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This article was published in 1999 and has not been updated or revised. BEYONDBEYOND DISCOVERYDISCOVERYTM

THE PATH FROM RESEARCH TO HUMAN BENEFIT WHEN THE EARTH MOVES AND PLATE

arly on the morning of Wednesday, April 18, the fault had moved, spanning nearly 300 miles, from 1906, people in a 700-mile stretch of the West San Juan Bautista in San Benito County to the south E Coast of the United States—from Coos Bay, of San Francisco to the Upper Mattole River in Oregon, to , —were wakened by Humboldt County to the north, as well as westward the ground shaking. But in San Francisco the ground some distance out to sea. The scale of this movement did more than shake. A police officer on patrol in the was unheard of. The explanation would take some six city’s produce district heard a low rumble and saw the decades to emerge, coming only with the advent of the street undulate in front of him, “as if the waves of the theory of . were coming toward me, billowing as they came.” One of the great achievements of modern science, Although the Richter Scale of magnitude was not plate tectonics describes the surface of Earth as being devised until 1935, scientists have since estimated that divided into huge plates whose slow movements carry the the 1906 San Francisco earthquake would have had a continents on a slow drift around the globe. Where the 7.8 Richter reading. Later that morning the disaster plates come in contact with one another, they may cause of crushed and crumbled buildings was compounded by catastrophic events, such as volcanic eruptions and earth- fires that broke out all over the shattered city. Some 700 quakes, which in turn can trigger the destructive ocean people were killed, another 250,000 were left homeless, waves known as . Plate tectonics became widely and 28,000 buildings were destroyed. Financial losses accepted by earth scientists starting only in the 1960s. were estimated at $500 million, almost $9 billion today. As the following article relates, investigators pursuing The earthquake that struck San Francisco that basic questions in ocean and terrestrial science gradually morning would go down in history not only for its arrived at an understanding of how the planet’s crust destructiveness but also for works, an understanding what seemed at the time to be that now allows us to pre- its inexplicable characteristics. pare for times when the very Scientists of the period knew ground beneath our feet about the nearby San undulates like waves on Andreas fault and were the ocean. familiar with the idea that one side of such a crack in the planet’s crust could shift up Shattered by a powerful or down against the other to earthquake on April 18, cause a localized earthquake. 1906, the ruins of San But as geologists began exam- Francisco’s City Hall stand surrounded by thousands of ining this event, they were at tons of brick rubble. (Photo a loss to explain its magni- courtesy of the Museum of the tude. A very long section of City of San Francisco.)

