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2 Earth’s Interior A s planets age they slowly evolve as the heat tion 6 deals with Earth materials properties, which trapped and generated in the interior is trans- control many of the internal processes discussed in ported to the surface. The internal planetary this chapter. processes that move this heat—including volcanism and convection—have a huge influence on the nature of QUESTION 4: HOW DOES EARTH’S planetary surfaces. Yet the vast interior is inaccessible to INTERIOR WORK, AND HOW DOES IT direct study and must be understood with geophysical AFFECT THE SURFACE? observations, experimental studies of materials under deep-Earth conditions, and theoretical models. For The previous chapter discussed evidence that Earth over a century, seismic wave, geomagnetic, and gravity and the Moon, and by extension the other terrestrial measurements made at the surface have been improv- planets, started out with high internal temperatures ing our characterization of Earth’s internal structure. about 4.5 billion years ago. Once the planetary accre- Experimental and theoretical determinations of mate- tion process tails off, the planets cool, first through a rial properties at high temperatures and pressures and period of active geological processes and ultimately to numerical modeling of mantle and core heat transport a state of geological quiescence. When the planet is and convection over very long timescales also play key geologically active, evidence of that activity is reflected roles in studies of internal dynamics. However, despite in the nature of its surface and atmosphere and perhaps continuing advances, we still cannot uniquely describe the existence of a magnetic field. After the interior Earth’s mantle structure or explain in any detail how cools and its viscosity increases sufficiently, geological the core and mantle work, why Earth differs from other activity grinds to a halt, and the planet’s surface stops planets, or how it may change in the future. regenerating. Thereafter, only external processes, such The three questions included in this chapter de- as bombardment with asteroids, further modify the scribe scientific challenges for understanding Earth’s surface. evolution and internal dynamics. Question 4 addresses Some planetary bodies, like the Moon, cooled deep-Earth dynamical processes, from the inner metal- quickly and have been geologically inactive for bil- lic core at the center of Earth to the convecting mantle lions of years. Despite rapid cooling after the Moon- to the volcanoes at the surface. Question 5 focuses on forming impact (Questions 1 and 2), Earth produced the near-surface features of Earth—old continents, and retained enough heat to power geological activity young ocean basins, and plate tectonics—that make until the present, and it is likely to do so for several Earth unique among Solar System planets and that also billion more years. However, both the amount of seem inextricably linked to the presence of water and Earth’s cooling and resulting changes in the internal the preservation of life-sustaining conditions. Ques- dynamics and surface environment are still poorly 35

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36 ORIGIN AND EVOLUTION OF EARTH known. Although we know that heat is transported by mantle and then sinks again in a turbulent pattern that mantle convection, we do not yet have the capability is affected by rotation and the magnetic field the flow to exactly describe these convective patterns, calculate generates. By contrast, mantle motions are ponderous. with confidence how different they were in the past, or Typical velocities are about 5 cm/yr (based on geodetic, predict how they will change in the future. Resolving magnetic, seismic, and geological measurements), and the critical questions about planetary evolution will at this rate the nominal “round-trip” journey of a mantle require much more advanced knowledge of planetary wide convection cell—across the surface for 5,000 km, materials and how they affect convection (Question down 2,900 km to the bottom of the mantle, and back 6), better constraints from seismology on the present to the surface again—would take about 300 million configuration of mantle flow at both large and small years. This rate of travel is consistent with simple ther- scales, and significant advances in mathematical mod- mal convection models that treat the mantle as if it were eling of convection that is driven by both temperature a liquid with a viscosity (estimated from postglacial and chemical variations. rebound rates) of about 1021 Pa-s. The configuration of convection in Earth’s mantle provides the primary Convection and Heat Flow control on how Earth cools, mainly because the mantle makes up roughly two-thirds of Earth’s mass and 85 About 43 TW (1012 J/s) of heat flows from Earth’s percent of its volume (Figure 2.1). interior through its surface at present, based on global Mantle motions carry hot material from deep heat flow measurements and thermal models for cool- inside Earth toward the surface, where heat is lost to ing oceanic lithosphere. Sources of this surface heat the atmosphere and ultimately to space, and also carry flow include the slow cooling of the mantle and core cold surface rocks down to great depths. Unresolved over the history of the planet; heating produced by issues concerning mantle convection arise from un- radioactive decay of U, Th, and K; and minor sources certainties about material properties at high pressures such as tidal heating. The exact contribution of each and temperatures. Experiments and field evidence to the planet’s heat flow is uncertain. For example, we show that mantle rock becomes soft enough to flow do not know how much U, Th, and K are contained over geological time periods at depths of just 30 to 60 inside Earth and how these elements are distributed km, where the temperature surpasses 700°C and pres- (McDonough, 2007). These elements are more effec- sure reaches several thousand atmospheres. At higher tive at keeping Earth hot if they are located deep within temperature—above 1200°C—the viscosity of mantle the mantle, or even to some degree in the core, rather rock is low enough that it behaves much like a thick than near the surface. As a result of these uncertainties, liquid; almost all of the mantle is hotter than 1200°C. we cannot yet answer the simple question: How fast is Mantle viscosity exerts the primary control on the form Earth cooling? of convection and the efficiency at which heat is moved The primary mechanism for transporting heat toward Earth’s surface. However, other factors also are within Earth’s interior is convection. It was once be- important. For example, viscous dissipation associated lieved that mantle convection was impossible because with deformation of stiff lithospheric plates at subduc- the mantle was demonstrably solid. But much like a tion zones strongly affects the form of convection and glacier, the mantle can behave like both a brittle solid the relationship between convective vigor and surface and a liquid: it fractures when deformed rapidly but heat flow. The largest uncertainties are for the lower flows on long timescales. We now know that both mantle. Seismological data suggest that the flow pat- the mantle and the outer core circulate in a complex tern there is complex. Other observations suggest that pattern of large- and small-scale flows. In the molten viscosity increases in the lower mantle, and numerical outer core, which has very low viscosity (some estimates models indicate that flow velocities in the lower mantle suggest a value similar to that of liquid mercury), con- may be much slower than plate velocities such that the vection is rapid. Hot liquid metal circulates up to the overturn time is a billion years or more (Kellogg et al., top of the core where it loses heat to the base of the 1999; Ren et al., 2007).

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EARTH’S INTERIOR 37 FIGURE 2.1  Cutaway view of Earth’s interior showing major layers (oceanic and continental crust, upper mantle, lower mantle, outer core, inner core) and features (mantle plumes, subduction zones, midocean ridges, convection currents, magnetic field). SOURCE: Lamb and Sington (1998). Copyright 1998 Princeton University Press. Reprinted with permission.

