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

This essential reference for graduate students and researchers provides a unified treatment of earthquakes and faulting as two aspects of brittle tectonics at different timescales. The intimate connection between the two is manifested in their scaling laws and populations, which evolve from fracture growth and interactions between fractures. The connection between faults and the seismicity generated is governed by the rate and state dependent friction laws - producing distinctive seismic styles of faulting and a gamut of earthquake phenomena including aftershocks, afterslip, earthquake triggering, and slow slip events. The third edition of this classic treatise presents a wealth of new topics and new observations. These include slow earthquake phenomena; friction of phyllosilicates, and at high sliding velocities; fault structures; relative roles of strong and seismogenic versus weak and creeping faults; dynamic triggering of earthquakes; oceanic earthquakes; megathrust earthquakes in subduction zones; deep earthquakes; and new observations of earthquake precursory phenomena.

The Mechanics of Earthquakes and Faulting

3.01 – Composition of the Continental Crust

The overall mechanics of fold-and-thrust belts and accretionary wedges along compressive plate boundaries is considered to be analogous to that of a wedge of soil or snow in front of a moving bulldozer. The material within the wedge deforms until a critical taper is attained, after which it slides stably, continuing to grow at constant taper as additional material is encountered at the toe. The critical taper is the shape for which the wedge is on the verge of failure under horizontal compression everywhere, including the basal decollement. A wedge of less than critical taper will not slide when pushed but will deform internally, steepening its surface slope until the critical taper is attained. Common silicate sediments and rocks in the upper 10-15 km of the crust have pressure-dependent brittle compressive strengths which can be approximately represented by the empirical Coulomb failure criterion, modified to account for the weakening effects of pore fluid pressure. A simple analytical theory that predicts the critical tapers of subaerial and submarine Coulomb wedges is developed and tested quantitatively in three ways: First, laboratory model experiments with dry sand match the theory. Second, the known surface slope, basal dip, and pore fluid pressures in the active fold-and-thrust belt of western Taiwan are used to determine the effective coefficient of internal friction within the wedge,/x = 1.03, consistent with Byerlee's empirical law of sliding friction,/at, = 0.85, on the base. This excess of internal strength over basal friction suggests that although the Taiwan wedge is highly deformed by imbricate thrusting, it is not so pervasively fractured that frictional sliding is always possible on surfaces of optimum orientation. Instead, the overall internal strength apparently is controlled by frictional sliding along suboptimally oriented planes and by the need to fracture some parts of the observed geometrically complex structure for continued deformation. Third, using the above values of/at, and/x, we predict Hubbert-Rubey fluid pressure ratios X = Xt, for a number of other active subaerial and submarine accretionary wedges based on their observed tapers, finding values everywhere in excess of hydrostatic. These predicted overpressures are reasonable in light of petroleum drilling experience in general and agree with nearby fragmentary well data in specific wedges where they are available. The pressure-dependent Coulomb wedge theory developed here is expected to break down if the decollement exhibits pressure-independent plastic behavior because of either temperature or rock type. The effects of this breakdown are observed in the abrupt decrease in taper where wedge thicknesses exceed about 15 km, which is the predicted depth of the brittle-plastic transition in quartz-rich rocks for typical geothermal gradients. We conclude that fold-and-thrust belts and accretionary wedges have the mechanics of bulldozer wedges in compression and that normal laboratory fracture and frictional strengths are appropriate to mountain-building processes in the upper crust, above the brittle-plastic transition.

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Mechanics of fold-and-thrust belts and accretionary wedges

Major and trace element composition of the depleted MORB mantle (DMM)

High-temperature creep of olivine.

To investigate high-temperature creep and kinetic decomposition of nickel orthosilicate (Ni2SiO4), aggregates containing ∼3 vol% amorphous SiO2 have been deformed in uniaxial compression at a total pressure of one atomsphere. Twenty-three samples with grain sizes (d) from 9 to 30 μm were deformed at temperatures (T) from 1573 to 1813 K, differential stresses (σ) from 3 to 20 MPa, and oxygen fugacities (f
o
2) from 10-1 to 105 Pa. At temperatures up to 1773 K, the steady-state creep rate (ɛ) can be described by the flow law 
$$\dot \varepsilon = 4.0{\text{ x }}10^8 \frac{{\sigma ^{1.1} }}{{d^{2.7} }}f_{O2}^0 {\text{ exp }}\left( {\frac{{ - 410kJ/mol}}{{RT}}} \right)\left[ {s^{ - 1} } \right]$$

 Under these conditions, the dominant deformation mechanism is Coble creep, rate-limited by grain boundary diffusion of silicon. At temperatures between 1803 and 1813 K, the samples of Ni2SiO4 plus amorphous SiO2 exhibit enhanced creep rates and increased ductility. This behavior is associated with fast diffusion through a thin intergranular film produced by kinetic decomposition of Ni2SiO4 under the imposed stress gradient.

High-temperature creep and kinetic decomposition of Ni2SiO4

The high temperature plastic deformation of (Ti, V)C alloys was studied by load relaxation experiments. The deformation experiments were performed at 1200 to 1600° C on fully dense, single phase, polycrystalline TiC, TiC-25% VC, TiC-50% VC and TiC-75% VC, prepared by hot-pressing. The analysis of logσ-log$\mathop \varepsilon \limits^\cdot $ data along with the evaluation of strain rate sensitivity ∼ 0.9 to 1.5 and the grain size dependence ∼ 2.6 indicates that the high temperature plastic deformation of these carbides occurs by grain-boundary diffusion, which is in accordance with hot-pressing results. Also, transmission electron microscopy of deformed samples, revealing little evidence for dislocation activity, supported this conclusion. The diffusion coefficients obtained in this study are in good agreement with those obtained for boundary diffusion, with the activation energy ∼ 390 kJ mol−1 for TiC and ∼ 307±13 kJ mol−1 for other compositions. A sharp decrease in the strength of (Ti, V)C alloys with the addition of VC is associated with the softening and/or melting of second phase material accumulated along the grain boundaries.

Inelastic deformation of (Ti, V)C alloys

An H2O2-assisted, sol−gel synthesis route to precursors to magnesium silicate thin films is described. Films having ratios of Mg to Si from 1 to 3 were prepared by spin-coating. On substrates such ...

David L. Kohlstedt

Papers

High-temperature creep of olivine.

High-temperature creep and kinetic decomposition of Ni2SiO4

Inelastic deformation of (Ti, V)C alloys

Sol−Gel Synthesis and Characterization of Magnesium Silicate Thin Films

Constricted segments of faulted dipoles in germanium