NATIONAL ACADEMY OF SCIENCES But seismologists soon realized that the instruments also offered a way to probe the mysterious interior of From Earthquakes to the planet. By the start of World War I, a succession of researchers had studied the behavior of seismic waves to infer a planetary structure composed of concentric In the early 1890s, John Milne, a geologist teaching layers: an inner core (although there was disagreement at the Imperial College of Engineering in Tokyo, devel- as to whether it was solid or fluid) covered by an inter- oped with colleagues the first accurate seismograph, an mediate layer of dense rock, the mantle, that began instrument used to record ground shaking, a frequent about 30 miles beneath the outermost surface crust. and sometimes devastating occurrence in Japan. A few Against this background of knowledge, a German years later a fire destroyed Milne’s home, his scientific meteorologist caused an uproar in the world of geolo- observatory, and the earthquake data he had collected gy with his bold theory about the of Earth’s over more than a decade while working in Japan. surface. In 1915, published The Discouraged but not defeated, Milne returned to his Origins of Continents and , in which he native Britain, where by the turn of the century he had addressed the puzzling match-up between the bulge established a bigger and bolder approach to the study of Brazil and the dent of southwestern Africa. He of earthquakes—a network of 27 instruments through- argued that the two continents had once been joined out the British empire. By the time Milne died in and had drifted apart. As additional evidence for 1913, 40 stations around the world were beginning continental displacement, or continental “drift” as to define the global pattern of earthquake location. the original German word was translated, Wegener A seismograph records vibrations produced by cited fossils of the Mesosaur, a 270-million-year-old sudden motion along a fault that generates several reptile found only in eastern South America and kinds of seismic, or “shaking,” waves—vibrations back western Africa. Most geologists of his generation and forth, side to side, and up and down. The first explained these similarities by postulating a connecting seismologists could distinguish two kinds of seismic land bridge that had later sunk out of sight into the waves, which move at different speeds. Primary, or P ocean. Wegener postulated, rather, that the Mesosaur waves, which alternately compress and dilate the mat- bones were found in these distant places because those ter in their path, arrive at the instrument first, inscrib- regions had come apart about 125 million years ago, ing a wavy line on a chart. Secondary, or S waves, slowly separating the groups of Mesosaur fossils. The which tend to oscillate snakelike, at right angles to continents of today drifted apart from a super-conti- their direction of motion, travel more slowly and have nent, which he called . a more ragged seismographic signature. The interval The meteorologist was unsure as to how these between the arrival of the two types of waves can be huge slabs move around, but suggested that they used to calculate the distance of the monitoring sta- were propelled through the by Earth’s tion from the earthquake’s epicenter, the point on centrifugal force and the gravitational pull of the Sun Earth’s surface above the subterranean hypocenter, and Moon. Many influential geophysicists argued or source of the shock. The distances from three convincingly that such mechanisms were insufficient seismographic stations can be used to triangulate to the task. However, in 1929, one supporter, Arthur the hypocenter and place it precisely on the map. Holmes of England, suggested that convective flow Milne’s network ushered in the ability to detect of heated rock in the mantle beneath the crust might and locate earthquakes by remote sensing, a signifi- provide the required driving force—that is, as rocky cant contribution to science and society in itself. material deep in the mantle heats, it becomes less dense and rises toward the surface, where it cools and In the early 1890s, sinks, to be reheated and rise again. Without more British geologist John evidence of such a mechanism, however, the theory Milne was instru- of continental drift gained few adherents. mental in developing an accurate seismo- graph for recording the vibrations pro- duced by earthquakes. Magnetic Clues (Photo courtesy of the Evidence was hard to come by, but in the mid-1950s Science and Society Picture Library.) clues began accumulating in studies of magnetism in

2 BEYOND DISCOVERY This article was published in 1999 and has not been updated or revised. In 1915, German meteorologist Alfred Wegener published what came to be called the theory of con- tinental drift, in which he suggested that the continents had once been part of a supercontinent he named Pangaea. But without a convinc- ing mechanism for moving the continents around the globe, Wegener’s theory had few champi- ons for more than 40 years. (Figure adapted from Earth: Past and Present by Graham R. Thompson, Jonathan Turk, and Harold L. Levin, copyright 1995, by Harcourt, Inc., reproduced by permission of the publisher. Photo courtesy of the Alfred Wegener Institute of Polar and Marine Research.)