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38 ORIGIN AND EVOLUTION OF EARTH How Are Mantle Convection and Earth’s Thermal understanding is insufficient to explain many of the Evolution Related? most important geological and geochemical features of our planet. We know that mantle convection is driven by the heat We are even further from understanding the in- of Earth’s interior, but what controls Earth’s tem- ternal evolution of other rocky bodies of our Solar perature? The current understanding is that the mantle System, where we have fewer data, and interactions itself acts as Earth’s primary “temperature regulator,” between thermal evolution and orbital evolution pro- and its actions depend on the atomic-scale properties vide additional complications (see Box 2.1). Earth of mantle minerals that determine viscosity. The ef- (and possibly Venus) has apparently maintained a high fective viscosity of the mantle depends on the rate at enough internal temperature to ensure continued geo- which the mineral grains can deform in response to an logical activity. However, on smaller planetary bodies, applied stress, which in turn is strongly dependent on geological surface activity has either long since stopped temperature. Laboratory data indicate that for a given (Moon) or slowed greatly (Mars). It is believed that stress a 100°C temperature increase lowers the viscosity the mantles of other terrestrial planets should function by about a factor of 10. Consequently, if Earth were to in the same way as Earth’s, unless there are different heat up, it would convect more vigorously and lose heat amounts of radioactive elements or different amounts faster. As heat is lost, temperature drops and convection of water dissolved in the mantle minerals. The addi- slows, decreasing the rate of heat loss. This tempera- tion of tiny amounts of water to mantle minerals would ture-viscosity feedback should keep Earth’s internal lower both the viscosity of the mantle and the melting temperature well regulated. The temperature at which temperature (Question 6) and may prolong a planet’s the thermostat is most likely to be set is just below the geologically active life. melting point of mantle rock because there is an even faster decrease in viscosity with temperature once the What Do Mantle Plumes Tell Us About mantle begins to melt. Convection and Heat Transport? The temperature-viscosity feedback model is useful but it implies a steady system that undergoes only slow The viscosity of Earth’s mantle is sufficiently low changes over long periods of time. This implication and sensitive to temperature that convection can in- is at odds with much of what we know and suspect clude complex small-scale currents. Evidence of this about mantle materials and geological history. For small-scale convection is provided by hot spots—large example, the continents, which are an end product of clusters of volcanoes, the most active of which are in Earth’s evolution, show evidence of rapid growth spurts Hawaii, Iceland, the Galapagos Islands, Yellowstone, (Question 5), which may or may not be associated and Réunion (Indian Ocean). Hot spots are usually with accelerated plate tectonics (Hoffman and Bow- explained as the surface outpourings of magma formed ring, 1984). The seafloor of the western Pacific Ocean in mantle plumes, which are cylindrical upwellings of contains enormous volcanic mountain ranges, which hot (and hence low viscosity) rock that are thought to suggests that the Cretaceous Period (65 to 150 Ma) form near the base of the mantle and rise to the surface was a time of exceptionally intense volcanic activity at rates much faster than plate velocities (Figure 2.2). and possibly also fast seafloor spreading (Engebretson Mantle plumes should form as a consequence of heat et al., 1992). We also know that the Cretaceous was a entering the bottom of the mantle from the much hot- period of exceptional global warmth and high sea level ter outer core. (Question 7) and stability of Earth’s magnetic field. Mantle plumes may also be responsible for large These observations as well as theoretical considerations igneous provinces, which are vast basalt lava plateaus on raise the question of whether Earth’s thermal evolution continents and the ocean floor. The best current expla- and internal processes are adequately described by our nation is that they form when the bulbous top of a new (quasi-) steady state models or whether the evolution plume approaches Earth’s surface (Figure 2.2), then has been unsteady and punctuated by catastrophic re- spreads out and causes widespread melting (Ernst et configurations. Thus, even though we understand the al., 2005). These large, rapid lava outpourings may have most basic features of mantle convection, our level of caused major perturbations to Earth’s climate (Ques-

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EARTH’S INTERIOR 39 BOX 2.1 Planetary Comparisons Our understanding of our planetary neighbors has advanced substantially over the last several decades through spacecraft exploration and analysis of lunar samples and meteorites from Mars and the Moon. The other terrestrial planetary bodies (Venus, Mars, Mercury, and the Moon) formed by the same processes as Earth (see Question 1) and are governed by the same physical and chemical laws and processes. Nevertheless, each has taken a distinct evolutionary track, deepening the questions we pose for how Earth works the way it does. Venus, at 0.8 Earth masses, is sometimes called Earth’s “sister planet,” but its massive carbon dioxide atmosphere (90-bar surface pressure) and global cloud cover have led to a runaway greenhouse, a surface temperature of 470°C, and the loss of most water from the atmosphere. Venus also lacks Earth-like plate tectonics, but the planet has been subjected to resurfacing—probably by some form of lithospheric recycling not understood—at least once and perhaps multiple times. The density of impact craters indicates that the surface has an average age between several hundred million years and 1 billion years. There are mountain belts and pervasively deformed plateaus, both of which are stratigraphically older than the widespread volcanic plains, known to be basaltic from spacecraft lander measurements. Unlike Earth, Venus has no detectable internal magnetic field. A strong correlation of long-wavelength gravity and topography in the plains is the signature of ongoing mantle convection. Rifting and volcanism have occurred more recently than the average surface age, and the planet is likely to be volcanically and tectonically active at present. Mars, at 0.1 Earth masses, evolved more rapidly than Earth or Venus. Isotopic evidence from Martian meteorites indicates that Mars formed its core, mantle, and most of its crust within a few tens of millions of years after the beginning of Solar System formation, probably without any plate tectonics era. Large segments of the most ancient crust on Mars are strongly magnetized, relics of a core dynamo that began early in Martian history but probably died out after several hundred million years. The Martian surface has seen a mix of plains volcanism and more focused magmatism in regional centers, dominated by the Tharsis volcanic province, largely constructed before 4 Ga (billion years ago). Fluvial landforms, widespread chemical alteration, and sedimentary deposits visited by surface rovers all indicate that water was an important agent of geological change early in Martian history. At about 4 Ga, Mars lost its global magnetic field, its carbon dioxide atmosphere was substantially thinned by solar wind stripping, the climate cooled, and water lost its dominant role in surface change. Martian volcanism continued at generally declining rates, and the planet may still be active at low levels today. The Moon and Mercury, at 0.01 and 0.05 Earth masses, respectively, have heavily cratered surfaces and only extremely tenuous atmospheres, but their similarities end there. The Moon began largely molten, presumably the result of the accumulation of hot ejecta from a giant impact on the early Earth. Cooling and solidification of the resulting magma ocean led to formation of the crust and the mantle source regions of later volcanic lavas. Those lavas erupted to partially fill the lunar maria, mostly on the lunar nearside at 3 to 4 Ga, but there are also isolated younger volcanic deposits. The Moon may have a small iron-rich core, but if so it is no more than a few percent by mass. Lunar rocks from 3 to 4 Ga are magnetized, but whether the magnetizing field was a central core dynamo or transient field generated during surface impacts is an open question. The Moon is seismically active at low levels today. Shallow moonquakes are probably the signature of interior cooling, whereas deep moonquakes occur in clusters and appear to be triggered by tidal stresses. Mercury, in contrast, has such a high bulk density that its iron-rich core comprises at least 60 percent of the planet’s mass. Mercury has a global magnetic field, dipolar like that of Earth, and the outer core is known to be molten on the basis of the amplitude of the planet’s libration forced by solar torques as Mercury progresses along its elliptical orbit. The planet has an ancient, heavily cratered crust, as well as somewhat younger plains units that may be volcanic in origin. The surface composition is poorly known, but Earth-based measurements indicate that surface silicate materials have little or no ferrous iron. The dominant tectonic landforms on Mercury are high-relief lobate scarps, the surface expressions of large-offset thrust faults. Because of the extensive distribution and apparently random orientation of these features, the lobate scarps have been interpreted to record an extended period of global contraction, the result of some combination of interior cooling and solidification of an inner core. tion 7) and perhaps even major extinctions (Question 200 to 600 km, while others seem to extend almost to 8). Other indications of plumes include broad bulges the core-mantle boundary. However, there is abun- in the ocean floor, such as those around Hawaii, and dant evidence for much larger, domical or irregularly the tremendously excessive amount of lava produced at shaped low-velocity features in the lower mantle that Iceland in comparison to other places along the Mid- are sometimes called superplumes (Figure 2.3). Does Atlantic ridge. this mean that thermal plumes do not exist in the Although there is good geological evidence that lower mantle or that the seismic resolution is still too mantle plumes exist, seismological evidence for the low to make them out? Seismic data suggest that the existence of narrow, hot, cylindrical upwellings in large low-seismic-velocity regions near the base of the the lower mantle is only equivocal. Some cylindrical mantle are anomalously dense, which is contrary to regions of low velocity appear to extend downward to expectations for buoyant thermal upwellings (Ishii and

