+ a sº e A * | *-*. oco º-A- U.S. Department of the Interior / U.S. Geological Survey This NDynamic Earth: the Story of Plate Tºclonics LääAºs MAR 0 || 1333 | DEPOSITED BY TED STATES OF AMºnº -------- Front cover: View of planet Earth from the Apollo spacecraft. The Red Sea, which separates Saudi Arabia from the continent of Africa, is clearly visible at top. (Photograph courtesy of NASA.) Conversion of Units Metric units of measurement are used in this book, because they are the standard in virtually every country in the world except the United States. For readers who wish to convert measurements from metric units to English units, the conversion factors are listed below. Multiply By To obtain Length centimeter (cm) 0.39 inch meter (m) 3.28 foot meter (m) 1.09 yard kilometer (km) 0.62 mile Temperature degree Centigrade 1.80 then degree Fahrenheit (°C) add 32 (°F) A simple guideline for approximate conversions: 1 inch equals about 2.5 centimeters; 1 yard equals about 1 meter Z This O Dynamic Earth: : the Story of Plate Tectonics by * W. Jacquelyne Kious and • Robert I. Tilling Oldoinyo Lengai, an active volcano in the East African Rift Zone, where Africa is being pulled apart by plate-tectomic processes. (Photograph by Jorg Keller, Albert-Ludwigs-Universität Freiburg, Germany.) Book designed and coordinated by Martha Kiger Illustrations and production by Jane Russell N º º/ / - º ſ - º/ -Equator Preface In the early 1960s, the emergence of the theory of plate tectomics started a revolution in the earth sciences. Since then, scientists have veri- fied and refined this theory, and now have a much better understanding of how our planet has been shaped by plate-tectonic processes. We now know that, directly or indirectly, plate tec- tonics influences nearly all geologic processes, past and present. Indeed, the notion that the entire Earth's surface is continually shifting has profoundly changed the way we view our world. People benefit from, and are at the mercy of, the forces and consequences of plate tectonics. With little or no warning, an earthquake or vol- canic eruption can unleash bursts of energy far more powerful than anything we can generate. While we have no control over plate-tectonic processes, we now have the knowledge to learn from them. The more we know about plate PERMIAN 225 million years ago TRIASSIC 200 million years ago tectonics, the better we can appreciate the grandeur and beauty of the land upon which we live, as well as the occasional violent displays of the Earth's awesome power. This booklet gives a brief introduction to the concept of plate tectonics and complements the visual and written information in This Dynamic Planet (see Further reading), a map published in 1994 by the U.S. Geological Survey (USGS) and the Smithsonian Institution. The booklet high- lights some of the people and discoveries that advanced the development of the theory and traces its progress since its proposal. Although the general idea of plate tectonics is now widely accepted, many aspects still continue to con- found and challenge scientists. The earth- science revolution launched by the theory of plate tectonics is not finished. -Equator JURASSIC 135 million years ago ºf vº - Z Historical perspecti In geologic terms, a plate is a large, rigid slab (meaning “all lands” in Greek), which figured of solid rock. The word tectonics comes from the prominently in the theory of continental drift Greek root “to build.” Putting these two words (p. 5)—the forerunner to the theory of plate together, we get the term plate tectomics, which tectonics. refers to how the Earth's surface is built of Plate tectonics is a relatively new scientific plates. The theory of plate tectomics states that the concept, introduced some 30 years ago, but it Earth's outermost layer is fragmented into a has revolutionized our understanding of the dozen or more large and small plates that are dynamic planet upon which we live. The theory moving relative to one another as they ride atop has unified the study of the Earth by drawing hotter, more mobile material. Before the advent together many branches of the earth sciences, of plate tectonics, however, some people already from paleontology (the study of fossils) to seis- believed that the present-day continents were the mology (the study of earthquakes). It has pro- fragmented pieces of preexisting larger land- vided explanations to questions that scientists masses (“supercontinents”). The diagrams below had speculated upon for centuries—such as why show the break-up of the supercontinent Pangaea earthquakes and volcanic eruptions occur in very -->-7- ASIA - F4– & eºſºpia wºº ND {) According to the continental drift theory, Equator – the supercontinent Pangaea began to break up about 225–200 million years ago, eventually fragmenting into the AMERICA ſ \ / - _-7– - - - º - - ANTARCTICA - = - _* CRETACEOUS PRESENT DAY 65 million years ago continents as we know them today. N EURASIAN PLATE A PHILIPPINE PLATE º C. C. NORTH AMERICAN S- PLATE JUAN DE FUCA § PLATE COCOS PLATE EOUATOR PACIFIC NAZCA PLATE PLATE PLATE ANTARCTIC º -- - - CARIBBEAN PLATE - SOUTH AMERICAN PLATE SCOTIA PLATE ** ARABIAN PLATE Aracan." PLATE EURASIAN PLATE § T INDIAN PLATE AUSTRALIAN PLATE The layer of the Earth we live on is broken into a dozen or so rigid slabs (called tectonic plates by geologists) that are moving relative to one another 2 specific areas around the world, and how and why great mountain ranges like the Alps and Himalayas formed. Why is the Earth so restless? What causes the ground to shake violently, volcanoes to erupt with explosive force, and great mountain ranges to rise to incredible heights? Scientists, philoso- phers, and theologians have wrestled with ques- tions such as these for centuries. Until the 1700s, most Europeans thought that a Biblical Flood played a major role in shaping the Earth's surface. This way of thinking was known as “cata- strophism,” and geology (the study of the Earth) AVANT L.A. SEPARATION was based on the belief that all earthly changes were sudden and caused by a series of catastro- phes. However, by the mid-19th century, cata- strophism gave way to “uniformitarianism," a new way of thinking centered around the “Uniformi- tarian Principle" proposed in 1 785 by James Hutton, a Scottish geologist. This principle is commonly stated as follows: The present is the key to the past. Those holding this viewpoint assume that the geologic forces and processes—gradual as well as catastrophic—acting on the Earth today are the same as those that have acted in the geologic past. A PRES L. : : - . Tº In 1858, geographer Antonio Snider- Pellegrini made these two maps showing his version of how the American and African continents may once have fit together, then later separated. Left: The formerly joined continents before (avant) their separation. Right: The continents after (aprés) the separation. (Reproduc- tions of the original maps courtesy of University of California, Berkeley.) N Cutaway views showing the internal structure of the Earth. Below: This view drawn to scale demonstrates that the Earth's crust literally is only skin deep. Below right: A view not drawn to scale to show the Earth's three main layers (crust, mantle, and core) in more detail (see text). To scale Inside the Earth The size of the Earth—about 12,750 kilome- ters (km) in diameter—was known by the ancient Greeks, but it was not until the turn of the 20th century that scientists determined that our planet is made up of three main layers: crust, mantle, and core. This layered structure can be com- pared to that of a boiled egg. The crust, the out- ermost layer, is rigid and verythin compared with the other two. Beneath the oceans, the crust varies little in thickness, generally extending only to about 5 km. The thickness of the crust beneath continents is much more variable but averages about 30 km; under large mountain ranges, such as the Alps or the Sierra Nevada, however, the Crust 0–100 km thick / Mantle Crust 2,900 km Outer core Inner core Not to scale Lithosphere (crust and upper- most solid mantle) Mantle 6,378 km base of the crust can be as deep as 100 km. Like the shell of an egg, the Earth's crust is brittle and can break. Below the crust is the mantle, a dense, hot layer of semi-solid rock approximately 2,900 km thick. The mantle, which contains more iron, magnesium, and calcium than the crust, is hotter and denser because temperature and pressure inside the Earth increase with depth. As a com- parison, the mantle might be thought of as the white of a boiled egg. At the center of the Earth lies the core, which is nearly twice as dense as the mantle because its composition is metallic (iron-nickel alloy) rather than stony. Unlike the yolk of an egg, however, the Earth's core is actu- ally made up of two distinct parts: a 2,200 km- thick liquid outer core and a 1,250 km-thick solid inner core. As the Earth rotates, the liquid outer core spins, creating the Earth's magnetic field. Not surprisingly, the Earth's internal structure influences plate tectonics. The upper part of the mantle is cooler and more rigid than the deep mantle; in many ways, it behaves like the overly- ing crust. Together they form a rigid layer of rock called the lithosphere (from lithos, Greek for stone). The lithosphere tends to be thinnest under the oceans and in volcanically active conti- nental areas, such as the Western United States. Averaging at least 80 km in thickness over much of the Earth, the lithosphere has been broken up into the moving plates that contain the world's continents and oceans. Scientists believe that below the lithosphere is a relatively narrow, mobile zone in the mantle called the astheno- sphere (from asthenes, Greek for weak). This zone is composed of hot, semi-solid material, which can soften and flow after being subjected to high temperature and pressure over geologic time. The rigid lithosphere is thought to "float" or move about on the slowly flowing astheno- sphere. /2 The belief that continents have not always been fixed in their present positions was suspect- ed long before the 20th century; this notion was first suggested as early as 1596 by the Dutch map maker Abraham Ortelius in his work Thesaurus Geographicus. Ortelius suggested that the Ameri- cas were “torn away from Europe and Africa...by earthquakes and floods” and went on to say: “The vestiges of the rupture reveal themselves, if someone brings forward a map of the world and considers carefully the coasts of the three [conti- ments].” Ortelius' idea surfaced again in the 19th century. However, it was not until 1912 that the idea of moving continents was seriously con- sidered as a full-blown scientific theory—called Continental Drift—introduced in two articles pub- lished by a 32-year-old German meteorologist named Alfred Lothar Wegener. He contended that, around 200 million years ago, the supercon- tinent Pangaea began to split apart. Alexander Du Toit, Professor of Geology at Johannesburg University and one of Wegener's staunchest sup- porters, proposed that Pangaea first broke into two large continental landmasses, Laurasia in the northern hemisphere and Gondwanaland in the southern hemisphere. Laurasia and Gondwana- land then continued to break apart into the vari- ous smaller continents that exist today. Wegener's theory was based in part on what appeared to him to be the remarkable fit of the South American and African continents, first noted by Abraham Ortelius three centuries earli- er. Wegener was also intrigued by the occur- rences of unusual geologic structures and of plant and animal fossils found on the matching coastlines of South America and Africa, which are now widely separated by the Atlantic Ocean. He reasoned that it was physically impossible for most of these organisms to have swum or have been transported across the vast oceans. To him, the presence of identical fossil species along the coastal parts of Africa and South America was the most compelling evidence that the two conti- ments were once joined. In Wegener's mind, the drifting of continents after the break-up of Pangaea explained not only the matching fossil occurrences but also the evi- dence of dramatic climate changes on some con- tinents. For example, the discovery of fossils of tropical plants (in the form of coal deposits) in