PubTalk 7/2004 — Secrets in Stone

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The Role of Paleomagnetism in the Evolution of Plate Tectonic Theory Video Presentation

Presentation of the award-winning USGS video "Secrets in Stone" (35 minutes), introduced by Jack Hillhouse, Research Geophysicist, and followed by a tour of the USGS Paleomagnetics Laboratory

  • Crucial discoveries in the early 1960.s were made in a "tar-paper shack" at the USGS Menlo Park campus
  • Learn how the old idea of .continental drift. was surprisingly revived by these discoveries and woven into the new theory of plate tectonics
  • Hear about the strange behavior of the Earth.s magnetic field: drift and occasional reversals of North and South poles
  • Do we know when the next flip (reversal) of the Earth's magnetic field will happen?
  • Tour the USGS Paleomagnetics Laboratory (Building 16 - see map on back) after the presentation and learn about the current research taking place there. All welcome.

Details

Date Taken:

Length: 01:05:25

Location Taken: Menlo Park, CA, US

Transcript

Good afternoon. My name is Leslie Gordon, and welcome to another preview of tonight’s public lecture. In fact, rather, it’s not a lecture, but we’re going to have a film screening this afternoon. You all know that the USGS is celebrating not only our 125th anniversary this year, but we in Menlo Park are celebrating our 50th anniversary. And, as part of that 50th anniversary, many of our lectures in this – in this monthly series deal with some aspect of our history and about our scientific accomplishments in the past 50 years here in Menlo Park. And today, we’re going to be talking about some landmark work done in the field of paleomagnetism. The film that you’re going to see today was actually created when an old termite-infested tarpaper shack was actually listed on the national – as a place on the National Historic Register. And we needed to preserve a bit of its history. So that’s here on this film. And it’s a wonderful story. To introduce the film is Jack Hillhouse, geophysicist here at the USGS. Jack will not only introduce the film, he’ll be around for questions afterward. And then we will also have the current rock magnetics laboratory, which is Building 16 – the funny brick-red- colored building in the back of campus. That building will be open for tours immediately after this movie. So let me tell you just a bit about Jack Hillhouse. He’s a native from the Bay Area. He didn’t go very far. He went across the East Bay and got his undergraduate degree at UC-Berkeley. But then he was on this side of the bay at you know where, Stanford, to get his Ph.D. When he first enrolled in the doctorate program there, he said he intended to be a seismologist. But then an opportunity sprang up for him to accompany Richard Leakey to East Africa and study human evolution. Well, that did it. That opportunity – he changed his field to geomagnetism and has been at it ever since. He’s been with the U.S. Geological Survey in Menlo Park since 1975 in the paleomagnetics laboratory. His work today continues to deal with dating young – geologically young rocks, that is, using paleomagnetism and solving plate tectonic puzzles. So let me introduce Jack Hillhouse, who will introduce the movie Secrets in Stone.

