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Solid-Earth Sciences and Society 2 Understanding Our Active Planet ESSAY: THE DYNAMIC EARTH Understanding earth processes requires broad and eclectic thinking. The earth system is complex, with open channels between interacting boundaries the norm rather than the exception. Many researchers think of the solid-Earth as an engine driven by radioactive decay, while others expand this view to include the whole earth system and consider the added processes driven by solar energy. Others see the Earth as a system of geochemical cycles with interchanges spanning ranges of time and space that extend back to the birth of the solar system. Finally, some scientists regard the planet as a series of concentric domains with ill-defined layers distinguished by the transfer of mass and energy. The Earth is all of these and more. The accelerated understanding of the earth system that characterized the past few decades is attributable to problem-solving strategies based on integration of these various interpretations. Contributions from geochemistry support theories developed from seismological data, structural geology depends on investigations in physics, and organic chemistry offers potential explanations for problems encountered in both resource extraction and waste management. Since their adoption of this expanded tool kit for investigating the implications of plate tectonics, earth scientists have made unprecedented progress. The Earth began over 4.5-billion-years ago with the accretion of material orbiting around the Sun, supplemented by the capture of other bodies from intersecting orbits. Early in the process of consolidation, proto-Earth collided with a Mars-sized body and the material from both reorganized into the Earth-Moon system. Soon—in a geological sense—after that event, convection cells became established within the Earth's mantle, a crust developed, free water entered the atmosphere, plates diverged, ocean basins evolved, and mountains rose through tectonic forces at work along plate boundaries. These distinctive earth phenomena are both cause and effect in a multiscaled arrangement of interacting processes.
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Solid-Earth Sciences and Society From the perspective of geochemical cycles, there are two end-member processes, differentiation and mixing; two end-member domains, the exterior Earth environment and the interior; and two end-member time frames, hundreds of million of years and the instant. Within each end-member pair, there is a continuum of possibilities. Generally, surface domain processes occur very quickly and interior processes endure over long intervals, although there are exceptions. Some continental material has endured for billions of years near the surface, and mantle plumes may erupt at the surface with no detectable warning, after migrating from the core-mantle boundary over mere millions of years. And while differentiation and mixing of large volumes continue for eons within the mantle, incremental changes within small volumes can take place quickly both at the surface and within the interior. The domains that extend above and beneath the surface contain the Earth's fluid envelopes. Water vapor in the atmosphere condenses and falls as rain. At the surface, water weathers the rocks physically and chemically: physically by impact and by freeze-thaw action and chemically by solution and the introduction of ions that foster reactions with rock minerals. Particles and solutions from crustal rocks wash downstream and enter the great water reservoirs of river, lake, ocean, and groundwater—settling out as detrital sediment and as precipitates. At ocean spreading centers and in volcanic environments, water may aid in the precipitation of mineral concentrations that become valuable resources when discovered in accessible terrain. Magmas and other fluids that move through the crust have the potential of becoming significant sources of minerals and energy. Along subduction zones, hydrated crust and water-saturated ocean-bottom sediments descend into the interior beneath a mantle wedge that extends over the sinking plate. At high-temperature and pressure these rocks dehydrate, which leads to melting. Volatile-rich magmas rise and interact with crustal rocks to generate the type of gas-charged magmas that erupt at the surface with devastating violence. The volatility is most pronounced along Cordilleran arcs of continents; the volcanoes along the South and North American Cordillera erupt in explosions that may literally blow them apart, as Mount St. Helens did in 1980. Volcanoes that build over hot-spots, such as those in the Hawaiian Islands, erupt magmas that flow rather placidly because of their chemical makeup. They contain smaller proportions of silica, and gases escape readily. By the time the magma reaches the surface it is less explosive and sticky, so it flows easily. Eruptions from hot-spots produce large volumes of basaltic lava spreading over extensive areas in layered sheets that may accumulate to great thicknesses; the Hawaiian volcanoes reach heights over 9,000 m above the deep ocean floor and nearly 4,000 m above sea level. Hot-spots originate deep within the Earth. They are the result of plumes that reach the surface after rising through mantle and crust. The source of these plumes may be within the mantle or from the mantle-core boundary. They may originate at both depths, and researchers are not yet able to recognize evidence that could characterize distinct sources.
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Solid-Earth Sciences and Society Researchers are also investigating the possible causes of mantle plumes. Physical anomalies along the core-mantle boundary and chemical anomalies attributable to recycled surface material are two intriguing possibilities. Whatever the cause or the source of mantle plumes, they bring to the surface basaltic lavas with chemical clues about the deep mantle material from which they were extracted. The movement of plumes through the mantle represents chemical and physical links between the interior and exterior. The mantle itself is flowing in a complicated pattern of convection. This pattern manifests itself at the surface as spreading centers and subduction zones, where vast slabs of lithosphere can be seismically traced along descending arms of the convecting system. The convection, which governs plate dynamics, may be limited to an outer layer of the mantle, possibly complementing another convection system delivering energy and material through an inner mantle. An alternative possibility suggests cells that convect through the whole of the mantle, directly linking the bottom boundary along the core to the surface characteristics of plate tectonics. There are two layers to the Earth's core, recognizable from the distinctive behavior of seismic waves. The outer core is fluid. Only compressional waves propagate through it, while shear waves can be detected propagating within the inner core. The core is the nucleus of the internal domain, 2,900 km below the surface. Even from that remote depth it affects the crust and the atmosphere: the core is the source of the magnetic field. Core, mantle, crust; lithosphere, hydrosphere, atmosphere, magnetosphere—every layer, every component of the earth system can be defined independently. But to understand the meaning of those definitions, the significance of the components, and the nature of the whole earth system requires consideration that transcends the specific. Exchanges between the innermost center and the outermost reaches of the earth system are ubiquitous and continuous. Earth scientists are discovering both explanations of the past and implications for the future by adopting this grand scale—the whole earth system—in their ongoing inquiries. This expansive perspective solves old problems and presents new ones. For example, 25 years ago plate structure was recognized as a characteristic that specifically defined the lithosphere. Lately, motion of the lithospheric plates has gained a prominent position as a factor in processes that affect both mantle heterogeneity and global climate. Seismic studies have traced hot areas associated with spreading centers deep into the mantle and recently have detected slabs of cool lithospheric material descending deep beneath subduction zones. This cooler material persists over long periods; cool-temperature anomalies found in the mantle today are remnants of the breakup of the ancient continent of Gondwanaland about 150-million-years ago. The breakup of Gondwana also caused drastic changes in climate patterns. As Africa, India, and South America drew away from Antarctica and encountered the landmasses of the north, the equatorial currents of the Tethys Sea were interrupted and deflected. After India collided with Asia 45-million-years ago, cold deep water began to accumulate off