Antarctica led to the conclusion that this frozen land previously must have been situated closer to the equator, in a more temperate climate where lush, swampy vegetation could grow. Other mis- matches of geology and climate included distinc- tive fossil ferns (Glossopteris) discovered in now- polar regions, and the occurrence of glacial deposits in present-day arid Africa, such as the Vaal River valley of South Africa. The theory of continental drift would become the spark that ignited a new way of viewing the Earth. But at the time Wegener introduced his theory, the scientific community firmly believed the continents and oceans to be permanent fea- tures on the Earth's surface. Not surprisingly, his proposal was not well received, even though it seemed to agree with the scientific information available at the time. A fatal weakness in Wegener's theory was that it could not satisfacto- rily answer the most fundamental question raised by his critics: What kind of forces could be strong enough to move such large masses of solid rock over such great distances? Wegener suggested that the continents simply plowed (text continued on page 9) N What is a tectomic plate? A tectonic plate (also called lithospheric plate) is a massive, irregularly shaped slab of solid rock, generally composed of both continental and oceanic lithosphere. Plate size can vary greatly, from a few hundred to thousands of kilometers across; the Pacific and Antarctic Plates are among the largest. Plate thickness also varies greatly, ranging from less than 15 km for young oceanic lithosphere to about 200 km or more for ancient continental lithosphere (for example, the interior parts of North and South America). How do these massive slabs of solid rock float despite their tremendous weight? The answer lies in the composition of the rocks. Continental crust is composed of granitic rocks which are made up of relatively lightweight minerals such as quartz and feldspar. By contrast, oceanic crust is composed of basaltic rocks, which are much denser and heavier. The variations in plate thick- ness are nature's way of partly compensating for the imbalance in the weight and density of the two types of crust. Because continental rocks are much lighter, the crust under the continents is much thicker (as much as 100 km) whereas the crust under the oceans is generally only about 5 km thick. Like icebergs, only the tips of which are visible above water, continents have deep "roots" to support their elevations. Most of the boundaries between individual plates cannot be seen, because they are hidden beneath the oceans. Yet oceanic plate bound- aries can be mapped accurately from outer space by measurements from GEOSAT satellites. Earthquake and volcanic activity is concentrated near these boundaries. Tectonic plates probably developed very early in the Earth's 4.6-billion- year history, and they have been drifting about on the surface ever since—like slow-moving bumper cars repeatedly clustering together and then separating. Like many features on the Earth's surface, plates change over time. Those composed partly or entirely of oceanic lithosphere can sink under another plate, usually a lighter, mostly continen- tal plate, and eventually disappear completely. This process is happening now off the coast of Oregon and Washington. The small Juan de Fuca Plate, a remnant of the formerly much larger oceanic Farallon Plate, will someday be entirely consumed as it continues to sink beneath the North American Plate. These four diagrams illustrate the shrinking of the for- merly very large Farallon Plate, as it was progressively consumed beneath the North American and Caribbean Plates, leaving only the present-day Juan de Fuca, Rivera, and Cocos Plates as small remnants (see text). Large solid arrows show the present-day sense of relative movement between the Pacific and North American Plates. (Modified from USGS Professional Paper 1515). Z 30 million | 20 million years ago years ago ocw. LL. H. <ſ —l Cl- LL. LL. H. H. <ſ PACIFIC <ſ —l – PACIFIC D- PLATE | | Cl- PLATE Los Angeles 2 Los Angeles? 2 <ſ R <ſ O O Cº. Dr. LL. LL > s <ſ <ſ 2 T I O H. H. —l Dr. Dr. —l O O <ſ 2 2 Dr. <ſ COCOS LL PLATE Trench Trench 10 million years ago PACIFIC PLATE | | Los Angelº Trench |- Present JUANDE FUCA PLATE PACIFIC | PLATE RIVERA 5%; ºg Himalayas Rising Tibetan Plateau º, 7 8. º:..º. Fº M ...” § - ... . . . :: ſ , : % * *; £º - % {_*º. Tibetan: ºx: º: º: Z *re--. * * * . º, Plateau & - º - º, %, ...ſº º ... " "...º. "Z sº º * Ganges Plain tº ſº? Fº &. ſº-º-º-º: Mt. Everest * º º: º, ... }, ºf ſº * INDIAN *" ºf ſº, º PLATE /~ vº º A º, --- * : 3. * ...: - º º ..” #! **** -* . § -- * 2 +... n. * : *. INDIAN OCEAN * 40 Z The Himalayas: Two continents collide Among the most dramatic and visible cre- ations of plate-tectonic forces are the lofty Himalayas, which stretch 2,900 km along the bor- der between India and Tibet. This immense mountain range began to form between 40 and 50 million years ago, when two large landmass- es, India and Eurasia, driven by plate movement, collided. Because both these continental land- masses have about the same rock density, one plate could not be subducted under the other. The pressure of the impinging plates could only be relieved by thrusting skyward, contorting the collision zone, and forming the jagged Himalayan peaks. About 225 million years ago, India was a large island still situated off the Australian coast, and a vast ocean (called Tethys Sea) separated India from the Asian continent. When Pangaea broke apart about 200 million years ago, India began to forge northward. By studying the history—and ultimately the closing—of the Tethys, scientists have reconstructed India's northward journey. About 80 million years ago, India was located roughly 6,400 km south of the Asian continent, moving northward at a rate of about 9 m a century. When India rammed into Asia about 40 to 50 million years ago, its northward advance slowed by about half. The collision and associ- ated decrease in the rate of plate movement are interpreted to mark the beginning of the rapid uplift of the Himalayas. E U R A S I A N P. L. A. T E \ 10 million – years ago TX /*-* ! - / I - \ Q \ I SRI LANKA - \ - N-7 } T-----, 38 million –– \ ^ ~~ ſ \ , I \ * years ago v_ _ _ – 2. ~ – Equator –A-Z 2- / I / T /* - *- / N N _Y \ - Z. N. \_ 2^ - / \ `s / | -- / v_0 TT, 55 million —— zº I ~ -- / years ago _2^ ſ 2–~~ º 2- 24 INDIAN 00EAN A 92. | y^ - / Y | rº ( / \ \ --- **) –71 million years ago 2 * \ / "INDIA." \ / Land mass \ - - zº SRI LANKA { / -- The º of the India landmass (Indian Plate) before its colli- sion with Asia (Eurasian Plate) about 40 to 50 million years ago (see text). India was once situated well south of the Equator, near the continent of Australia. 41 Sunset view of towering, snow-capped Mt. Everest, from the village of Lobuche (Solu-khumbu), Nepal. (Photograph by Gimmy Park Li.) The Himalayas and the Tibetan Plateau to the north have risen very rapidly. In just 50 million years, peaks such as Mt. Everest have risen to heights of more than 9 km. The impinging of the two landmasses has yet to end. The Himalayas continue to rise more than 1 cm a year—a growth rate of 10 km in a million years! If that is so, why aren't the Himalayas even higher? Scientists believe that the Eurasian Plate may now be stretching out rather than thrusting up, and such stretching would result in some subsidence due to gravity. Fifty kilometers north of Lhasa (the capital of Tibet), scientists found layers of pink sandstone containing grains of magnetic minerals (mag- netite) that have recorded the pattern of the Earth's flip-flopping magnetic field. These sand- stones also contain plant and animal fossils that were deposited when the Tethys Sea periodically flooded the region. The study of these fossils has revealed not only their geologic age but also the type of environment and climate in which they formed. For example, such studies indicate that the fossils lived under a relatively mild, wet envi- ronment about 105 million years ago, when Tibet was closer to the equator. Today, Tibet's climate is much more arid, reflecting the region's uplift and northward shift of nearly 2,000 km. Fossils found in the sandstone layers offer dramatic evi- dence of the climate change in the Tibetan region due to plate movement over the past 100 million years. At present, the movement of India continues to put enormous pressure on the Asian continent, and Tibet in turn presses on the landmass to the north that is hemming it in. The net effect of plate-tectonics forces acting on this geologically complicated region is to squeeze parts of Asia eastward toward the Pacific Ocean. One serious CO e of these processes is a deadly "domino"-effect: tremendous stresses build up within the Earth's crust, which are relieved peri- odically by earthquakes along the numerous faults that scar the landscape. Some of the world's most destructive earthquakes in history are related to continuing tectonic processes that began some 50 million years ago when the Indian and Eurasian continents first met. 42 Z Transform boundaries The zone between two plates sliding horizon- tally past one another is called a transform-fault boundary, or simply a transform boundary. The concept of transform faults originated with Canadian geophysicist J. Tuzo Wilson, who pro- posed that these large faults or fracture zones con- nect two spreading centers (divergent plate boundaries) or, less º ..". Most transform faults are found on the ocean floor. They commonly offset the active spreading ridges, producing zig- zag plate margins, and are generally defined by shallow earthquakes. However, a few occur on land, for example the San Andreas fault zone in California. This transform fault connects the East Pacific Rise, a divergent boundary to the south, with the South Gorda-Juan de Fuca- Explorer Ridge, another divergent boundary to the north. The San Andreas fault zone, which is about 1,300 km long and in places tens of kilometers wide, slices through two thirds of the length of California. Along it, the Pacific Plate has been grinding horizontally past the North American Plate for 10 million years, at an average rate of about 5 cm/yr. Land on the west side of the fault zone (on the Pacific Plate) is moving in a northwesterly direction relative to the land on the east side of the fault zone (on the North American Plate). Oceanic fracture zones are ocean-floor valleys that horizontally offset spreading ridges; some of these zones are hundreds to thousands of kilo- meters long and as much as 8 km deep. Examples of these large scars include the Explorer ridge CANADA Juan —- N T- de Fuca ridge Blanco × fracture ZOne § UNITED STATES Mendocino fracture zone San Francisco 5 O merican Plate Murra - Tºº-sk Relative motion Los • of Pacific Plate "99% --- N- Molokai The Blanco, Mendocino, Murray, and Molokai fracture zones are some of the many fracture zones (transform faults) that scar the ocean floor and offset ridges (see text). The San Andreas is one of the few transform faults exposed on land. Aerial view of the San Andreas fault slicing through the Carrizo Plain in the Temblor Range east of the city of San Luis Obispo. (Photograph by Robert E. Wallace, USGS.) Clarion, Molokai, and Pioneer fracture zones in the Northeast Pacific off the coast of California and Mexico. These zones are presently inactive, but the offsets of the patterns of magnetic strip- ing provide evidence of their previous transform- fault activity. Plate-boundary zones Not all plate boundaries are as simple as the main types discussed above. In some regions, the boundaries are not well defined because the plate-movement deformation occurring there extends over a broad belt (called a plate-boundary zone). One of these zones marks the Mediter- ranean-Alpine region between the Eurasian and African Plates, within which several smaller frag- ments of plates (microplates) have been recog- nized. Because plate-boundary zones involve at least two large plates and one or more micro- plates caught up between them, they tend to have complicated geological structures and earthquake patterns. - 44 Z Rates of motion We can measure how fast tectonic plates are moving today, but how do scientists know what the rates of plate movement have been over geo- logic time? The oceans hold