- Okay, thank you, Leslie. Well, good afternoon, everybody. I’m glad to see there’s some history buffs here in Menlo Park. The movie I’m going to show, Secrets in Stone, is the story of how a rather obscure research project here at the USGS played a key role in the acceptance of plate tectonics, which is the unifying theory of Earth science. Now, the project began in 1958 when three USGS scientists were involved in a study of rock magnetism, chemical dating of rocks, and polarity reversals of the magnetic field. When these three scientists started the project, little did they know that they would be put on the forefront of the plate tectonics revolution. Two of them, Allan Cox and Dick Doell, would be awarded the Vetlesen Prize by Columbia University in 1971. And many consider this award to be the most prestigious in Earth science. Having studied with Professor Allan Cox, I was well aware that this scientific discovery required very keen intellect, a lot of intuition, and just – and a good creative environment to work in, plus just plain luck to be at the right place at the right time studying the right kind of thing. Now, the film attempts to explain the science and circumstances behind geomagnetic reversal discoveries in Menlo Park and how these discoveries moved continental drift from what was considered pretty much a wild and largely discredited idea to that theory of plate tectonics, which is now our unifying theory of how ocean basins form and mountains are built and continents move. Now, the primary historical source for this film is Bill Glen’s excellent book, The Road to Jaramillo, which was published in 1982 by Stanford University Press. And, out of necessity to be brief, we took some artistic license in doing this film, and so it glosses over many of the intricacies and details that are in Bill Glen’s book. But the book is really the definitive source of information on this. And if you’re interested in this topic about the early days of paleomagnetism and plate tectonics, I recommend that book to you. So, before starting the film, I was going to give a little introduction of some of the key concepts of paleomagnetism and briefly review the discoveries that were made in the period from 1959 to 1965 in rock magnetism. And it’s all kind of viewed from my perspective as being a graduate student working with Allan Cox in the early 1970s just after many of these discoveries were made. The film runs about 35 minutes, and it’ll be followed by a short period of questions from you. And then I invite you to come over to the paleomagnetics lab, Building 16, which is just out the front door and to the right. It’s the red building with the dome-shaped roof. And we’ll have that open until about 2:00 today to see some of the exhibits that are in there from the historic period, plus our current research we’re doing there. I’m looking at my audience here, and I’m trying to decide whether I should – for the evening lecture, I have some fundamental stuff about geomagnetism in the talk. And I see – there are some people that maybe haven’t studied the subject here, so I think I’ll do the full talk rather than abbreviate it. Well, what is a geomagnetic reversal, some people may be asking. But first thing we have to do is define what geomagnetism is, and that’s the magnetic field of the – it’s produced within the molten core of the Earth. Now, the liquid iron part of the Earth’s core, which is – starts about 2,000 miles below our feet, there is a mechanism there called the geodynamo. And it combines thermal energy and electrical currents, all under the influence of the Earth’s rotation to generate the magnetic field that we measure and that we see on the Earth’s surface. The form of the magnetic field varies with time because it’s a fluid source. And it’s somewhat complicated locally, but in general, it is really a simple dipole field with poles – magnetic poles near the Earth’s geographic north and south poles. And if you want to see the effect of a dipole field, the easiest way to do that is to take some iron filings and sprinkle them on a piece of paper, put a bar magnet underneath, and you’ll see kind of – the particles line up in an oval pattern around the magnet. And that’s reflecting the force that the magnet’s – the dipole field is producing. Similarly, if you go to the – with a magnetic compass outside, you will get a reading that is parallel to the local magnetic field. Well, at infrequent intervals in the past, the magnetic field has reversed polarity so that a compass would show a 180-degree shift in magnetic direction. So ships, planes, and maybe a few Boy Scouts navigating with a magnetic compass would be led in exactly the wrong direction if there had been a magnetic reversal. Magnetic north becomes magnetic south. And, of course, the Earth itself does not shift its position, but the mechanism that generates the field is capable to work in the opposite direction. Now, fortunately, magnetic reversals occur very infrequently – only about five reversals per million years is the average. And the process takes about 5,000 years to complete, we think. For the past 780,000 years, the magnetic field has pretty much kept to its present form, which we call normal polarity. So we’re kind of in an unusually long period of stable single polarity right now. And some of you might have read in a recent New York Times article that there is some thought that the magnetic field might be actually in the early stages of a reversal right now. For the last several hundred years, measurements have shown that the field strength has been declining at a fairly rapid rate and that, perhaps in 500 years or so, it could actually go to zero if it continued on that decline. Now, of course, we know that the magnetic field strength fluctuates, so it could actually reverse that trend and go back increasing at some point in the future. And so we really can’t say that it’s even probable, or even likely, that we’re in a magnetic reversal. And we really wouldn’t know until maybe 20 generations of humans had passed anyway to know the answer for sure to that question. Well, how do we know what the magnetic field was doing in the very distant past? And that’s the question is the – sort of the heart of this film and the focus of our film. So we need a means for recording the magnetic field in the past. And we need a measure – a method for measuring when that event occurred – some kind of dating method, well before humans kept any records or there were any scientific measurements made. And paleomagnetism and istopic dating provide the answers to those question. Paleomagnetism is the ancient signal that is preserved in rock, such as a lava, when it first comes out on the Earth’s surface and cools, and it acquires the magnetic direction of the Earth at that point. We can get the strength of the magnetic field from measurements. We can also get the direction of the past magnetic field at the spot where and when the rock first formed. Isotopic dating is a method for measuring the age of certain kinds of rocks – usually volcanic rocks or granite. And, you know, most of these rocks contain unstable elements that – radioactive elements that decay to daughter products. And the more daughter product you measure in a rock, the older it is, is basically how radiometric dating works. The rate of decay – radioactive decay can be precisely measured. So events that occurred millions of years ago can be dated rather precisely now. And we’re going to talk about, in the film, potassium-argon dating, which is the method of choice for dating geomagnetic reversals for the last 5 million years. Well, what took place at Menlo Park in the rock magnetism lab some 45 years ago? Well, two scientists – Allan Cox and Dick Doell – joined the USGS to work on paleomagnetism. And they undertook an extensive literature search at the time – this was about 1959, I believe. There wasn’t much published on the subject. And they delved into it and wrote a long review article. And they decided that studying rocks that had reverse magnetic polarity – in other words, a magnetic direction that was opposite of the present Earth’s field – was a good field worthy of study. And reverse polarity had been measured in volcanic rocks for quite a long time going back to the beginning of the 20th century. But the question remained, was the reversal thing in the rocks due to a global reversal of the Earth’s magnetic field? Or was it something in the rock – some chemical process in the rock that was causing it? And Cox and Doell set out to settle that question with a lot of energy and a lot of dedication. Now, to prove that the reversal was a global phenomenon and not what we call self-reversal, they needed to show that rocks of the same age and same polarity were found throughout the world. And so this was a – this proposed project, then, was a perfect opportunity to travel to exotic and geologically exciting places. Because large stacks of volcanic rocks needed for these kinds of studies happen to occur on islands like Hawaii, the Galápagos, and the Aleutians, and so forth. And Allan Cox once told me that rocks are everywhere, so you might as well go someplace to study them where it’s exciting. So that was his advice to me. Of course, budget-conscious managers at the USGS weren’t thrilled about the idea of sending people all over the world for these kinds of studies. But Cox and Doell had outside funds from a U.S.