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by William J. Crornie Rapidly developing technologies are adding to the ways earth scientists can examine their enigmatic subject. 28 MOSAIC Geologists are getting deeper into their work. Once they were confined largely to the earth's surface; now new techniques are opening windows that let them see all the way to the planet's center. The new views are startling, if somewhat blurry. They show internal details that are reflected in the formation and drifting of continents, in the making of mountains and islands, and in volcanism and earthquakes. These details eliminate many cherished explanations of such processes, and they set narrower limits to explanations that are possible. As recently as 1960 most geologists believed that the continents had remained fixed throughout the 4.5-billionyear history of the earth. They thought that mountain building, earthquakes, and volcanism involved vertical movements, up and down adjustments of an elastic interior to changes in mass at the surface. Geologists then described the planet as having an outer crust of solid rock varying in thickness from 10 kilometers under the oceans to 70 kilometers under the continents. They compared this crust to the shell of a hard-boiled egg u n d e r which lies a white (the mantle) about 2,800 kilometers thick. The yolk (core) extends from 2,900 to 6,400 kilometers and consists of a 1,100-kilometer-thick, semifluid envelope enclosing a solid, iron™ dominated inner core. By 1970, it was difficult to find a geologist who did not believe that oceans open and close, and that continents split apart, drift, rotate, and collide with each other. Virtually every earth scientist championed the scenario demanded by the theory of plate tectonics, which holds that the earth's shell, rather than being seamless, is broken into 20 or so plates, the largest of which consist of continents attached to huge sections of ocean floor. Oceans open and close and continents drift because the plates move with respect to each other. Heat escaping from the mantle causes molten material, or magma, to rise and rift the thin crust that underlies the oceans. This creates submarine valleys and ridges made of the youngest rocks on earth. The peaks of the ridges sometimes thrust through the surface as volcanic islands. The ridges also mark the inner boundaries of plates that are moving away from each other. The outer, leading edges of spreading plates collide with each other. When they converge, the edge of one plate may override the other, forcing the edge of one plate down into the mantle and producing mountains, volcanism, and earthquakes. Converging plates also slide by each other. California is split by a plate boundary, visible at the surface as a series of faults running from the Gulf of California to San Francisco. Stresses along the faults relieve themselves as earthquakes. The National Academy of Sciences' Committee on Opportunities for Research in the Geological Sciences has called the theory of plate tectonics "a compellingly attractive hypothesis, simple, elegant, and potentially able to explain a wide range of diverse observations." The theory explains continental drift, the appearance and disappearance of oceans, and the observation that no rocks older than 200 million years have ever been dredged from the deep sea. (Two hundred million years is the maximum time it takes for a rock born at a mid-ocean ridge to be destroyed or changed by plate convergence.) Plate tectonics theory also accounts for the positions and surface features of continents dating back at least 600 million years. Beyond this time, the history of the earth becomes, in the words of Charles L. Drake, a geologist at Dartmouth College, "more theological and less geological." The major reasons involve the ages of continents. Radioactive dating shows that continental rocks are as old as 3.8 billion years. In contrast to the sea floor's straightforward creation, spreading, and destruction, the continents have been repeatedly compressed, extended, split apart, rotated, drifted, and subjected to wave after wave of collisions with other continents, islands, a n d slabs of ocean floor. To unravel this chaotic history, geologists need to extend the plate-tectonics revolution from sea to shore, from 600 million to 4 billion years, and from the crust to the core of the planet. (See ''Tectonics in an Archean Earth," in this Mosaic.) A deep hole Deep drilling provides the most direct view of the earth's third and fourth dimensions, depth and time. It provides only a shallow view, although Soviet scientists have shown that it can be a highly revealing one. The Soviet Union's exploratory drilling program includes the deepest hole drilled to date. Spudded in the Kola Peninsula east of Finland, the hole reportedly goes down 13 kilometers, 85 percent of the way to its targeted depth. Although the United States leads in deep-ocean and commercial drilling, its deepest hole on land, drilled by the United States oil industry, bottomed at 9.6 kilometers. The Kola bore hole is one of a series being drilled to obtain both practical information and basic knowledge of the earth. Since its drill bit first chewed into rock in 1970, the Kola effort has been producing data that cast doubt on traditional theories of the structure of the continental crust, heat flow, and mineral formation. Textbooks describe the continental crust as being composed of silicon- and aluminum-rich granitic rocks. These overlie denser basaltic rocks, which are lower in silicon and higher in magne™ sium. Geologists draw this picture on the basis of a sharp increase in the velocity of earthquake waves as the waves travel downward into the crust. Soviet researchers intended to penetrate the so-called Conrad discontinuity between the two rock types and bring up samples