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

The functioning and stability of terrestrial ecosystems are determined by plant biodiversity and species composition1,2,3,4,5 However, the ecological mechanisms by which plant biodiversity and species composition are regulated and maintained are not well understood These mechanisms need to be identified to ensure successful management for conservation and restoration of diverse natural ecosystems Here we show, by using two independent, but complementary, ecological experiments, that below-ground diversity of arbuscular mycorrhizal fungi (AMF) is a major factor contributing to the maintenance of plant biodiversity and to ecosystem functioning At low AMF diversity, the plant species composition and overall structure of microcosms that simulate European calcareous grassland fluctuate greatly when the AMF taxa that are present are changed Plant biodiversity, nutrient capture and productivity in macrocosms that simulate North American old-fields increase significantly with increasing AMF-species richness These results emphasize the need to protect AMF and to consider these fungi in future management practices in order to maintain diverse ecosystems Our results also show that microbial interactions can drive ecosystem functions such as plant biodiversity, productivity and variability

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Mycorrhizal fungal diversity determines plant biodiversity, ecosystem variability and productivity

3.01 – Composition of the Continental Crust

Geochronological, structural, and sedimentological data provide the basis for a regional synthesis of the evolution of the Cordilleran retroarc thrust belt and foreland basin system in the western USA In this region, the Cordilleran orogenic belt became tectonically consolidated during Late Jurassic time ( 155 Ma) with the closure of marginal oceanic basins and accretion of fringing arcs along the western edge of the North American plate Over the ensuing 100 Myr, contractile deformation propagated approximately 1000 kilometers eastward, culminating in the formation of the Laramide Rocky Mountain ranges At the peak of its development, the retroarc side of the Cordillera was divided into five tectonomorphic zones, including from west to east the Luning-Fencemaker thrust belt; the central Nevada (or Eureka) thrust belt; a high-elevation plateau (the "Nevadaplano"); the topographically rugged Sevier fold-thrust belt; and the Laramide zone of intraforeland basement uplifts and basins Mid-crustal rocks beneath the Nevadaplano experienced high-grade metamorphism and shortening during Late Jurassic and mid- to Late Cretaceous time, and the locus of major, upper crustal thrust faulting migrated sporadically eastward By Late Cretaceous time, the middle crust beneath the Nevadaplano was experiencing decompression and cooling, perhaps in response to large-magnitude ductile extension and isostatic exhumation, concurrent with ongoing thrusting in the frontal Sevier belt The tectonic history of the Sevier belt was remarkably consistent along strike of the orogenic belt, with emplacement of regional-scale Proterozoic and Paleozoic mega- thrust sheets during Early Cretaceous time and multiple, more closely spaced, Paleo- zoic and Mesozoic thrust sheets during Late Cretaceous-Paleocene time Coeval with emplacement of the frontal thrust sheets, large structural culminations in Archean- Proterozoic crystalline basement developed along the basement step formed by Neoproterozoic rifting A complex foreland basin system evolved in concert with the orogenic wedge During its early and late history ( 155 - 110 Ma and 70 - 55 Ma) the basin was dominated by nonmarine deposition, whereas marine waters inundated the basin during its midlife ( 110 - 70 Ma) Late Jurassic basin development was controlled by both flexural and dynamic subsidence From Early Cretaceous through early Late Creta- ceous time the basin was dominated by flexural subsidence From Late Cretaceous to mid-Cenozoic time the basin was increasingly partitioned by basement-involved Laramide structures Linkages between Late Jurassic and Late Cretaceous Cordilleran arc- magmatism and westward underthrusting of North American continental lithosphere beneath the arc are not plainly demonstrable from the geological record in the Cordilleran thrust belt A significant lag-time ( 20 Myr) between shortening and coeval underthrust- ing, on the one hand, and generation of arc melts, on the other, is required for any linkage to exist However, inferred Late Jurassic lithospheric delamination may have provided a necessary precondition to allow relatively rapid Early Cretaceous continen- tal underthrusting, which in turn could have catalyzed the Late Cretaceous arc flare-up

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Late Jurassic to Eocene evolution of the Cordilleran thrust belt and foreland basin system, western U.S.A.

