography and geology of California, thereby proving to geologists that plate tectonics mattered to their work.
IN DECEMBER 1968, the Geological Society of America convened a five-day meeting at the Asilomar Conference Grounds in Pacific Grove, California, to discuss “the new global tectonics.” Atwater was the only woman invited to speak.
“I might have been the only woman in the room,” she recalled. Dressed in her usual flowers and beads, she stood in front of the jampacked room to present her findings. When she rambled over the time limit, the moderator tried to stop her. But a voice rose from the rapt audience: “Let her go on! This is great stuff.”
Atwater kept going. When she finished, a skeptic challenged her. Could they really believe the San Andreas was that young? It was contrary to the opinions of many eminent geologists. Atwater was scrambling for a reply when another scientist shouted, “It’s true! Believe it!” Ken Hsu had been aboard the Glomar Challenger for the deep-sea drilling expedition. He defended Atwater’s findings with “all the passion of the newly convinced,” she recalled.
The audience was stunned. Atwater was the first person to use the newly revealed secrets of the seafloor to explain a geological feature on land. She showed her colleagues that anywhere you had magnetic data, you could reconstruct the movement of Earth’s tectonic plates across millions of years.
The movement of plates — what J. Tuzo Wilson called “the dance of the continents” — explained not just the San Andreas Fault, but California’s pattern of earthquakes, the birth of the Coast Ranges, volcanism in the Cascades and the opening of the Gulf of California. No wonder she ran overtime. She tracked this story from the start of the Cenozoic Era, 66 million years ago, from the extinction of dinosaurs, to the rise of mammals, through the rending and suturing of continents and seas.
“I felt I had just been witness to the end of an era,” one listener later recalled, “and the beginning of a totally new approach to understanding the dynamics of the Earth.”
SPEAKING INVITATIONS POURING IN. Atwater’s ability to translate ocean science into earth science packed lecture halls. “I gave a bazillion talks everywhere,” she said. “Everybody came, partly to see the girl geophysicist, but also to hear what the revolution could mean for them.”
A year after she spoke at Asilomar, Atwater published her findings in The Bulletin of the Geological Society of America. Geology students everywhere were assigned to read it. Atwater was still a student herself, but she didn’t find her new notoriety awkward.
“I knew I deserved it,” she said.
Extra pages were stuffed in the backs of geology textbooks; then, new editions appeared with plate tectonics woven into every chapter. The movement of the plates gave an underlying logic to once-mysterious phenomena, such as earthquakes and volcanoes. It was no longer heresy to envision very different Earths in the deep past.
Atwater earned her Ph.D. in oceanography from Scripps in 1972. Two years later, her son, Alyosha, was born. It’s a part of her story she thinks women need to hear. “When I was coming through,” Atwater said, “I thought I had to choose: a family or a career. I had no idea that you could manage both.”
She taught for seven years at MIT, then went back West to spend the rest of her career at the University of California-Santa Barbara, in view of the arid, pine-studded Santa Ynez Mountains, a section of the Transverse Ranges that rises between the Pacific and the San Andreas Fault. She descended to the ocean floor 12 times in a 6-foot submersible, the Alvin. Those day-long trips were launched from a catamaran where Atwater had to sleep on the floor until she convinced her colleagues it was OK to share the tiny bedroom with men. “There’s still some of the same silliness,” Atwater admitted, speaking about the experiences of women in science. But, she added, “for many years, I owned the ladies’ room,” at American Geophysical Union meetings. “Now I have to stand in line.”
Atwater credits her parents for the conviction that she could be anything she wanted. “An awful lot of women didn’t get that message,” she said. She once worried about having switched majors so often. Now she feels that every step she took led her in the right direction. “You have to have some faith that it’s going to work out.”
Now a professor emeritus, she tags along on university field trips, sharing her love of geology with students across California’s rugged mountains and jagged coastline. Since the mid-’90s, Atwater has used her artistic knack to create digital animations that explain plate movements, “showing all the things that I can see in my head.” The animations explore Earth’s history, from the global breakup of the supercontinent Pangea to the creation of California’s Transverse Ranges and Channel Islands. She lives in the center of that gorgeous geography, which was caught between jostling plates 18 million years ago and twirled clockwise 110 degrees, a rotation that continues to this day.
Science, like the Earth, isn’t static. Atwater keeps waiting for some new idea to overturn the theory of plate tectonics. “But it hasn’t,” she said, with a tinge of surprise in her voice. “We make more measurements and it’s more precise. We never know for sure; we can never prove anything is true. But this is about as sure as you can get.
“It’s a relief,” she added, “since I spent my whole life on it.”
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This article appeared in the January 2026 print edition of the magazine with the headline “Continental Shift.”
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