of each. Minister of Geology Yevgeny A. Kozlovsky has r e p o r t e d , however, that "with increasing depth In the Kola hole, the expected increase In rock densities was not recorded. Neither was any increase in the speed of propagation of seismic waves nor any other changes in the physical properties of the rocks detected. Thus the traditional idea that geophysical data obtained from the surface can be directly correlated with MOSAIC 29 Eurasian Crustal plates jostle each other on earth's surface. Spikes point in the direction of subduction where they meet. geological materials in the deep crust must be reexamined/ "It's a matter of interpretation/' says Jack Oliver, a seismologist at Cornell University. "All rocks are compressible, and at those depths pressure may be responsible for giving certain kinds of granitic rocks seismic characteristics not unlike those of basaltic rocks." And, he cautions, "you must always take mistakes in measurement into account." "What really surprised us about the Kola hole results," Oliver says, "is that they found open fractures with fluids flowing through them at depths down to at least 11.5 kilometers. Many scientists expected that pressure would close all open space below three kilometers." Fluids in the open spaces may carry valuable minerals, which precipitate out as the liquids rise and cool. Soviet drillers encountered what they say are commercial quantities of copper, cobalt, zinc, lead, and iron between depths of 1.7 and 6.5 kilometers. The same fluids bring up unanticipated amounts of heat. ' T h e temperature gradient measured down to three kilometers corresponded to that expected, i.e. one degree Celsius inCromie, executive director of the Council for the Advancement of Science Writing, writes frequently on the earth sciences. 30 MOSAIC crease per 100 meters depth," Kozlovky reported. "Below this, the gradient increased to 2.5 degrees per 100 meters, and at ten kilometers the temperature was 180 degrees Celsius instead of the expected 100 degrees." Evidently enough fluid flows to transfer heat by convection, a much more efficient process than conduction through solid rock. "We once believed that heat was transferred through the ocean crust entirely by conduction," the Cornell seismologist says. "Now we know that the major transfer occurs through convection at mid-ocean ridges. Confirming Kola hole results may mean that the same process operates in the continental crust/ Mountain building Scientists in the United States hope to get their own deep continental drilling program started by 1986. The White House Office of Science and Technology Policy has recommended that funds for site surveys and drilling be appropriated in fiscal years 1985 and 1986. The Continental Scientific Drilling Committee of the National Academy of Sciences has assigned the highest priority to a ten-kilometer hole in the southern Appalachian Mountains to test a new concept of mountain building. Geologists suspect that the Appala- chians and many other mountain belts were thrown u p w h e n plate convergence caused huge slabs of rock to be thrust up and over a continent. Some of the overthrust sheets evidently traveled hundreds of kilometers, bulldozing seafloor sediments and continental margins into coastal mountains. (See "Old Rock on Young Rock" and "Seismic-reflection profiling" by Henry Simmons, Mosaic, Volume 14, Number 2.) The idea that lateral thrusting may have happened at m a n y places and times in the earth's history has existed since the last century. Evidence that it actually occurred, however, was seen only recently through another window into the planet: seismic-reflection profiling. This technique involves beaming long trains of sound waves, or vibrations, of varying frequencies into the earth. The frequencies vary from 8 to 40 cycles per second. The wave trains reflect vertically from faults and rock boundaries and are picked u p by an antenna-like array of geophones implanted at the surface. In 1974, a group of researchers from Cornell and other universities organized the Consortium for Continental Reflection Profiling, or Cocorp, to apply this technique to study the deep basements of continents. To date the consortium has completed more than 5,500 kilo- meters of seismic surveys in the United States. Maps of reflection horizons across the Appalachians in the Carolinas and New England yield evidence of multiple, shallow, eastward-dipping faults along which sheets of rock were thrust westward by the closing of the Atlantic and the convergence of continents between 250 million and 500 million years ago. "Because the Appalachians are considered a representative mountain belt, it is likely that thin-skinned overthrusting took place in many other locations/' Jack Oliver states. These locations include the Ouachita Mountains in Arkansas, the Rocky Mountains, the Alps, the Himalayas, and mountains in northwest Africa that appear to be mirror images of the Appalachians. To be positively identified, a reflection boundary must be traced downward from the surface or sampled by drilling. "The process seems so general that it is very important to test the interpretations based on the seismic data with a deep drill hole," says Oliver. Mosaic continents Overthrusting and buildup of continental margins also occurs on a lessthan-continental scale. The inexorable, conveyor-like movement of ocean floors toward destruction u n d e r continents carries pieces of many sizes, including chains of islands, sea m o u n t s , submerged plateaus, and miniature continents. Geologists envision the western part of North America as having been formed by such a tectonic conveyor belt. "Large-scale, overlapping thrust sheets are well documented from Alaska to southern California," says Allan Cox, a geophysicist at Stanford University. "The amount of differential movement along