rocks. Patrick M. S. Blackett (who received the 1948 Hess, a geologist at Princeton University, aided the Nobel Prize in physics for his work on nuclear physics U.S. military with one hand and geoscience with the and cosmic rays) at Imperial College, Stanley Keith other. As the commander of an attack transport Runcorn at Cambridge University, and Edward during World War II, Hess had the most powerful Bullard at the National Physical Laboratory in England model of an echo sounder available, and he ran it were studying magnetism in rocks as part of their almost constantly during his Pacific missions, intent research into the nature of Earth’s magnetic field. on expanding on what little was known about the Scientists knew that newly formed crustal rock would configuration of the seafloor. By sending out pulses record an imprint of the strength and orientation of of sound and listening for return echoes from the Earth’s magnetic field at the time of the rock’s forma- seafloor, a sonar instrument can measure the dis- tion. Looking to see whether the rock’s magnetism tance between a ship and the ocean bottom. Hess varied directionally, Blackett, Runcorn, Bullard, combined measurements from several passages to and their students found considerable evidence that produce a rough contour map of the seafloor and in throughout geological history rocks had somehow the course of his war service discovered and mapped moved relative to the planet’s magnetic poles. Two about 100 flat-topped underwater mountains. Later, interpretations of this evidence were possible: either back at Princeton, Hess theorized that these moun- Earth’s poles had wandered relative to the continents tains originated as pointed-top volcanoes that subse- or the continents had moved relative to the poles. quently had been flattened by erosion. This led him By the mid-1950s, bolstered by paleomagnetic to consider the life cycle of underwater mountains, data gathered by Edward Irving at the Australian an interest he pursued throughout the 1950s. National University in Canberra, Blackett, Runcorn, Meanwhile, had become the and Bullard were convinced that Wegener had been home base for a vigorous marine research right. The rocks showed distinctly different apparent program, headed by Maurice Ewing. During the polar wandering paths for different continents, paths early 1950s, the research ships of Columbia’s Lamont that were consistent with the positions of the conti- Geological Observatory (now named Lamont-Doherty nents as suggested by Wegener’s drift theory. Earth Observatory) collected numerous depth sound- ings taken across the Atlantic Ocean, and in 1952 the Lamont researchers began assembling a map derived Insights from the Ocean from these soundings. One of the features of the Atlantic seafloor known Despite the magnetic evidence that the conti- since the mid-1870s was an undersea nents had moved over geological history, proponents called the Mid-Atlantic Ridge. Rising from a broad of the theory were still lacking proof of a mecha- flat plain to either side, the range had peaks that jutted nism. However, additional support was gathering up 10,000 feet from the ocean floor. The Lamont on the high seas. researchers discovered startling new information about With the help of war-driven improvements in the it, however. Not only was the Mid-Atlantic Ridge technique of echo location, or sonar (for sound navi- high, it was long. Running some 9,000 miles, nearly gation and ranging), for submarine detection, Harry the full length of the ocean from north of Greenland

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This article was published in 1999 and has not been updated or revised. to south of Africa, the ridge stretched farther than that nearly ring the Pacific Ocean and are found at the the and the Andes combined. The northeastern boundary of the Indian Ocean. Lamont researchers also found that the crest of the These were fundamental new discoveries, and ridge system is virtually bare of sediments, as com- Harry Hess, who had kept abreast of new data about pared with the thick layer of sediment on the plains the seafloor, was eager to explore their implications. near the continental margins, which can reach a thick- In 1960, he gleaned from Bruce Heezen the idea that ness of several miles. Perhaps the most unexpected Earth was “coming apart at the seams”—that is, at feature of the Mid-Atlantic Ridge was the deep valley the ridges. Given the youthful nature of the samples running down its spine. This rift, as it is called, from the rift bottom, Heezen argued that volcanic descended an average of 6,000 feet from the ridge rock——was welling up from below the crust. crest and ranged in width from 8 to 30 miles— From this hint of a mechanism that could explain the dimensions that could comfortably contain the Grand midocean ridges, Hess developed a new synthesis of Canyon of the Colorado River, which is at most about earth science in his well-known 1962 paper, “The 18 miles wide. Bottom samples taken from the rift History of Ocean Basins.” Although he characterized revealed the seafloor there to be made of extremely the essay as “geopoetry,” as if to warn other scientists young, dark volcanic rock. that not every concept could be proven, the work A map of the North Atlantic showing features stimulated thinking in the field. of the midocean ridge was published by Lamont Echoing the insights of seismologists, Hess posited researchers Bruce Heezen, Marie Tharp, and Maurice a multilayered interior for the planet. By this time Ewing in 1959. As it happened, echo soundings made researchers had refined their notions of Earth’s inner- elsewhere had produced similar profiles of the seafloor most structure. Rather than a single iron core, they throughout the world, and now an extraordinary now described a solid inner core of iron and a fluid pattern emerged. The soundings revealed the entire outer core of metal alloy, mostly iron. Surrounding midocean ridge system to be 37,200 miles in length, this was the mantle, overlain by the outermost thin long enough to circle the equator one and one-half oceanic and thick continental crust. Hess then elabo- times. It was one of the planet’s three dominant phys- rated on the evolution of the planet’s architecture. ical features, along with the continents and the oceans The crust was made up of iron-poor rock that had themselves. Researchers had also mapped a system of risen to the surface when heated and deep trenches—the deepest parts of the ocean basins— melted rocks in the interior of the newly condensed

Discoveries in Plate Tectonics and Seafloor Spreading

This chronology shows the chain of basic research that led to key insights about plate tectonics and related processes.