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40 ORIGIN AND EVOLUTION OF EARTH FIGURE 2.2  Sketch of mantle convec- tion and structure based on inferences from fluid mechanics and seismologi- cal data. SOURCE: Courtesy of Geoff Davies, Australian National University. After Davies (1999). Copyright 1999 by Elsevier Science and Technology Jour- nals. Reproduced with permission. Figure 2.2.eps Tromp, 1999). However, it is becoming better appreci- or in layers. Models, geochemical analyses of mantle ated that temperature variations may not be the only magmas erupted on the surface, and interpretations of source of buoyant upwellings in the mantle. Chemical seismic waves that have passed through Earth have all variations may be large enough to affect large-scale yielded different answers. In general, mantle models mantle flow, and mantle plumes can have both thermal based on geochemistry suggest that mantle convection and chemical components to their buoyancy (Davaille, occurs in two layers, whereas most geophysical evidence 1999; Farnetani and Samuel, 2003). and numerical models strongly support whole-mantle convection. Reconciling these differences is impor- Does Convection Occur Through the Whole tant for understanding Earth’s volcanic and thermal Mantle or in Layers? evolution. A key question about the modern form of mantle flow Geochemical analysis. Interactions of the mantle with is whether convection occurs through the whole mantle the core and surface chemically alter the upper and lower boundary regions of the mantle (discussed be- low). Convection then stirs this altered material back into the main volume of the mantle. The chemical com- position of lavas derived from the mantle provides clues about the extent to which these heterogeneities persist in time and hence about the nature of mantle convec- tion (Van Keken et al., 2002). Lavas (and most other rocks) contain every one of the 90 naturally occurring elements in the periodic table, although about 75 are present in small abundances. With new techniques the concentration of each of the 90 elements and the rela- tive amounts of isotopes of about half of the elements can be measured precisely. The isotopes formed by ra- dioactive decay (206Pb, 207Pb, 208Pb, 87Sr, 143Nd, 230Th, 226Ra, and others) provide detailed information about mantle evolution as well as the processes that produce FIGURE 2.3  Representation of large-scale seismic velocity and transport magma. structure of the mantle. Red zones have relatively slow P-wave velocity and blue zones are relatively fast. Slow velocities Low-abundance trace elements and isotopes of are thought to represent hotter parts of the mantle. SOURCE: Pb, Sr, Nd, Hf, He, and Os show large, nonrandom . See also variations among volcanic rocks. Basalt lavas erupted Su et al. (1994). Used with permission. at midocean ridges differ systematically from those

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EARTH’S INTERIOR 41 erupted at hot spots. Midocean ridge lavas also vary bulk composition derived from meteorites (Ques- from ridge to ridge and along individual ridges (Figure tion 1) and with the distribution of heterogeneities 2.4). The chemical differences between hot spots and at depth. midocean ridges have long been considered evidence that the lower mantle (whence mantle plumes presum- Seismic interpretation. The most direct observations ably come) is different, and convects separately, from available for inferring the present-day configuration the upper mantle (Hofmann, 1997). of mantle convection are provided by seismicity in Nevertheless, there are complications in the isoto- subduction zones and three-dimensional seismic pic data. For example, 3He data suggest that parts of the tomography models of the interior. Seismic velocity mantle are relatively unaltered, or at least less degassed, variations are caused by changes in pressure, tem- but other isotopes (Nd, Sr, Pb, Hf ) tell a different story perature, composition, and mineral alignment, so (Moreira et al., 2001). The mantle overall does not interpretation of the models requires information seem to have an Nd or Hf isotopic composition that from mineral physics (Question 6) and geodynamics. properly complements that of the continental crust. High-seismic-velocity features corresponding to cold Many such chemical clues must be sorted out before sinking oceanic lithosphere are clearly observed in re- we can develop a model for mantle convection and the gional and global seismic tomography models (Figure mantle-crust system that agrees with models for Earth’s 2.5). Low-velocity features (presumably signifying FIGURE 2.4 (Left) Bathymetry of the Mid-Atlantic ridge and topography of adjacent continents. SOURCE: . (Right) Variations of neodymium isotopic composition in basalt lavas from along the Mid-Atlantic ridge plotted against latitude. Zero on the epsilon scale corresponds to the bulk Earth value, which assumes Earth has the same Sm/ Nd ratio as chondritic meteorites. The high degree of heterogeneity indicates that diverse materials are generated in the mantle by melting and subduction and that these heterogeneities are not homogenized by convection. SOURCE: Data from the online database PetDB, averaged by ridge segment by Su (2002).