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Solid-Earth Sciences and Society Antarctica. Circulation around that continent became firmly established when the Drake Passage between South America and Antarctica was finally breached about 30-million-years ago, and glaciation advanced over eastern Antarctica. After Arabia collided with Asia 15-million-years ago, circulation of the warm saline waters southward out of the Indian Ocean ceased and the West Antarctic ice sheet grew. Finally, following the emergence of the Isthmus of Panama and complete interruption of extensive east-west equatorial ocean circulation, Arctic glaciation began and eventually grew to cover vast areas of North America and Eurasia. This modern glacial mode persists, despite the relatively small glaciated areas of the current interglacial period. Climate changes result from ocean circulation changes. Ocean circulation in turn follows paths established by variations in plate distribution, according to the vagaries of plate tectonics. Evidence of plate tectonics is restricted to Earth, despite rigorous surveys of similar planets and planet-like bodies. The mantle convection that results in plate tectonics originated soon after establishment of the Earth-Moon system 4.5-billion-years ago. Whatever the source of this remarkable process that renews much of the Earth's surface every few hundred-million-years, the result is a surface environment supporting the only known life in the universe. Understanding this dominant earth process is essential for maintaining the surface environment. ORIGIN OF THE EARTH Any theory of the Earth should consider its origin, and any well-rounded program in the earth sciences should consider it a component of the solar system. This consideration addresses scientific issues such as the formation of the solar system, the processes causing evolution of the planets, and the relationship of these processes to its structure and history. Knowledge about the origin of the solar system comes from studies of astronomical data on forming stars and planetary properties as well as from meteorites. We can expect advances in the application of chemical physics to the interpretation of change in cosmic material in its journey from the stars and the interstellar medium to its eventual resting place in the Earth, the planets, and other solar system bodies. This will require a multidisciplinary approach. Laboratory work using innovative experimentation and analytical techniques, observational astronomy, and theoretical efforts is involved. Geochemical and cosmochemical studies of meteorites, comets, or their analogs can become more intimately related to the origin of the Earth. The Earth formed some 4.5-billion-years ago. Its age is known from differences in its radiogenic lead isotope ratios from those of the Moon and meteorites. Technical advances allow detailed computer simulations of early solar system evolution. The simulations apply the physical laws that control orbits, collision frequency, and accumulation mechanics as small objects grow to the size of planets. One conclusion of this work is that planet growth through the accumulation of small objects occurs relatively rapidly. To go from a multitude of dust grains in a solar nebula collapsing under its own gravitational attraction to a limited number of planets circling a central star requires only a few tens of millions of years. The most recently obtained isotopic dates from meteorites suggest that this process happened about 4.55-billion-years ago in our solar system. The simulations also indicate that as the size of colliding bodies grows, more kinetic energy from the incoming body is deposited deep in the planet. For a planet like the Earth, which grows to full size in only tens of millions of years, this heat cannot be transferred to its surface, and then into space, fast enough to keep the interior from reaching very high-temperatures. Recently, attention has focused on the hypothesis that a collision between the proto-Earth and an
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Solid-Earth Sciences and Society FIGURE 2.1 Model of the origin of the Earth's Moon by impact of a Mars-sized object on Earth. object roughly the size of Mars, approximately one-tenth the mass of the Earth, may have been responsible for the formation of the Earth's moon (Figure 2.1). This impact would have been sufficiently energetic—comparable to a trillion 1-megaton atomic explosions—to propel material into Earth orbit, forming the Moon. The giant-impact origin for the Moon seems capable of explaining both the large relative size of the Moon and the geochemical similarities between it and the Earth that have been deduced from analyses of lunar samples returned by the Apollo missions. Experiments on and calculations of the effects of impacts during accretion should lead to better understanding of the early history of the Earth, the role of a magma ocean in differentiation, and the history of outgassing and its relationship to the formation of the atmosphere and oceans. Comparative Planetology Many processes of the solid-Earth must also occur on other planets but may frequently have rather different consequences. The proposal that the extinction of dinosaurs and many other species near the Cretaceous-Tertiary boundary was caused by the catastrophic impact of a 10-km asteroid or comet has sparked heated debate among geologists. But cosmochemists and solar system dynamicists argue that such events are probable, and researchers studying impact craters on the Moon, Mars, and Mercury say it is consistent with their data. Study of lunar samples has led to new insights into the evolution of Earth. The earliest history of the Earth has been destroyed by plate tectonic processes. Early Earth history must be deduced from analyses of lunar and meteorite material and from exploration of other planets. Detailed chemical analyses of lunar samples suggest that a largely molten Moon crystallized to form a thick crust composed of the low-density aluminum-rich mineral-plagioclase-overlying a dense magnesium and iron-rich interior. The lunar crust appears to have been formed by gravitational sorting according to the relative density of the minerals crystallizing from an original magma ocean. Age determinations show that much of the chemical differentiation of the Moon was complete by 4.35-billion-years ago, or within 200-million-years of lunar formation.
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Solid-Earth Sciences and Society Debate continues about many issues concerning the evolution of the Earth. These issues include the degree of outgassing, the cooling of the planet, the heterogeneity of the mantle, the conditions necessary for plate tectonics, continental growth and crustal recycling, the creation and persistence of stable continental cratons, and the generation of the magnetic field. In the face of this array of questions, it would help greatly to have at least one other planet to use for comparison. The planet Venus differs less than 20 percent from the Earth in mass; in mean density; and, as far as can be detected, in content of chemically active volatiles. Yet in secondary properties related to its interior, it is very different from the Earth. Radar imagery from the recent Magellan probe reveals a planet surface scarred by volcanic activity and bombardment from space, as well as deformation, but plate motions are not evident and the mountain-building processes contrast greatly with those of the Earth. Faults can be traced, and broad elliptical areas spanning hundreds of kilometers may represent mantle plumes rising beneath the Venerian lithosphere. With no apparent plate tectonics, heat may have to escape from deep within Venus in other ways. The comparative study has just begun, and it should be greatly enhanced by continuing analyses of data from the Magellan project. Early Earth Evolution and Great Impacts Obviously, any extraterrestrial impact capable of creating the Moon would have major consequences for the early evolution of the Earth. Computer simulations of giant impacts indicate that the energy released is sufficient to raise the temperature of the whole Earth by 3000 to 10000°C—more than enough to cause total melting of the planet. Recent advances in high-pressure instrumentation have made possible studies of phase equilibria and element partitioning at pressures as high as 500 Gigapascals—5 million times atmospheric pressure—and temperatures up to 6000°C. These new limits allow experimental analyses over pressure and temperature ranges covering the entire interior of the Earth. In the next decade, integration of high-pressure experimental studies of earth materials and enhanced theoretical understanding of the dynamic behavior of the planet should provide dramatic advances in understanding these earliest stages of the Earth's evolution. A hot early Earth accommodates speculations about formation of the Earth's large iron-metal core. Compared to the silicate minerals that make up the major portion of Earth, iron metal has a relatively low melting point. During the growth of the early Earth, the molten iron coalesced into masses of increasing size that eventually began to sink toward the center because of their high densities. The excess heat energy stored in the core is released slowly as the molten core crystallizes. At the present time, only about 5 percent of the core has crystallized, which indicates that continued crystallization of the liquid outer core may be a significant source of heat within the Earth. In addition, the heat released as the outer core crystallizes, and the transfer of that heat into the overlying mantle may provide the driving force for the convection in the outer core that is responsible for producing the magnetic field. During their early histories, the Earth and Moon were subject to a high flux of relatively large impactors. Because the Earth is an active planet, no record of this flux is recorded, but evidence from the Moon suggests that the flux had died away by about the time of preservation of the oldest rocks exposed on Earth. The discovery of high iridium contents in some rocks a few hundred-million-years younger than the 3.8-billion-year age usually considered to mark a sharp drop in impactor flux indicates that this question may need reexamination. Since that time in the earliest recorded history of the Earth, the flux of impactors has been slow, although the record of impacts is too poor to show whether it has been anything other than steady. Roughly 100 craters more than 1 km in diameter have been identified on the continents. The precise count depends on which criteria are regarded as strong evidence of impact. Some impacts are as old as 2-billion-years, and the largest craters are 100 km or more across, perhaps indicating the impact of a 10-km-diameter object. Both asteroids and comets are likely to have been involved. The distribution, characteristics, and possible consequences of impact in the more recent geological record are all active topics of research. The innovative suggestion about a decade ago that the great biological extinction 66-million-years ago, which included extermination of the dinosaurs, resulted from impact has proved very stimulating. Evidence of impact at that time in the form of widespread high iridium concentrations, shocked quartz, and wildfire is persuasive. The giant crater at Chixulub in Yucatan is a strong candidate for the main impact. The possibility that other large craters, such as that at Manson in Iowa, are associated