1963 1890s 1959 Fred Vine and Drummond John Milne develops the 1915 Bruce Heezen, Marie Tharp, Matthews at Cambridge seismograph, establishes German scientist Alfred and Maurice Ewing of Columbia University conceive of a a network of seismo- Wegener publishes his University publish the first connection between graphic stations in the then-radical theory of detailed map diagrams of seafloor spreading and British empire. continental drift in The the globe-girdling midocean the “stripes” of magnetic Origin of Continents ridge system. reversal in the seafloor. and Oceans.

Early 1900s 1962 Scientists use seismographic Mid-1950s In seeking to explain the midocean research to probe Earth’s inner Patrick M. S. Blackett, S. Keith ridges, Harry Hess of Princeton structure, developing a model Runcorn, and Edward Irving produce University proposes that the of concentric layers of core, paleomagnetic data from several seafloor is slowly spreading away mantle, and surface crust. continents showing apparent from the ridges, driven by convec- polar wandering consistent with tion currents in the mantle. Wegener’s drift theory.

This article was published in 1999 and has not been updated or revised. planet; this crust had once formed a single continental “normal,” north-pointing, as it is today; at other times land mass. In the presence of continued heating in the geomagnetic field was reversed. In 1963, Allan the planet’s interior, a convection “loop” of rising and Cox, Richard Doell, and Brent Dalrymple of the U.S. sinking material would be created in the mantle—just Geological Survey and Ian McDougall at Australian as Arthur Holmes had suggested in 1929. National University began to establish a quantitative Hess theorized that once the planet had formed, timescale of reversals by measuring the magnetic this mantle convection subdivided into numerous directions in lava flows on land and determining their separate circulating loops extending from the core. ages by radioactive methods. It was a painstaking Where the currents rise to the surface, molten material process, but by 1966 researchers had charted the oozes out, building up the midocean ridges and form- reversal timescale for the past 3.5 million years. ing new oceanic crust; as magma continues to flow, At sea, meanwhile, researchers were finding older seafloor is carried away in either direction from an intriguing pattern. Magnetic surveying of the the ridge by mantle convection. Where the convec- seafloor had been developed during World War II tion currents descend, cooled older oceanic crust as an outgrowth, like echo sounding, of the effort plunges back into the mantle at deep ocean trenches. to improve detection of submarines. In 1961, Arthur Hess thus subordinated the configuration of the Raff and Ronald Mason of the Scripps Institution oceans and continents to the motions of the spread- of noted magnetic anomalies in the ing, moving seafloor. While seafloor spreading was pattern of stripes on the ocean floor off the coast a compelling vision, it seemed untestable. Hess was of Washington state. One year later geophysicist proposing that it occurred at about the rate at which Drummond Matthews of Cambridge University, who a fingernail grows. The proof would have to come had gathered magnetic surveys of an undersea ridge indirectly—from magnetism, as it turned out. in the Indian Ocean, also noticed a distinct and curi- ous pattern of magnetic stripes—stronger and weaker magnetic signals in parallel bands on either side of the Of Magnetism and Time ridge crest. Back in England he talked over his find- ings with Fred Vine, a graduate student at Cambridge Scientists had known since the 1920s that rock University who was focusing on marine . from different geological times could show opposite The two of them hypothesized that the seafloor had magnetic polarities. Sometimes the orientation was recorded Earth’s magnetic field orientation at the