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42 ORIGIN AND EVOLUTION OF EARTH FIGURE 2.5  Seismic tomography data indicating that in some areas subduct- ed slabs extend through the 660-km discontinuity and well down into the lower mantle. Blue shading indicates higher seismic body wave (P) and shear wave (S) velocity, both of which should correlate with lower temperature. The thickness of the cold slab, however, is only about 50 to 100 km, whereas the thickness of the high-velocity (blue) zone is close to 500 km in the lower mantle. The greater thickness in the lower mantle could be due to deformation of the slab or to decreased spatial resolution of the image at greater depth. SOURCE: After Trampert and van der Hilst (2005). Copyright 2005 American Geophysical Union. Reproduced with permission. Figure 2.5.eps relatively high temperature) underlie ocean ridges, back bitmap image clearly penetrating the 660-km boundary. The variable arc basins, and tectonically active areas of continents. depth of lithospheric slab subduction is not easily un- Continental cratonic areas are underlain by high-seis- derstood in the context of simple thermal convection mic-velocity regions extending 250 to 350 km deep, in- and is the primary observation driving consideration dicating fundamental differences between oceanic and of more complex convection models involving both continental plates (Question 5). Deeper seismic-veloc- thermal and chemical effects. ity structures are less easily related to surface tectonics, with very large scale structures tending to dominate in Models. Mantle convection models have progressed the transition zone from 410 to 660 km deep, and in the from simplified two-dimensional models to complex lowermost mantle above the core-mantle boundary. For three-dimensional simulations, in concert with increas- several decades the resolution of seismic tomography ing computing power and improving knowledge of models has been improving, and this is guiding numeri- mantle material properties (Figure 2.6; see also Cohen, cal modeling of mantle flow processes. 2005). Comparison with seismological models allows Seismic evidence shows a large velocity disconti- some parameters in convection models to be tested, but nuity 660 km below the surface, which is thought to many issues are still unresolved. Among the challenges involve mineral phase transformations that tend to of simulating mantle convection are the strong depen- impede flow through the transition depth. However, dence of viscosity on temperature and composition, seismological data also show some subducted slabs ex- mineralogical heterogeneity in the mantle on both large tending to depths greater than 1,000 km (Figure 2.5), and small scales, departures from simple fluid behavior,

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EARTH’S INTERIOR 43 FIGURE 2.6  Computer simulation of mantle convection in two dimensions. Red-green-blue color scale depicts temperatures from 4000°C to 0°C. Fine-scale features, which arise from extreme variations in material properties at small length scales, are not well Figure 2.6.eps represented in this simulation, but hot upwellings from the core-mantle boundary region, and cold downwellings (analogous to subduc- tion) from the cold surface boundary layer, are prominent features. SOURCE: Butler et al. (2005). Copyright 2005 by Elsevier Science and Technology Journals. Reproduced with permission. and the effects of melting and phase changes on mate- seismological and geodynamic results tend to favor rial properties. Although the simulations are guided by an intermediate model of mantle convection that observations and experimental measurements, these are is neither strictly layered nor simple whole-mantle often indirect or subject to varying interpretations as a convection. result of the difficulty in specifying material properties at conditions of high pressure and temperature. For When Did Earth’s Inner Core Form? example, the uppermost mantle is mostly made of three minerals: olivine, orthopyroxene, and clinopyroxene. Earth’s thermal evolution is reflected in and strongly In the lower mantle these minerals are transformed by influenced by the temperature of the liquid outer core. pressure into higher density forms, and the size and The fact that Earth’s outer core is liquid rather than even the composition of the mineral grains are poorly solid is evidence for the hot origin of Earth, and the fact known. It is the deformation characteristics of these that the core has not completely solidified over Earth’s mineral aggregates that determine the nature of mantle 4.5-billion-year history means that it has been pre- convection. Because the grain size and other proper- vented from losing heat too quickly. Laboratory experi- ties of the deep mantle have yet to be determined, our ments suggest that the top of the core is about 1500°C ideas about convection in the lower mantle involve large hotter than the deep mantle (Figure 2.7). Therefore, extrapolations of the properties we can determine for heat must be flowing from the outer core into the lower Earth materials at lower pressure and temperature (see mantle, and the core must be cooling. The core must Question 6). also be close to its solidification temperature because Numerical simulations of mantle convection show the inner core is solid. As the core cools, it solidifies that even with phase transitions inhibiting flow and from the bottom up, so we deduce that the solid inner a viscosity increase in the lower mantle, it is plausible core is growing and the liquid outer core is shrinking. that large-scale transport of material between the The inner core–outer core boundary must have a upper and lower mantle does occur. All in all, current temperature exactly equal to the melting temperature of

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44 ORIGIN AND EVOLUTION OF EARTH FIGURE 2.7  Schematic representation of average temperature in Earth’s interior versus depth. Viscosity estimates are also shown. Temperature is highly uncertain below about 500-km depth. The average mantle temperature (red line) is based on an adiabatic gradient and a temperature of 1350°C at a pressure of 1 bar. Higher and lower temperatures for plumes and subducted slabs are approximate but close to estimates. Temperature in the core and the large temperature drop at the base of the mantle are poorly constrained. See van der Hilst et al. (2007) for a recent estimate of temperature at the core-mantle boundary and Bunge (2005) for a discussion of nonadiabatic temperature structure in the mantle. the core at the corresponding pressure. The core melting started, it would slow cooling of the core because temperature is uncertain because the core contains mi- crystallization releases heat. It has recently been nor elements other than Fe, and it is not known exactly inferred from convection models that the inner core which elements and how much of them. Hence, the may be relatively young; it may have begun forming melting temperature of the core is likely to be a complex about 1.5 billion to 2 billion years ago (Labrosse et function of both composition and material properties al., 2001). This idea, however, is inconsistent with at high temperature and pressure. Ongoing research is theoretical models that suggest the presence of a examining the possibility that heat-producing elements solid inner core may be important for the strength (e.g., potassium) may be present in the core and may of the magnetic dipole field and for the occurrence contribute to a slowing of core cooling. of reversals. Moreover, there is evidence that Earth’s How long the inner core has existed, its rate of magnetic field is older than 2 billion years (Tarduno growth, and why the core has not fully solidified are et al., 2007). This apparent conundrum may be partly fundamental unresolved issues (Butler et al., 2005). a consequence of our still poor understanding of the Part of the answer seems to be that the core has been characteristics and processes near the core-mantle kept in a molten state by the mantle, which because boundary, including the values of the temperature of its much higher viscosity does not remove heat fast contrast and the amount of heat flowing across the enough. Also, once crystallization of the inner core boundary (e.g., Bunge, 2005).