-Japan cooperative project. And that gave them some latitude to do some extra – do the extra travel. And they were pretty lucky to have a very sympathetic and foresighted branch chief, James Balsley, who would support their travels. Now, the favored status of Cox and Doell back in those days subjected them to a lot of good-natured ribbing, especially in the annual Pick and Hammer Show, which was an annual event that featured parodies and songs that would sort of deflate the egos of the rising scientific stars at the Survey and also, you know, jab the penny-pinching managers at the same time. And so, in the 1963 Pick and Hammer Show, Cox and Doell were lampooned as Smallpox and Dipole – were their names. [laughter] And they were the subject of several parodies, including one called Around the World for Free. And I’m not sure what the tune was. It might have been Around the World in 80 Days. I don’t know. But Smallpox – or Allan Cox – sings, you think you’ve had it really grand. I flew Pan Am to old Japan, that lovely geisha land. And then I pulled another trick. I sold old Andy – and that was C.A. Anderson, the chief geologist – on a junket up to Reykjavík. I think if I put up a fuss, what do you know, I’ll get to go to the Galápagos. Remnant magnetism is the key. It surely beats geology. Remnant magnetism is the key to get around the world for free. [laughter] So this was the age-old battle between the field men and the lab men that was going on back in the early ’60s. Well, in 1959, Cox and Doell needed a laboratory, but space was kind of hard to find on the Menlo Park campus. But there was a row of old, dilapidated buildings out in the back lot here. And they were once part of the then- decommissioned Dibble Army Hospital. And Allan Cox liked to tell this story, and it’s also told in William Glen’s book, Road to Jaramillo. Allan said that he went to one of the old buildings with the facilities manager at the time, and he noted that the building would be perfect for his new paleomagnetics laboratory. And the manager said, well, that building is not available. But then Allan noticed that the manager dropped a key and walked away. And Allan contemplated the significance of this for quite some time, and … [laughter] And he picked up the key, and it fit the lock, so he moved in. You know, Allan always liked to add drama to these kinds of stories, and I’m not sure it really happened. We’ve never been able to prove this really happened, but it’s a good story. And I might add that that old dilapidated hospital building, which was built more than 60 years ago, has been called Temporary Structure B. And we were still using that in 1995 – it’s quite a temporary building – when we finally had it torn down. So, after toiling away in the old place for many years, I’m sure that my colleagues Sherman Grommé, Eddie Mankinen, Duane Champion, Jon Hagstrum, and certainly I don’t miss that building. We don’t miss the dust and the asbestos and the animal droppings and miserable lavatory that it had. [laughs] It was – also, when we tore it down, we noticed that the walls were completely full of acorns. Because decades of woodpeckers had been pecking holes and putting acorns in the walls in that building. So Temporary Shack B was also known as the tarpaper shack to many people. And it’s kind of the USGS equivalent to the Hewlett Packard garage, I think. It was located in what is now a parking lot out in front of the new paleomagnetics laboratory. But, because of the import of the scientific work that went on in that building in the early ’60s, the lab was entered onto the Register of Historic Places for Science and Technology back in 1995. But really, the costs of preserving the building were so huge and impractical that we were able to tear it down if we were to make a video documentary and a photographic documentary of the building and the work that took place there. And that’s what Secrets in Stone is – that documentary. And David Howell, a geologist here, and filmmaker David Donnenfield used that opportunity of the documentary to tell the story of plate tectonics as well as document the historic work that the lab had done during that timeframe. So in the new lab, we have an exhibit dedicated to the history of the lab. We preserved most of the original instruments that were used in the early ’60s, including the flight tube for the Reynolds mass spectrometer that was used in the dating experiments. And we have a sort of a replica – part of the shack and some of the original windows and some photographs of the – of that timeframe. So you are welcome to visit the lab after this film is shown. We’ll keep it open until about 2:00 today. Well, in 1965, Brent Dalrymple, who is now the – is the third member of this rock magnetics team came to my high school here in the peninsula. And he addressed our science seminar. And his topic was this newly discovered thing, geomagnetic reversals. And that was my first introduction to the topic. And I guess USGS outreach has some effect, because 40 years later, I’m still studying geomagnetic reversal. Brent had joined the project to bring potassium-argon dating, which was being developed at UC-Berkeley at the time. And Brent’s arrival gave the project a key ingredient that had been missing, and that is to have capacity to date a lot of rocks quickly, to date the polarity samples that Cox and Doell were collecting from around the world. And, by that time, in 1963, there was intense competition between the USGS laboratory and other laboratories, one especially in Australia, to answer the reversal question. And there was a rush to publish the early polarity time scales. But Cox and Doell and Dalrymple got there first with the time scale they published – Time Scale 1 in 1963 in Nature magazine. And it was a short article – rather kind of a obscure title and short article that supported the hypothesis that the field was a global – was undergoing global reversals in the past. And then, at about the same time, researchers in Canada and in England had been puzzling over some prominent striped features on the seafloor near submarine ridges. And, in 1965, Brent Dalrymple presented the latest version of the USGS polarity time scale at an International Congress of Geophysicists. And Fred Vine and Drummond Matthews from Cambridge University were attending that meeting. And when they saw that refined time scale, which included an event called the Jaramillo event, which is a normal – short, normal polarity event that occurred about a million years ago, it immediately struck Vine and Matthews as, this was a perfect match to the seafloor record. So the record that was being seen on land from isolated lava flows was also being duplicated on the seafloor. And the conclusion was obvious – that the seafloor was spreading from these central ridges. And that, as new material came up and was magnetized, it was recording the magnetic field reversals. And you could see this polarity pattern stepping out from the ridges. Well, that immediately turned people around to believing that the seafloor had the capability of spreading. And that gave a mechanism for continental drift. And it led to global plate tectonics, which is going to be – that story is pretty well told in the film. But the science aside, this is the story of how people – scientists are caught up in the excitement of a great discovery and the random twists and turns that had occurred during that. And Allan Cox always acknowledged that luck and timing played a major role in the discoveries leading to the geomagnetic reversals and plate tectonics. It was just good timing. As scientists around the world were seeking answers to what caused the magnetic stripes on the seafloor, why were there rocks carrying reversed polarity? How could rocks be dated accurately? And, if the continents had moved apart, how could they do so through the solid rock of the Earth’s crust? And World War II had produced the technology to be able to measure these weak magnetic fields on the crust and on the seafloor, and potassium-argon dating was just coming into being at Berkeley. And Cox and Doell had just become interested in this arcane aspect of rock magnetism reversals. So the timing was just right for these scientific threads to come together and be woven into a great discovery for Earth science. And, in the six years from 1960 to 1966, the study of geology really changed dramatically. Faulting, volcanism, mountain-building, all had a new meaning in the context of plate tectonics. And, by 1970, geology went from largely a descriptive science to a science based on this unifying theory. The great success and impact of the rock magnetics project – those were completely unexpected. Cox said that it was a perfect case of serendipity. No central committee planning the future of Earth science could conceivably have guessed that this would happen. Well, after Allan Cox left the USGS for Stanford in about 1969, and Dick Doell retired from the field after working on the Apollo 11 moon rocks, Sherman Grommé and Ed Mankinen carried on the paleomagnetic work here at the USGS. And Duane Champion and Jon Hagstrum and I joined about in the early ’70s – mid-’70s. And, over those years, we worked on refining the paleo time scale even more, using secular variation, or the fine-scale changes in the magnetic field, to date volcanic rocks. We’ve been looking at what happens to the magnetic field during reversals – what transition – field directions. I did a lot of work on the tectonic assembly of Alaska – the southern part of Alaska. And dated evolution of man in East Africa. And Sherman Grommé spent a lot of time working on the Grand Canyon, on the paleomagnetism of how the North American continent was assembled. So many of the original instruments now in the lab have been replaced by modern electronics and cryogenic magnetometers and improved shielding – magnetic shielding. We had a – at one time, we had a gifted machinist, Major Lillard, who turned out custom parts on a – on the project lathe. And Nat Sherrill was the electronics man who built many of the electronic systems. And that kind of technical support was a real luxury – would be considered a real luxury today, as it was in those days when these guys were working there. But today we buy most of that equipment now off the shelf from manufacturers. But all the modern equipment in our laboratory really goes back to the legacy of these engineers and scientists at USGS some 40 years ago. So, with that, we can roll the film. Or, spin the DVD, as this case is.