some of these faults must certainly exceed several h u n d r e d kilometers." Cox counts 150 slabs and blocks of "varying size that are as different from each other as they are from land farther east/ The Basin and Range, a region that lies between Utah's Wasatch Range and the Sierra Nevada, now appears to be undergoing extension along the same type of low-angle faults that probably welded it to the continent by compression hundreds of millions of years ago. At that time, a large sheet of rock evidently slipped or was pulled back down an inclined fault without breaking, something thought previously to be impossible. Surveyors for Cocorp have traced such a fault 70 kilometers into the crust, slanting down to a depth of 25 kilometers. Such a situation provides an exceptionally clear look through the seismic window; geologists do not have to guess what kind of boundary reflects the seismic energy because boundary rock is exposed at the surface. The Cocorp researchers believe that the continental crust has stretched along these faults to double its original area, adding as much as 300 kilometers to the distance between Salt Lake City and Reno. This stretching is accompanied and may be caused by the rise of magma from below. Surface swelling and the behavior of seismic waves indicate the presence of chambers of molten material under New Mexico near Socorro, under Death Valley, and under Long Valley in the Sierra Nevada of eastern California. Geologists blame movement of magma for recent earthquakes in the Long Valley area. The magma pushed the valley floor upward 45 centimeters in five years and prompted the U.S. Geological Survey to issue a volcanohazard alert in 1982. To open the seismic window wider, reflection profiling is used in combination with refraction, a technique that measures the speed of shock waves that travel horizontally through rocks rather than being reflected from boundaries between them. These waves eventually are bent or refracted back to surface recorders. Their velocities depend on the type of rock traversed, and the velocities can help identify different rock types. Refracted waves travel more slowly in the mantle beneath the Basin and Range region than they do under the more stable crust to the east. Some researchers interpret this to mean that the western part of the crust is stretching along a flat, semiliquid layer that marks the crust-mantle b o u n d a r y at a d e p t h of about 25 kilometers in this region. They picture blobs and chimneys of magma spreading out along this boundary after rising from a layer of soft or lowviscosity material underlying all the plates at a depth between 50 and 100 kilometers. Heat may cause the Mohorovicic seismic discontinuity, or Moho, the crust-mantle boundary, to flatten out in this area like hot asphalt. K. Douglas Nelson, a geologist at Cornell University, speculates that the Moho may also represent the maximum height to which basaltic magma from under the plates normally rises. The less-dense crustal material above acts as a lid, causing the rising magma to spread out at the base of the crust. Where the crust is extending, however, magma can continue to work its way up from the Moho to the surface. Still to be answered is the question of whether rising magma is the cause or the result of crustal extension. Hot spots and undulations Shallow magma bodies, like the ones under Death Valley and central New Mexico, move with the continents. Plumes, like the 300-kilometer-deep one that generates the geysers of Yellowstone National Park, remain relatively stationary as the plates drift over them, thereby serving as references for measurement of the motions of the plates. Earth scientists have mapped 42 hot spots, most of them lying under the oceans and Africa. Typically the hot spots are crowned by volcanoes and sit near broad domes or swells in the crust. The island of Hawaii, for example, with its recently active volcanoes, sits on the edge of such an uplift about 1,500 kilometers wide" and one kilometer high. Many geologists believe that magma rising from the mantle in these areas initiates rifts that break apart continents and ouen UD new oceans. (See "The Earth's Foundations of Fire/' by Patrick Young, Mosaic, Volume 11, Number 3.) Hot-spot swells and other bumps and depressions in the surface can be mapped through a window opened by earth satellites. Radar altimeters on orbiting spacecraft record small- and large-scale variations in sea level. These variations are reflections of gravity anomalies, and they can be correlated with changes in the elevation of the ocean floor. Uplifts in the vicinity of the Hawaiian Islands and other hot spots add extra mass to the surface and create positive anomalies. Negative anomalies occur where cold, dense material causes depressions. The anomalies can be smaller or larger than the l,500-to-3,000-kilometerwide hot-spot swells. William F. Haxby and Jeffrey K. Weissel, geophysicists at Lamont-Doherty Geological Observatory in New York, examined data from a radar altimeter aboard the Seasat spacecraft and found smaller-scale anomalies MOSAIC 31 beneath the Pacific and Indian oceans. High and low anomalies alternate, with a wavelength of 150 to 220 kilometers. Haxby and Weissel attribute these to convective motion in the relatively soft upper layer of the mantle on which the plates move. The negative anomalies may be attributable to cold material sinking from the bottom of the plates. This material would be replaced by hotter, less-dense material, which would create the positive anomalies. Evidence that the upper part of the mantle is soft enough to permit this kind of motion includes the fact that seismic waves, the speed of which decreases with decreased density, slow d o w n in this zone. Called the asthenosphere (from, the Greek word asthenos, meaning without strength) this zone