one of the key pieces to the puzzle. Because the ocean-floor magnetic striping records the flip-flops in the Earth's magnetic field, scientists, knowing the approximate duration of the reversal, can calcu- late the average rate of plate movement during a given time span. These average rates of plate South Pacific about 3, the fastest rate (more than Evidence of past rates o ement also can be obtained from geologic mapping studies. If a rock formation of known age—with distinc- tive composition, structure, or fossils—mapped on one side of a plate boundary can be matched with the same formation on the other side of the boundary, then measuring the distance that the formation has been offset can give an estimate of the average rate of plate motion. This simple but effective technique has been used to deter- mine the rates of plate motion at divergent boundaries, for example the Mid-Atlantic Ridge, and transform boundaries, such as the San Andreas Fault. Current plate movement can be tracked directly by means of ground-based or space-based geodetic measurements; geodesy is the science of the size and shape of the Earth. Ground-based measurements are taken with conventional but very precise ground-surveying techniques, using laser-electronic instruments. However, because plate motions are global in scale, they are best measured by satellite-based methods. The late 1970s witnessed the rapid growth of space geodesy, a term applied to space-based techniques for - - - - - One of the mysterious, imposing stone monoliths—some standing 5 m tall and weighing 14 tons—on Easter Island (Chile), carved by ancient Polynesians out of volcanic rock. Easter Island, which lies on the Nazca Plate close to the East Pacific Rise, is moving eastward toward South America by seafloor spread- ing at the fastest rate known in the world (see text). (Photograph by Carlos Capurro, U.S. Embassy, Santiago, Chile.) 45 Right: Artist's conception of a Global Positioning System (GPS) satellite in orbit. (Illustration courtesy of NASA.) Below: A GPS ground receiver—here set up on the flank of Augustine Volcano (Cook Inlet, Alaska)—recording the sig- mals sent by four or more of the orbiting GPS satellites. (Photograph by Jerry Swarc, USGS.) taking precise, repeated measurements of care- fully chosen points on the Earth's surface sepa- rated by hundreds to thousands of kilometers. The three most commonly used space-geodetic techniques—very long baseline interferometry (VLBI), satellite laser ranging (SLR), and the Global Positioning System (GPS)—are based on technologies developed for military and aero- space research, notably radio astronomy and satellite tracking. Among the three techniques, to date the GPS has been the most useful for studying the Earth's crustal movements. Twenty-one satellites are cur- rently in orbit 20,000 km above the Earth as part of the NavStar system of the U.S. Department of Defense. These satellites continuously transmit radio signals back to Earth. To determine its precise position on Earth (longitude, latitude, elevation), each GPS ground site must simultane- ously receive signals from at least four satellites, recording the exact time and location of each satellite when its signal was received. By repeat- edly measuring distances between specific points, geologists can determine if there has been active movement along faults or between plates. The separations between GPS sites are already being measured regularly around the Pacific basin. By monitoring the interaction between the Pacific Plate and the surrounding, largely continental plates, scientists hope to learn more about the events building up to earthquakes and volcanic eruptions in the circum-Pacific Ring of Fire. Space-geodetic data have already confirmed that the rates and direction of plate movement, aver- aged over several years, compare well with rates and direction of plate movement averaged over millions of years. 46 Z “Hotspots”. Mantle thermal plumes The vast majority of earthquakes and volcanic eruptions occur near plate boundaries, but there are some exceptions. For example, the Hawaiian Islands, which are entirely of volcanic origin, have formed in the middle of the Pacific Ocean more than 3,200 km from the nearest plate boundary. How do the Hawaiian Islands and other volcanoes that form in the interior of plates fit into the plate-tectonics picture? In 1963, J. Tuzo Wilson, the Canadian geo- physicist who discovered transform faults, came up with an ingenious idea that became known as the “hotspot” theory. Wilson noted that in cer- tain locations around the world, such as Hawaii, volcanism has been active for very long periods of time. This could only happen, he reasoned, if relatively small, long-lasting, and exceptionally hot regions—called hotspots—existed below the plates that would provide localized sources of high heat energy (thermal plumes) to sustain vol- canism. Specifically, Wilson hypothesized that the distinctive linear shape of the Hawaiian Island-Emperor Seamounts chain resulted from the Pacific Plate moving over a deep, stationary hotspot in the mantle, located beneath the pre- sent-day position of the Island of Hawaii. Heat from this hotspot produced a persistent source of magma by partly melting the overriding Pacific Plate. The magma, which is lighter than the surrounding solid rock, then rises through the mantle and crust to erupt onto the seafloor, Space Shuttle photograph of the Hawaiian Islands, the southern- most part of the long volcanic trail of the “Hawaiian hotspot” (see text). Kauai is in the lower right corner (edge) and the Big Island of Hawaii in the upper left corner. Note the curvature of the Earth (top edge). (Photograph courtesy of NASA.) 47 forming an active seamount. Over time, count- less eruptions cause the seamount to grow until it finally emerges above sea level to form an island volcano. Wilson suggested that continu- ing plate movement eventually carries the island beyond the hotspot, cutting it off from the magma source, and volcanism ceases. As one island volcano becomes extinct, another devel- ops over the hotspot, and the cycle is repeated. This process of volcano growth and death, over many millions of years, has left a long trail of vol- canic islands and seamounts across the Pacific Ocean floor. According to Wilson's hotspot theory, the vol- canoes of the Hawaiian chain should get progres- sively older and become more eroded the farther they travel beyond the hotspot. The oldest vol- canic rocks on Kauai, the northwesternmost inhabited Hawaiian island, are about 5.5 million years old and are deeply eroded. By comparison, magma on the “Big Island” of Hawaii–southeasternmost formation. in the chain and presumably still positioned over Sº - the hotspot—the oldest exposed rocks are less than 0.7 million years old and new volcanic rock is continually being formed. Above: Artist's conception of the movement of the Pacific Plate over the fixed Hawaiian "Hot Spot,” illustrating the formation of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from a drawing provided by Maurice Krafft, Centre de Volcanologie, France). Left: J. Tuzo Wilson's original diagram (slightly modified), published in 1963, to show his pro- posed origin of the Hawaiian Islands. (Reproduced with permission of the (a) Section (b) Plan Canadian Journal of Physics.) 48 Z Canadian geophysicist J. Tuzo Wilson was also pivotal in advancing the plate-tectonics theo- ry. Intrigued by Wegener's notion of a mobile Earth and influenced by Harry Hess' exciting ideas, Wilson was eager to convert others to the revolution brewing in the earth sciences in the early 1960s. Wilson had known Hess in the late 1930s, when he was studying for his doctorate at Princeton University, where Hess was a dynamic young lecturer. In 1963, Wilson developed a concept crucial to the plate-tectonics theory. He suggested that the Hawaiian and other volcanic island chains may have formed due to the movement of a plate over a stationary "hotspot" in the mantle. This hypothesis eliminated an apparent contradiction to the plate-tectonics theory—the occurrence of active volcanoes located many thousands of kilo- meters from the nearest plate boundary. Hun- dreds of subsequent studies have proven Wilson right. However, in the early 1960s, his idea was considered so radical that his "hotspot" manu- script was rejected by all the major international scientific journals. This manuscript ultimately was published in 1963 in a relatively obscure publication, the Canadian Journal of Physics, and became a milestone in plate tectonics. Another of Wilson's important contributions to the development of the plate-tectonics theory was published two years later. He proposed that there must be a third type of plate boundary to connect the oceanic ridges and trenches, which he noted can end abruptly and "transform" into major faults that slip horizontally. A well-known example of such a transform-fault boundary is the San Andreas Fault zone. Unlike ridges and trenches, transform faults offset the crust hori- zontally, without creating or destroying crust. J. Tuzo Wilson: Discovering transforms and hotspots Wilson was a professor of geophysics at the University of Toronto from 1946 until 1974, when he retired from teaching and became the Director of the Ontario Science Centre. He was a tireless lecturer and traveller until his death in 1993. Like Hess, Wilson was able to see his concepts of hotspots and transform faults confirmed, as knowledge of the dynamics and seismicity of the ocean floor increased dramatically. Wilson and other scientists, including Robert Dietz, Harry Hess, Drummond Matthews, and Frederick Vine, were the principal architects in the early develop- ment of plate tectonics during the mid-1960s—a theory that is as vibrant and exciting today as it was when it first began to evolve less than 30 years ago. Interestingly, Wilson was in his mid- fifties, at the peak of his scientific career, when he made his insightful contributions to the plate-tec- tonics theory. If Alfred Wegener had not died at age 50 in his scientific prime, the plate tectonics revolution may have begun sooner. J. Tuzo Wilson (1908-1993) made major contributions to the development of the plate-tectonics theory in the 1960s and 1970s. He remained a dominant force in the Canadian scientific scene until his death. (Photograph courtesy of the Ontario Science Centre.) 49 N The possibility that the Hawaiian Islands become younger to the southeast was suspected by the ancient Hawaiians, long before any scien- tific studies were done. During their voyages, sea-faring Hawaiians noticed the differences in erosion, soil formation, and vegetation and rec- ognized that the islands to the northwest (Niihau and Kauai) were older than those to the south- east (Maui and Hawaii). This idea was handed down from generation to generation in the legends of Pele, the fiery Goddess of Volcanoes. Pele originally lived on Kauai. When her older sister Namakaokahai, the Goddess of the Sea, attacked her, Pele fled to the Island of Oahu. When she was forced by Namakaokahai to flee again, Pele moved southeast to Maui and finally to Hawaii, where she now lives in the Halemau- mau Crater at the summit of Kilauea Volcano. The mythical flight of Pele from Kauai to Hawaii, which alludes to the eternal struggle between the growth of volcanic islands from eruptions and their later erosion by ocean waves, is consistent with geologic evidence obtained centuries later that clearly shows the islands becoming younger from northwest to southeast. EXPLANATION Divergent plate boundaries— Where new crust is generated as the plates pull away from each other. . y - *—- Convergent plate boundaries— Where crust is consumed in the Earth's interior as one plate dives under another. - Nº. ------------------ Transform plate boundaries— Where crust is neither produced nor destroyed as plates slide horizontally past each other. N Plate boundary zones—Broad belts in which deformation is diffuse and boundaries are not well defined. Selected prominent hotspots - - - N World map showing the locations of selected prominent hotspots; those labelled are mentioned in the text. (Modified from the map This Dynamic Planet.) 