[Video starts]

- For as long as humans have cared to look about and see, really see, the diversity of form and features making up the landscape, we speculated on how things became the way they are. How did a desert become a desert? And why is it where it is? How are mountains formed? And valleys? What accounts for the deep ocean basins? And how do you explain islands sprouting up out of them? And if you’ll forgive the pun, what gives rise to volcanoes? Some people didn’t stop with the land’s features. They wondered about continents. About the entire crust of the Earth itself. Why are continents and oceans where they are? Had they always been there? The very vastness – the enormity of these speculations made answering them seem unlikely, if not impossible. Forever a secret of the Earth alone. Menlo Park is a suburban community 30 miles south of San Francisco. It’s also home to the Western Regional Headquarters of the U.S. Geological Survey, maybe best known for its surveying, mapmaking, and volcano and earthquake monitoring activities. But, in the 1960s, a basic research project on the Earth’s magnetic field by three young scientists – Richard Doell, Allan Cox, and Brent Dalrymple – provided important insights into some of the Earth’s deepest secrets. In the end, it laid the foundation for what became a revolution in the Earth sciences – a revolution we now call plate tectonics.

- Plate tectonics has reshaped our entire view of how the Earth works. It shows us that, by the movement of the broken pieces of the Earth’s shell, like an eggshell, pulling away, sliding past each other, bunking into each other, the mountain are formed, the deep-sea trenches are formed, volcanoes are born. Great faults in the Earth produce movements that trigger gigantic earthquakes. In short, all of the action – all the action of the body Earth that threatens humans and poses sources of danger, and all the major features of the Earth, which only 30 years ago, we didn’t understand, are all produced by the major – by the major movements of these plates over time. In short, all of Earth’s history is a history of the movement of these plates.