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EARTH’S INTERIOR 45 How Has Earth’s Magnetic Field Evolved Through Time? It has long been recognized that the main part of the geomagnetic field is sustained by fluid motions in Earth’s electrically conducting outer core. These mo- tions cause the magnetic field to change over many timescales, from diurnal to annual to geological time- scales. However, a unified picture of how the geody- namo and the core fit into the Earth system has not yet emerged. Important questions about the internal op- eration of the geodynamo and the relationship between the geodynamo and other Earth processes remain unanswered. These include: How do the geodynamo, mantle convection, and plate tectonics interact? What role did the geodynamo play in Earth’s early history? The age of the magnetic field is of interest because the magnetosphere may help keep Earth habitable. For example, the magnetosphere may have been necessary to help Earth retain its atmosphere against the eroding powers of the solar wind, and it partly shields Earth’s FIGURE 2.8  A snapshot of a three-dimensional computer surface against radiation from space. How important simulation of the geodynamo. The magnetic field is illustrated using lines of force; blue lines represent the inward directed field the latter is in preserving life or in modulating the and yellow lines represent the outward directed field. The field is rates of evolution is not agreed upon. New insights on intense and complicated in the model’s fluid iron core, where it is these questions will come from continued satellite and generated by fluid motions. Like Earth’s field, the simulated field has a dominantly dipolar structure outside the core. SOURCE: ground-based observations of the geomagnetic field Courtesy of Gary Glatzmaier, University of California, Santa and the paleomagnetic field, dynamical interpreta- Cruz; and Paul Roberts, University of California, Los Angeles. tions of the core’s seismic structure, and sophisticated numerical dynamos (e.g., Figure 2.8) and models of core evolution. carbon dioxide, and other gases, continually renew- What Are the Chemical Consequences of Mantle ing the oceans and atmosphere. Mountain building, Convection? erosion, and subduction, which also reflect the effects of mantle convection, remove these same materials The mantle interacts with Earth’s surface environ- and tend to recycle them into the deepest parts of the ment through volcanism, heat and mass exchange at mantle. At the core-mantle boundary we infer there is midocean ridges, and subduction. The mantle may also mainly heat exchange, but there is tantalizing evidence exchange material with Earth’s outer core. Overall, the of chemical interaction as well (Brandon et al., 1999). mantle mediates a grand-scale circulation of materials Still unknown are whether the processes that mediate that may extend from the core-mantle boundary to the these exchanges were different in the past. An inter- surface and back again. The nature of this mantle cir- esting possibility is that the nature of continents and culation and the processes that produce the interactions oceans that support a habitable surface environment at the mantle boundaries are critical to understanding today reflect only a particular phase of Earth’s cooling how Earth’s chemistry is continually modified. For ex- and hence might have been absent or much different ample, volcanism builds oceanic and continental crust in the past and might also be much different in the (Question 5) and releases to the atmosphere water, future.

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60 ORIGIN AND EVOLUTION OF EARTH quantitatively predict the dependence of erosion rate and chemistry at the extreme conditions of planetary on other variables and the strength and deformation interiors and at the smallest scales of mineral surfaces properties of rocks in the lower continental crust and and nanoparticles. Advances at the other end of the the upper mantle. spectrum, when the scale is extremely large and/or the processes are extremely slow, will require advances QUESTION 6: HOW ARE EARTH in experiment, theory, computation, and observation. PROCESSES CONTROLLED BY MATERIAL Only the combination of all four is likely to bring PROPERTIES? progress. Geology is founded on the central insight that rocks What Minerals Comprise Planetary Interiors? can be read as a record of Earth’s history. Rocks and minerals are produced and altered by geological As noted in Questions 4 and 5, the nature of the con- processes—melting, eruption, weathering, erosion, vection and deformation that affect Earth’s mantle and deformation, and metamorphism. Therefore, deci- crust, and hence models for plate tectonics and Earth’s phering the secrets of the rock record begins with an temperature history, depends directly on the material understanding of large-scale geological processes. The properties of rocks and minerals at the high tempera- keys to understanding these processes are the basic tures and pressures of planetary interiors. The pressure physics and chemistry of the materials that make up is 136 GPa (1.36 million atmospheres) at the base of the planet. Scientists now recognize that macroscale the mantle and 364 GPa at Earth’s center, while the behaviors—plate tectonics, volcanism, and so on—arise temperature reaches 4000 K at the base of the mantle from the microscale composition of Earth materials and 6000 K at Earth’s center (similar to the temperature and indeed from the smallest details of their atomic at the surface of the Sun; Figure 2.17). structures. Understanding materials at this microscale is essential for comprehending Earth’s history (NRC, Phase transformations. The pressure in Earth’s interior 1987) and making reasonable predictions about how is so enormous that it alters the fundamental properties things may change in the future. of elements; for example, it can convert insulators to The high pressures and temperatures of Earth’s metals and cause magnetism to collapse (Figure 2.18). interior, the enormous size of Earth and its structures, Such changes occur because pressure compresses and the long expanse of geological time, and the vast di- distorts the electron orbitals, thereby changing the most versity of materials and properties present challenges basic properties of the materials. Changing pressures to investigation. Moreover, minerals are complicated bring about many kinds of phase transformations. The solids that generally contain not only their essential most familiar of these are melting and freezing, but chemical constituents but also trace amounts of almost many more complex phase transformations have been every element known in nature. Although we can learn identified. Structural phase transitions are also com- much about Earth from the study of pure compounds mon. The transition from graphite to diamond is well that approximate real minerals, we also know that even known, but more important for Earth processes is how minute amounts of other chemical elements can radi- mantle olivine and pyroxenes change at high pressure. cally change a mineral’s behavior. High-pressure mineral transformations, and their Fortunately, the surge of interest in understanding dependence on temperature, allow us to estimate the Earth materials at the atomic level has been accom- temperature of the deep Earth and provide constraints panied by rapid development of new tools, including on how mantle convection works. Temperatures inside new synchrotron sources that bring the ability to probe Earth can be estimated by comparing the pressure and the atomic structure of minerals and liquids (Figure temperature conditions at which mineral transforma- 2.16); high-pressure devices to simulate the distortion tions occur in the laboratory to the depths at which of atomic arrangements under huge pressures; and sudden changes in the physical properties of the mantle advanced quantum mechanical theory, which prom- and core occur (Figure 2.19). We know, for example, ises major advances in our understanding of physics that the boundary between the liquid outer core and the

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EARTH’S INTERIOR 61 FIGURE 2.16  (Top) Aerial view of the storage ring at the Advanced Photon source. Such third-generation synchrotron sources have revolutionized the study of Earth materials by dramatically increasing spatial and temporal resolution of experimental measurements and allowing for the study of much smaller samples than had been possible. A similar qualitative advance is expected when the first fourth-generation synchrotron sources (X-ray-free electron lasers) come online in 2009. ����������������������������������� SOURCE: . ���������������������������������������������������������������������������������������������������������� Courtesy of Argonne National Laboratory, managed and operated by the University of Chicago, Argonne, LLC, for the U.S. Department of Energy. (Bottom) Results of a quantum mechanical computation based on density function theory, showing the predicted structure and distribution of electrons in SiO2 at high pressure. Such computational methods can provide estimates of material properties over the vast range of pressures and temperatures encountered in planetary interiors. SOURCE: Oganov et al. (2005). Reprinted with permission. Copyright 2005 by the American Physical Society. solid inner core must be at the melting temperature of or enhance the sinking of subducted slabs or change the core (Question 4), although the temperature is not the size and shape of mantle plumes as they rise. A known precisely due to uncertainty in the composition previously unknown phase transformation was recently of the core and the difficulties of exploring these high discovered at pressures well beyond those previously temperatures in the laboratory. The temperature of the probed (Murakami et al., 2004). The new transforma- most important changes of seismic wave velocity in tion, from perovskite, the main mineral structure of the the mantle, which happen at depths of about 400 and deep mantle, to a higher pressure postperovskite form, 660 km, is well constrained by laboratory studies of the occurs at the top of the D″ region, an anomalous zone conversion of olivine and pyroxene to higher density above the core-mantle boundary (corresponding to minerals. These phase transformations are so drastic some 100-GPa pressure) that exhibits intriguing and that they can influence mantle convection; a phase highly variable seismological features (see Question 4), transformation that causes a large change in density some of which may be caused by the transformations. can work either for or against the thermal buoyancy that drives convection. What is the melting temperature of rocks under pressure? Although the effects of phase transitions on mantle Much of what we know about how Earth’s interior convection are generally appreciated, we still do not works is based on knowledge of the melting temperature know how the natural system actually works—for ex- of rock and metal, and how this temperature changes ample, the extent to which the phase transitions impede with pressure (Question 4). To expand this knowledge,