extends downward to about 250 kilometers. The 150-to-220-kilometer-long anomalies exist in areas adjacent to ocean ridges, or diverging plate boundaries. Between Central America a n d the Hawaiian Islands, they a p p e a r to be elongated, or smeared out, in the direction of plate motion, or west-northwest relative to the hot spot under the island of Hawaii. Haxby and Weissel find such anomalies in the southeastern Pacific Ocean and the Indian Ocean, too. These also are areas where y o u n g crust is being made at active ocean rifts. According to Haxby, 'The best explanation that satisfies ail of the observations is that the gravity-anomaly undulations reflect small-scale longitudinal convection produced in a thin, weak, low-viscosity layer of the upper mantle by shear imparted by the moving p l a t e s / ' Convection is driven by temperature differences, explains Barry Parsons of the Massachusetts Institute of Technology. "The shear aligns convection with the direction of plate motion," he says. Not all earth scientists agree with this explanation of the anomalies. What does seem acceptable to everyone, however, is that material from the crust sinks where plates converge. The material descends, melts, and rises to the surface again at mid-ocean ridges. On the way it may get swept up in swellings 1,500 to 3,000 kilometers wide that are associated w i t h hot spots, or in the smaller-scale undulations postulated by Haxby and Weissel. Depressions are associated with thick slabs of cold crust sinking into the mantle where plates converge. Highs, however, do not coincide with hot mate- 32 MOSAIC rial rising in the ocean ridges. Rather they are centered over the equator, the Atlantic-Africa region, and the central Pacific. Lows are concentrated in a broad band that Includes northeastern Canada, the western Atlantic Ocean, Brazil, Antarctica, the Indian Ocean, and Siberia. This band includes 60 percent of the earth's total land area. One theory is that continents slide from high to low areas. Don L. Anderson, a California Institute of Technology seismologist, proposes a model in which the thick continents prevent heat from escaping from the mantle. The heat buildup causes crustal bulges. Eventually hot spots associated with the upwarping split the crust. Continents break apart, and the pieces slide downhill to low areas overlying sunken slabs of old crust. The east coast of North America may overlie the former Pacific Ocean floor, which subducted under the westward-moving continent. In the future the Yellowstone plume may split the North American continent. Tomography Earth scientists obtain another view of gravity anomalies through a new window that gives them a look at the earth In three dimensions. The technique is similar to computer-assisted tomography (a term based on the Greek word tomos, meaning section.), wherein a computer assembles a series of X-ray images having common points Into a three-dimensional picture. Earth tomography replaces x rays with earthquake-generated waves that pass through the planet or sweep its surface. The longer the wavelength of surface waves, the deeper they penetrate. Don Anderson and his colleagues at Caltech's Seismological Laboratory use surface waves with periods of u p to 250 seconds to take tomographic slices down to 400 kilometers. These waves travel rapidly through negative anomalies, or anomalous lows, and more slowly through positive anomalies, or highs. This difference is interpreted to mean that the lows are cold areas of higherdensity rocks. Slow travel indicates hotter, lower-density material. Sluggish travel occurs around the Yellowstone plume and around hot-spot swells such as the one under East Africa, where up swelling material is splitting apart the continent and opening the Red Sea. A l t h o u g h mid-ocean ridges do not produce surface highs, seismic waves travel slowly down to 200 between and 400 kilometers below these features, indicating that hot material extends to those depths. Surface waves from large earthquakes also provide information about the direction of flow of convecting material in the mantle. Seismic energy propagates at different speeds in different directions, a phenomenon called anisotropy. The anisotropy is attributed to the preferred orientation of crystals of olivine, generally accepted as a principal rockforming mineral in the upper mantle. Orientation indicates direction of flow, Confusion range House range Sevier desert Canyon range and the seismic waves travel faster through crystals aligned in this direction. Such data are interpreted to show that material moves upward along the MidAtlantic Ridge, the East Pacific Rise, and spreading ridges in the Indian Ocean and the southern ocean around Antarctica. At 450 kilometers, flow direction and propagation velocities suggest that the flow is horizontal and the mantle is cold and dense. A more penetrating wiew Body waves, which pass completely through the earth, can be used to study deeper parts of the mantle. Two Harvard University geophysicists, Adam M. Dziewonski and John H. Woodhouse, combine data from surface and body waves to construct composite images down to 2,900 kilometers, the mantle-core boundary. Because of the locations of seismometers, resolution with this technique is limited to about 3,000 kilometers, and only large-scale anomalies can be mapped. "Nevertheless," Dziewonski says, "we have a tool with which we are seeing things we did not know existed a year ago." "The most important conclusion," he says, "is that below 400 kilometers, maps of the mantle bear little relation to surface features/' Indications of continents and ocean ridges begin to disappear at 200 to 250 kilometers. Various ridges and sections of ridges can be traced to different depths, and the data suggest that ridges can be fed from the sides rather than from directly