- The revolution at its origin and the outlandish theories advanced several decades earlier by the visionary scientist Alfred Wegener. He proposed the idea that the Earth’s continents were once joined as one huge continent and that they had broken up and moved across the face of the globe to their present positions. Mobile continents? Many considered Wegener’s proposal preposterous and dismissed him as a fantasist. At the time, scientific thought was as immovable as the continents appeared to be. Geology as a formal science is not very old. A founder of modern geology, Scotsman James Hutton, proposed in 1785 that the forces which formed the Earth’s geologic features in the past are the very same forces currently at work. Erupting volcanoes, for example, provide crustal material for new landforms, while the relentless erosive forces of weather and water wear them away. What may seem obvious today conflicted with the predominant views of the time. It was widely held that miraculous events were responsible for the Earth’s features. Moreover, the Earth itself was thought to be no more than 6,000 years old. This didn’t match the conclusions Hutton reached from his own observations. His reasoned approach opened the way to a more comprehensive study and understanding of geologic processes. In the years that followed, the Earth sciences were the subject of intense interest and activity. Geologists made great strides in explaining the dynamic forces responsible for the Earth’s features. Paleontologists – scientists who study fossils – divided a large part of the Earth’s 4-1/2 billion-year history into an unfolding story of meaningful chapters. But scientists had yet to answer some of the most intriguing questions about Earth processes. One persistent mystery was how the land masses attained their current positions on the globe. Through the years, various observers had noticed how certain continents, notably South America and Africa, had coastlines that, if pushed together, seemed to fit. Was this jigsaw-puzzle- like correspondence coincidence? Or was it evidence that the continents had, at one time in the past, been joined? To most scientists, this conclusion was unfounded and a fantastic leap of imagination. The notion of mobile continents remained outside serious scientific debate until Alfred Wegener, a German meteorologist with interest in a wide range of disciplines, published The Origin of Continents and Oceans in 1915. In his book, Wegener marshaled an impressive array of evidence to support his theory of continental drift. He argued that, at one time in the past, the present continents were joined in one supercontinent he called Pangea. This explained why now-distant continents shared similar geological structures. Like the mountain range in Argentina that bears a strong resemblance to one in South Africa. Or how frozen Antarctica could have supported the plant life to form coal deposits there. Or why ancient plants and animals inhabited lands now separated by an expanse of ocean. All of these baffling phenomena could be accounted for by Wegener’s theory of continental drift. According to Wegener, Pangea split apart, and the continents slowly moved to their present positions. But what caused Pangea to fragment into continents? And what force could move such massive structures through the more dense oceanic crust? Wegener’s inability to provide an acceptable reason, or mechanism, for continental breakup and movement led to his theory’s rejection by most scientists. He labored unsuccessfully for years to have his theory accepted, but when he died tragically in 1930 on an expedition in Greenland, his theory faded with him. [Multiple children speaking] The Earth behaves much like a bar magnet. Its weak magnetic field extends out from the poles to surround the Earth. Anyone who has played with a simple magnet and iron filings has seen a magnetic field displayed. Since at least the 17th century, an interesting phenomenon was noticed about pottery made with certain clays that were fired at high temperatures. When placed close to a compass, the cool ceramic revealed it had been magnetized by the Earth’s magnetic field. Somehow, during the firing process, the clay had recorded the direction of the Earth’s field, which the hardened pottery now retained. No one knew why. In 1906, two French researchers, Bernard Brunhes and Pierre David, came across rocks – volcanic rocks with a magnetism opposite to that of the Earth’s. Surprisingly, in these rocks, the north and south poles were reversed. Soon after the discovery in France, others found rocks with reverse magnetic polarity in other parts of the world. We’re here in the foothills of the Sierra Nevada mountain range of California in an area of past volcanic activity. Now, there’s something unusual right underfoot. But without a magnetometer to measure the magnetic field, you wouldn’t know it. Let me show you. Now, we’ve determined that this is the rock’s north-facing edge. As I bring it close to the magnetometer, watch the instrument’s needle. It’s swinging into the negative zone. This rock isn’t just magnetized. It’s reversely magnetized. It has a magnetic field opposite to that of the Earth’s. Yet, not far from here are volcanic rocks that have a normal magnetism. These discrepancies, or anomalies, are what puzzled Brunhes, David, and others. What could possibly account for this variation in one of the Earth’s fundamental forces – magnetism? Scientists arrived at two possible explanations. The first was that there was something intrinsic to certain rocks that caused the magnetic minerals in them to spontaneously reverse polarity. Although researchers didn’t know what that property was, this explanation at least seemed more plausible than the alternative. In the second theory, scientists suggested that the Earth’s magnetic field itself changed polarity. Could hot volcanic rock, at the time of its formation, acquire the Earth’s magnetic field, like the potter’s fired clay? As it cooled, the rock would, in effect, record the Earth’s magnetic field, whether normal, as it is today, or reversed. But the image of the Earth’s field switching north and south magnetic poles was just too audacious for most scientists. Still, neither of the explanations could be proved nor disproved at the time. So the mystery remained. The 1950s and ’60s were an especially exciting time for the Earth sciences and science in general. The world had emerged from the shadows of World War II. Geologists and geophysicists were reaping the benefits of wartime technology put to peaceful use. More powerful and more sensitive instrumentation was extending the researcher’s insight into hidden processes. Meanwhile, the old debate about continental drift had ignited again. In the 1950s, with new research data accumulating, Wegener’s theory attracted renewed interest. The topic seemed to fire the imagination of Earth science students who were too young to remember the hostility and derision with which Wegener’s ideas were first received. Richard Doell was one of those students in the geology department at the University of California at Berkeley. He had been fascinated by continental drift while still an undergraduate. As a graduate student, he was exposed to the idea that paleomagnetism might be instrumental to resolving the matter of drift. Paleomagnetism was a relatively new branch of the Earth sciences, dealing with the ancient magnetism retained in rocks. Allan Cox came to the university a few years after Doell. As an Earth science student, Cox was also attracted to the topic of continental drift. But later became particularly intrigued by the phenomenon of reversely magnetized rocks. He as well was optimistic that paleomagnetism could solve some of these enigmas of Earth science. So much was awaiting discovery and explanation, with the answers seemingly just beyond scientific reach. Cox and Doell knew this all too well from the extensive survey of paleomagnetism they began while Cox was still a graduate student. They agreed that, if the opportunity arose, they would devise a research project to answer some of these questions together. That opportunity arose in 1958 at the U.S. Geological Survey while Cox was still a student and Doell was an assistant professor at MIT. Doell had been asked by James Balsley, the chief of the geophysics branch, to establish a laboratory for paleomagnetic studies there at the Survey. At the time, there weren’t many facilities available at the USGS Menlo Park campus to house the lab. The most likely candidate proved to be a tarpaper-and-wood shack built during World War II as a support unit to the hospital there. The humble structure was, in a way, well-suited to the research work that would be conducted there. Away from traffic and other disturbances, there was little to interfere with the delicate magnetic measurements. And the temporary nature of the building made modifications to it easy.

- We were always very thankful that this was a tarpaper shack and not a – not a big official building. Because, since it really wasn’t a recognized USGS building, that meant we could do almost anything we wanted in it. If we needed to knock a hole in the wall or put in more power or change a sewer line, we could simply do that without getting the government involved in some kind of a big project. So, in terms of being a facility for forefront research, where you have to make a lot of changes in a hurry if you’re going to progress, this was the ideal building.