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Solid-Earth Sciences and Society with the same event may require a cometary rather than an asteroidal encounter. Current astronomical estimates of the flux of impactors make it clear that the number of impacts recognized on most continents is improbably low. This is also true of the number of impacts recognized within the continental stratigraphic record. Thus, much remains to be done in locating and studying ancient impact craters. STRUCTURE AND DYNAMICS OF THE SOLID EARTH One of the most significant advances in understanding the solid-Earth took place within the past 30 years with the general acceptance that the solid interior is in motion and that movement of the surface plates is an expression of that motion. Everyday experience suggests that rocks are solid, but geological investigations reveal that earth materials behave very differently on long time scales and on human time scales. On a geological time scale, the solid mantle behaves like a fluid and convects. It is convection, not conduction, that is the main means of heat transfer within the Earth. This is the process by which heat is most effectively transported from the deep interiors of planets. With this realization, it was no longer possible to view earthquakes, volcanic activity, mountain belts, sedimentary basins, or the general division between oceans and continents as isolated surficial phenomena. Temperature variations within the Earth control the convection that ultimately produces the magnetic field, surface topography, and active geology. Interactions between the rigid surface plates cause earthquakes and the majority of volcanic activity and provide the stresses leading to mountain building and basin formation. The plates are driven by the slow convective processes of the mantle. There is little question that subducted oceanic crustal plates penetrate at least a third of the way through the mantle to depths of 670 km. Some lines of evidence suggest that these plates may travel all the way through the mantle to form a layer around the core. Heat flowing out of the core may disturb the thermal boundary layer separating it from the convecting mantle to produce narrow plumes of uprising solid material that produce surface volcanism in settings like Hawaii and Iceland. The exact style of convection is a subject of active research. Advances in understanding this process are being made through three-dimensional seismic imagery, increasingly sophisticated computer models, and laboratory simulations. Several avenues of research promise major breakthroughs in understanding the thermal state and evolution of the interior. It is now clear that the surface characteristics of the Earth originate in, and are being continually modified by, a complex interplay between the mobile surface plates and the dynamic interior of the planet. Seismic Determinations of Earth Structure The structure of the interior has been examined in detail for over 50 years, through the application of seismology—the study of natural and artificially generated vibrations traveling through earth material. The emphasis has been on determining the variations in physical properties, especially velocity, refraction, and reflection behavior, as a function of depth. These variations reveal major changes in composition with depth. For example, the core, an iron-rich alloy, is more than four times denser than the crust, which is made mainly of aluminosilicates. Because of advances in seismic instrumentation and analysis, the interior can be viewed in three dimensions from the surface to the center. Thus, the processes underlying near-surface geological phenomena can be mapped and understood. A primary feature of present-day models of the interior (Figure 2.2) is the asthenosphere, a region of low seismic shear wave velocity in the upper few hundred kilometers of the mantle, where materials approach their melting point and where mantle flow may be concentrated. At a 400-km depth the first of the deep mantle discontinuities in seismic wave velocity occurs, followed by an even larger discontinuity at the 670-km depth. Current debate centers on the nature of these discontinuities. One view is that they represent phase changes in a mantle of constant composition. In this scenario the increasing pressure causes silicate minerals to convert to more dense crystal structures (Figure 2.3) with depth. Other evidence suggests that these seismic discontinuities mark more than phase transitions and may be regions where the chemical composition of the mantle changes. If only the phase and not the composition changes, the discontinuities would not necessarily be barriers to convection. If, however, the discontinuities reflect compositionally induced density differences of sufficient magnitude, they would inhibit flow across them. In this case convection would be confined to a series of layers within the Earth. These two distinct types of convection have drastically different implications for the compositional and thermal evolution of the interior.
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Solid-Earth Sciences and Society FIGURE 2.2 Cross sections of the Earth and its properties. The upper panel shows the seismologically determined regions and pressures as a function of depth (100 GPa = 1 Mbar = 1 million atmospheres). The bottom panel shows the average elastic parameters as a function of depth as determined by seismological analyses.
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Solid-Earth Sciences and Society FIGURE 2.3 The bulk of the material making up the lower mantle is believed to have the perovskite structure. At present, most physical property measurements have been carried out at room temperature and pressure. Extrapolation of these measurements to appropriate pressures and temperatures for seismic discontinuities carries sufficient uncertainties to allow either explanation for their origin. Only recently, with the development of several new approaches for high-pressure and high-temperature experimental apparatus, has it become possible to provide precise measurements of parameters such as density, compressional and shear wave velocities, and seismic wave attenuation. A new effort in mineral physics at high-pressure and temperature, coordinated with interpretation of high-resolution seismic images of the mantle from advanced seismic instrumentation programs, such as those administered by the Incorporated Research Institutions for Seismology (IRIS), is enabling earth scientists to better understand the structure of the mantle. Seismologists have used a wide variety of techniques to create three-dimensional images of the Earth's interior structure. One method is tomography, based on the same principle that is exploited in medical x-ray and ultrasonic imaging devices. It concentrates on small variations in the observed arrival times of seismic waves following earthquakes. A limitation of this technique is that it requires extremely dense arrays of seismic recording stations to provide high-resolution images. This limitation has led to the development of sophisticated methods that utilize more of the complicated signals arriving at seismic stations during and after earthquakes. Large earthquakes excite free oscillations that are sensitive to the largest scales of heterogeneity and give direct constraints on lateral variations in density. Surface wave arrivals give good lateral resolution of upper-mantle structure but need to be augmented with other data to give good depth resolution. Body wave arrivals provide the best resolution in depth and can be used to map the topography of internal discontinuities. Present-day images of the interior are of low resolution and uncertain accuracy. They reveal distinctive heterogeneities over horizontal distances extending thousands of kilometers. The heterogeneity is strongest near the top and bottom of the mantle, decreasing from 2 to 10 percent in the upper mantle to about 1 percent throughout the bulk of the lower mantle. The lowermost 100 to 300 km of the mantle, called the D'' (D double prime) layer, is also significantly variable, by 5 percent or more. Thus, the strongest lateral variations in physical properties are associated with the major boundaries of the Earth: the surface and the interface between the mantle and the core. This result supports the assumption that the top and bottom of the mantle are two regions in which material moves horizontally, with little vertical motion. From a dynamic perspective, the seismically produced images suggest direct associations between these heterogeneities and the mantle's convective flow. The instability of these boundary layers ultimately produces crustal deformation through the forces of subduction and plumes. Thus, seismic tomography in principle can map out the underlying motions that drive plate tectonics at the surface. One dramatic example of this type of mapping inside the Earth is the detection of cold slabs of lithosphere, the crust and uppermost mantle, sinking into the mantle beneath subduction zones. Because the slabs are cold, they appear as regions with anomalously fast seismic velocities relative to the velocities in the hot surrounding mantle. The presence of slabs in the mantle has been used to explain the existence of deep-focus earthquakes along the postulated extensions of near-surface subduction zones. Now actual images defining the dimensions of thermal anomalies—cold slabs—are being produced by tomography. Therefore, it is possible to detect subduction at depth, even in places where a slab may be seismically quiet. The present location of cold slabs at depth is an indication of past subduction because rock changes temperature very