50 /2 Although Hawaii is perhaps the best known hotspot, others are thought to exist beneath the oceans and continents. More than a hundred hotspots beneath the Earth's crust have been active during the past 10 million years. Most of these are located under plate interiors (for example, the African Plate), but some occur near diverging plate boundaries. Some are concen- trated near the mid-oceanic ridge system, such as beneath Iceland, the Azores, and the Galapagos Islands. A few hotspots are thought to exist below the North American Plate. Perhaps the best known is the hotspot presumed to exist under the conti- mental crust in the region of Yellowstone Nation- al Park in northwestern Wyoming. Here are sev- eral calderas (large craters formed by the ground collapse accompanying explosive volcanism) that were produced by three gigantic eruptions dur- ing the past two million years, the most recent of which occurred about 600,000 years ago. Ash deposits from these powerful eruptions have been mapped as far away as Iowa, Missouri, Texas, and even northern Mexico. The thermal energy of the presumed Yellowstone hotspot fuels more than 10,000 hot pools and springs, geysers (like Old Faithful), and bubbling mudpots (pools of boiling mud). A large body of magma, capped by a hydrothermal system (a zone of pres- surized steam and hot water), still exists beneath the caldera. Recent surveys demonstrate that parts of the Yellowstone region rise and fall by as much as 1 cm each year, indicating the area is still geologically restless. However, these measur- able ground movements, which most likely reflect hydrothermal pressure changes, do not necessarily signal renewed volcanic activity in the area. While the existence of the Hawaiian hotspot is now widely accepted, little scientific agreement exists on the precise number and locations of the other presumed hotspots, despite recent studies. Even the presumed hotspot beneath Yellowstone, one of the best candidates for a con- tinental hotspot, is still considered to be highly speculative, especially by the scientists most knowledgeable about the geology of the Western United States. Snow-capped 4, 169-m-high Mauna Loa Volcano, Island of Hawaii, seen from the USGS Hawaiian Volcano Observatory. Built by Hawaiian hotspot volcanism, Mauna Loa—the largest mountain in the world—is a classic example of a shield volcano. (Photograph by Robert I. Tilling, USGS.) - N Map of part of the Pacific basin showing the volcanic trail of the Hawaiian hotspot–6,000-km-long Hawaiian Ridge-Emperor Seamounts chain. (Base map reprinted by permission from World Ocean Floor by Bruce C. Heezen and Marie Tharp, Copyright 1977.) The long trail of the Hawaiian hotspot Over the past 70 million years, the combined processes of magma formation, volcano eruption and growth, and continued movement of the Pacific Plate over the stationary Hawaiian "hot- spot" have left a long trail of volcanoes across the Pacific Ocean floor. The Hawaiian Ridge- Emperor Seamounts chain extends some 6,000 km from the "Big Island" of Hawaii to the Aleutian Trench off Alaska. The Hawaiian Islands themselves are a very small part of the chain and are the youngest islands in the immense, mostly submarine mountain chain composed of more than 80 volcanoes. The length of the Hawaiian Ridge segment alone, from the Big Island north- west to Midway Island, is about equal to the dis- tance from Washington, D.C. to Denver, Colorado (2,600 km). The amount of lava erupted to form the Hawaiian-Emperor chain is calculated to be at least 750,000 cubic kilometers—more than enough to blanket the entire State of California with a layer of lava roughly 1.5 km thick. A sharp bend in the chain indicates that the motion of the Pacific Plate abruptly changed about 43 million years ago, as it took a more westerly turn from its earlier northerly direction. Why the Pacific Plate changed direction is not known, but the change may be related in some way to the collision of India into the Asian conti nent, which began about the same time. As the Pacific Plate continues to move west- northwest, the Island of Hawaii will be carried beyond the hotspot by plate motion, setting the stage for the formation of a new volcanic island in its place. In fact, this process may be under way. Loihi Seamount, an active submarine vol- cano, is forming about 35 km off the southern coast of Hawaii. Loihi already has risen about 3 km above the ocean floor to within 1 km of the ocean surface. According to the hotspot theory, assuming Loihi continues to grow, it will become the next island in the Hawaiian chain. In the geo- logic future, Loihi may eventually become fused with the Island of Hawaii, which itself is com- posed of five volcanoes knitted together—Kohala, Mauna Kea, Hualalai, Mauna Loa, and Kilauea. /2 Some unanswered questions The tectonic plates do not randomly drift or wander about the Earth's surface; they are driven by definite yet unseen forces. Although scientists can neither precisely describe nor fully under- stand the forces, most believe that the relatively shallow forces driving the lithospheric plates are coupled with forces originating much deeper in the Earth. What drives the plates? From seismic and other geophysical evidence and laboratory experiments, scientists generally agree with Harry Hess' theory that the plate-dri- ving force is the slow movement of hot, softened mantle that lies below the rigid plates. This idea was first considered in the 1930s by Arthur Holmes, the English geologist who later influ- enced Harry Hess' thinking about seafloor spreading. Holmes speculated that the circular motion of the mantle carried the continents along in much the same way as a conveyor belt. However, at the time that Wegener proposed his theory of continental drift, most scientists still believed the Earth was a solid, motionless body. We now know better. As J. Tuzo Wilson elo- quently stated in 1968, “The earth, instead of appearing as an inert statue, is a living, mobile thing.” Both the Earth's surface and its interior are in motion. Below the lithospheric plates, at some depth the mantle is partially molten and can flow, albeit slowly, in response to steady forces applied for long periods of time. Just as a solid metal like steel, when exposed to heat and pressure, can be softened and take different shapes, so too can solid rock in the mantle when subjected to heat and pressure in the Earth's interior over millions of years. The mobile rock beneath the rigid plates is believed to be moving in a circular manner somewhat like a pot of thick soup when heated to boiling. The heated soup rises to the surface, spreads and begins to cool, and then sinks back to the bottom of the pot where it is reheated and rises again. This cycle is repeated over and over to generate what scientists call a convection cell or convective flow. While convective flow can be observed easily in a pot of boiling soup, the idea of such a process stirring up the Earth's interior is much more difficult to grasp. While we know that convective motion in the Earth is much, much slower than that of boiling soup, many unanswered questions remain: How many con- vection cells exist? Where and how do they origi- nate? What is their structure? Convection cannot take place without a source of heat. Heat within the Earth comes from two main sources: radioactive decay and resid- ual heat. Radioactive decay, a spontaneous process that is the basis of “isotopic clocks” used to date rocks, involves the loss of particles from the nucleus of an isotope (the parent) to form an isotope of a new element (the daughter). The 53 N Right: Conceptual drawing of assumed convection cells in the mantle (see text). Below a depth of about 700 km, the descending slab begins to soften and flow, losing its form. Below: Sketch showing convection cells commonly seen in boiling water or soup. This analogy, however, does not take into account the huge diſ- ferences in the size and the flow rates of these cells. º \ Trench *s Outer core Inner core radioactive decay of naturally occurring chemical elements—most notably uranium, thorium, and potassium—releases energy in the form of heat, which slowly migrates toward the Earth's surface. Residual heat is gravitational energy left over from the formation of the Earth— 4.6 billion years ago—by the “falling together” and com- pression of cosmic debris. How and why the escape of interior heat becomes concentrated in certain regions to form convection cells remains a mystery. Until the 1990s, prevailing explanations about what drives plate tectonics have emphasized mantle convection, and most earth scientists believed that seafloor spreading was the primary mechanism. Cold, denser material convects downward and hotter, lighter material rises because of gravity; this movement of material is an essential part of convection. In addition to the convective forces, some geologists argue that the intrusion of magma into the spreading ridge provides an additional force (called “ridge push”) to propel and maintain plate movement. Thus, subduction processes are considered to be sec- ondary, a logical but largely passive consequence of seafloor spreading. In recent years however, the tide has turned. Most scientists now favor the notion that forces associated with subduction are more important than seafloor spreading. Professor Seiya Uyeda (Tokai University, Japan), a world-renowned expert in plate tectonics, con- cluded in his keynote address at a major scientiſ- ic conference on subduction processes in June 1994 that “subduction...plays a more fundamen- tal role than seafloor spreading in shaping the earth's surface features” and “running the plate tectonic machinery.” The gravity-controlled sink- ing of a cold, denser oceanic slab into the sub- duction zone (called “slab pull")—dragging the rest of the plate along with it—is now considered to be the driving force of plate tectonics. We know that forces at work deep within the Earth's interior drive plate motion, but we may never fully understand the details. At present, none of the proposed mechanisms can explain all the facets of plate movement; because these forces are buried so deeply, no mechanism can be tested directly and proven beyond reasonable doubt. The fact that the tectonic plates have moved in the past and are still moving today is beyond dispute, but the details of why and how they move will continue to challenge scientists far into the future. 54 Z What went on before the break-up of Pangaea? Plate-tectonic movements since the break-up of the supercontinent Pangaea are now fairly well understood. Most scientists believe that similar processes must also have occurred earlier. However, the pre-Pangaea history of plate tecton- ics is very difficult to decipher, because nearly all of the evidence has been obscured by later geo- logic and plate-tectonic processes, including the subduction of older oceanic crust, which carried with it the record of magnetic reversals and hotspot traces. The clues to past plate tectonics can only be found on the present-day continents—in rocks, fossils, and structures older than about 200 mil- lion years. This is because the average age of the present-day oceanic crust is about 55 million years; the oldest parts are about 180 million years old, indicating that oceanic crust is entirely re- cycled every 150 million years or so. By contrast, the average age of the present-day continental crust is about 2.3 billion years, with the oldest known rocks (other than meteorites) dating back 3.96 billion years; these oldest rocks in turn con- tain minerals (zircons) derived from older rocks, possibly as old as 4.3 billion years. Continents are built of blocks of crust varying in age, size, rock composition, structure, and fos- sil assemblage (fauna and flora). In general, most continents have stable, older interiors (called cratons), while the zones bordering the cratons typically consist of younger, structurally more complicated rocks. Some bordering zones are composed of remnants of ancient oceanic lithosphere, volcanic arcs, or mountain ranges— reasonably interpreted to be products of pre- Pangaea plate tectonics—that have attached themselves to the cratons. In other zones, how- ever, the geological arrangement of these attached remnants seemed totally chaotic, defy- ing reasonable explanation by geologists until recently. For example, one remnant characterized by a specific kind of rock or fossil of distinctive age may lie next to, or be surrounded by, other remnants characterized by entirely different groups of rocks or fossils, even though they may be similar in geologic age. With the plate- tectonics model, it is now possible to provide more rational explanations for these zones of oddly juxtaposed crustal remnants. Scientists now recognize that continental mar- gins are often a mosaic of lithosphere fragments that have been added as a result of plates impinging on one another during movement. The process by which the lithospheric fragments, actually pieces of other plates, became attached to the continents is called accretion. Such frag- ments can be either continental or oceanic in ori- gin; if they are sufficiently large and share similar geologic characteristics, these fragments are called terranes. Terranes that seem out of place geologically, called exotic or suspect terranes, are composed of pieces of plates that have broken off and then drifted great distances before attaching (accreting) to some other terrane or continental landmass. Western North America is an example of a complex geologic region that is best inter- preted as a patchwork of several far-travelled ter- ranes that accreted together after the break-up of Pangaea. 