- Doell and Cox prepared an ambitious research agenda to tackle the persistent mysteries confronting Earth science that they had been thinking about. Among the topics, they wanted to resolve the issue of polarity reversals in rocks. They suspected that settling the matter of polarity reversals could play a role in settling the debate over continental drift. For this work, and other experiments, they needed volcanic samples from distant places around the globe. They mounted expeditions to collect recently formed volcanic rocks in Hawaii, Alaska, New Mexico, and the Sierra Nevada in California. The work was physically arduous, climbing over rugged lava with packs filled with drilling and calibration equipment, much of it improvised. In the laboratory, the volcanic samples would first be put through a series of treatments to strip away all but the rock’s original remnant magnetization. In a sense, what it was born with. The measured orientation of that magnetization would then be used to determine if it was normal or reversed. If these sample rocks, of roughly the same age, all showed the same polarity, polarity reversal was due to reversal of the Earth’s magnetic field and not some property of the rock itself. The consistency was evident. It didn’t matter where they were from. Most of those rocks thought to be somewhat older than the beginning of the Ice Ages, about a million years ago, had a remnant magnetization reversed from what the Earth is presently. North was south, and south was north. Incredibly, the logical explanation was that the Earth’s field itself was reversed when the rocks were formed. But were they measuring a single event or a frequent occurrence? If reversals were common, did they happen at regular or irregular intervals? For that matter, how long did a reversal last? To answer these questions, Doell and Cox needed precise dates. Relative ages based upon the order in which the rocks were laid down wouldn’t be of much help. At the time Cox and Doell were students at UC-Berkeley, Dr. John Reynolds of the physics department was perfecting an instrument that would later prove critical to their research. In order to calculate the age of a rock, scientists must measure the proportion of specific elements in them. They use an instrument called a mass spectrometer for this. And the one John Reynolds made was much more sensitive than any previously developed. The dating procedure works because volcanic rocks contain a radioactive element – potassium. When postassium decays, it produces another element – argon. What’s important is that the decay occurs at a known rate. Since scientists know the rate of decay, measuring the amounts of potassium and argon allows them to pinpoint the age of a rock. But argon is a gas, and extracting it is a tricky process, requiring great skill and an elaborate laboratory. Neither Cox nor Doell were trained in the radiometric dating of rocks. They needed someone else who could set up a lab and extract argon gas from the samples for use in dating with the Reynolds-type mass spectrometer. Brent Dalrymple was one of the first students at the University of California at Berkeley trained in the use of the Reynolds mass spectrometer. He happened to meet Doell and Cox while doing field research in the Sierra Nevada for his thesis. During late-night discussions at their campsite, Cox related the mysterious phenomenon of magnetic reversals and how the riddle might be solved. Dalrymple was, of course, keen to learn that precise dating of rocks was crucial to such an experiment. After graduation, Dalrymple was asked to join the rock magnetics project by Doell to perform argon extraction from the rock samples for precise dating. Because the project didn’t have its own mass spectrometer, the samples were sent to other USGS labs for analysis. The results were slow to return. The first set of data published by the USGS team in June of 1963 was consistent with an assumption that the geomagnetic field was reversing with regularity every one or every half-million years. However, with only three polarity intervals known, each lasting about a million years, it was too early to know if reversals typically lasted for that span of time. But one had to wonder if the Earth’s magnetic field alternated with a regular pulse. At the time Brent Dalrymple was at Berkeley, he was one of only a handful of students learning the Reynolds mass spectrometer for rock dating. Another was a young Australian, Ian McDougall, who, after his training, returned home to a newly completed rock dating lab assembled by Professor Jack Evernden of Berkeley. It was an exact replication of Berkeley’s lab, right down to its Reynolds mass spectrometer. Not surprisingly, the next edition to the time scale didn’t come from the rock magnetics lab, but from Ian McDougall’s group in Australia. They found a different time boundary for the onset of the most recent magnetically reversed interval, which pushed the duration out another half-million years. The Australians also found evidence for two additional polarity reversals further back in time. Maybe the reversals occurred with more complexity than suspected. The work going on in Australia pointed up how desperately the USGS team needed its own radiometric rock dating lab and Reynolds mass spectrometer. But setting up such a facility was no small task. It could take a year or more. Much of the complex equipment had to be designed and built from scratch. Brent Dalrymple worked day and night with two technicians, Major Lillard and Nat Sherrill, to construct the lab. The glass flight tube for the mass spectrometer came from the university in Berkeley.

- This is the tube part of a Reynolds mass spectrometer. This is the mass spectrometer we actually used in the magnetic field experiments. The gas would come into this end of the tube, and it would go through the source area, represented by these metal plates, where the gas is ionized – that is, each atom picked up an electrical charge. Then the ion beam would go down through this part of the tube, where it’s bent, and there would be a magnet on either side that would separate that ion beam into ion beams of differing mass at this end. By varying the strength of the magnet, we could any ion beam we wished into the collector. And, as each ion beam went into the collector, it created an electrical signal, and the strength of that electrical signal was a direct indication of the relative proportions of the different argon isotopes in that particular sample. And that information was then used as part of the age calculation.