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Solid-Earth Sciences and Society slowly. For the first time, observers can determine how far the slabs penetrate into the mantle, providing first-order information on the deeper aspects of the convection patterns associated with plate tectonics. In addition, the shape of the slab is a direct reflection of the tectonic forces associated with convection. Observations of significant distortion of slabs in certain regions give evidence for variations in rock properties with depth and for background flows in the mantle convection pattern. This conclusion is supported by evidence of changes in the distribution and focal mechanisms of earthquakes with depth. All indications are that tectonic forces do vary along subduction zones, but the patterns are still indistinct. Mantle Convection Although many aspects of mantle convection are reasonably well understood, major scientific questions remain unresolved. These include the vertical structure and multiple scales of mantle convection and the efficiency of mantle mixing. Plate tectonics is the surface manifestation of mantle convection. The rigid plates are the uppermost thermal boundary layers of mantle convection. These cool layers are rigid on geological time scales and behave as plates. But the plates become denser because of thermal contraction. Eventually, they become gravitationally unstable and founder into the mantle at subduction zones, defined by the ocean trenches. The weight of descending plates is a major force driving plate tectonics. There is direct seismic evidence that the slabs of material subducted into the mantle at ocean trenches descend to depths of 670 km. An unresolved question is whether the downward limbs can penetrate this depth. The evidence is contradictory, and the views of experts are divided. If thermal convection penetrates this density barrier, whole-mantle convection occurs. If convection does not penetrate, then separate convection cells develop in the upper and lower mantle and mantle convection is layered. If mantle layers do not mix, significant variations in chemical composition and temperature could characterize the interior. There is also the possibility that both styles of convection can coexist. The amount of material transported across the entire mantle is currently the single largest uncertainty in understanding the Earth's thermal and chemical evolution. Just as the plates are thermal boundary layers at the top of the convecting mantle, there should be boundary layers at the base of the convecting system. For layered mantle convection this boundary layer would be the result of heat transfer from the lower mantle; for whole mantle convection, it would be the result of heat transfer from the core. The gravitational instabilities in these lower boundary layers may generate ascending mantle plumes that are responsible for intraplate volcanism, such as that in Hawaii. The fate of plates that founder into the mantle at ocean trenches has also been a subject of controversy. These plates are layered. The basaltic ocean crust extends to a mean thickness of about 6 km. Beneath that crust is a zone that has been depleted of basalt and is primarily composed of the refractory mineral olivine. This layer, which has a thickness of approximately 50 to 100 km, is gravitationally buoyant. Simple mass balance calculations show that ocean crust must be recycled through the mantle on a time scale of about 1.7-billion-years or less. Therefore, present-day basaltic ocean crust has been processed through the plate tectonic cycle several times. One hypothesis for the fate of the subducted ocean crust is that convection stirs it into the bulk of the mantle until it is nearly homogenous. Another hypothesis suggests that significant density differences between the basaltic ocean crust, which transforms to a dense phase called eclogite at depth, and the olivine-rich mantle result in gravitational segregation, with the depleted mantle rock overlying the crustal rock. The essential question that must be answered is whether convective mixing can homogenize the mantle before the buoyancy differences can cause layering. The effect of an increase in temperature is to decrease the density of rock by a fractional amount—a few percent per 1000°C. Density variations caused by lateral temperature variations drive mantle convection. By the same token, the convective flows induce temperature differences. The flow field and temperature field are coupled. Assuming that lateral variations of seismic velocities in the deep mantle can be ascribed to temperature variations, laboratory measurements of the changes in acoustic velocity with temperature can be used to infer the density variations in the mantle. In this way the buoyancy forces associated with mantle convection are obtained directly from seismic tomography. Variations of the external gravity field can be calculated from the inferred density variations, taking into account not only the density distribution within the convecting mantle but dynamic topog-
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Solid-Earth Sciences and Society raphy as well. Dynamic topography is the deformation of the surface and internal boundaries, such as that between the mantle and core, that occurs because of convective flow in the interior. By comparing the calculations with the observed variations in gravity determined from the analysis of satellite orbits, it has been found that the large-scale features of the Earth's external gravity are reproduced closely by the convection patterns deduced from seismic tomography. On a scale of 1,000 km, therefore, the seismic heterogeneity appears to be caused largely by temperature variations driving flow in the mantle. This analysis suggests that the large-scale pattern of convection and the associated buoyancy forces are being accurately imaged by seismic tomography. Flow in the mantle is dominated by viscous forces, so inertially dominated turbulence is absent. The convective flow is amenable to both numerical and experimental modeling. Numerical models of mantle convection have improved with advances in the speed of computers and in computational techniques and with better understanding of the processes governing creeping flow in the Earth. Two-dimensional calculations now include temperature and pressure dependence as well as chemical and phase boundaries with realistic values for mantle parameters. Three-dimensional cartesian and spherical calculations with constant-property material have recently been completed. In the atmospheric sciences, general circulation models (GCMs) have become credible enough to be used for routine simulations by researchers in atmospheric chemistry and climatology—subjects far removed from solid-earth fluid dynamics. But within the next decade, mantle convection models should reach this level of acceptance, and applications should become widespread. Core Dynamics and Geomagnetism The outstanding geophysical problem involving the Earth's core is the generation of a magnetic field. The magnetic field is a product of dynamo action in the electrically conducting fluid outer core. A majority of planets in the solar system and an overwhelming majority of stars possess magnetism, and all of these magnetic fields are a consequence of dynamo action. Despite this universality, our understanding of the Earth's dynamo remains rudimentary. Several key theoretical issues remain unexplained, including the physics of magnetic field equilibration and proper characterization of the energy sources in terms of fluid motions. Core convection may be driven by thermal buoyancy produced by heat loss to the mantle or by crystallization of the inner core. Core fluid dynamics is characterized by a wide spectrum of frequencies and spatial length scales. Filtering by the mantle allows only the lowest-frequency variations in the magnetic field to reach the surface. Thus, even the most informed theories are based on a heavily filtered, and therefore distorted, image of core processes. At the present time it is not possible to develop consistent models of the geomagnetic dynamo. The relatively simple flows that are computed in numerical convection models generally do not work as dynamos. The successful theoretical dynamos strongly suggest that large-scale magnetic fields are produced by a broadband spectrum of fluid velocities that are chaotic and turbulent. In short, the difficulty is that simple flows do not produce simple dynamos—they produce no dynamo. Complex flows are required to produce dynamos; therefore, models of the dynamo process are necessarily complex. The flow in the core required to produce the dynamo must be complex. An analogy with the winds in the atmosphere that determine weather patterns may help in comprehending core flow complexity. Scaling analyses show that 1 year of flow in the core corresponds to about 1 hour in the atmosphere; researchers are just now able to see the equivalent of 1 week's worth of weather in the core. To understand the magnetic field on a geological time scale requires the equivalent of understanding long-term climatic patterns in the flow of the outer core. One outstanding question is why the magnetic field varies with time. A suitable solution must explain the reversals in polarity that have occurred frequently—at roughly 1-million-year intervals—throughout geological history. These variations have proved useful for applications in geology and geophysics, most notably in paleomagnetic documentation of tectonic motions of the crust. Mapping magnetic orientations of rocks has illustrated continental growth by the assembly of preexisting blocks. Among the most exciting results of the past few years are the first reliable observations of changes in the magnetic field during a polarity reversal. Both the pattern and the strength of the field appear to change rapidly over 10,000 years, the duration of a single reversal. In fact, observations of the same reversal, as recorded in rocks from widely separated locations around the globe, are just becoming avail-