55 PACIFIC PLATE JUAN DE FUCA PLATE EXPLANATION NORTH AMERICAN Island arc PLATE | Submarine deposits Ancient ocean floor Attached fragments Ancient continental interior (craton) Sonoma terrane UNITED STATES Divergent boundary -A-A- Convergent boundary M MEXICO - Transform boundary F- East Pacific Rise In recent years, the study of terranes (called “terrane tectonics" or "terrane analysis") has become a specialized field within plate-tectonics research. Such studies suggest that plate tecton- ics has been operating in some fashion since very early in the Earth's history, perhaps as early as 3.8 billion years ago. An intriguing, but sketchy, picture seems to be emerging: There have been several cycles of supercontinent formation, each followed by break-up and subsequent drifting of the fragmented parts. Pangaea itself may have been formed by the aggregation of separate con- tinents that drifted back together after the break- up of an older supercontinent that existed about 550 million years ago. Dr. David G. Howell (USGS, Menlo Park, California), a specialist in terrane analysis, likens such movement of continents—as the plates join and separate again and again throughout the Earth's history—to the motion of "lithospheric bumper cars." There are several important differ- ences, however: this imaginative comparison ignores the fact that electric bumper cars at amusement parks can each move independently, rather than being parts of an integrated system. And their average speeds are at least 500 million times faster than those of tectonic plates! Western North America showing some important plate- tectomics features and the mosaic of far-travelled exotic terranes plastered against the long-lived, stable interior of the continent (see text). (Modified from illustration provided by Oceanus Magazine, original figure by Jack Cook, Woods Hole Oceanographic Institution.) Extraterrestrial plate tectonics? The Earth may be unique in our solar system because it appears to be the only planet that is still volcanically and tectonically active; our plan- et therefore remains very much alive, while the others apparently have long ceased activity. Volcanic activity requires a source of internal heat, and it is the escape of this heat that fuels plate tectonics. While volcanism played a major role in the early history of Mars, the Moon, and probably Mercury, their small sizes relative to Earth resulted in the loss of internal heat at a much faster rate. They have been inactive globes for the last billion years or so. Venus may still be active, though the evidence is questionable. In 1979, the Pioneer-Venus spacecraft measured a high amount of sulfur in the upper atmosphere of the planet; the sulfur amount then decreased over the next few years. This observation suggested that the high sulfur concentration measured in 1979 may have re- sulted from a catastrophic event, perhaps a vol- canic eruption. Beginning in 1990, radar images made by the Magellan spacecraft revealed dra- matic volcanic features and long, deep valleys similar in size and shape to oceanic trenches on Earth. The Voyager spacecraft discovered several vol- canic plumes rising many hundreds of kilometers above the surface of Io, one of the moons of Jupiter and about the size of our Moon. Sci- entists speculate that large pools of liquid sulfur may exist on Io, possibly heated by tidal forces resulting from gravitational attraction between Io and Jupiter. The thermal energy generated by such tidal forces may be enough to produce Top: A computer-generated image of the Aleutian Trench (in violet); "warm" col- ors (yellow to red) indicate topographic highs, and “cool” colors (green to blue) represent lower elevations. Bottom: The topography of Artemis Corona, a trench-like feature on Venus, shown at the same vertical and horizontal scale as the Aleutian Trench. (Imagery courtesy of David T. Sandwell, Scripps Institution of Oceanography.) 57 N convection in Io's interior, although no one has clearly recognized any surface feature that may have formed from such convection. The surface of Ganymede, another moon of Jupiter and about the size of Mercury, is broken into many plate-like blocks, with long narrow A volcanic plume of sulfur dioxide (SO2) gas rising about 150 km above the surface of Io. This computer- enhanced image was captured “live” by the Voyager 2 spacecraft on 4 March 1979. (Imagery courtesy of NASA.) depressions between some of them. Whether these surface features represent ancient “fossil” plate tectonics, or are actively forming, remains to be answered. Crucial to determining whether plate tectonics is occurring on Ganymede is the search for evidence of a deep ocean beneath its icy surface. Such a body of water, if it exists, might contribute to internal convection. The rate of heat loss is critical to a planet's tectonic activity. Size is one determining factor: larger bodies lose heat more slowly and will therefore remain active longer. Another factor is composition, which influences the ability of a body to convect. For example, a liquid interior, such as may exist within Ganymede, is more likely to convect and drive plate tectonics than the “stony” interiors of the Moon, Mercury, Venus, and Mars. The amount of radioactive elements present in the planet's composition also affects the likelihood of internal convection, because the decay of these elements produces heat. Apparently, the interiors of the Moon, Mercury, and Mars are either too rigid or have lost too much of their internal heat to convect and drive plate tectonics. Eventually the Earth, too, will lose so much heat that its interior will stop convecting. Earth- quake and volcanic activity will then cease. No new mountains will form, and the geologic cycle of mountain building, erosion, sedimentation, and soil formation will be disrupted and also will cease. Exactly how a cooled-down Earth will change surface conditions—and whether our planet will still be habitable—nobody knows. Fortunately, these changes will not happen for many billions of years! 58 Z– Plate tectomics and people Over geologic time, plate movements in con- cert with other geologic processes, such as glacial and stream erosion, have created some of na- ture's most magnificent scenery. The Himalayas, the Swiss Alps, and the Andes are some spectacu- lar examples. Yet violent earthquakes related to plate tectonics have caused terrible catastro- phes—such as the magnitude-7.7 earthquake that struck the Chinese province of Haicheng in 1976 and killed as many as 800,000 people. Natural hazards Most earthquakes and volcanic eruptions do not strike randomly but occur in specific areas, such as along plate boundaries. One such area is the circum-Pacific Ring of Fire, where the Pacific Plate meets many surrounding plates. The Ring of Fire is the most seismically and volcanically active zone in the world. Earthquakes Because many major population centers are located near active fault zones, such as the San Andreas, millions of people have suffered per- sonal and economic losses as a result of destruc- tive earthquakes, and even more have experi- enced earthquake motions. Not surprisingly, some people believe that, when the “Big One” hits, California will suddenly “break off” and “fall into the Pacific,” or that the Earth will “open up” along the fault and “swallow” people, cars, and houses. Such beliefs have no scientific basis whatsoever. Although ground slippage common- ly takes place in a large earthquake, the Earth will not open up. Nor will California fall into the sea, because the fault zone only extends about 15 km deep, which is only about a quarter of the thickness of the continental crust. Furthermore, California is composed of continental crust, whose relatively low density keeps it riding high, like an iceberg above the ocean. Like all transform plate boundaries, the San Andreas is a strike-slip fault, movement along which is dominantly horizontal. Specifically, the San Andreas fault zone separates the Pacific and North American Plates, which are slowly grind- ing past each other in a roughly north-south direction. The Pacific Plate (western side of the fault) is moving horizontally in a northerly direc- tion relative to the North American Plate (east- ern side of the fault). Evidence of the sideways shift of these two landmasses can be found all along the fault zone, as seen from the differ- ences in topography, geologic structures, and, sometimes, vegetation of the terrain from one side of the fault to the other. For example, the San Andreas runs directly along Crystal Springs Reservoir on the San Francisco Peninsula. Aerial view, looking north toward San Francisco, of Crystal Springs Reservoir, which follows the San Andreas fault zone. (Photograph by Robert E. Wallace, USGS.) - - - - - - - EXPLANATION — Fault [-] Locked segments N - \ HAYWARD N. 2 \ FAULT N. \ \ | | | | - Creeping segments | | | | Map of the San Andreas and a few of the other faults in California, segments of which display different behavior locked or creeping (see text). (Simplified from USGS Professional Paper 1515.) Topographically, this reservoir fills a long, straight, narrow valley that was formed by ero- sion of the easily erodible rocks mashed within the fault zone. Movement along the San Andreas can occur either in sudden jolts or in a slow, steady motion called creep. Fault segments that are actively creeping experience many small to moderate earthquakes that cause little or no damage. These creeping segments are separated by seg- ments of infrequent earthquake activity (called seismic gaps), areas that are stuck or locked in place within the fault zone. Locked segments of the fault store a tremendous amount of energy that can build up for decades, or even centuries, before being unleashed in devastating earth- quakes. For example, the Great San Francisco Earthquake (8.3-magnitude) in 1906 ruptured along a previously locked 430 km-long segment of the San Andreas, extending from Cape Men- docino south to San Juan Bautista. The stresses that accumulate along a locked segment of the fault and the sudden release can be visualized by bending a stick until it breaks. The stick will bend fairly easily, up to a certain point, until the stress becomes too great and it snaps. The vibrations felt when the stick breaks represent the sudden release of the stored-up energy. Similarly, the seismic vibrations pro- duced when the ground suddenly ruptures radi- ate out through the Earth's interior from the rupture point, called the earthquake focus. The geographic point directly above the focus is called the earthquake epicenter. In a major earth- quake, the energy released can cause damage hundreds to thousands of kilometers away from the epicenter. 