- By the early months of 1964, it was quite clear that, from about 1 to 2-1/2 million years ago, the Earth’s field had, indeed, reversed its polarity. And so it was that self-reversal became a footnote in history. While the paleomagnetists were chasing the mystery of magnetic reversals, marine geophysicists were pondering some unusual data collected from the seafloor. In 1961, English scientist Drummond Matthews took magnetic measurements over a small patch of seafloor in the Indian Ocean. His instruments revealed a series of alternating magnetic bands of greater and lesser magnitude across the ocean bottom near a submarine feature called the Carlsberg Ridge. This banding, like the stripes on a zebra, was unique to the seafloor and, in fact, had been detected earlier in other parts of the world’s oceans. Various hypotheses were proposed to account for these peculiar magnetic anomalies, but none were conclusive. But Matthews’ doctoral student, Fred Vine, hit upon a novel explanation, which, coincidentally, was also proposed by Lawrence Morley of the Canadian Geological Survey. It was based on the unproved concept of seafloor spreading proposed some years earlier by Harry Hess of Princeton University. Winding around the Earth, much like the seam on a baseball, is a remarkable feature called the Mid-Ocean Ridge. Hess’ idea was that the seafloor was spreading outward on either side from the Mid-Ocean Ridge. He suggested that the ridge provided new seafloor material as molten rock welled up from inside the Earth. Vine and Matthews, and Morley independently, deduced that the magnetic stripes were normal- and reversely magnetized blocks of the seafloor due to the Earth changing the direction of its magnetic field, as some had suggested. This idea led Vine to conclude in 1963 that the normal and reverse banding represented a kind of fossilized record of past periods of seafloor spreading. Compared to each other, the pattern of banding Vine saw imprinted on the seafloor was reminiscent of the polarity reversal time scale that Doell, Cox, and Dalrymple were developing from terrestrial samples. At the time, though, Vine didn’t see a correspondence. The time scale was just too crude. Meanwhile, just six months after they had begun, the USGS team had its own rock dating lab up and running. With a mass spectrometer on site, work progressed at an accelerated pace. Cox, Doell, and Dalrymple published a new time scale in June of 1964. It defined two new short periods when the Earth’s magnetic field was opposite to the usual polarity for the period. More and more, the pattern of reversals that was emerging looked irregular. Additional testing of young rock samples confirmed or refined the June time scale. Three succeeding versions didn’t change much in appearance. But one last area on the time scale stubbornly resisted explanation. The data indicated that something may have happened between 700,000 and 1 million years ago. Could it be another previously undetected reversal? Samples taken from the Jemez Mountains of New Mexico provided the key that unlocked a virtual vault of revelations. Around 900,000 years ago, during a reversed period, the Earth’s magnetic field switched to a normal orientation. They called it the Jaramillo event, after the creek near the location where the samples were taken. And its discovery at last snapped the picture of magnetic reversals for the past 4 million years into sharp focus. This new time scale revealed a record of global-wide polarity reversals occurring irregularly every 1 to 1-1/2 million years, with a few very short periods of opposing polarity. The random nature of the pattern was a critical feature. It produced a pattern as unique as a person’s fingerprint. Just as fingerprints are used to solve crimes, the time scale pattern helped solve the mystery on the seafloor – and more. In late 1965, Brent Dalrymple presented the rock magnetic lab findings at a meeting of the Geological Society of America. Fred Vine was also at the meeting to report recent work on seafloor magnetic anomalies. When the latest time scale incorporating the Jaramillo event was unveiled, Vine saw, in its intervals of time, an exact proportional match to the mysterious seafloor bands. It was a revelation and the start of a revolution in Earth science.

- The definition of the reversal time scale, and in particular, the final piece of the jigsaw to be put in place – the Jaramillo event – was absolutely crucial to the vindication of my idea – the verification of my idea, and hence seafloor spreading, which ultimately led to plate tectonics.

- Applied to the seafloor, the time scale’s pattern provided a unique view into the Earth’s dynamic past. As it formed along the Mid-Ocean Ridge, the seafloor on either side was like a conveyor belt, bearing away at an almost constant rate – a mirror-image, fossilized imprint of the Earth’s magnetic orientation at the time of the seafloor’s creation. As a strip of ocean floor formed during an normal epoch, it would be adjoined to one formed while the Earth’s polarity was reversed, leading, over time, to a unique pattern of alternating normal- and reversely magnetized bands of seafloor rock. Discovery of the Jaramillo event, combined with Vine’s hypothesis, caused a revolution in our thinking about the Earth and became the underpinnings for an entirely new model of how the Earth works – what we now call plate tectonics. The mid-ocean ridges, along with the deep ocean trenches, and young fold mountain ranges, such as the Himalayas, mark the boundaries of vast crustal plates that move in relation to each other. The slow, inexorable, and sometimes violet movement of these plates creates the surface features of our planet. Alfred Wegener was right after all. The continents do move. But not by themselves, as he had proposed. Instead, they sit astride the floating crustal plates, hitching a ride, as it were. Modern computer reconstructions of the fit among the continents as they once existed in the single land mass of Pangea have replaced the imaginative drawings by observers of an earlier age. But, to theirs and Wegener’s credit, they are striking in their similarity. At the time of their investigations, Richard Doell, Allan Cox, and Brent Dalrymple couldn’t have anticipated the impact their work would have. To them, polarity reversals of the Earth’s magnetic field was an overlooked, but promising, area of scientific inquiry. Who could have guessed that their time scale would be the Rosetta Stone that unlocked some of the Earth’s deepest secrets? That it would trigger a revolution in our perceptions of the Earth? But then, revolutions have to begin somewhere. Why not in a tarpaper shack?

[Video stops]

- I’ll answer questions if you have any. Yeah?

- I understand that a lot of the mapping was a result of the sonic mapping of minesweepers during the war. And a lot of this was recorded, and somebody picked this up and started plotting [inaudible]. Was that [inaudible]?

- Right. That was – that was true. The seafloor was mapped in some detail, and that’s when the oceanic ridges were discovered. And people began to puzzle what those might mean …

- Well, that was a forerunner [inaudible] …

- That was – that was before, actually, that the – you know, probably the sonic mapping and the magnetic mapping were going on simultaneously.

- Yeah. Right. But it was recognized that there’s some relation.

- Yeah. It was all – it had to do with submarine warfare, really, I think.

- Yeah, right, yeah.

- You know, it was looking for – using magnetometers to detect submarines and using seafloor features to hide submarines.

- Maybe Bush will do something in Iraq for us. [laughter]

- Yes?

- I recently read an article in National Geographic about the [inaudible] sun. I was wondering if there’s any comparison between [inaudible].

- Well, it’s all – it’s a dynamo effect on the sun as well. It’s a different thing. In the Earth, you’ve got molten iron as the source, but in the sun, you’ve got hydrogen, you know, as the – as the conductor. But, on the sun, the reversals – it’s a multi-polar structure. And it happens very rapidly – like, every few – every few decades, or 11 years or so, there are a lot of magnetic reversals. It’s much more active on the sun, actually. But it’s a similar process. The planets that have magnetic fields all have things in common. They rotate. They have some kind of conducting liquid core – liquid core to produce a magnetic field. Mars no longer has a magnetic field. It might have had one in the past, but apparently, the interior is – can’t support a magnetic field now on Mars. And so one of the consequences of losing the magnetic field is there’s more erosion of the atmosphere due to the solar wind. And eventually the planet loses its atmosphere over, probably, billions of years. And that’s where we are with Mars today.

