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

Chemical characteristics of fluid phase released from a subducted lithosphere and origin of arc magmas: Evidence from high-pressure experiments and natural rocks

Ultramafic inclusions from San Carlos, Arizona: Petrologic and geochemical data bearing on their petrogenesis

This handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures emphasizes terrestrial impact structures, field geology, and particularly the recognition and petrographic study of shock-metamorphic effects in terrestrial rocks. Individual chapters include: 1) Landscapes with Craters: Meteorite Impacts, Earth, and the Solar System; 2) Target Earth: Present, Past and Future; 3) Formation of Impact Craters; 4) Shock-Metamorphic Effects in Rocks and Minerals; 5) Shock-Metamorphosed Rocks (Impactities) in Impact Structures; 6) Impact Melts; 7) How to Find Impact Structures; and 8) What Next? Current Problems and Future Investigations.

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Traces of Catastrophe: A Handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures

Natural disturbances such as lava flows, landslides and glacial moraines, and human-damaged sites such as pavement, road edges and mine wastes often leave little or no soil or biological legacy. This 2003 book provided the first comprehensive summary of how plant, animal and microbial communities develop under the harsh conditions following such dramatic disturbances. The authors examine the basic principles that determine ecosystem development and apply the general rules to the urgent practical need for promoting the reclamation of damaged lands. Written for ecologists concerned with disturbance, landscape dynamics, restoration, life histories, invasions, modeling, soil formation and community or population dynamics, this book will also serve as an authoritative text for graduate students and a valuable reference for professionals involved in land management.

/pdf/primary-succession-and-ecosystem-rehabilitation-46xfnlnrj7.pdf

Primary Succession and Ecosystem Rehabilitation

Environmental effects of off-road vehicles.

Environmental effects of off-road vehicles. Impacts and management in arid regions.

Metasomatism of the Upper Mantle and the Genesis of Kimberlites and Alkali Basalts

Peridotite inclusions in basanite at Dish Hill and Siberia craters near Ludlow, California, contain abundant amphibole, smaller quantities of phlogopite and apatite, and rare plagioclase. These minerals occur as veins transecting the peridotite and as interstitial components. Veins of undeformed amphibole cut across wehrlite-lherzolite layering and metamorphic foliation in therzolite and offset kink bands in olivine, indicating that the amphibole postdates both anhydrous mineral layering and plastic deformation of the peridotite. Gradations in textural behavior of arirphibole frorn veins to interstitial modes of occurrence suggest that all of the amphibole and associated minerals are post-consolidation additions to the peridotite, unrelated to its original crystallization. This conclusion is supported by the presence of amphibole with the same textural relations in all types of peridotite, irrespective of their modal composition or degree of deformation and recrystallization. The source of the secondary hydrous minerals is not known. They may predate the basanite and have contributed to its formation, or they may have been derived from the basanite or related magma. The absence of amphibole phenocrysts in the basanite, as well as similar occurrences of amphibole in alpine-t1pe peridotites that were not carried into the crust by mafic alkaline magma, support the former view. Both alternatives are currently under investigation.

Howard G. Wilshire

Papers

Environmental effects of off-road vehicles.

Environmental effects of off-road vehicles. Impacts and management in arid regions.

Metasomatism of the Upper Mantle and the Genesis of Kimberlites and Alkali Basalts

Structural and textural relationships of amphibole and phlogopite in peridotite inclusions, Dish Hill, California

Recovery of soils and vegetation in a Mojave desert ghost town, Nevada, U.S.A