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Solid-Earth Sciences and Society The extreme elevation of Tibet has affected atmospheric circulation over much of Asia, perhaps influencing not only the development of the Asian monsoon but also the formation of the Sahara and even the onset of Northern Hemisphere glaciations. The extensive high ground in western North America may have played a similar role, but other tectonic events, such as the closing of the Panama Isthmus and the Zagros Ocean, have modified oceanic circulation and may have been more significant. Both kinds of major barriers to warm equatorial circulation would have generated profound climatic and environmental changes within the past 15-million-years. Recognition of tectonic influences on the evolution of the Earth's climate demonstrates the potential for intellectual breakthroughs resulting from the study of the earth system. Continental collision is a major research interest in the solid-earth sciences. Although most attention focuses on the active Alpine/Himalayan belt, older collisions, especially those recorded in the ancient Precambrian rocks, yield complementary information. Rocks from high-temperature and high-pressure environments buried deep within the modern mountain chains are preserved and well exposed for study at the surface in the old belts. Traditionally, these provocative exposures provide both information and inspiration to earth scientists. Growth of the Continents Through Time Much research remains to be undertaken to test and modify the simple picture of continents assembled by the process of arc collision and modified by the cordilleran, continental collisional, impact, hotspot, rifting, flooding, and erosional processes. Establishment of the history of the continents through time will provide an important test of how they have evolved. Additions to the continental crust are known to have occurred throughout recorded geological time, from about 4-billion-years ago to the present. With the average age of continental surface rocks around 2-billion-years, and with a wide variety of ages spanning most of recorded time, it seems possible that the continental crust has grown in volume through the history of the Earth. However, the details of this growth are still uncertain, and determination of a crustal volume versus age curve is of considerable importance for understanding the Earth's evolution. There is geological and geochemical evidence suggestive of periods of enhanced crustal growth, although this picture may be clouded by the geographically patchy distribution of the data. Large volumes of continental crust give isotopic signatures indicating that the material from which they formed became fractionated from the mantle (by the processes of partial melting that take place at divergent plate boundaries and beneath volcanic arcs) relatively recently. For example, much of the crystalline basement of Arabia, Egypt, and eastern Sudan formed from the mantle some 600 million to 900-million-years ago. Estimates of the present-day rate of crustal addition in island arcs fall short of the rate of average growth of the continents (about 2 km3/year, a figure obtained by dividing the present volume of the continents by the age of the Earth). If island arc addition has been the main way of making continents, rates must have been far higher in the past. We have a clear picture of the present distribution of continental material today in the large bodies of Eurasia, Africa, North and South America, Australia, and Antarctica and in smaller objects like Greenland, New Zealand, Madagascar, Japan, and the Seychelles. The motions of these fragments over the past 200-million-years since Pangea began to break up are reasonably well known from the history of the intervening ocean basins. Some idea of how continental fragments were assembled into Pangea, between the assembly of Gondwanaland about 600-million-years ago and its final collision with Laurasia about 290-million-years ago, has also emerged, but no clear picture has yet been obtained of how continental material was distributed about the surface in earlier times. Ancient latitudinal indicators for these older times have provided a confused picture. Determining when and how all the pieces of all the continents were formed and how they came to be in their present positions is proving a substantial research exercise. It looks as though the greater part of North America was assembled into one piece by 1.7-billion-years ago, but Asia is a continent put together only within the past 600-million-years. We do not yet know whether these temporal and geographic differences record stochastic operation of plate tectonic processes or whether they reflect systematic changes in the Earth's behavior in space and time. The central part of North America—with perhaps 75 percent of the present continental area—was put together long ago. Establishing a subsequent history dominated by the peripheral addition of relatively small exotic blocks and fragments is an active research frontier. Questions such as where the blocks came from and how they were incorporated into North America are rendered more challenging
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Solid-Earth Sciences and Society by the violent changes wrought on these objects at and after collision. Only the western side of North America has wholly escaped episodes of collision by major continents over the past billion years. These huge collisions and subsequent ruptures have made the story even harder to unravel in such areas as the Canadian Arctic, the Appalachians, and the Ouachitas. One intriguing idea is that the cycles of ocean opening and closing that have operated throughout earth history have caused the continents to have episodically come together to form a huge single landmass. After a few hundred-million-years, these supercontinents have been torn apart into fragments that drifted away from each other, much as the modern continents were dispersed from Pangea. One hypothesis for why this occurs is that the cold thick continental fragments are pushed away from areas of active upwelling of the underlying mantle, much as the skin on a pan of heated milk moves away from violently boiling areas. Eventually, this retreat from hot mantle brings the continental fragments together where they collide and weld again. At a certain size, however, the thick continent hinders heat transport out of the mantle that underlies it. Eventually, the underlying mantle heats up to a point at which it forms uprising plumes, which, if of sufficient size, break through the overlying continent. If continued, this plume activity evolves into a general upwelling of the mantle, which again causes the breakup of the supercontinent and the formation of an ocean basin. Determining long-term feedback relations between the elements of mantle convection, including plate formation and subduction as well as mantle plumes, and the elements of surface geology, including sea level change, continental assemblies, and continental rupture, is a major interdisciplinary challenge. Regional geology, mantle geophysics, and geochemistry need to be used together in a new way. The identification of cold regions in the mantle, interpreted as subducted slabs over 100-million-years old, marks an important step in this new direction. GEOCHEMICAL CYCLES Only some 20 to 25 years ago was it realized that surface materials could be returned to the Earth's interior through the process of plate subduction. The importance of recycled crustal materials in modifying mantle composition has been revealed only in the past 5 to 10 years. The consequences of crustal recycling and the involvement of crustal material in mantle convection are still being worked out (Figure 2.15). The crustal material includes water, carbon dioxide, and elements that are significant in the biogeochemical cycles occurring at and near the surface, and the interchange between biogeochemical cycles and the deep interior needs to be established. Cycling of material from the reservoirs of the deep interior through the Earth's surface systems typically occurs on time scales ranging from tens of millions to billions of years, but cycles in the near-surface systems driven by solar energy can be as short as a year or as long as hundreds of millions of years. Interchanges have to be recognized as happening on a large range of time scales. With realization of the significance of the interchange, it has become clear that the Earth as a whole—its interior, crust, oceans, and atmosphere—all interact through chemical cycling. To understand the characteristics of one reservoir, the nature of the exchange with the other components of the earth system must be understood and taken into account. The near-surface cycles are dealt with in Chapters 3 and 4. Here, emphasis is on geochemical cycles in the mantle and crust with reference to their connections with the hydrosphere and atmosphere. Extraction of the continents from the mantle has left an identifiable chemical imprint on the interior, causing the mantle, as sampled by the basalts erupted along the worldwide ocean ridge system, to be depleted of the same elements as those by which the continental crust is enriched. Isotopic data for oceanic basalts erupted throughout earth history show that the composition of their mantle source was modified early in earth history. The original modification could have been caused by the extraction of continental crust prior to 4.0-billion-years ago. Significant volumes of continental crust of this age, however, are not preserved. If the early chemical differentiation of the mantle was caused instead by removal of a crust more like oceanic than continental crust, it is possible that this ancient crust was subducted into the mantle and is no longer visible at the surface. An additional increment of chemical modification of the mantle would then have occurred when the majority of continental crust was formed, roughly 2.5 billion to 3-billion-years ago. This second step in chemical differentiation of the mantle, in fact, is observed in the isotopic record of mantle-derived rocks and crustal sediments. One of the major recent achievements of mantle geochemistry has been the identification of recycled surface materials as an intrinsic part of the mantle. The first major consequence of crustal cycling is its
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Solid-Earth Sciences and Society FIGURE 2.15 Schematic diagram illustrating plate recycling. instigation of volcanism landward of subduction zones. Arc volcanic rocks display a chemical signature best explained by the contributions of subducted oceanic sediments and altered oceanic crust to their source regions in the mantle. The recent discovery of an isotope of beryllium in arc volcanoes, but in no other volcanic system on Earth, provides clear evidence that even the upper few meters of sediment on the ocean floor are transported to a depth of at least 150 km in subduction zones, the depths at which the processes of dehydration and perhaps partial melting initiate the generation of arc magmas. There is a direct link with cycles in the atmosphere and hydrosphere. The radioactive isotope, 10Be, is transferred from the upper atmosphere by rain and becomes concentrated in ocean sediments. The fact that it decays relatively rapidly places constraints on the time interval between raining from the atmosphere, subduction in sediments, and emergence at the surface in volcanic lavas. A distinctive feature of arc lavas is their relatively high water content, which is largely derived from the rocks of the subducted oceanic plate. The combination of experimental-phase equilibrium studies on source rocks and lavas with geophysical modeling of temperatures at depth confirms that most of the subducted water is removed by dehydration or melting by a depth of 150 km or so, whence it returns to the surface for involvement in the shallow cycles. However, under some conditions there is opportunity for small amounts of water to escape the magmatic processes and to be transported to greater depths for long-term storage within the mantle. Recent theoretical and experimental studies at very high pressures have led to proposals that the lower mantle, deeper than 700 km, may contain water in significant quantities: at least 0.3 percent by