below. All evidence of ridges and the distinction between continents and ocean basins, however, is gone at between 350 and 450 kilometers. Below these depths a broad region of high velocity underlies South America, the South Atlantic, and West Africa, suggesting the presence of cold, subducted rock. Seismic waves also travel rapidly below the western Pacific, but low speeds persist in the central and eastern Pacific. Only hot spots, such as the one associated with the northwestern Indian Ocean, retain their character below 500 kilometers. ' T h e hot spot associated with Hawaii may extend all the way to the core-mantle boundary," Dziewonski notes. "In both the upper and lower mantle, velocity anomalies are significantly correlated with the long-wavelength anomalies." "The simplest assumption," Woodhouse says, "is that the velocity anomalies are due to temperature differences, but we cannot rule out chemical differences." Many researchers believe that the major cause of a velocity Increase at about 450 kilometers is a pressureinduced rearrangement of atoms in olivine. The rearrangement produces a denser structure, resembling the mineral spinel. "It is not clear yet," says Dziewonski, "whether the disappear- ance of surface characteristics In this depth range is related to this p h a s e transformation." Earthquake waves experience another, even more marked jump In velocity at a depth of 670 kilometers. This can also be explained as a phase change in which atoms are squeezed Into a structure resembling that of the mineral perovskite. Spinel has each of its silicon atoms s u r r o u n d e d by four oxygen atoms; perovskite is even denser, packing six oxygens around each silicon. A fundamental change The 670-kilometer discontinuity apparently marks a fundamental change In the earth. Most if not all earthquake generation stops at this depth. Seismic waves from shallower temblors increase their speed at this boundary, but few if any earthquakes originate beyond it. The abrupt cutoff appears w h e n geo- MOSAIC 33 physicists map earthquakes generated by the edges of sinking plates. The foci of these shocks descend at an angle under the overriding plate, then suddenly stop at 670 kilometers. The simplest explanation is that the sinking slab meets resistance so strong it cannot move deeper. As with most simple explanations of what goes on in the deep earth, not everyone agrees with this one. Geophysicist Thomas H. Jordan of MIT examined the travel times of earthquake waves that p a s s e d beneath the Sea of Okhotsk, where a plate carrying the Kamchatka Peninsula is plunging under the northeastern part of Siberia. He insists that these data "require penetration by the Kamchatka slab to depths of at least 1,000 kilometers, although penetration d e p t h s much greater cannot be excluded/' Bradford Hager, a Caltech geophysicist, takes the middle road: that the slabs meet resistance but do go through the barrier. Woodhouse and Domenico Giardini at Harvard University have studied earthquakes in the Tonga Island area, where the Pacific plate sinks under the Australian plate. They conclude that the sinking slab breaks up at the 67Q-kilo~ meter discontinuity. "But/' they add, "the pieces may go through and not cause deeper quakes because of changes in material properties with pressure and temperature." Existence of this structural or chemical discontinuity is central to the question of whether it prevents movement of material from the lower to the upper mantle. The geological community is divided on the issue. There are those researchers who believe that convection takes place in cells that extend from the top of the core to the bottom of the crust. Others maintain that the discontinuity separates convection In the lower 2,100 or 2,200 kilometers of the mantle from that In the upper 670 kilometers. They agree that heat moves across the barrier, but Insist that rockforming materials do not. Tomographic Images do not resolve the issue. Surface waves reach the limit of their resolution at 650 to 700 kilometers, and body-wave data are least certain at the same depth. "The hot spot near Hawaii seems to extend through the barrier," Dziewonski says. "Also, the low-velocity anomaly associated with the Red Sea-Gulf of Aden hot spot extends below the discontinuity and appears to merge with other deep, low- 34 MOSAIC velocity anomalies that continue obliquely to the core." These data, therefore, do not show evidence of an interruption In mantle convection. However, frustrating uncertainties in resolution and interpretation of the data persist, making it impossible to rule out twolayer convection. (See "Mantle Geochemistry: More Than One Riddle," by Blanchard Hlatt, Mosaic, Volume 12, Number 2.) Another frustration involves the obvious lack of a clear relationship between the lateral extent of postulated convection cells and motions of the plates. Cold crust sinks at convergence zones, but rising hot material is not confined to spreading centers between diverging plates. Convection may not occur in simple circular or elliptical cells; rather it may Involve eddies within eddies within even larger eddies. Once geophysicists accept convection, they face the question of whether it controls or is controlled by plate position. In the latter case, insulation of the mantle by the overlying continents causes overheating, which initiates an episode of continental rifting. When the continents come to rest, the earth enters a long period of stability, perhaps without convection. (See "What Drives the Earth's Plates," by Patrick Young, Mosaic, Volume 10, Number 5.) If convection controls plate motion, it may carry the plates along like a conveyor belt that p u s h e s at mid-ocean ridges and pulls at convergence zones. Studies of plate motion with respect to hot spots reveal that continents and ocean floors drift in a way