1965 Vine and J. Tuzo Wilson of the University of Toronto 1977 bolster the Vine-Matthews The National hypothesis with additional Late 1960s Earthquake Reduction magnetic reversal evidence Wilson coins the term Program is established from the seafloor at the “plate” in developing to reduce earthquake Juan de Fuca Ridge. Later the concept of seafloor hazards throughout the that year deep-sea cores spreading into a new United States. collected by Columbia’s scientific worldview— Neil Opdyke show a match- plate tectonics. ing timescale. Seafloor 1968 spreading is confirmed. Bryan Isacks, Jack Oliver, and Lynn 1977 Skyes recognize that The first seafloor hot slabs of material springs are discovered Late 1960s rigid enough to sus- at the Galápagos Rift in an expedition led by Jack Xavier Le Pichon, Dan tain earthquakes are 1963—1966 Corliss of Oregon State McKenzie, and W. Jason descending into deep Allan Cox, Richard Doell, and Brent University and Robert Morgan define the shapes trenches, creating Dalrymple of the U. S. Geological Ballard of Woods Hole of the plates and how their earthquake zones. Survey and Ian McDougall of the Oceanographic Institution. movement and location on Australian National University The scientists observe the globe could be determine the timescale of reversals unique and unexpected described by elementary of Earth’s magnetic field from animal communities living spherical geometry. measurements of lava flows on land. in the seafloor vents.

This article was published in 1999 and has not been updated or revised. Studies of seafloor magnetism in the early 1960s began establishing a timescale of reversals in Earth’s geomagnetic polarity, from north- pointing to south-pointing. The reversals showed up as a striped pat- tern of normal and oppositely mag- netized materials on either side of undersea ridges, such as Reykjanes Ridge off the southwestern coast of Iceland, an early clue to the move- ment of the seafloor. (Reprinted with permission from 1966, Research. 13: 427, as repro- duced in Seibold and Berger, The Sea Floor, 2nd edition revised, 1993, and U.S. Geological Survey.)

time the new molten rock oozed up from the mantle. samples matched those determined from lava flows on If spreading of the ocean floor occurs as Harry Hess land and from seafloor magnetic stripes. suggested, these blocks of alternately normal and reversely magnetized material would be carried away parallel to either side of the ridge. The Vine-Matthews hypothesis, published in the Plates in Motion fall of 1963, garnered little support in the geophysical Scientists now held the key to a whole new way of community, partly because the magnetic reversal understanding planet Earth. Tuzo Wilson, in an effort timescale was not yet complete, so the seafloor anom- to explain seafloor fault lines, was the first to tackle the aly data matched their hypothesis only poorly. But far-reaching implications of seafloor spreading. two years later, in 1965, Fred Vine found himself Around the globe, researchers had noted faults— in the company of Harry Hess, who had arrived at fractures perpendicular to the midocean spreading Cambridge on sabbatical leave, and J. Tuzo Wilson, ridges that cross whole oceans and break the ridges up there from the University of Toronto, continuing into segments. When Wilson took up the question, the some of his own research on midocean ridges. favored interpretation was that the faults were evidence Wilson was examining Raff and Mason’s maps of the tearing of the ocean crust from edge to edge. of the seafloor area off the coast of Vancouver Island The ridges were assumed to have started out as contin- and south to California, and he suggested that the uous features that were later fragmented and offset by maps showed a seafloor spreading ridge. Vine and the faults. Wilson disagreed. Yes, the faults were evi- Wilson published a paper in October 1965 proposing dence of crustal tearing, but only between the spread- a model for seafloor spreading in the northeastern ing ridge segments, segments that had always been off- Pacific, using as evidence bands of reversed magnet- set. This new view suggested that active deformation ism that marched out from either side of the ridge. is concentrated at the ridges and along their connecting Shortly thereafter, a slight discrepancy between the faults and that the rest of the ocean crust simply drifts seafloor reversal bands and the timing of known along, unbroken. Wilson gave the name “plate” to field reversals on land was smoothed out by a new these large masses of moving rock. He further pro- land-based field reversal discovered by Doell and posed that Earth’s surface was divided into about seven Dalrymple. With this addition, the two sets of data large crustal plates and several smaller ones. matched astonishingly well. Wilson’s ideas about oceanic faults and plates were The confirmation of seafloor spreading was sup- easily tested by the emerging earthquake location data ported by other observations made in 1965 and set and Lynn Sykes, working at Lamont, was quick to 1966. Key among them were ocean sediment sam- try this test. Wilson’s theories passed with flying colors. ples analyzed by Lamont’s Neil Opdyke. The samples Sykes found that oceanic earthquakes were, indeed, were from vertical cores, 16 to 40 feet long, taken concentrated along the midocean ridges and their from the ocean floor in the South Pacific. The timing connecting faults, and that the interiors of the oceanic and pattern of magnetic reversals in Opdyke’s core “plates” were nearly aseismic or earthquake-free.