60 The magnitude-7.1 Loma Prieta earthquake of October 1989 occurred along a segment of the San Andreas Fault which had been locked since the great 1906 San Francisco earthquake. Even though the earthquake's focus (approxi- mately 80 km south of San Francisco) was cen- tered in a sparsely populated part of the Santa Cruz Mountains, the earthquake still caused 62 deaths and nearly $6 billion in damage. Follow- ing the Loma Prieta earthquake, the fault remains locked from Pt. Arena, where it enters California from the ocean, south through San Francisco and the peninsula west of San Fran- cisco Bay, thus posing the threat of a potential destructive earthquake occurring in a much more densely populated area. A dramatic photograph of horses killed by falling debris during the Great San Francisco Earthquake of 1906, when a locked segment of the San Andreas fault suddenly lurched, causing a devastating magnitude-8.3 earthquake. (Photograph by Edith Irvine, courtesy of Brigham Young University Library, Provo, Utah.) 61 N The lesser known Hayward Fault running east of San Francisco Bay, however, may pose a poten- tial threat as great as, or perhaps even greater than, the San Andreas. From the televised scenes of the damage caused by the 7.2-magni- tude earthquake that struck Kobe, Japan, on 16 January 1995, Bay Area residents saw the possible devastation that could occur if a comparable size earthquake were to strike along the Hayward Fault. This is because the Hayward and the Nojima fault that produced the Kobe earthquake are quite similar in several ways. Not only are they of the same type (strike-slip), they are also about the same length (60–80 km) and both cut through densely populated urban areas, with many buildings, freeways, and other structures built on unstable bay landfill. On 17 January 1994, one of the costliest nat- ural disasters in United States history struck southern California. A magnitude-6.6 earth- quake hit near Northridge, a city located in the populous San Fernando Valley just north of Los Angeles, California. This disaster, which killed more than 60 people, caused an estimated $30 billion in damage, nearly five times that resulting from the Loma Prieta earthquake. The North- ridge earthquake did not directly involve move- ment along one of the strands of the San Andreas Fault system. It instead occurred along the Santa Monica Mountains Thrust Fault, one of several smaller, concealed faults (called blind thrust faults) south of the San Andreas Fault zone where it bends to the east, roughly paralleling the Transverse Mountain Range. With a thrust fault, whose plane is inclined to the Earth's sur- face, one side moves upward over the other. Movement along a blind thrust fault does not break the ground surface, thus making it diffi- cult or impossible to map these hidden but potentially dangerous faults. Although scientists have found measurable uplift at several places in the Transverse Range, they have not found any conclusive evidence of ground rupture from the 1994 Northridge earthquake. Similar earth- quakes struck the region in 1971 and 1987; the San Fernando earthquake (1971) caused sub- stantial damage, including the collapse of a hos- pital and several freeway overpasses. Not all fault movement is as violent and destructive. Near the city of Hollister in central California, the Calaveras Fault bends toward the San Andreas. Here, the Calaveras fault creeps at a slow, steady pace, posing little danger. Much of the Calaveras fault creeps at an average rate of 5 to 6 mm/yr. On average, Hollister has some 20,000 earthquakes a year, most of which are too small to be felt by residents. It is rare for an area undergoing creep to experience an earthquake with a magnitude greater than 6.0 because stress is continually being relieved and, therefore, does not accumulate. Fault-creep movement general- ly is non-threatening, resulting only in gradual offset of roads, fences, sidewalks, pipelines, and other structures that cross the fault. However, the persistence of fault creep does pose a costly nuisance in terms of maintenance and repair. Mid-plate earthquakes—those occurring in the interiors of plates—are much less frequent than those along plate boundaries and more dif- ficult to explain. Earthquakes along the Atlantic seaboard of the United States are most likely related in some way to the westward movement 62 of the North American Plate away from the Mid- Atlantic Ridge, a continuing process begun with the break-up of Pangaea. However, the causes of these infrequent earthquakes are still not under- stood. East Coast earthquakes, such as the one that struck Charleston, South Carolina, in 1886 are felt over a much larger area than earthquakes occurring on the West Coast, because the eastern half of the country is mainly composed of older rock that has not been fractured and cracked by frequent earthquake activity in the recent geo- logic past. Rock that is highly fractured and crushed absorbs more seismic energy than rock that is less fractured. The Charleston earth- quake, with an estimated magnitude of about 7.0, was felt as far away as Chicago, more than 1,300 km to the northwest, whereas the 7.1-mag- nitude Loma Prieta earthquakes was felt no far- ther than Los Angeles, about 500 km south. The most widely felt earthquakes ever to strike the United States were centered near the town of New Madrid, Missouri, in 1811 and 1812. Three earthquakes, felt as far away as Washington D.C., were each estimated to be above 8.0 in magni- tude. Most of us do not associate earthquakes with New York City, but beneath Manhattan is a network of intersecting faults, a few of which are capable of causing earthquakes. The most recent earthquake to strike New York City occurred in 1985 and measured 4.0 in magnitude, and a pair of earthquakes (magnitude 4.0 and 4.5) shook Reading, Pennsylvania, in January 1994 causing minor damage. Above: Creeping along the Calaveras fault has bent the retaining wall and offset the sidewalk along 5th Street in Hollister, California (about 75 km south- southeast of San Jose). Right: Close-up of the offset of the curb. (Photographs by W. Jacquelyne Kious.) N Time-exposure photograph of the electronic-laser, ground-motion measure- ment system in operation at Parkfield, California, to track movement along the San Andreas fault (see text). (Photo- graph by John Nakata, USGS.) We know in general how most earthquakes occur, but can we predict when they will strike? This question has challenged and frustrated sci- entists studying likely precursors to moderate and large earthquakes. Since the early 1980s, geologists and seismologists have been intensive- ly studying a segment of the San Andreas near the small town of Parkfield, located about half- way between San Francisco and Los Angeles, to try to detect the physical and chemical changes that might take place—both above and below ground—before an earthquake strikes. The USGS and State and local agencies have blanket- ed Parkfield and the surrounding countryside with seismographs, creep meters, stress meters, and other ground-motion measurement devices. The Parkfield segment has experienced earth- quakes measuring magnitude 6.0 about every 22 years on average since 1881. During the most recent two earthquakes (1934, 1966), the same section of the fault slipped and the amount of slippage was about the same. In 1983, this evi- dence, in addition to the earlier recorded history of earthquake activity, led the USGS to predict 64 /2 that there was a 95 percent chance of a 6.0 earth- quake striking Parkfield before 1993. But the anticipated earthquake of magnitude 6.0 or greater did not materialize. The Parkfield exper- iment is continuing, and its primary goals remain unchanged: to issue a short-term predic- tion; to monitor and analyze geophysical and geochemical effects before, during, and after the anticipated earthquake; and to develop effective communications between scientists, emergency- management officials, and the public in respond- ing to earthquake hazards. While scientists are studying and identifying possible precursors leading to the next Parkfield earthquake, they also are looking at these same precursors to see if they may be occurring along other segments of the fault. Studies of past earthquakes, together with data and experience gained from the Parkfield experiment, have been used by geoscientists to estimate the proba- bilities of major earthquakes occurring along the entire San Andreas Fault system. In 1988, the USGS identified six segments of the San Andreas as most likely to be hit by a magnitude 6.5 or larger earthquake within the next thirty years (1988-2018). The Loma Prieta earthquake in 1989 occurred along one of these six segments. The Parkfield experiment and other studies car- ried out by the USGS as part of the National Earthquake Hazards Reduction Program have led to an increased official and public awareness of the inevitability of future earthquake activity in California. Consequently, residents and State and local officials have become more diligent in planning and preparing for the next big earth- quake. Volcanic eruptions As with earthquakes, volcanic activity is linked to plate-tectonic processes. Most of the world's active above-sea volcanoes are located near con- vergent plate boundaries where subduction is occurring, particularly around the Pacific basin. However, much more volcanism—producing about three quarters of all lava erupted on Earth—takes place unseen beneath the ocean, mostly along the oceanic spreading centers, such as the Mid-Atlantic Ridge and the East Pacific Rise. bduction-zone volcanoes like Mount St. Helens (in Washington State) and Mount Pina- tubo (Luzon, Philippines), are called composite comes and typically erupt withº fºLºsº, - because the magma is too stiff to allow easy escape of volcanic gases. As a consequence, tremendous internal pressures mount as the trapped gases expand during ascent, before the pent-up pressure is suddenly released in a violent eruption. Such an explosive process can be com- pared to putting your thumb over an opened bottle of a carbonated drink, shaking it vigorous- ly, and then quickly removing the thumb. The shaking action separates the gases from the liq- uid to form bubbles, increasing the internal pres- sure. Quick release of the thumb allows the gases and liquid to gush out with explosive speed and force. 65 An 18 km-high volcanic plume from one of a series of explosive eruptions of Mount Pinatubo beginning on 12 June 1991, viewed from Clark Air Base (about 20 km east of the volcano). Three days later, the most powerful erup- tion produced a plume that rose nearly 40 km, penetrating well into the stratos- phere. (Photograph by David H. Harlow, USGS.) In 1991, two volcanoes on the western edge of the Philippine Plate produced major erup- tions. On June 15, Mount Pinatubo spewed ash 40 km into the air and produced huge ash flows (also called pyroclastic flows) and mudflows that devastated a large area around the volcano. Pinatubo, located 90 km from Manila, had been dormant for 600 years before the 1991 eruption, which ranks as one of the largest eruptions in this century. Also in 1991, Japan's Unzen Volcano, located on the Island of Kyushu about 40 km east of Nagasaki, awakened from its 200- year slumber to produce a new lava dome at its summit. Beginning in June, repeated collapses of this active dome generated destructive ash flows that swept down its slopes at speeds as high as 200 km per hour. Unzen is one of more than 75 active volcanoes in Japan; its eruption in 1792 killed more than 15,000 people—the worst vol- canic disaster in the country's history. While the Unzen eruptions have caused deaths and considerable local damage, the impact of the June 1991 eruption of Mount Pinatubo was global. Slightly cooler than usual 66 temperatures recorded worldwide and the bril- 30 liant sunsets and sunrises have been attributed to this eruption that sent fine ash and gases high into the stratosphere, forming a large volcanic Stratosphere cloud that drifted around the world. The sulfur 5 20 - dioxide (SO2) in this cloud—about 22 million C | . . . . . . . . . . . . tons—combined with water to form droplets of s sulfuric acid, blocking some of the sunlight from E reaching the Earth and thereby cooling tempera- £ Tropopause tures in some regions by as much as 0.5 °C. An 2ſ. 10 - eruption the size of Mount Pinatubo could affect AIR = TRAFFIC the weather for a few years. A similar phenome- non occurred in April of 1815 with the cata- Troposphere clysmic eruption of Tambora Volcano in Indo- Sea nesia, the most powerful eruption in recorded level history. Tambora's