consistent with motion away from the ridges and toward zones of subductlon: Plates with a large part of their boundary sinking Into the mantle move most rapidly, about eight centimeters a year. Those without substantial subducting borders poke along at only about two centimeters a year. The slow movers Include all the plates carrying large continents. Windows in store Unknowns about the earth's structure and dynamics also can be approached by way of the chemistry of deep rocks brought to the surface by volcanism and other tectonic upheavals. Particularly helpful are studies of Isotopes whose ratios in the rock are not affected by high-temperature reactions that alter materials on their journey u p w a r d . Such reactions melt or partly melt rocks, causing separation of light and heavy fractions. The continents, for example, were formed from lighter material that rose to the surface of a partially molten mantle. Separated fractions, however, carry isotoplc fingerprints that enable researchers to trace their origins. In addition radioactive Isotope ratios act as calendars that date the separations. Fractionation, for example, concentrated elements such as the rare earth neodymium and the alkali metal rubidium In the crust, while the rare earth samarium and the alkali metal strontium remained In the mantle. Radioactive samarium 147 decays to neodymium 143, which Is thereby increased in amount, while the quantity of neodymium 144 remains constant. This activity occurs in both the crust and mantle, but the ratio of neodymium 143 to neodymium 144 grows more rapidly in the mantle, because more samarium is decaying there. Conversely the ratio of strontium 87 (whose parent is rubidium 87) to strontium 86 is greatest in the continents. Advanced mass spectrometers enable researchers to measure the ratio of isotopes in a rock relative to their ratio in the whole, undifferentiated earth. The whole-earth ratio Is impossible to determine directly, so geochemists use the average composition of a class of stonv meteorites, carbonaceous chondrites, as a reference. Geologists assume that the planets formed from the same cosmic materials as these meteorites. The materials undoubtedly were altered during the formation of the planets, but the proportions of refractory elements, such as neodymium and samarium, are assumed to have remained the same. Rocks from different parts of the crust show different variations from the whole-earth standard, Indicating that they did not come from the same part of the mantle. This fact supports the Idea of a layered mantle because, if the whole mantle Is converting, differences would be eliminated. Whole-mantle convection, however, may have existed In the early history of the earth, when the planet was much hotter. Caltech's Don Anderson believes that vigorous stirring, produced by more Intense thermal gradients between the core and the surface, would prevent formation of a permanent crust until the planet cooled and convection slowed. He cites this as the reason that no rocks older than 3.8 billion years have been found on a planet estimated to be 4.5 billion years old. As the earth cooled, low-density fractions of the mantle formed a stable crust. Geochemists argue that this crust could have formed from the upper third of the mantle, leaving the lower twothirds unaffected. Those who believe it happened this way see the mantle below a 670-kilometer depth as a reservoir of primitive material that has existed separately from the upper mantle for two or three billion years. Geoalchemy The separation, however, does not rule out an exchange of material. Some earth models call for material to rise from the lower mantle in the form of diapirs, or huge blobs, that pass unaltered through the upper mantle and break through weak spots in the crust. Massive outpourings of basalt between 15 million and 60 million years ago, in what now is the Columbia River basin of Washington and Oregon, for example, bear isotopic fingerprints similar to those of reference meteorities. Stanley R. Hart, who lists his affiliation as "the Center for Geoalchemy, Massachusetts Institute of Technology/' maintains that "no primitive mantle ex- ists/' and no material can make the journey from the lower mantle unaltered. He believes that mixing in the upper mantle, as well as reactions with solutions and vapors, produces a blend of isotopes that sometimes mimic those in the meteorites. Many blends exist. Hot spots, which may extend into the lower mantle, have isotopic signatures that are different MOSAIC 35 from the Columbia River basalts and from each other. Isotopic ratios in volcanic rocks from Hawaii are intermediate between those that erupt along ocean ridges and those that characterize a primitive earth. Rocks from Kerguelen Island in the southern Indian Ocean show an enrichment of neodymium and rubidium more typical of the crust than the mantle. Such confused identities may result from different depths or origin; they might also derive from combinations of contaminants from the upper mantle and crust. Isotopic fingerprints of volcanic rocks on some but not all island arcs at the edges of subducting plates are totally different from those on intraplate islands. The island-arc rocks hold amounts of strontium typical of continental rocks. Geochemists believe that water dissolves strontium out of the continental rocks and carries it to the sea. Strontium-laced seawater eventually mixes with basalts at the ocean ridges and becomes incorporated into oceanic crust. When this crust finally becomes the sinking edge of a plate, it mixes with sediments accumulated on the ocean floor and with mantle material above the sinking slab. This combination melts and erupts in island-arc volcanoes. Subsequent crustal compression welds t h e s e islands to the continents, creating mountain ranges such as the Andes and the Cascade Range. 