- When you bore these cores …

- Yeah.

- … you’re kind of drilling these sort of random? And how do you make sure that you’ve got – you recognize the orientation of the …

- Well, when you drill a core, you use a tool to orient the core. We’re using a sun compass or a magnetic compass – usually a sun compass is better.

- [inaudible] …

- And we measure the angle relative to the horizontal. So we come back with a complete orientation we can reconstruct in the lab from where we collect it in the field.

- How close are you?

- It’s probably within a degree – the orientation. Or better. Half a degree. Yeah. If you drill a deep enough core, you can make a very accurate reading on its orientation. Yeah? Gary.

- Jack, is it possible to describe simply the mechanism by which reversals happen?

- [laughs] Albert Einstein said it was one of the great unsolved mysteries of science. But …

- I’m asking you.

- Yeah. [laughs] I’m no Einstein, that’s for sure. But what happens is, because there’s a lot of eddy currents and sort of chaotic fluid flow in the core, apparently a small region will become – you know, its polarity will become reversed relative to the rest of the core. And that area begins to flood. And, as the field collapses, the whole dipole mechanism breaks down. And then, as soon as conditions are favorable for the field to grow back, it either grows back in the original orientation, or it goes in the opposite. It can work either way. Recently, we finally had the computing power – not us here, but laboratories have been doing simulations of the conditions in the Earth’s core. And they put in all the electrical and mechanical forces and thermal forces they think are there. And they can actually simulate a dipole field that does reverse on kind of the kind of timeframe that we’re seeing now. So a lot of progress has been made in that area of how the dynamo works.

- What temperature is [inaudible] point? When it solidifies?

- For lavas, it’s about 580 degrees centigrade is when the magnetization starts to lock in.

- I’m glad you used the term “centigrade.”

- Yes. And, if you’re dealing with magnetite, that’s the temperature. If you’ve got hematite, which is a different kind of magnetic mineral, it’s higher. It’s, like, 680 degrees centigrade. But then the magnetization is acquired all through the lower temperatures until you get to room temperature. But it’s – if you heat it to 580, you lose it all – lose all the remnant magnetization.

- Are there any other questions this afternoon? Yes. Go ahead.

- I have a quick question.

- Yeah.

- You said early on there was a alternative hypothesis that maybe there would be localized reversals …

- Right. Self-reversal.

- And you were talking about some maybe chemical process.

- Yeah.

- Was there evidence for such a process? Or was that completely – you know, the ether.

- It just so happens that there was a rare class of volcanic rocks – some dacites are self-reversal – self-reversers. And one of the ones they just happened to pull off the shelf back in the ’50s was a self-reverser. It was the Haruna dacite in Japan. And that led people off on this other tangent.

- What do you mean by self-reversing?

- Well, the magnetic minerals in the rock sort of form in two crystal structures. And, when you heat the rock, one dominates over the other. And they just happened to be opposed in their magnetization. That’s the way the energy works out. So you can have some self-reversing rocks. Highly oxidized, very quartz-rich volcanic rocks do that. But basalts don’t, and that’s where the bulk of this data came from.

- So there’s still not a good theory yet on the actual reversal process, right?

- I think the dynamo models are pretty accurate as to – as to what happens.

- Well, [inaudible] why the reversal?

- But why – well, it’s really a random process. It’s just the – there’s a very fine balance that keeps the magnetic field going. And any disturbance in the fluid flow of the core can upset that balance, and the field collapses. And it’ll grow back, but it can grow back in either direction.

- Why a complete reverse?

- Well, I guess the currents work in either direction. And the field wants to be aligned with the Earth’s rotation axis. So clearly the spin of the Earth has something – you know, controls the magnetic field. And it stays parallel to the spin axis for long periods of time. But when it – when the field begins to collapse, the dipole feature kind of goes away, and it’s kind of a multi-pole thing. And then suddenly the dipole grows back, and it can grow back opposite. Yeah?

- It is possible that the weakening of the Earth’s magnetic field could occur on a more regular basis than the reversals? So the – you know, you say that it’s a random thing as to whether or not [inaudible] …

- Yeah. Well, actually, if you – if you monitor the field strength using our paleo-intensity techniques going back to the last 10,000 years, you see quite an oscillation in the field that’s – so there’s a – there’s a fine-scale oscillation that goes on – field rising and falling. So it just happens that, if the field is on a low tick, and then suddenly something else happens, and it drops it all the way down to zero, then the field collapses. I don’t know if you – any of you have watched the movie, The Core, but [chuckles] it takes into account, you know, what some of the effects might be if a magnetic field collapsed. And of course, it exaggerates everything, you know, a million-fold, but [laughs] there are some effects like that. Yeah.

- What about manmade – like, lightning, too, and things like that? Don’t they disturb this?

- Oh, yeah. Lightning is a big problem with collecting samples in the field because the lightning bolt will re-magnetize the rock completely. So we try to avoid areas that have been hit directly by a lightning bolt. And we can prospect with an instrument to do that. Because – and the other thing is that, you know, you – we want to avoid power lines and all those kinds of things too.

- In the lab, this must be a problem. You have to organize things.

- Yeah. And we try to work in a low-field environment in the lab. So these specs don’t ruin the results.

- I was going to say, maybe that’s the perfect segue – a question about how they have a low-field environment in the lab. So if there’s not another very pressing question, maybe this is the appropriate time to walk over to the paleomagnetic lab, and you can see how they do that and account for the other fields.

- Yeah. Okay, so just go out to the front of the building and turn right, and you’ll see a red building with a domed roof, and that’s the lab. And I’ll be heading over there in a minute.

- Thank you, Jack. That was – delightful stories.

[Applause]

- Thank you.

- And thank you all for coming. Good afternoon.

- Let me get rid of my microphone first.