volcanic cloud lowered global Diagram showing the lower two layers of the atmosphere: the troposphere and the stratos- temperatures by as much as 3 °C. Even a year phere. The tropopause—the boundary between these two layers—varies in altitude from after the eruption, most of the northern hemi- 8 to 18 km (dashed white lines), depending on Earth latitude and season of the year. The sphere experienced sharply cooler temperatures summit of Mt. Everest (inset photograph) and the altitudes commonly flown by commercial during the summer months. In part of Europe jetliners are given for reference. (Photograph by David G. Howell, USGS.) and in North America, 1816 was known as “the year without a summer.” Apart frompossibly affecting climate, vol. noes. Pyroclastic flows, also called nuées ardemtes canic clouds from explosive eruptions also pose a ("glowing clouds” in French), are fast-moving, hazard to aviation safety. During the past two avalanche-like, ground-hugging incandescent decades, more than 60 airplanes, mostly com- mixtures of hot volcanic debris, ash, and gases mercial jetliners, have been damaged by in-flight that can travel at speeds in excess of 150 km per encounters with volcanic ash. Some of these hour. Approximately 30,000 people were killed encounters have resulted in the power loss of all by pyroclastic flows during the 1902 eruption of engines, necessitating emergency landings. Mont Pelée on the Island of Martinique in the Luckily, to date no crashes have happened be- Caribbean. In March-April 1982, three explosive cause of jet aircraft flying into volcanic ash. eruptions of El Chichón Volcano in the State of Since the year A.D. 1600, nearly 300,000 peo- Chiapas, southeastern Mexico, caused the worst ple have been killed by volcanic eruptions. * volcanic disaster in that country's history. Vil- deaths were caused by pyroclas . flows and mud- lages within 8 km of the volcano were destroyed flows, deadly hazards which often accompany by pyroclastic flows, killing more than 2,000 explosive eruptions of subduction-zone volca- people. N Aerial view of the city of Armero, Colombia, devastated by mudflows trig- gered by the eruption of Nevado del Ruiz in November 1985. The mudflows destroyed everything in their paths and killed about 25,000 people. (Photograph by Darrell G. Herd, USGS.) Mudflows (also called debris flows or lahars, an Indonesian term for volcanic mudflows) are mix- tures of volcanic debris and water. The water usually comes from two sources: rainfall or the melting of snow and ice by hot volcanic debris. Depending on the proportion of water to vol- canic material, mudflows can range from Soupy floods to thick flows that have the consistency of wet cement. As mudflows sweep down the steep sides of composite volcanoes, they have the strength and speed to flatten or bury everything in their paths. Hot ash and pyroclastic flows from the eruption of the Nevado del Ruiz Volcano in Colombia, South America, melted snow and ice atop the 5,390-m-high Andean peak; the ensuing mudflows buried the city of Armero, killing 25,000 people. Eruptions of Hawaiian and most other mid- plate volcanoes differ greatly from those of com- posite cones. Mauna Loa and Kilauea, on the island of Hawaii, are known as º because they resemble the wide, rot pe of an ancient warrior's shield. Shield volcanoes tend to erupt non-explosively, mainly pouring out huge volumes of fluid lava. Hawaiian-type eruptions are rarely life threatening because the 68 lava advances slowly enough to allow safe evacua- tion of people, but large lava flows can cause considerable economic loss by destroying proper- ty and agricultural lands. For example, lava from the ongoing eruption of Kilauea, which began in January 1983, has destroyed more than 200 struc- tures, buried kilometers of highways, and dis- rupted the daily lives of local residents. Because Hawaiian volcanoes erupt frequently and pose little danger to humans, they provide an ideal natural laboratory to safely study volcanic phe- nomena at close range. The USGS Hawaiian Volcano Observatory, on the rim of Kilauea, was among the world's first modern volcano observa- tories, established early in this century. In recorded history, explosive eruptions at subduction-zone (convergent-boundary) volca- noes have posed the greatest hazard to civiliza- tions. Yet scientists have estimated that about three quarters of the material erupted on Earth each year originates at spreading mid-ocean ridges. However, no deep submarine eruption has yet been observed “live” by scientists. Be- cause the great water depths preclude easy obser- vation, few detailed studies have been made of the numerous possible eruption sites along the tremendous length (50,000 km) of the global mid-oceanic ridge system. Recently however, repeated surveys of specific sites along the Juan de Fuca Ridge, off the coast of the Oregon and Washington, have mapped deposits of fresh lava, which must have been erupted sometime between the surveys. In June 1993, seismic sig- mals typically associated with submarine erup- tions—called Tphases—were detected along part of the spreading Juan de Fuca Ridge and inter- preted as being caused by eruptive activity. Iceland, where the Mid-Atlantic Ridge is exposed on land, is a different story. It is easy to see many Icelandic volcanoes erupt non- explosively from fissure vents, in similar fashion to typical Hawaiian eruptions; others, like Hekla Volcano, erupt explosively. (After Hekla's cata- strophic eruption in 1104, it was thought in the Christian world to be the “Mouth to Hell.”) The voluminous, but mostly non-explosive, eruption at Lakagigar (Laki), Iceland, in 1783, resulted in one of the world’s worst volcanic disasters. About 9,000 people—almost 20% of the coun- try's population at the time—died of starvation after the eruption, because their livestock had perished from grazing on grass contaminated by fluorine-rich gases emitted during this eight month-long eruption. Wahaula Visitor Center, Hawaii Volcanoes National Park, was one of more than 200 structures overrun by lava flows (foreground) from the 1983- present eruption at Kilauea Volcano. (Photograph by J.D. Griggs, USGS.) 69 N Tsunamis Major earthquakes occurring along subduc- tion zones are especially hazardous, because they can trigger tsunamis (from the Japanese word tsunami meaning “harbor wave”) and pose a potential danger to coastal communities and islands that dot the Pacific. Tsunamis are often mistakenly called "tidal waves when, in fact, they have nothing to do with tidal action. Rather, tsunamis are seismic sea waves caused by earthquakes, submarine landslides, and, infre- quently, by eruptions of island volcanoes. During a major earthquake, the seafloor can move by several meters and an enormous amount of water is suddenly set into motion, sloshing back and forth for several hours. The result is a series A giant wave engulfs the pier at Hilo, Hawaii, during the 1946 tsunami, which killed 159 people. The arrow points to a man who was swept away seconds later (Retouched photograph courtesy of NOAA/EDIS.) of waves that race across the ocean at speeds of more than 800 km per hour, comparable to those of commercial jetliners. The energy and momentum of these trans-oceanic waves can take them thousands of kilometers from their origin before slamming into far-distant islands or coastal areas. To someone on a ship in the open ocean, the passage of a tsunami wave would barely elevate the water surface. However, when it reaches shallower water near the coastline and “touches bottom,” the tsunami wave increases in height, piling up into an enormous wall of water. As a tsunami approaches the shore, the water near shore commonly recedes for several minutes— long enough for someone to be lured out to col- lect exposed sea shells, fish, etc.—before sudden- ly rushing back toward land with frightening speed and height. The 1883 eruption of Krakatau Volcano, located in the Sunda Straits between the islands of Sumatra and Java, Indonesia, provides an excellent example of an eruption-caused tsuna- mi. A series of tsunamis washed away 165 coastal villages on Java and Sumatra, killing 36,000 peo- ple. The larger tsunamis were recorded by tide gauges as far away as the southern coast of the Arabian Peninsula—more than 7,000 km from Krakataul Because of past killer tsunamis, which have caused hundreds of deaths on the Island of Hawaii and elsewhere, the International Tsunami Information Center was created in 1965. This center issues tsunami warnings based on earth- quake and wave-height information gathered from seismic and tide-gauge stations located around the Pacific Ocean basin and on Hawaii. 70 - Valdez, Alaska º º earthquake Z 2. - - Z * ~ º Hawaiian / ^ - Islands/_- ºs. / Z - - º - - - ºn tº s - º dº - º tº º º ºx -- ºn º º º - Cºle - \ \ earthquake º ż \ º - - \ - - - - - The Hawaiian Islands are especially vulnerable to destructive tsunamis generated by major earthquakes in the circum-Pacific Ring of Fire. Travel times (in hours) are shown for the tsunamis produced by the 1960 Concepción, Chile, earthquake (purple curves) and by the 1964 Good Friday, Valdez (Anchorage), Alaska earthquake (red curves). The 1960 tsunamis killed 61 people and caused about $24 million in damage. - N Natural resources Many of the Earth's natural resources of ener- gy, minerals, and soil are concentrated near past or present plate boundaries. The utilization of these readily available resources have sustained human civilizations, both now and in the past. Fertile soils Volcanoes can clearly cause much damage and destruction, but in the long term they also have benefited people. Over thousands to mil- lions of years, the physical breakdown and chem- ical weathering of volcanic rocks have formed some of the most fertile soils on Earth. In tropi- cal, rainy regions, such as the windward (north- eastern) side of the Island of Hawaii, the forma- tion of fertile soil and growth of lush vegetation following an eruption can be as fast as a few hun- dred years. Some of the earliest civilizations (for example, Greek, Etruscan, and Roman) settled on the rich, fertile volcanic soils in the Mediter- ranean-Aegean region. Some of the best rice- growing regions of Indonesia are in the shadow of active volcanoes. Similarly, many prime agri- cultural regions in the western United States have fertile soils wholly or largely of volcanic origin. Ore deposits Most of the metallic minerals mined in the world, such as copper, gold, silver, lead, and zinc, are associated with magmas found deep within the roots of extinct volcanoes located above sub- duction zones. Rising magma does not always reach the surface to erupt; instead it may slowly cool and harden beneath the volcano to form a wide variety of crystalline rocks (generally called plutonic or granitic rocks). Some of the best examples of such deep-seated granitic rocks, later exposed by erosion, are magnificently dis- played in California's Yosemite National Park. Ore deposits commonly form around the magma bodies that feed volcanoes because there is a ready supply of heat, which convectively moves and circulates ore-bearing fluids. The metals, originally scattered in trace amounts in magma or surrounding solid rocks, become concentrat- ed by circulating hot fluids and can be redeposit- ed, under favorable temperature and pressure conditions, to form rich mineral veins. The active volcanic vents along the spreading mid-ocean ridges create ideal environments for the circulation of fluids rich in minerals and for ore deposition. Water as hot as 380 "C gushes out of geothermal springs along the spreading centers. The water has been heated during cir- culation by contact with the hot volcanic rocks forming the ridge. Deep-sea hot springs contain- ing an abundance of dark-colored ore minerals (sulfides) of iron, copper, zinc, nickel, and other metals are called “black smokers.” On rare occa- sions, such deep-sea ore deposits are later exposed in remnants of ancient oceanic crust that have been scraped off and left (“beached”) on top of continental crust during past subduc- tion processes. The Troodos Massif on the Island of Cyprus is perhaps the best known example of such ancient oceanic crust. Cyprus was an important source of copper in the ancient world, and Romans called copper the “Cyprian metal"; the Latin word for copper is cyprium. Fossil fuels Oil and natural gas are the products of the deep burial and decomposition of accumulated organic material in geologic basins that flank mountain ranges formed by plate-tectonic processes. Heat and pressure at depth transform the decomposed organic material into tiny pock- ets of gas and liquid petroleum, which then migrate through the pore spaces and larger openings in the surrounding rocks and collect in reservoirs, generally within 5 km of the Earth's surface. Coal is also a product of accumulated decomposed plant debris, later buried and com- pacted beneath overlying sediments. Most coal originated as peat in ancient swamps created many millions of years ago, associated with the draining and flooding of landmasses caused by changes in sea level related to plate tectonics and other geologic processes. For example, the Appalachian coal deposits formed about 300 mil- lion years ago in a low-lying basin that was alter- nately flooded and drained. º Half Dome as viewed from Glacier Point, Yosemite National Park, rises more than a kilometer above the valley floor. The granitic rocks that form Half Dome and other spectacular Park features represent unerupt- ed magma later exposed by deep erosion and glaciation. (Photograph by Carroll Ann Hodges, USGS.) Geothermal powerplant at The Geysers near the city of Santa Rosa in northern California. The Geysers area is the largest geothermal development in the world. (Photograph by Julie Donnelly- Nolan, USGS.) Geothermal energy Geothermal energy can be harnessed from the Earth's natural heat associated with active volcanoes or geologically young inactive volca- noes still giving off heat at depth. Steam from high-temperature geothermal fluids can be used to drive turbines and generate electrical power, while lower temperature fluids provide hot water for space-heating purposes, heat for greenhouses and industrial uses, and hot or warm springs at resort spas. For example, geothermal heat warms more than 70 percent of the homes in Iceland, and The Geysers geothermal field in Northern California produces enough electricity to meet the power demands of San Francisco. In addition to being an energy resource, some geo- thermal waters also contain sulfur, gold, silver, and mercury that can be recovered as a byprod- uct of energy production. Z- A formidable challenge As global population increases and more countries become industrialized, the world demand for mineral and energy resources will continue to grow. Because people have been using natural resources for millennia, most of the easily located mineral, fossil-fuel, and geo- thermal resources have already been tapped. By necessity, the world's focus has turned to the more remote and inaccessible regions of the world, such as the ocean floor, the polar conti- ments, and the resources that lie deeper in the Earth's crust. Finding and developing such resources without damage to the environment will present a formidable challenge in the com- ing decades. An improved knowledge of the relationship between plate tectonics and natural resources is essential to meeting this challenge. The long-term benefits of plate tectonics should serve as a constant reminder to us that the planet Earth occupies a unique niche in our solar system. Appreciation of the concept of plate tectonics and its consequences has rein- forced the notion that the Earth is an integrated whole, not a random collection of isolated parts. The global effort to better understand this revo- lutionary concept has helped to unite the earth- sciences community and to underscore the link- ages between the many different scientific disci- plines. As we enter the 21st century, when the Earth's finite resources will be further strained by explosive population growth, earth scientists must strive to better understand our dynamic planet. We must become more resourceful in reaping the long-term benefits of plate tectonics, while coping with its short-term adverse impacts, such as earthquakes and volcanic eruptions. Farmer plowing a lush rice paddy in central Java, Indonesia; Sundoro Volcano looms in the background. The most highly prized rice-growing areas have fertile soils formed from the break- down of young volcanic deposits. (Photograph by Robert I. Tilling, USGS.) 75 N Further reading These works listed furnish additional information on topics not covered, or only briefly discussed, in the booklet. Attenborough, David, 1986, The Living Planet: British Broadcasting Corporation, 320 p. (An informative, narrative version of the highly success- ful television series about how the Earth works.) Coch, N.K., and Ludman, Allan, 1991, Physical Geology: Macmillan Publishing Company, New York, 678 p. (Well-illustrated college textbook that contains excellent chapters on topics related to Earth dynamics and plate tectonics.) Cone, Joseph, 1991, Fire Under the Sea: William Morrow and Company, Inc., New York, 285 p. (paperback). (A readable summary of oceano- graphic exploration and the discovery of volcanic hot springs on the ocean floor.) Decker, Robert, and Decker, Barbara, 1989, Volcanoes: W.H. Freeman and Company, New York, 285 p. (paperback). (An excellent introduc- tion to the study of volcanoes written in an easy-to- read style.) Duffield, W.A., Sass, J.H., and Sorey, M.L., 1994, Tapping the Earth's Natural Heat: U.S. Geological Survey Circular 1125, 63 p. (A full-color book that describes, in non-technical terms, USGS studies of geothermal resources—one of the benefits of plate tectonics—as a sustainable and relatively nonpollut- ing energy source.) Ernst, W.G., 1990, The Dynamic Planet: Columbia University Press, New York, 280 p. (A comprehen- sive college-level textbook that includes good chap- ters on plate tectonics and related topics.) Heliker, Christina, 1990, Volcanic and seismic hazards of the Island of Hawaii: U.S. Geological Survey general-interest publication, 48 p. (A full-color booklet summarizing the volcanic, seismic, and tsunami hazards.) Krafft, Maurice, 1993, Volcanoes: Fire from the Earth: Harry N. Abrams, New York, 207 p. (paper- back). (A well-illustrated, non-technical primer on volcanoes; Maurice Krafft and his wife Katia were the world's foremost photographers of volcanoes before they were killed during the June 1991 erup- tion of Unzen Volcano, Japan.) Lindh, A.G., 1990, Earthquake prediction comes of age; Technology Review, Feb/March, p. 42-51. (A good introduction to the basis and techniques used by scientists in attempting to predict earthquakes.) McNutt, Steve, 1990, Loma Prieta earthquake, October 17, 1989: An overview: California Geology, v. 43, no. 1, p. 3-7. (Along with the com- panion article by D.D. Montgomery, gives the essential information about this destructive earth- quake along the San Andreas Fault.) McPhee, John, 1993, Assembling California: Farrar, Straus, & Giroux, New York, 303 p. (A fascinating account of the role of plate tectonics in the geology of California, told in the typical McPhee style of conversations with scientists.) Montgomery, D.D., 1990, Effects of the Loma Prieta earthquake, October 17, 1989: California Geology, v. 43, no. 1, p. 8-13. (Along with the companion article by Steve McNutt, gives the essential informa- tion about this destructive earthquake along the San Andreas Fault.) Ritchie, David, 1981, The Ring of Fire: New American Library, New York, 204 p. (paperback). (A popularized account of earthquakes, volcanoes, and tsunamis that frequently strike the circum- Pacific regions.) 76 2 Schulz, S.S., and Wallace, R.E., 1989, The San Tilling, R.I., Topinka, Lyn, and Swanson, D.A., 1990, Andreas Fault: U.S. Geological Survey general- Eruptions of Mount St. Helens: Past, present, and interest publication, 16 p. (This little booklet pro- future: U.S. Geological Survey general-interest pub- vides the basic information about the San Andreas lication, 56 p. (A nontechnical summary, illus- Fault Zone, including a good discussion of earth- trated by many color photographs and diagrams, of quakes that occur frequently along it.) the abundant scientific data available for the vol- Simkin, Tom, Unger, J.D., Tilling, R.I., Vogt, P.R., and cano, with emphasis on the catastrophic eruption Spall, Henry, compilers, 1994, This Dynamic of 18 May 1980; similar in format to this book.) Planet: World map of volcanoes, earthquakes, Time-Life Books Inc., 1982, Volcano: 1983, impact craters and plate tectonics: 1 sheet, U.S. Continents in Collision, in Planet Earth Series: Geological Survey (USGS). (In addition to the Alexandria, Virginia, Time-Life Books, 176 p. each. map's visually obvious physiographic features that (Informative and general surveys of volcanism and relate to plate tectonics, the explanatory text gives plate tectonics.) a concise summary of how plate tectonics work.) Wright, T.L., and Pierson, T.C., 1992, Living with vol Sullivan, Water, 1991, Continents in Motion: McGraw- canoes: U.S. Geological Survey Circular 1073, 57 Hill Book Co., New York, 430 p. (A comprehensive p. (A non-technical summary of the USGS’ review of the developments that culminated in the Volcano Hazards Program, highlighting the scien- plate tectonics theory. Science Editor of the New tific studies used in forecasting eruptions and York Times, Sullivan is widely regarded as the assessing volcanic hazards, in the United States and “dean" of America's science writers.) abroad.) Tarbuck, Edward, and Lutgens, Frederick, 1985, Earth Science: Charles E. Merrill Publishing Co., Columbus, Ohio, 561 p. (A college-level geology textbook that contains good chapters on plate tec- tonics and related topics.) Tilling, R.I., 1991, Born of fire: Volcanoes and igneous rocks: Enslow Publishers, Inc., Hillside, This publication is one of a series of general interest publications New Jersey, 64 p. (An introductory text about the prepared by the U.S. Geological Survey to provide information about kinds of vol d their d and h the earth sciences, natural resources, and the environment. To inds of volcanoes and their products an aZ- obtain a catalog of additional titles in the series "General Interest ardous impacts—aimed at approximately junior Publications of the U.S. Geological Survey," write: high- to high-school level.) i. º ...'s - ---- - - ranch of intormation Services Tilling, R.I., Heliker, C., and Wright, T.L., 1987, P.O. Box 25286 Eruptions of Hawaiian Volcanoes: Past, present, Denver, CO 80225 and future: U.S. Geological Survey general-interest publication, 54 p. (A nontechnical summary, illus- trated by many color photographs, of the abundant data on Hawaiian volcanism; similar in format to this book.) university OF MICH3AN THE UNIVERSITY OF MICHIGAN HE NIVERSITY DATE DUE —dictiºn— MAR 8 1998 00T 01:1998 DEC 17 2000 DEC 26 2007. Now 3 tº DEC 20 2004 MAR 07 2006 00T 0 9 2007 JAN 29 200 ſº - - º - ** - - - - - - Snow-clad Mt. Rainier, a 4,392 m-high volcano built by plate-tectonic processes, dominates the pastoral scene around Orting, Washington. This valley is an inviting place for people to live, work, and play, but it is also highly vulnerable to destructive mudflows that could be generated by renewed eruptive activity at Mt. Rainier Society must learn to “co-exist” intelligently with active volcanoes. (Photograph by David E. Wieprecht, USGS.) For sale by the U.S. Government Printing Office Superintendent of Documents, Mail Stop: SSOP. Washington, DC 20402-9328 ISBN 0-1 6-048220-8 As the Nation's principal conser- vation agency, the Department of the Interior has responsibility for most of our nationally owned pub- lic lands and natural and cultural resources. This includes fostering sound use of our land and water resources; pro- tecting our fish, wildlife, and biological diversity; preserving the environmental and cultural values of our national parks and historical places; and providing for the enjoyment of life through out- door recreation. The Department assesses our energy and mineral resources and works to ensure that their development is in the best inter- ests of all our people by encouraging stewardship and citizen participation in their care. The Department also has a major responsibility for American Indian reservation communities and for people who live in island territories under U.S. administration. ISBN 0-16-048220–8 | | | 90 000 917801 60"482205