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Solid-Earth Sciences and Society weight, the equivalent of about two masses of ocean water. The recent synthesis of iron hydride at high pressures corresponding to conditions within the Earth's core has raised the question of how much hydrogen and oxygen may be present within the core. Thus, water may be involved in earth processes all the way from the core to the atmosphere. Another example of the depth of penetration of subducted material is provided by diamonds. Diamonds crystallize only at depths greater than about 150 km in the mantle. A significant fraction of diamonds have distinct carbon isotopic compositions, like that found for organic carbon-rich sediments at the surface. Thus, some diamonds may once have been living matter at the surface that was subducted with an oceanic plate to depths where the elevated pressure and temperature turned the dispersed carbon into diamond. Small, but observable, heterogeneities in the chemical and isotopic composition of the mantle have been documented by detailed studies of the basalts erupted in the ocean basins. The important feature of this heterogeneity is that it appears to reflect mixing between a depleted residual mantle, formed by extraction of continental and oceanic crust, and two to three other more "enriched" components. Two of the enriched components are believed to be basaltic ocean crust and continentally derived sediments. These materials are injected into the mantle in subduction zones; are carried along by the general mantle circulation, during which time they partially mix with surrounding mantle; and eventually return to the surface beneath ocean ridges and in mantle plumes. Estimates of the time necessary to travel from subduction zone to ridge are on the order of 1.7-billion-years. The complete implications of this process represent an area of active research that links geochemical and seismic observations of the mantle with theoretical treatments of the nature of mantle convection and the efficiency with which convective "stirring" can rehomogenize the injected components. What was once viewed as a one-way transfer of material from the mantle to the crust must now be seen as a continuation of global geochemical cycles, in this case including all of the planet. The ability to recycle surface components back into the mantle may be responsible for keeping the Earth's interior unexpectedly close to its original composition. Recycling of crust may be unique to the Earth as a consequence of its active plate tectonic system and may explain its continuing vigor. Some of the key questions in chemical geodynamics or deep geochemical cycling include: What is the mass balance of the cycling? Has all of Earth's interior once been at the surface, or is some material reaching the surface for the first time? Can the age and geographical distribution of returning recycled oceanic crust be used to trace flow patterns and velocities in the mantle and provide clues for the locations of past subduction? What is the balance of fluid transport? Are the oceans growing with time, or is water being subducted into the mantle faster than it is emitted at ocean ridges? What fraction of the water in arc volcanism is from the mantle, and what fraction is recycled from the oceans? Is subduction responsible for continent formation? Are an active plate tectonic system and a liquid ocean necessary prerequisites for the development of Earth-like continents on a planet? How has recycling of chemically differentiated material from the Earth's surface affected the composition of the mantle? Is the Earth still an active planet because its attempts at internal chemical differentiation were reversed by the rehomogenization accompanying recycling and convective mixing? Key areas for understanding deep geochemical cycles, and their connections with the near-surface biogeochemical cycles, are the two tectonic environments where most material enters the interior and where most material emerges from the interior: subduction zones and oceanic ridges. Detailed multidisciplinary investigations of the fluxes of energy and matter in these and related environments (e.g., continental rifts) are of fundamental importance. Major parts of the biogeochemical cycles involve material transferred relatively rapidly in and out of the biosphere and material transferred between rocks and the fluid envelopes during weathering processes. INTERACTION BETWEEN THE SOLID EARTH AND ITS FLUID ENVELOPES The surface water of the Earth and to a lesser extent the atmosphere play a critical part in the recycling of material that is one of the planet's most distinctive features. The outer layer of solid-Earth is in contact with the fluid envelopes of the hydro-
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Solid-Earth Sciences and Society sphere and the atmosphere. Seventy-one percent of the solid surface is innundated by ocean water, and the remaining 29 percent, the continental surface, is flushed by water at intervals with a frequency governed by climatic zone. The crust can be thought of as a catalytic bed of minerals and fluids of great diversity consisting of several trillion square kilometers of surface area. Most of this surface consists of the interface between minerals and water. The reactions that occur at these interfaces, some of which are catalytic in nature, directly affect the planet we live on and the way we live. They play critical roles in determining the quality of our fresh water supply, the development of soils and the distribution of plant nutrients within them, the genesis of certain types of ore and hydrocarbon deposits, and the geochemical cycling of elements. Mineral-water interface geochemistry is not a new field. For example, observations relevant to silicate mineral dissolution were made more than 150 years ago. However, it was not until the past 10 or 20 years that the instrumental means were developed to directly study mineral surfaces and the reactions that occur at mineral-water interfaces at the molecular level. Much of the technology needed has been provided by developments in the field of surface science, which has traditionally been in the domains of chemistry (heterogeneous catalysis, electrochemistry) and applied physics (semiconductors and integrated circuits). The transfer of this technology by geochemists is relatively recent (within the past few years), but it is already leading to new fundamental knowledge about how minerals dissolve and undergo reduction or oxidation, how chemical species partition from fluids to mineral surfaces, and how the hydrosphere interacts with crustal rocks. An immediate application of this knowledge to a problem of societal relevance is the development of more robust and accurate models for predicting the transport of contaminants in groundwater. Certain contaminants can be strongly chemisorbed on mineral surfaces under certain conditions, thus removing them from the fluid phase. However, we must understand the molecular-level mechanisms for such reactions to model them properly; incorrect assumptions about the stoichiometry of sorption reactions can lead to errors of several orders of magnitude in predictions of the partitioning behavior of chemical species. Errors of this magnitude simply cannot be tolerated in predicting how a contaminant plume disperses. Although the field of interface geochemistry is in its infancy, it is already leading to changes in the way we think about many geochemical and mineralogical processes. Continuing studies in this rapidly growing field will undoubtedly lead to a more fundamental understanding of the chemistry and physics of mineral-water interfacial phenomena and how chemical species are partitioned between minerals and aqueous fluids in the crust. RESEARCH OPPORTUNITIES The Research Framework (Table 2.1) summarizes the research opportunities identified in this chapter and in the relevant panel reports, with reference also to other disciplinary reports and recommendations. These topics, representing significant selection and thus prioritization from a large array of research projects, are described briefly in the following section. Processes operating near the surface are, for the most part, reserved for Chapter 3, although there is no sharp boundary between the deep-seated processes and surficial geology. The research areas are interrelated. There is continuity between the processes occurring at the core-mantle boundary, mantle convection, the physical deformation of the lithosphere and mountain building in the crust, and the geochemical transfer of material from mantle to crust. In the crust the material is exposed to physical and chemical interaction with the atmosphere, the oceans, and the hydrosphere, which transgresses the two fluid envelopes and the crust. Research opportunities arise when methods are developed to explore new regions, such as the inaccessible ocean floor, continental lower crust, and the even more inaccessible deep interior. Research Area II: Global Geochemical and Biogeochemical Cycles Evolution of the Crust and Its Relationship to the Mantle Examination of the Earth's crust through the use of isotopes, trace elements, and rare gases continues to be a fundamental frontier topic. Study of the growth rates of the continents can provide a long-term view of the Earth's evolution, and these rates should be investigated by a concerted effort that couples detailed field studies with high-precision geochronology and isotopic studies to determine mantle separation ages for different continental terrains. Within the continental rocks lies an historical record of mantle convection, changes in