6 BEYOND DISCOVERY This article was published in 1999 and has not been updated or revised. New seafloor forms as molten rock rises up through layers of the mantle and spreads outward from crustal rifts at midocean ridges. Old seafloor descends back down into the mantle at deep trenches that occur where two tectonic plates collide— regions where earthquakes are common. Perpendicular to the midocean ridges are fractures called transform faults, where the ocean crust tears apart as tectonic plates move. (Figure adapted from Earth: Past and Present by Graham R. Thompson, Jonathan Turk, and

Cold plate Harold L. Levin, copyright 1995, sinking into mantle by Harcourt, Inc., reproduced by permission of the publisher.)

Earthquake studies also supplied a crucial step Morgan at Princeton University went on to define the for our understanding of zones. By the shapes of the contiguous plates and how their move- 1940s, Kiyoo Wadati in Japan and Hugo Benioff at ment and location on the globe could be described the California Institute of Technology observed that by elementary spherical geometry, not only for the deep earthquakes were on a plane dipping beneath the present but for the past and future, too. In a speech ocean floor and were concentrated in areas around the before a convention of his colleagues in 1967, Tuzo edges of oceans close to volcanoes on land. Studies in Wilson declared that seafloor spreading and plate tec- the 1950s showed that those ocean areas were also the tonics “could be as important to geology as Harvey’s location of deep trenches, which Harry Hess invoked discovery of the circulation of the blood was to physi- in his model of seafloor spreading. The deep trenches ology or evolution to biology.” and the quakes associated with them had puzzled seis- mologists. Some of these earthquakes occurred very deep in the mantle, where high temperatures should have softened anything rigid so that rocks would flow Putting Geophysical Insight rather than being so hard and brittle that they fracture to Work easily in earthquakes. What changed that view was work by Lamont’s Jack Today, long-standing human concerns for public Oliver, Bryan Isacks, and Lynn Sykes, who examined safety have benefited from an understanding of plate earthquake activity in a trench near the South Pacific tectonics. For example, since the advent of plate tec- island of Tonga. Beginning in 1964, they collected tonic theory in the late 1960s, scientists have recog- seismic data to identify the subterranean sources, or nized that the in California is the foci, of earthquakes there. They observed, as had boundary separating two plates that are slowly sliding Benioff and Wadati, that the foci outlined a plane tilting past each other. The , the largest in the down from the ocean floor at about 45 degrees. But world, is moving in a northwesterly direction past the Lamont team was the first to recognize that this North America, dragging the rim of the continent plane was a slab of descending material cool and hard with it. Californians have long known that their land enough to sustain earthquakes and, moreover, that the was earthquake-prone. The understanding that they slab—containing the seafloor itself—was being bent are living on a major plate boundary clarifies that down into the trench, creating the earthquake zone. earthquakes are inevitable, that it is only a question The descending slab of seafloor, they determined, had of “when,” not “if” they will occur. considerable thickness, some 60 miles. What was mov- Although scientists cannot yet predict when earth- ing was not just the surface of the seafloor, or the crust quakes will occur, understanding the rate of plate alone, but a thicker block. It seemed fair to call this movement and the pattern of earthquakes associated moving block what Wilson had—a plate. with the San Andreas fault has prompted the California As the 1960s drew to a close, Xavier Le Pichon state government to take specific precautionary mea- at Lamont, Dan McKenzie at Scripps, and W. Jason sures. In 1975, California established a Seismic Safety