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62 ORIGIN AND EVOLUTION OF EARTH FIGURE 2.17  Diamond-anvil apparatus (top). The sample is placed between two opposed diamond anvils, the tips of which range from 0.01 to 1 mm across, depending on the pressure range of interest. The vertically oriented strip is a metal gasket that prevents the sample from extruding. Diamond is ideal for high-pressure studies because is it strong, chemically inert, and transparent to most light. SOURCE: . Used with permission. A shock wave experiment can be carried out using a gun (right), magnetic drive, or laser. The projectile can produce pressures and temperatures that exceed those at Earth’s center (like a diamond anvil cell) but for very short periods of time (in contrast to static anvil experiments). New methods combine both static and dynamic approaches to reach pressure-temperature domains (Jeanloz et al., 2007). SOURCE: . Used with permission. (a) HS Maj A. HS Maj O (c) LS Maj C. LS Maj O O O O O Fe Fe O O O O O O FIGURE 2.18  Influence of pressure on the iron atom. Shown is the predicted charge density of the doubly charged iron cation (Fe) Figure 2.18.eps in the mineral ferropericlase (Mg,Fe)O, in which it is surrounded by six oxygens (O). (Left) At low pressure the spins of the d electrons are maximally aligned, producing a net magnetic moment on each iron atom (called the high-spin or HS state) and the magnetic properties that we are familiar with, such as the tendency of magnetic minerals to align with the magnetic north pole. (Right) At high pressures characteristic of Earth’s deep mantle the spins pair (called the low-spin or LS state), the atomic magnetic moments vanish, and iron-bearing minerals are nonmagnetic. The figures show that the size and shape of the iron cation also change across the high-spin to low-spin transition: iron is smaller (by about 10 percent in volume) and less spherical in the low-spin state, which should produce a change in density and other physical properties of iron-bearing minerals. SOURCE: Tsuchiya et al. (2006). Reprinted with permission. Copyright 2006 by the American Physical Society.

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EARTH’S INTERIOR 63 also increases with pressure but more slowly. Although there is so far only scant experimental and theoretical evidence, it suggests that magma can be denser than mantle rock deep inside Earth (Figure 2.20; Miller et al., 1991). The consequences of this for Earth’s evolu- tion would be profound. If silicate melt sinks instead of rising toward the surface, it could be stored at depth for long periods, where it would be kept hot. The geo- chemical consequences of this inverted gravitational separation could also be important, but little is known about the distribution of trace elements between solids and liquids at high pressures. Iron-rich liquid would likely exist as a separate, denser phase than Earth’s silicate fraction and sink to the center, forming the core FIGURE 2.19 Photograph looking into a diamond cell at a (Question 2). The timescale of this descent and the par- 100-micron blue single crystal of hydrous ringwoodite (ideally Mg2SiO4 composition) held in situ at 30 GPa, corresponding titioning of elements between the iron-rich and silicate to a depth of 800 km in Earth. The brown spots indicate where portions during core formation are still uncertain and the sample has been heated with a laser to a few thousand have profound implications for the chemical composi- degrees, causing a phase transformation to the assemblage MgSiO3 perovskite + MgO periclase that is thought to comprise tion of the core and the origin of the geomagnetic field most of Earth’s mantle below a depth of 660 km. SOURCE: (Question 4). Courtesy of Steven Jacobsen, Northwestern University. Used There is confirming evidence that liquid may be with permission. present in the deep mantle, especially near the core- mantle boundary. Seismologists have identified thin layers of extremely low shear wave velocity at the base we need to understand the processes that control the of the mantle, a characteristic of liquid. It has been sug- melting and freezing of rocks and minerals in the gested that this region could be made of dense, partially planetary interior. Melting of rocks involves complex solidified magma and that it could even be a remnant chemistry, because rocks are typically composed of four of the Hadean planetary magma ocean (Williams and or more mineral phases, none of which are pure. As Garnero, 1996; see Question 2). If U, Th, and K are rock melts, the composition and density of the liquid concentrated in this deep liquid, it could mean that the portion are different from those of the solid, and thus, base of the mantle produces extra heat from radioactiv- with the help of gravity, one can segregate from the ity, which would affect how we think about the core other. For example, the lava that erupts from volcanoes dynamo and about the overall chemical composition of is both less dense and compositionally different from the mantle. If mantle liquid is in contact with the liquid the parent mantle rock. Over Earth’s long history the outer core, it would also mean that chemical exchange repeated processes of melting, melt ascent due to buoy- across the boundary would be much more effective than ancy, and eruption onto the surface have completely if the mantle is solid; this would change the way we rearranged many of its chemical elements. This process think about the origin of chemical heterogeneity in the of planetary differentiation, making chemically distinct mantle (Question 4). To resolve these issues we need to domains out of a homogeneous starting material, is one know much more about the properties of silicate liquids of the most fundamental features of planetary evolution and solids at very high pressures and temperatures. Re- (Questions 2, 4, and 5). cent experimental advances, including measurements One of the more intriguing questions about melt- of liquid structure in situ at high pressure (Shen et al., ing is whether, under some conditions, magma may 2004), will work hand in hand with theoretical and be denser than the surrounding solid mantle. Magma computer modeling. Modeling of high-pressure prop- is highly compressible, so its density must increase erties (Figure 2.20), using the principles of quantum rapidly with increasing pressure. The density of solids mechanics, shows promise, although at present only a

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64 ORIGIN AND EVOLUTION OF EARTH be seen in the inner core, where longitudinal seismic waves travel 3 percent faster along the rotational axis than in the equatorial plane. This difference may be due to alignment of iron crystals in the core, although the mechanism for producing the alignment is still uncer- tain (Stixrude and Brown, 1998). Understanding the origin of this alignment is likely to tell us a great deal about the dynamics at Earth’s center, the history of the core, and the origin of the geomagnetic field. FIGURE 2.20  Predicted atomic-scale structure of a model magma (MgSiO3 composition) showing that the large compress- How Much Water Is in the Solid Earth? ibility of liquids is caused by rearrangement of the structure from an open configuration near zero pressure (left) to a much more Earth is unique in the Solar System for its abundant compact and highly coordinated structure at the pressure of surface water, and most models for the early Earth the core-mantle boundary (right). Silicon-oxygen coordination polyhedra are shown in blue and magnesium ions in yellow. suggest that the source of this water was the mantle via SOURCE: Stixrude and Karki (2005). Reprinted with permission volcanic eruptions. Based on recent research, it seems from the American Association for the Advancement of Science likely that the interior continues to be a major reser- (AAAS). voir of both water and carbon dioxide (Williams and Hemley, 2001). Earth is so massive that if the mantle is only 0.03 percent water, it would hold the equivalent of small number of atoms can be modeled, which means all the water in the modern oceans. Upwelling mantle that it is not yet possible to use this approach to explore material at midocean ridges appears to contain about how trace elements behave. this much water, so at present Earth’s interior has at least one ocean’s worth of water. How much more it Can seismic waves be used to uniquely determine mantle might have and how this amount has changed over mineralogy? Material properties and seismology are in- Earth’s history are outstanding questions. terdependent in a fundamental way. Seismologists can We do not know whether Earth has always had measure the speed at which seismic waves traverse the the present amount of water at its surface, but the mantle and use this information to construct pictures of answer has implications for a variety of processes. To the deep Earth in a process analogous to a medical CAT reach the answer, we need a deeper understanding scan. At the same time, pictures of the deep mantle of where water and carbon dioxide are stored in the cannot be interpreted without information about mantle. We know of two potential reservoirs of water: mantle minerals and rocks, just as radiologists need hydrous phases, such as clays that contain predictable to know how bone and other types of tissue transmit amounts of water within their crystal structures, and X-rays. The changes in seismic wave velocity through nominally anhydrous phases, such as olivine (the most different structures in the deep Earth are small—about abundant mineral in the upper mantle), which include 1 percent—so the elastic properties of the minerals hydrogen as defects (Figure 2.21). Knowing more about need to be known precisely to interpret the changes. these reservoirs may frame our view of the long-term As these properties become better known, geologists evolution of the hydrosphere, including formation of hope to use seismic images to map the temperature and the oceans (Question 2). Understanding the evolution composition variations in the mantle and perhaps even of the deep hydrosphere is also central to our view of the pattern of convection. The latter is possible because mantle dynamics, since even small amounts of hydro- seismic wave velocity is dependent on direction, or gen can change the viscosity of the mantle by orders anisotropy, and can be related to flow patterns if there of magnitude and the melting temperature of rocks is sufficient knowledge of the elasticity of minerals and by hundreds of degrees (Question 4). For example, if the mechanisms by which they deform (Karato, 1998). the mantle has more water, it might convect faster and A striking example of anisotropy inside Earth may produce more volcanism, by which it loses water to the

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EARTH’S INTERIOR 65 about how minerals and fluids react. Recent studies have shown that reactivity is exquisitely sensitive to the finest details of surface structure. For example, the rate of exchange with water for oxygen atoms on distinct but structurally similar sites on an aluminum hydroxide surface may vary by seven orders of magnitude (Phillips et al., 2000). In addition, a major new realization is that most of the mineral surface area in the environment may be in the form of nanophases: extremely small mineral particles, 1 to 100 nm in size, orders of magnitude too small to see with the naked eye. These very small mineral grains have dramatically different physical and chemical properties than larger ones (Banfield and Zhang, 2001). The surface energy of nanophases is so important that it can stabilize structures that do not ex- ist in bulk material (Navrotsky, 2004). These structures may have unique reactive sites, adsorptive properties, FIGURE 2.21 An example of how water might be stored in and reaction kinetics. The structures of nanophases Earth’s interior. Shown is the predicted structure of a nominally also vary depending on whether they are surrounded by anhydrous mantle mineral (stishovite, ideally SiO2) with trace amounts of hydrogen incorporated via the replacement: Si4+ = water, air, or organic ligands. Nanophases are important Al3+ + H+. Dark- and light-blue polyhedra are SiO6 and AlO6 for their role as a unique reactive surface area, and they coordination environments, respectively; red spheres are oxygen also help us understand how minerals form, since all atoms; and the green sphere is a hydrogen atom. The solubil- minerals start out as nanophases in the form of small ity of water in this mineral reaches a few percent at conditions typical of the shallow lower mantle. SOURCE: Courtesy of Lars nucleation centers (Figure 2.22). Stixrude, University of Michigan. At and near Earth’s surface, the formation and dissolution of minerals take place in the presence of microorganisms, and there is a growing awareness that surface. If the mantle loses too much water, volcanism biology plays a significant role in mediating chemical might slow down until enough water is returned to the reactions at mineral surfaces (Question 8). In addition, mantle by subduction. This type of feedback may help many minerals are formed entirely by living organisms, regulate Earth’s surface environment and the water both large and small. Limestone, for example, is almost content of the mantle (see also Question 7). entirely formed as calcium carbonate shell material by small marine organisms. Much of the modern study of mineral formation lies at the interface of biology, How Do Minerals and Fluids React? chemistry, and geology. With new analytical techniques Chemical reactions between minerals and water enable it is becoming possible to study how minerals are made the oceans and atmosphere to exchange chemicals with by organisms and to compare biological and inorganic the rocks of the crust and mantle. These chemical reac- processes. For example, it is possible that an organism tions control the mineral weathering that accompanies can produce a microenvironment that causes calcite erosion and ultimately affect the composition of sea- to be precipitated essentially by inorganic processes. water, the bioavailability of nutrients and toxins in the By altering the microenvironment, the organism can environment, and the amount of carbon dioxide in the control the particular form, and hence trace element atmosphere (Question 7). All of this chemistry occurs composition, of the mineral that is precipitated (Bentov in the microenvironment at the surfaces of minerals. and Erez, 2006). We may have much to learn about how New data about natural materials, especially about the minerals form by carefully watching how organisms microstructure of mineral surfaces, are changing ideas make them (Figures 2.22 and 2.23).

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66 ORIGIN AND EVOLUTION OF EARTH FIGURE 2.22  Necklace of titania nanocrystals that have aggregated spontaneously by oriented attachment. In this mineral growth pathway, crystals no more than a few nanometers in diameter aggregate and rotate so that adjacent surfaces share the same crys- tallographic orientation. The pair of adjacent interfaces is eliminated and the pair of nanoparticles is converted to a larger single crystal. Individual atoms are visible in the lower view. SOURCE: (Top) Penn and Banfield (1999). Copyright 1999 by Elsevier Science and Technology Journals. Reproduced with permission. (Bottom) Courtesy of Lee Penn, University of Minnesota, and Jillian Banfield, University of California, Berkeley. Used with permission. Can Large-Domain, Multiscale, and Extremely to be concentrated in narrow zones rather than being Slow Earth Processes Be Predicted? widely distributed. Other feedbacks of this sort include thermal weakening and damage weakening (Bercovici Many properties and processes depend on length scale and Karato, 2002). In the latter, deformation either and timescale in ways that are difficult to predict. The reduces grain size or increases crack density, making general idea of scaling, or inferring the behavior of ma- the material easier to deform. There are many ways that terials at one scale from knowledge of those materials rocks can behave when stressed; these different defor- at another scale, underlies much of our thinking about mation processes affect one another; and the larger the Earth. For example, our understanding of mantle con- rock body under consideration, the more processes that vection is founded on our ability to relate planet-scale can come into play. Hence, predicting what will happen (large) and laboratory-scale (small) flows that have the at a large scale from information about what happens same ratio of buoyancy forces to viscous resisting forces at a small scale is a major challenge. (the Rayleigh number). Laboratory analogs are likely The behavior of faults raises many scale-related to be accurate for some aspects of mantle convection, fundamental questions: How are earthquakes (large but they have limits. For example, we know that the scale) generated and can we predict them using small- crust and uppermost mantle exhibit nonfluid behavior, scale models (Question 9)? What localized (small- or there would be no plate tectonics (Question 5). We scale) process and set of conditions trigger a (large) also know that most of the surface deformation caused fault to rupture a particular distance on a particular day? by plate tectonics takes place in narrow zones at the How much of continental deformation (large) is caused edges of the plates. The localization of deformation by slip on faults (small)? Some of the most influential probably has an origin in complex failure processes predictors of fault movement have been laboratory that are dependent on both size and timescale. Rocks measurements of rock strength: squeeze a rock in one and even magmas can exhibit a behavior called strain direction and eventually it will break or slide along softening, which means that as the amount or rate of preexisting faults, once friction is overcome. However, deformation increases, the resistance to deformation rock at the scale of a great earthquake rupture is much decreases, which increases the amount and rate of de- weaker than rock in the laboratory. One possible ex- formation further. Consequently, deformation is most planation is that water is pervasive in the crust and likely to continue wherever it has already started and weakens fault planes by acting as an easily sheared but

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EARTH’S INTERIOR 67 FIGURE 2.23  Orange, polymer-laden ferric iron oxyhydroxides from a submerged mine. The slime consists of colloidal aggregates of nanoparticles, mineralized cell products, and cells (left) of two bacteria. The twisted stalks are characteristic of iron-oxidizing bacteria belonging to the Gallionella genus, while sheathed elongate cells are typical of bacteria belonging to the iron-oxidizing Lepthothrix genus. The contrast is due to iron oxyhydroxide nanoparticles. (Right) A closeup of the nanoparticle aggregates reveals that while the individual particles are separated (white regions), they have been bio-assembled so that they are crystallographically oriented in the same direction. SOURCE: Banfield et al. (2000). Reprinted with permission from AAAS. incompressible lubricant that dramatically reduces the magma migration in the mantle and formation of the friction between the two rock surfaces (Figure 2.24). core (Questions 2 and 4; Holtzman et al., 2003). The But as noted above, there are many other possible ways mantle is made of solid minerals with varying strengths. to cause Earth’s crust to appear weak in comparison to Just as in the case of magma channelization, mantle rocks in the laboratory. convection may organize weaker and stronger miner- Another reason scaling is challenging is that als into layers (foliation), dramatically influencing the Earth is heterogeneous: material properties, including viscosity as well as the seismic signal and our interpre- viscosity, electrical and thermal conductivity, chemical tation of it in terms of composition, temperature, and diffusivity, and elasticity, may vary spatially by orders flow pattern. Chemical reaction of fluids and melts with of magnitude on scales ranging from nanometers to surrounding solids can also produce channels, which kilometers. Heterogeneity may dramatically influence can significantly influence the composition of the dynamics. Cappuccino drinkers are familiar with the magma and our inferences about its origin (Spiegelman fluid dynamical oddities of composites, seen in the and Kelemen, 2003; Figure 2.25). relative stiffness of milk foam as compared with its constituents, air and milk. Analogous phenomena The importance of time. The solid-like or fluid-like are common in nature. For example, as magma forms behavior of the mantle illustrates the importance of by melting inside Earth, it juxtaposes relatively fluid time in the material properties of large domains. The magma with mineral crystals that are essentially rigid. boundary between fluid-like and solid-like behavior The viscosity of crystal mush, which largely determines is set by the Maxwell relaxation time—the ratio of how fast it rises (or sinks), depends strongly and non- viscosity to shear modulus—which is on the order of linearly on the amount of suspended solid crystals it 1,000 years for the mantle. This means that we can contains. Deformation and/or dissolution of the solid only determine the viscosity of mantle materials in the matrix through which magma moves can also organize laboratory at extremely slow rates of deformation or solid and liquid fractions so that the liquid becomes at unrealistically high temperatures to bring the Max- channelized, dramatically increasing the rate of liq- well relaxation time within the window achievable by uid-solid segregation, with important implications for experiment. Just as solids behave like fluids on long

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68 ORIGIN AND EVOLUTION OF EARTH Figure 2.24 top.eps bitmap image FIGURE 2.24  Photograph (upper left) and thin section (upper right) of the Punch Bowl fault in southern California. The principal slip surface (pss) is thought to have accommodated several kilometers of slip. The slip is localized to a 1-mm (white) region, including a Figure 2.24 bottom.eps microshear zone with more intense shearing (dark) occurring within a few hundred microns. SOURCE: (Upper left) Chester and Chester type has been converted to paths (1998). Copyright 1998 Elsevier, reprinted with permission. (Upper right) Courtesy of Judith Chester, Texas A&M University. (Bottom) Results of experiments on fault slip in natural rocks showing that the friction coefficient depends on slip velocity and nearly vanishes for slip velocities similar to those of earthquakes (1 m/s). SOURCE: Di Toro et al. (2004). Reprinted by permission from Macmillan Publishers Ltd.: Nature, copyright 2004. timescales, fluids behave like solids and rupture on Summary short timescales. When magma is deformed very rap- idly—for example, during an eruption—it may fracture. Understanding how Earth works depends on knowl- Understanding this behavior is helping us sort out the edge of the properties of rocks and minerals. After dynamics of volcanic eruptions (Question 9) and how a period of steady progress, breakthroughs are now these depend on features such as magma composition at hand because of new analytical tools provided by (e.g., Gonnermann and Manga, 2003). advanced radiation sources (e.g., synchrotron, neutron, and laser facilities) and advanced computing. Much of

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EARTH’S INTERIOR 69 the essential physics and chemistry of Earth materials arises from structures and processes that occur at the atomic level. The new tools allow these small scales to be studied directly as well as simulated, bridging the gap between quantum mechanics and microscopy and paving the way for a new level of understanding of planetary processes at longer length scales. Earth materials present a challenge to understand- ing because of their complex chemical composition and the high pressures and temperatures of planetary inte- riors. The long timescales of geological processes also create difficulties because some of the critical processes that affect planetary evolution take place so slowly that they cannot be simulated in the laboratory and because they may be caused by mechanisms that are not impor- FIGURE 2.25 Simulation of the distribution of melt (as mea- tant or even perceptible at laboratory timescales. The sured by porosity) in a deforming, reacting matrix. The melt organizes itself into channels that vary in width, position, and physics of large domains, long timescales, and multiple melt content with time. SOURCE: Courtesy of Marc Spiegelman, interacting scales remains a major challenge in Earth Columbia University. Used with permission. See also Spiegelman science and one that will advance only with interdis- et al. (2001). ciplinary effort.

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