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Solid-Earth Sciences and Society TABLE 2.1 Research Opportunities Objectives Research Areas A. Understand Processes B C D I. II. Geochemical and Biogeochemical Cycles ■ Evolution of the crust and its relationship to the mantle ■ Fluxes along the global rift system ■ Fluxes at convergent plate margins ■ Mathematical modeling in geochemistry III. Fluids in and on the Earth ■ Mineral-water interface geochemistry ■ Role of pore fluids in active tectonic processes ■ Magma generation and migration IV. Dynamics of the Crust and Lithosphere ■ Oceanic lithosphere generation and accretion ■ Architecture and history of continental rift valleys ■ Sedimentary basins and continental margins ■ Continental-scale modeling ■ Recrystallization and metasomatism of the lithospheric mantle and lower crustal metamorphism ■ Thermal structure, physical nature, and thickness of the continental crust ■ The lithosphere at convergent plate boundaries ■ Tectonic and metamorphic history of mountain ranges ■ Quantitative understanding of earthquake rupture ■ Rates of recent geological processes ■ Real-time plate movements and near-surface deformations ■ Geological predictions ■ Modern geological maps V. Dynamics of the Core and Mantle ■ Origin of the magnetic field ■ Core-mantle boundary ■ Imaging the Earth's interior ■ Experimental determination of phase equilibria and the physical properties of Earth ■ Chemical geodynamics ■ Geodynamic modeling Facilities - Equipment - Data Bases ■ Remote sensing of the continental unit from satellites ■ Global digital seismic array, with broadband instruments ■ Portable seismic arrays ■ Ion microprobes ■ Accelerator mass spectrometers ■ Ultrahigh pressure temperature instrumentation ■ Microscale, in situ analytical instrumentation ■ Synchrotron radiation facilities ■ Ocean-bottom seismometers and other geophysical instruments ■ Instruments for micromagnetic measurements ■ Deep continental drilling ■ Parallel-processing computers ■ Data storage and distribution facilities crustal growth and destruction by recycling, temporal variations in the temperatures in the interior, variability in the nature of the plate tectonic process, and the compositional evolution of the mantle. Fluxes Along the Global Rift System The rift system transports energy and material from the interior to the lithosphere, hydrosphere, and biosphere. An important goal of research in
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Solid-Earth Sciences and Society this area is understanding the geophysical, geochemical, and geobiological causes and consequences of this transport. The flow of mantle material, the generation of melt, its emplacement along spreading centers, and its transformation into crystalline oceanic crust are primary problems. Aspects relevant to magmas at hot-spots beneath oceans and continents are a significant related concern. Fluxes at Convergent Plate Margins Convergent plate boundaries are the sites of material transport from the surface to its interior. It is necessary to determine the relevant mass and heat fluxes, treating the generation and flow of vapors and melts as part of a dynamical system. Mathematical Modeling in Geochemistry Practically all theoretical treatments of geochemical cycles use overly simple models, with each box representing a chemically homogeneous reservoir and the transfer of material being represented by fluxes between boxes. Intense efforts are required to improve theoretical modeling. There is an urgent need to determine the connections between the relatively near-surface short-term biogeochemical cycles covered in Chapter 3 and the longer-term geochemical cycles extending deep into the Earth's mantle. Research Area III: Fluids in and on the Earth Mineral-Water Interface Geochemistry This rapidly growing field uses technology transferred from the domains of surface chemistry and physics (catalysis and semiconductors). Continuing studies in this field will lead to a more fundamental understanding of the chemistry and physics of mineral-water interfacial phenomena, how chemical species are partitioned between minerals and aqueous fluids, and how the hydrosphere interacts with crustal rocks. Role of Pore Fluids in Active Tectonic Processes The fluid state may be most important in understanding the mechanics of deformation, including earthquakes, and insight can be gained from laboratory experiments of rock fracture and rock friction with fluids present. A program of continental drilling would greatly add to our empirical knowledge concerning pore fluids. Magma Generation and Migration Magma generation and migration are fundamental processes. Their analysis needs the coupling of phase equilibria and trace element partitioning data to fluid dynamical equations, with deformable solid matrices. Magma study provides research opportunities ranging from the thermodynamics and geochemistry of rocks in the molten state, through their role in chemical differentiation of the Earth, to their manner of emplacement in and on the crust. The physical processes associated with upward migration of the magma and the mechanisms of eruption are only partially understood. The transition from micro- to macropermeability is relevant. Magma-driven fractures are also likely to play an important role. Fractures are amenable to being characterized as fractal, and fracture geometrics is a promising area of active research. Research Area IV: Dynamics of the Crust and Lithosphere Oceanic Lithosphere Generation and Accretion Continued interdisciplinary studies of ocean ridges, with their associated magmatic processes and interaction with the hydrosphere, promise excellent scientific returns. The global correlation of ocean ridge basalt geochemistry with axial depth and crustal thickness resulting from temperature variation in the mantle is an example of recent interdisciplinary successes. Patching together diverse results from three decades of geophysical surveys and geochemical analyses has led to the formulation of a model for the detailed structural and magmatic segmentation of mid-ocean ridge spreading centers. Architecture and History of Continental Rift Valleys Better comprehension of continental separation and the formation of continental margins, where natural resources are commonly concentrated is needed. There has been a revolution in our understanding of extensional tectonics with the discovery of basement-penetrating normal fault systems. Determination of the rheology of the lithosphere requires deep seismic profiling and laboratory experiments on rock strength. Isotope and trace element
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Solid-Earth Sciences and Society analysis of rift valley magmas and xenoliths will test rift models deduced from independent data sets. Sedimentary Basins and Continental Margins Gravity data for many sedimentary basins, including both foreland and cratonic basins, indicates the presence of buried loads. The origin of these loads and their relation to thermal subsidence remain unexplained challenges. We need a better way of estimating the original masses of ancient sediments. Also, to improve modeling we need a much better inventory of sediments beneath present-day continental margins. Continental-Scale Modeling This area of computational tectonophysics has great potential. Its benefits would include the testing of qualitative hypotheses for orogenies, the discovery of new orogenic mechanisms (e.g., delamination, Moho buckling), and in situ determinations of the rheology of the lithosphere. Dynamical systems approaches (fractals, chaos) offer much promise for attacking major unsolved problems in crustal dynamics. Recrystallization and Metasomatism of the Lithospheric Mantle and Lower Crustal Metamorphism Intensive study should integrate the full complement of isotopic and trace element tools integrated with field work, geophysics, the study of deep crustal/upper mantle xenoliths—''meteorites from the mantle"—and the calibrations of experimental petrology, in terms of depth, temperature, and fluid compositions. Thermal Structure, Physical Nature, and Thickness of the Continental Crust These are key variables governing the state of stress and attenuation of seismic energy in the crust. Study of the ancient crust may help to establish the history of the thermal structure of the Earth. A fundamental area of research in active tectonics is the source, distribution, and propagation of the energy that drives tectonic changes. Heat flow studies, in situ stress measurements, and measurements of strain are among the elements in need of study. The opportunities range from field measurement of these elements to theoretical modeling of how they interact. The Lithosphere at Convergent Plate Boundaries Multidisciplinary studies of this environment are essential for understanding mantle dynamics and geochemistry as well as the chemical differentiation of the Earth. Tectonic and Metamorphic History of Mountain Ranges A new dimension, time, has recently been added to the depth-temperature framework provided by experimental petrology. Isotopic analyses yield ages for different stages of mineral growth, and another approach to "time" involves geophysical calculations on the thermal history of rocks. Quantitative Understanding of Earthquake Rupture The physical processes applicable to earthquake rupture can be approached by integrated seismological, geophysical, and geological studies of earthquakes and the faults on which they occur. Direct measurement of stress by drilling techniques for accessible faults is now feasible. Improvements in three-dimensional imaging capabilities are needed to map subsurface faults that are often nonplanar. Rates of Recent Geological Processes New insights into the rates at which processes take place can be expected, especially for those occurring over the less-than-1-million-year time scale. New and improved techniques for dating materials and events offer rewarding opportunities. There should be extensive dating of Quaternary landforms and sediments. Real-Time Plate Movements and Near-Surface Deformations The Global Positioning System (GPS), which is rapidly evolving into a powerful tool, is used to monitor these motions. Geodetic techniques, based on land and in space, provide some of the best direct evidence about short-term changes in the crust. Geological Predictions The revolutionary field of geological prediction in geological evolution within 1-million-year time frames requires increasing emphasis on surficial geology and neotectonics. The documentation and