NATIONAL ACADEMY OF SCIENCES 7 This article was published in 1999 and has not been updated or revised. Commission, which, among other responsibilities, submersibles have now collected more than 200 reviews and updates the state’s earthquake hazard entirely new species of worm, mollusk, and arthropod mitigation plan. from these hydrothermal vents—a far cry from the On a national level, the Earthquake Hazards barren desert that biologists had once imagined as the Reduction Act in 1977 established the National ocean floor. Thriving in the absence of sunlight, the Earthquake Hazards Reduction Program (NEHRP) vent creatures create their energy by oxidizing chemi- to reduce earthquake hazards throughout the United cal compounds emerging from the interior of the plan- States, something that earthquake specialists had et—primarily hydrogen sulfide, a poison for most life been calling for since the great Alaskan earthquake of on Earth. Among the most intriguing creatures to 1964. Research carried out by NEHRP participants be discovered in the vent systems are “hyperther- influences not only programs of public education but mophiles,” microorganisms adapted to temperatures also the development of seismic standards for the sometimes exceeding the normal boiling point of design and construction of buildings and other struc- water. Their peculiar metabolic properties have tures. One significant design change to the Uniform made them the subject of research in biotechnology. Building Code, for example, involves the principle of Plate tectonics explains virtually all of Earth’s base isolation, in which a structure is supported by a major geological features and makes sense of other- number of bearing pads that are placed between the wise inexplicable phenomena such as earthquakes, structure itself and its foundation, or base. As the volcanic eruptions, and the formation of mountain ground moves in one direction, a structure on a fixed ranges. Tuzo Wilson’s claim for the significance of base lags behind, effectively going in the opposite the theory would therefore seem to be well founded. direction; given the nature of earthquake ground But plate tectonics has also led to new discoveries— waves, this means a structure actually vibrates back such as that of the organisms and and forth, in many directions. If its base is isolated their implications for our understanding of life on the by intervening bearing pads, however, the pads planet—outcomes that investigators such as John themselves absorb the vibration and the building Milne, Alfred Wegener, or Harry Hess could not have essentially remains stationary. Because of similar geo- predicted as they followed their scientific curiosity. logic hazards both Japanese and Californians are leaders in developing strengthened construction codes for, among other things, public schools, This article, which was published in 1999 and has not bridges, hospitals, and dams. been updated or revised, was adapted from an article In addition to its relevance to public safety, the written by science writer Joseph Cone for the National advent of plate tectonics theory has been a boon Academy of Sciences’ series Beyond Discovery™: The Path to essential economic activities such as mining and from Research to Human Benefit. Assistance was petroleum exploration. Since the 1970s, prospecting provided by Drs. Tanya Atwater, Richard Bernknopf, for oil and natural gas, for example, has been Kenneth H. Brink, G. Brent Dalrymple, William enhanced by the development and continued refine- Ellsworth, W. G. Ernst, Richard McCarthy, Dan P. ment of paleogeographic (literally “fossil” geography) McKenzie, and Neil Opdyke. maps that can help identify ancient environments that The Academy, located in Washington, D.C., is a were conducive to the formation and preservation of society of distinguished scholars engaged in scientific petroleum sources. and engineering research, dedicated to the use of science and technology for the public welfare. For more than a century it has provided independent, objective scientific advice to the nation. Window on a New World

More unexpected, perhaps, is the new undersea Funding for this article was provided by the world discovered at hydrothermal vents, the hot National Science Foundation and the National springs created where seeps down into the Academy of Sciences. heated crust at spreading zones. In 1977, an expedi- tion to the Galápagos Rift discovered astonishing new ecosystems around the hot springs there; similar ecosystems were soon found in several oceans around the globe. Researchers using manned and unmanned © 1999 by the National Academy of Sciences October 1999

8 BEYOND DISCOVERY This article was published in 1999 and has not been updated or revised.