'Island arcs are factories that make new continental crust/' notes Stanley Hart. "Fractionation of the mantle above the sinking plate edge is the sole means of producing new continental material." Hart postulates that continents grow by this means at the expense of oceans. "An increase in the size of the continents is balanced by a decrease in ocean-ridge volume," he says. Geologist William S. Fyfe of the University of Western Ontario believes just as firmly that the continents are shrinking. He maintains that the loss of sediments eroded from the continents and the wearing away of continental edges at subduction zones exceeds the a m o u n t of new crust produced by island-arc volcanism. Isotope ratios Indicate that much of the crust separated from the mantle between 3.5 billion and 2.5 billion years ago. There are, however, no measurements to reveal how the continents were formed. They could have originated as a thin, globe-covering crust that broke u p and thickened into con- 36 MOSAIC tinents. Or they could have grown around cores that formed over heterogeneities in the mantle, where light fractions rose to the surface. Once plate tectonics began, however, the continents were able to grow by collision. Geologists, w h o search out suture zones that mark small and catastrophic collisions, trace plate tectonics back 1.3 billion years. N o n e of the basalts associated with subduction, mountain building and other tectonic events are older than one billion years, and their distribution hints that plate tectonics Is younger t h a n the continents. Some geologists claim that these rocks do not occur In the oldest central core areas of continents; therefore the continents preceded subduction and other events that characterize plate tectonics. Other geologists claim that so-called greenstone belts In the continental cores possess characteristics similar to volcanic rocks on modern island arcs. William Fyfe, w h o believes that continents predate plate tectonics, speculates that hot-spot tectonics dominated the sculpturing of the earth before plate tectonics. Temperature and composition However continents formed and plate tectonics began, their origins are attributable to the same processes that drive geological evolution today: density and heat differences. Studies of the propagation of seismic waves provide a good look at variations In density, but temperature gradients are more difficult to determine. Orson L. Anderson, a geophysicist at the University of California at Los Angeles, compares our knowledge of the earth to a three-legged stool. "One leg Is temperature; one, density; and one, p r e s s u r e / ' he says, "Density and pressure are known fairly well, but lack of data about temperatures makes the stool wobbly/ Heat now coming out at the surface must have originated from the formation of the planet and from the decay of radioactive elements. The problem is to determine the relative contribution of each source. Theories that the earth was born hot, for the most part, have replaced those proposing that It began cold. "All the evidence suggests that the earth accreted after metal and silicate particles condensed out of a cooling solar nebula/' says Raymond Jeanloz, a geophysicist at the University of California at Berkeley. These particles may have come together as a result of gravitational attraction, after which the material differentiated into a dense iron-rich core and lessdense silicate-rich mantle and crust. As iron particles sank to the center, they would release enough gravitational energy to raise the earth's temperature about 1,500 degrees Celsius. Alternatively metallic particles could have agglomerated into a core, which then attracted silicate materials. If the silicates accreted rapidly enough, heat would have accumulated faster than it could radiate into space. "An Increasing number of scientists believe that the major source of present temperature gradients is this primordial fire/' says Orson Anderson. Again, not everyone agrees. ''Enough radioactive elements, such as u r a n i u m , thorium and potassium, exist in the mantle to account for more than half of the earth's total heat loss," Hart says. Anderson, however, does not believe that this heat is needed to explain what goes on. He notes that helium and xenon gas dating from the earth's birth have been detected escaping from the interior through mid-ocean ridges. This indicates that both primordial gases and heat have been leaking out of the planet for 4.5 billion years. As the planet cooled, core material is believed to have crystallized, accounting for seismic evidence of a solid core 1,100 kilometers in diameter and enveloped by a molten outer core. As the outer core loses its heat through the mantle, the Inner core slowly increases In size. The two parts can be seen clearly through the seismic window, because compressional waves, which vibrate in the direction of travel, become severely attenuated by molten or liquid material. The velocity of seismic waves is consistent with the theory that the inner core consists mainly of Iron. Calculations of the earth's moment of inertia and of the cosmic abundance of elements, as well as the need for a metallic core to account for the planet's magnetic field, also suggest an iron core. Determining core composition is vital to putting knowledge about the earth on a stable footing, because the temperature of a molten outer core depends on the material of which it Is made. This temperature then becomes a reference for calculating the planet's heat balance. J. Michael Brown of Texas A. & M. .University and Robert G. McQueen of the Los Alamos National Laboratoryhave attempted to put an upper limit on this temperature by duplicating pressures and materials believed to be characteristic of the core. They compressed pure iron to a pressure of 2.5 