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Solid-Earth Sciences and Society understanding of rapid geological processes are an entirely new dimension for earth scientists to explore, with computers now making it possible. Modern Geological Maps Maps constitute an important data base in the solid-earth sciences. Field relations must be continually reexamined in the light of new theoretical concepts, and no substitute exists for continuing geological mapping and analysis of relations in the field. Maps should include three-dimensional data on geophysics and geochemistry and data from satellite-based remote sensing. Digitizing the different data sets that are to be used together is essential. Research Area V: Dynamics of the Core and Mantle Origin of the Magnetic Field Magnetic field generation is one of the universal processes in the cosmos, and the outstanding unsolved geophysical problem involving the core is generation of the geomagnetic field. Important advances in the near future will concentrate on more limited problems such as (a) the origin of the dipole inclination, secular variation, and the westward drift; (b) the role of the mantle in influencing magnetic field structure; and (c) a determination of the power source driving the dynamo. Core-Mantle Boundary With the deployment of a new broadband digital network of seismometers, the likelihood of deciphering the nature of the core-mantle boundary is excellent. There is the prospect that geomagnetic anomalies can be associated with seismological heterogeneities found at the base of the mantle, leading to the possibility of documenting changes in the core-mantle boundary through the geological past. Imaging the Earth's Interior The new digital recording seismometers with broad wavelength sensitivity and large dynamic range include both portable varieties and permanent stations that will be applicable to global studies. The three-dimensional distribution of velocity anomalies in the mantle obtained through these data can then be used to infer relative temperatures and compositions within the mantle. Experimental Determinations of Phase Equilibria and the Physical Properties of Earth Materials at High Pressure and Temperature New high-pressure apparatus extends the range of experimentation. Properties of materials (e.g., density, seismic velocity, melting temperature) can be measured in situ using, for example, high-intensity x-rays produced by synchrotron sources. Comparison of the high-resolution seismic images of the interior with direct experimental determinations of the physical properties of earth materials at high-pressure and temperature will advance the understanding of the interior's temperature and compositional structure to an unprecedented degree. This can yield better insight into how the high-temperatures drive internal motions that in turn determine the geological history of the Earth's surface. Chemical Geodynamics The combination of geochemistry and geophysics continues to reveal the scale of heterogeneities within the mantle. Isotopic variations in mantle-derived rocks provide time-dependent information about the creation of mantle heterogeneities by partial melting or lithosphere subduction and about the efficiency of convection in remixing the mantle components. The nature and source of the mantle plumes responsible for generation of at least some volcanic hot-spots remain a tantalizing problem, one that may link phenomena at the core-mantle boundary to massive volcanic eruptions. Geodynamic Modeling The recent success in quantitatively relating geoid anomalies and the results of seismic tomography, at least on scales of thousands of kilometers, prompts similar investigations over smaller distances. The amount of material transported across the entire mantle by convection is currently the single largest uncertainty in our understanding of the Earth's thermal and geological evolution. This transport is part of the major geochemical cycles of the Earth. FACILITIES, EQUIPMENT, AND DATA BASES The earth sciences offer special opportunities for the development and application of new technologies, as exemplified by the instrumentation that has recently been created for use in the field and in the
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Solid-Earth Sciences and Society laboratory. Technological developments in the earth sciences are at the forefront of research. The emphasis in geochemical instrumentation continues to be on attaining higher spatial resolution while maintaining high sensitivity and accuracy in isotopic and trace element analyses. One of the greatest needs in the field of isotope geochronology is to bring more ion microprobe instruments into operation. New generations of instrumentation include the super high-resolution ion microprobe (SHRIMP) and the x-ray microprobe. For cosmogenic nuclide geochronology the major goal is a better understanding of variations in production rates. Accelerator mass spectrometer measurements are especially well suited to studies of the interrelationships among the solid-earth, the atmosphere, and the hydrosphere. The development of new ultra-high-pressure/ temperature instrumentation for simulating the deep interior remains a major goal. This high-pressure research requires additional development of techniques for microscale in situ analysis of small samples or of physical properties (e.g., elastic constants). Continued access to synchrotron radiation facilities is important for precise crystallographic information as well as compositional data. Ocean bottom deployment of geophysical instruments is an area generally in need of technological development. Until this becomes routine, achieving global coverage from satellite measurements will continue to be a major problem. High-sensitivity, high-spatial-resolution analytical techniques are in demand for carrying out micromagnetic measurements on minerals and rocks as functions of temperature and field. The increasing availability of massive computational capabilities provides opportunities for attacking complex problems. Parallel-processing computers hold much promise for a wide variety of simulations in the earth sciences. Less powerful special-task computers are seeing increasing applications in the field and the laboratory. There is an urgent need to improve the capabilities for communications and storage of data. Networking between computers will be necessary, particularly for large-scale data transfer and high-speed interactive computing. BIBLIOGRAPHY National Research Council Reports NRC (1980). Studies in Geophysics: Continental Tectonics, Geophysics Study Committee, Board on Earth Sciences and Resources, National Research Council, National Academy Press, Washington, D.C., 197 pp. NRC (1982). Studies in Geophysics: Climate in Earth History, Geophysics Study Committee, Geophysics Research Board, National Research Council, National Academy Press, Washington, D.C., 198 pp. NRC (1983). Opportunities for Research in the Geological Sciences, Committee on Opportunities for Research in the Geological Sciences, Board on Earth Sciences, National Research Council, National Academy Press, Washington, D.C., 95 PP. NRC (1983). The Lithosphere: Report of a Workshop, U.S. Geodynamics Committee, Board on Earth Sciences, National Research Council, National Academy Press , Washington, D.C., 84 pp. NRC (1984). Studies in Geophysics: Explosive Volcanism: Inception, Evolution, and Hazards, Geophysics Study Committee, National Research Council, National Academy Press, Washington, D.C., 176 pp. NRC (1986). Studies in Geophysics—Active Tectonics, Geophysics Study Committee, Board on Earth Sciences and Resources, National Research Council, National Academy Press, Washington, D.C., 266 pp. NRC (1987). Earth Materials Research: Report of a Workshop on Physics and Chemistry of Earth Materials, Committee on Physics and Chemistry of Earth Materials, Board on Earth Sciences, National Research Council, National Academy Press, Washington, D.C., 122 pp. NRC (1987). Geologic Mapping in the U.S. Geological Survey, Committee Advisory to the U.S. Geological Survey, Board on Earth Sciences, National Research Council, National Academy Press, Washington, D.C., 22 pp. NRC (1988). The Mid-Oceanic Ridge: A Dynamic Global System, Ocean Studies Board, National Research Council, National Academy Press, Washington, D.C., 351 pp. NRC (1989). Margins: A Research Initiative for Interdisciplinary Studies of Processes Attending Lithospheric Extension and Convergence , Ocean Studies Board, National Research Council, National Academy Press, Washington, D.C., 285 pp. NRC (1990). Studies in Geophysics: The Role of Fluids in Crustal Processes, Geophysics Study Committee, Board on Earth Sciences and Resources, National Research Council, National Academy Press, Washington, D.C., 170 pp. NRC (1990). Studies in Geophysics: Sea Level Change, Geophysics Study Committee, Board on Earth Sciences and Resources, National Research Council, National Academy Press, Washington, D.C., 234 pp. NRC (1991). International Global Network of Fiducial Stations: Scientific and Implementation Issues, Committee on Geodesy, Board on Earth Sciences and Resources, National Research Council, National Academy Press, Washington, D.C., 129 pp. NRC (1991). Opportunities in the Hydrologic Sciences, Water Science and Technology Board, National Academy Press, Washington, D.C., 348 pp. Other Reports Basaltic Volcanism Study Project (1981). Basaltic Volcanism on the Terrestrial Planets, Pergamon Press, Inc., New York, 1,286 pp. Interagency Coordinating Group for Continental Scientific Drilling (1988). The Role of Continental Scientific Drilling in Modern Earth Sciences Scientific Rationale and Plan for the 1990's, 151 pp. Interagency Coordinating Group for Continental Scientific Drilling (1991). The United States Continental Scientific Drill
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Solid-Earth Sciences and Society ing Program, Third Annual Report to Congress, 35 pp. + appendices. Inter-Union Commission of the Lithosphere (1990). International Lithosphere Program, International Council of Scientific Unions, 119 pp. NASA (1991). Solid-Earth Science in the 1990s, Vol. 1. Program Plan, NASA Office of Space Science and Applications, Washington, D.C., 61 pp. USGS (1987). Volcanism in Hawaii, U.S. Geological Survey Professional Paper 1350, R. W. Decker, T. L. Wright, and P. H. Stauffer, eds., U.S. Government Printing Office, Washington, D.C., 1,667 pp. USGS (1990). The San Andreas Fault System, California, U.S. Geological Survey Professional Paper 1515, R. E. Wallace, ed., U.S. Government Printing Office, Washington, D.C., 283 pp.
Representative terms from entire chapter: