This chapter reviews the present-day composition of the continental crust, the methods employed to derive these estimates, and the implications of the continental crust composition for the formation of the continents, Earth differentiation, and its geochemical inventories. We review the composition of the upper, middle, and lower continental crust. We then examine the bulk crust composition and the implications of this composition for crust generation and modification processes. Finally, we compare the Earth's crust with those of the other terrestrial planets in our solar system and speculate about what unique processes on Earth have given rise to this unusual crustal distribution.

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Composition of the Continental Crust

Geophysical, petrological, and geochemical data provide important clues about the composition of the deep continental crust. On the basis of seismic refraction data, we divide the crust into type sections associated with different tectonic provinces. Each shows a three-layer crust consisting of upper, middle, and lower crust, in which P wave velocities increase progressively with depth. There is large variation in average P wave velocity of the lower crust between different type sections, but in general, lower crustal velocities are high (>6.9 km s−1) and average middle crustal velocities range between 6.3 and 6.7 km s−1. Heat-producing elements decrease with depth in the crust owing to their depletion in felsic rocks caused by granulite facies metamorphism and an increase in the proportion of mafic rocks with depth. Studies of crustal cross sections show that in Archean regions, 50–85% of the heat flowing from the surface of the Earth is generated within the crust. Granulite terrains that experienced isobaric cooling are representative of middle or lower crust and have higher proportions of mafic rocks than do granulite terrains that experienced isothermal decompression. The latter are probably not representative of the deep crust but are merely upper crustal rocks that have been through an orogenic cycle. Granulite xenoliths provide some of the deepest samples of the continental crust and are composed largely of mafic rock types. Ultrasonic velocity measurements for a wide variety of deep crustal rocks provide a link between crustal velocity and lithology. Meta-igneous felsic, intermediate and mafic granulite, and amphibolite facies rocks are distinguishable on the basis of P and S wave velocities, but metamorphosed shales (metapelites) have velocities that overlap the complete velocity range displayed by the meta-igneous lithologies. The high heat production of metapelites, coupled with their generally limited volumetric extent in granulite terrains and xenoliths, suggests they constitute only a small proportion of the lower crust. Using average P wave velocities derived from the crustal type sections, the estimated areal extent of each type of crust, and the average compositions of different types of granulites, we estimate the average lower and middle crust composition. The lower crust is composed of rocks in the granulite facies and is lithologically heterogeneous. Its average composition is mafic, approaching that of a primitive mantle-derived basalt, but it may range to intermediate bulk compositions in some regions. The middle crust is composed of rocks in the amphibolite facies and is intermediate in bulk composition, containing significant K, Th, and U contents. Average continental crust is intermediate in composition and contains a significant proportion of the bulk silicate Earth's incompatible trace element budget (35–55% of Rb, Ba, K, Pb, Th, and U).

Nature and composition of the continental crust: A lower crustal perspective

Two models, a simple cooling model and the plate model, have been advanced to account for the variation in depth and heat flow with increasing age of the ocean floor. The simple cooling model predicts a linear relation between depth and t½, and heat flow and 1/t½, where t is the age of the ocean floor. We show that the same t½ dependence is implicit in the solutions for the plate model for sufficiently young ocean floor. For larger ages these relations break down, and depth and heat flow decay exponentially to constant values. The two forms of the solution are developed to provide a simple method of inverting the data to give the model parameters. The empirical depth versus age relation for the North Pacific and North Atlantic has been extended out to 160 m.y. B.P. The depth initially increases as t½, but between 60 and 80 m.y. B.P. the variation of depth with age departs from this simple relation. For older ocean floor the depth decays exponentially with age toward a constant asymptotic value. Such characteristics would be produced by a thermal structure close to that of the plate model. Inverting the data gives a plate thickness of 125±10 km, a bottom boundary temperature of 1350°±275°C, and a thermal expansion coefficient of (3.2±1.1) × 10−5°C−1. Between 0 and 70 m.y. B.P. the depth can be represented by the relation d(t) = 2500 + 350t½ m, with t in m.y. B.P., and for regions older than 20 m.y. B.P. by the relation d(t) = 6400 - 3200 exp (−t/62.8) m. The heat flow data were treated in a similar, but less extensive manner. Although the data are compatible with the same model that accounts for the topography, their scatter prevents their use in the same quantitative fashion. Our analysis shows that the heat flow only responds to the bottom boundary at approximately twice the age at which the depth does. Within the scatter of the data, from 0 to 120 m.y. B.P., the heat flow pan be represented by the relation q(t) = 11.3/t½ μcal cm−2s−1. The previously accepted view that the heat flow observations approach a constant asymptotic value in the old ocean basins needs to be tested more stringently. The above results imply that a mechanism is required to supply heat at the base of the plate.

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An analysis of the variation of ocean floor bathymetry and heat flow with age

3.01 – Composition of the Continental Crust

Seismic techniques provide the highest-resolution measurements of the structure of the crust and have been conducted on a worldwide basis. We summarize the structure of the continental crust based on the results of seismic refraction profiles and infer crustal composition as a function of depth by comparing these results with high-pressure laboratory measurements of seismic velocity for a wide range of rocks that are commonly found in the crust. The thickness and velocity structure of the crust are well correlated with tectonic province, with extended crust showing an average thickness of 30.5 km and orogens an average of 46.3 km. Shields and platforms have an average crustal thickness nearly equal to the global average. We have corrected for the nonuniform geographical distribution of seismic refraction profiles by estimating the global area of each major crustal type. The weighted average crustal thickness based on these values is 41.1 km. This value is 10% to 20% greater than previous estimates which underrepresented shields, platforms, and orogens. The average compressional wave velocity of the crust is 6.45 km/s, and the average velocity of the uppermost mantle (Pn velocity) is 8.09 km/s. We summarize the velocity structure of the crust at 5-km depth intervals, both in the form of histograms and as an average velocity-depth curve, and compare these determinations with new measurements of compressional wave velocities and densities of over 3000 igneous and metamorphic rock cores made to confining pressures of 1 GPa. On the basis of petrographic studies and chemical analyses, the rocks have been classified into 29 groups. Average velocities, densities, and standard deviations are presented for each group at 5-km depth intervals to crustal depths of 50 km along three different geotherms. This allows us to develop a model for the composition of the continental crust. Velocities in the upper continental crust are matched by velocities of a large number of lithologies, including many low-grade metamorphic rocks and relatively silicic gneisses of amphibolite facies grade. In midcrustal regions, velocity gradients appear to originate from an increase in metamorphic grade, as well as a decrease in silica content. Tonalitic gneiss, granitic gneiss, and amphibolite are abundant midcrustal lithologies. Anisotropy due to preferred mineral orientation is likely to be significant in upper and midcrustal regions. The bulk of the lower continental crust is chemically equivalent to gabbro, with velocities in agreement with laboratory measurements of mafic granulite. Garnet becomes increasingly abundant with depth, and mafic garnet granulite is the dominant rock type immediately above the Mohorovicic discontinuity. Average compressional wave velocities of common crustal rock types show excellent correlations with density. The mean crustal density calculated from our model is 2830 kg/m3, and the average SiO2 content is 61.8%.

Seismic velocity structure and composition of the continental crust: A global view

New measurements of compressional and shear wave velocities to hydrostatic pressures of 1 GPa are summarized for 678 rocks. Emphasis was placed on obtaining high-accuracy velocity measurements, which are shown to be critical in calculating Poisson's ratios from velocities. The rocks have been divided into 29 major groups for which velocities, velocity ratios, and Poisson's ratios are presented at several pressures. Observed Poisson's ratios for the monomineralic rocks compare favorably with theoretical Poisson's ratios calculated from single-crystal elastic constants. Plagioclase feldspar composition is important in understanding rock Poisson's ratios, since Poisson's ratio of albite increases from 0.28 to a predicted value of 0.31 for anorthite. Fe substitution for Mg in pyroxene and olivine also increases Poisson's ratio. Plotting rock compressional wave velocities versus Poisson's ratios reveals a triangular distribution bounded by quartzite with low compressional wave velocity and low Poisson's ratio, dunite with high compressional wave velocity and intermediate Poisson's ratio, and serpentinite with low compressional wave velocity and high Poisson's ratio. For common plutonic igneous rocks, there is a clear trend relating Poisson's ratio to composition, in which Poisson's ratio for granitic rocks increases from 0.24 to 0.29 as composition changes to gabbro and then decreases with decreasing plagioclase and increasing olivine contents to 0.25 in dunite. Changes in Poisson's ratio with progressive metamorphism of mafic and pelitic rocks correlate reasonably well with mineral reactions. There is no simple correlation between Poisson's ratio and felsic and mafic rock compositions; however, a linear correlation of increasing Poisson's ratio with decreasing SiO2 content is observed for rocks with 55 to 75 wt % SiO2. Average Poisson's ratios for continental and oceanic crusts are estimated to be 0.265 and 0.30, respectively.

Poisson's ratio and crustal seismology

Formation of Anisotropy in Upper Mantle Peridotites ‐ A Review

Water and hydrous minerals play a key role in geodynamic processes at subduction zones, however there are few seismological constraints on the presence of water within the subducting plate. Audet et al. model converted teleseismic waves to constrain the seismological properties of subducted oceanic crust from the Cascadia continental margin to its intersection with the forearc mantle. Their observations suggest that water is pervasively present in fluid form at high pore pressures, indicating that the megathrust is a low-permeability boundary. These results may hold important implications for our understanding of seismogenesis, subduction-zone structure and the mechanism of episodic tremor and slip. This study models converted teleseismic waves to constrain the seismological properties of subducted oceanic crust from the Cascadia continental margin to its intersection with the forearc mantle. The observations suggest that water is pervasively present in fluid form at high pore pressures, indicating that the megathrust is a low-permeability boundary. These results may hold important implications for our understanding of seismogenesis, subduction-zone structure and the mechanism of episodic tremor and slip. Water and hydrous minerals play a key part in geodynamic processes at subduction zones1,2,3 by weakening the plate boundary, aiding slip and permitting subduction—and indeed plate tectonics—to occur4. The seismological signature of water within the forearc mantle wedge is evident in anomalies with low seismic shear velocity marking serpentinization5,6,7. However, seismological observations bearing on the presence of water within the subducting plate itself are less well documented. Here we use converted teleseismic waves to obtain observations of anomalously high Poisson’s ratios within the subducted oceanic crust from the Cascadia continental margin to its intersection with forearc mantle. On the basis of pressure, temperature and compositional considerations, the elevated Poisson’s ratios indicate that water is pervasively present in fluid form at pore pressures near lithostatic values. Combined with observations of a strong negative velocity contrast at the top of the oceanic crust, our results imply that the megathrust is a low-permeability boundary. The transition from a low- to high-permeability plate interface downdip into the mantle wedge is explained by hydrofracturing of the seal by volume changes across the interface caused by the onset of crustal eclogitization and mantle serpentinization. These results may have important implications for our understanding of seismogenesis, subduction zone structure and the mechanism of episodic tremor and slip.

Seismic evidence for overpressured subducted oceanic crust and megathrust fault sealing.

Vp from 6.g to 6.5 km/s within gO m.y.; both the mantle and layer 3 are statistically anisotropic. Dredge lithologies consist predominantly of serpentinized ultramafics and mafic igneous rocks ranging from basalt to gabbro, the gabbro often showing evidence of fractionation. Metamorphism of mafic rocks from zeolite to amphibolite facies grade is common. Velocities in oceanic serpentinites and basalts are generally lower than layer 3 refraction velocities. Unaltered gabbros have compressional wave velocities of approximately 7.0 km/s, which is high for layer 3, together with shear wave velocities V, of 3.g km/s and values of Poisson's ratio Uof 0.30. Metabasites containing hornblende and plagioclase have values of Vp = 6.g km/s, V, = 3.g km/s, and u = 0.2g, in good agreement with those of layer 3. On the basis of petrology and velocity it is suggested that layer 3 is composed of hornblende metagabbro underlain by normal gabbro. In a model consistent with geophysical observations of heat flow, seismicity, gravity, and seismic structure at the ridge it is proposed that layer 2 and the upper levels of layer 3 form near the median valley but that deeper levels of layer 3 thicken for 40 m.y. by intermittent offridge intrusion fed from the underlying anomalous mantle. Ophiolites in such a model represent segments of thin immature ridge crest obducted onto continental margins during subduction of a spreading ridge.

Nikolas I. Christensen

Papers

Seismic velocity structure and composition of the continental crust: A global view

Poisson's ratio and crustal seismology

Formation of Anisotropy in Upper Mantle Peridotites ‐ A Review

Seismic evidence for overpressured subducted oceanic crust and megathrust fault sealing.

Structure and constitution of the lower oceanic crust