megabars, or millions of atmospheres, and measured its melting temperature as 5,000 degrees Kelvin. By extrapolation they calculated that temperatures where the inner and outer cores meet reach about 5,800 degrees. "This is the first time temperature estimates near the earth's center have been obtained by experimental data," says Jeanloz. Brown and McQueen conclude that the inner core is principally pure iron and that the outer core contains an inhomogeneous mixture of iron alloyed with lighter elements. The lighter elements must be present, because calculations of the earth's density are inconsistent with a core made entirely of iron. Moderate amounts, some 8 to 10 percent by weight, of such elements such as oxygen, silicon, or sulfur must be present to produce a match with seismic and astronomical observations. Combining iron with a lighter element, however, lowers its melting point. Sulfur, for example, has a much larger effect on the melting point than oxygen does at lower pressures; if this difference persists at high pressures, an outer core with iron sulfide would be much cooler than one with iron oxide. The uncertainty about composition introduces an uncertainty of about 1,000 degrees Celsius in the temperature at the core-mantle boundary. Diamond window The next step for laboratory scientists is to determine the melting points of different alloys at pressures between 1.5 million and 3.5 megabars to see which melting point fits best as the third leg of the stool. Researchers can do this by squeezing a microscopic sample of material in a so-called diamond anvil cell. In such a cell, the sample sits between two flawless, flat-faced diamonds driven together by a screw mechanism attached to a sliding piston. By improving on this simple device, geophysicists at the Carnegie Institution of Washington have produced a pressure of up to 2.8 megabars in a chamber 200 microns in diameter and 50 microns in height (a feat that is included in the Guinness Book of World Records). Laser beams flashed through diamond windows in the cell can heat Bell. Guinness-record pressures. samples to 5,300 degrees Kelvin. "With such experiments, which touch the limit of technology, we are starting to duplicate conditions at the core-mantle boundary," says Carnegie's Peter M. Bell. "I have no doubt that in the near future we will be able to reach pressures and temperatures as high as those at earth's center." Such experiments already have changed established views of the planet's interior. Before diamond-cell work began in the early 1960s, geophysicists believed they could not account for density differences in the deep earth simply by putting crustal granites and basalts under high pressure. "Diamond-cell experiments at pressures of 200 kilobars and 1,500 degrees Celsius proved this wrong," Bell says. Under these conditions, believed to prevail at the 670-kilometer discontinuity, minerals found at the surface take on the crystal structure of perovskite,. the density of which can account for the behavior of seismic waves in the lower mantle. "Silicateperovskite is the densest known mixture of crustal silicates and oxides," the Carnegie scientist points out. "Such a structure may exist all the way to the core-mantle boundary." No one knows what happens at this boundary. Seismic data indicate a mushy, semifluid zone of reduced velocity similar to what would be expected from a slurry of crystals and liquid. "Solid-liquid phase transitions like this normally are accompanied by changes in chemical composition," Bell says. "It is difficult, however, to determine whether physical or chemical change dominates." Bell and his colleague Ho kwang (David) Mao propose that chemical reactions predominate, kicking iron out of the mantle and into the core. This concept, they speculate, could account for the presence of Iron oxide in the outer core. If phase change dominates, silicates could behave like metals because of the increased pressure. This concept raises the possibility that the core may contain other compounds besides Iron alloys. Raymond Jeanloz is using shock waves and diamond anvil cells to study the possibility that calcium oxide exists in a metallic phase in the core. Researchers have not found the amount of calcium that they were led to expect by its abundance in stony meteorities. "This missing calcium may have been squeezed into a super-dense phase and stored in 'the core," Bell suggests. Diamond anvil cells, like other windows to the earth's interior, Impose new limits on what Is possible and so constrain speculation. "Only 25 years ago, there were so many theories that placing your belief was like an act of faith," says Robin Brett of the U.S. Geological Survey. "Many different faiths coexisted because none could be proved or disproved." The plate-tectonics model destroyed many of these faiths and helped to unify those that survived. Brett says plate tectonics theory "sparked a revolution in geology that has been as important as the theory of the atom was to physics and the theory of evolution to biology." The theory, however, successfully accounts for the development of only the youngest twothirds of the earth's surface, the ocean basins. The technologies that are opening new observational windows are extending the revolution to the continents and to more distant time periods. Of course answers to fundamental questions still He In the future, as does the application of the revolutionary knowledge to finding mineral and energy resources, predicting earthquakes and volcanoes, and forecasting sea-level changes. Nonetheless, these new windows enable scientists to look at the earth with more Insight and to predict its future with more certainty. • The National Science Foundation contributes to the support of research discussed in this article through programs in its Division of Earth Sciences. MOSAIC 37