Chemical composition of the continental crust as revealed by studies in East China

Active and passive seismic experiments show that the southern Sierra, despite standing 1.8 to 2.8 kilometers above its surroundings, is underlain by crust of similar seismic thickness, about 30 to 40 kilometers. Thermobarometry of xenolith suites and magnetotelluric profiles indicate that the upper mantle is eclogitic to depths of 60 kilometers beneath the western and central parts of the range, but little subcrustal lithosphere is present beneath the eastern High Sierra and adjacent Basin and Range. These and other data imply the crust of both the High Sierra and Basin and Range thinned by a factor of 2 since 20 million years ago, at odds with purported late Cenozoic regional uplift of some 2 kilometers.

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Origin of high mountains in the continents: The Southern Sierra Nevada

Gravity data are now available for about 90 percent of the Sierra Nevada batholith, magnetic data for about 30 percent. About 1,000 density and 250 magnetic property measurements have been made on selected samples. Bouguer gravity anomalies decrease about 200 mgal, from −40 to −240 mgal, eastward across the batholith and gradually rise about 50 mgal on its east side. Gravity calculations show that these Bouguer anomalies are consistent with the general form of Bateman and Eaton's seismic model of the central Sierra Nevada and are inconsistent with the model of Carder and others. The calculations confirm Eaton's velocity-density relation for crystalline rocks. The major gravity step across the western Sierra Nevada is caused primarily by a mafic mountain root, not the batholith. Local anomalies of −5 to −40 mgal are associated with the most leucocratic felsic plutons. Correlation with available isotope ages shows the gravity anomalies to be largest and broadest over the youngest Cretaceous plutons, which suggests that they extend to greater depths than the older, exposed plutons.

Magnetic contrasts between plutons are generally much greater than density contrasts, and they provide a method for mapping some of the more magnetic plutons under some types of cover. The depth extent of magnetic plutons in Yosemite National Park is at least 7 km and probably greater. A regional magnetic low centered near the western edge of the Sierra Nevada correlates with a regional depression in the Curie isothermal surface, as evidenced by low surface heat flux. Because the depression in the isotherm allows for a greater thickness of material to be magnetic, the magnetic low is unexpected, and the relation is a noteworthy paradox.

Bouguer gravity anomalies correlate fairly well with initial Sr87/Sr86 ratios, and the magnetic field correlates with oxidation ratios of sampled granitic rocks. These data, together with physical property measurements of gneissic and peridotitic xenoliths discovered in volcanic pipes by J. P. Lockwood, form a foundation on which new models of the petrogenesis and structure of the batholith must be based.

Gravity and magnetic investigations of the Sierra Nevada batholith, California

The calibration of gravimeters has long been primarily the concern of geodesists involved in measuring large gravity differences, but recent developments suggest that the precision and stability of gravimeter calibrations may have greater geologic importance in the future. First, the use of high-speed computers and an increasing variety of supplemental data now make possible the geologic interpretation of gravity anomalies so small that they would not have been noticed in surveys made ten years ago. The kind of gravity interpretation that identifies small reefs and local accumulations of petroleum [McCulloh, 1967] often requires local increases in station density and the assurance that the calibration of meters used in all parts of a survey are compatible. Second, temporal changes of gravity have already been measured in connection with earthquakes [Barnes, 1966], volcanic eruptions [Iida et al, 1952], and the movement of ice caps [Behrendt, 1967]; they are now being considered for several other types of geologic processes.

Standardization of gravimeter calibrations in the geological survey

Bouguer gravity values at about 11,000 stations in east-central California range from −14 mgal near Merced to −274 mgal in Long Valley. Gravity lows in the west and south parts of the San Joaquin Valley and over local basins south and east of the Sierra Nevada are produced by large thicknesses of Upper Cretaceous and Cenozoic deposits. A large regional gravity low over the Sierra Nevada can be explained by isostatic compensation of the range together with the effect of the relatively low-density rocks of the Sierra Nevada batholith. A broad gravity ridge along the east side of the San Joaquin Valley shows excellent correlation with a similar magnetic ridge where the two sets of data are available, suggesting that both anomalies are caused by a dense, magnetic mass buried at an estimated depth of 5–10 miles. Seismic refraction measurements further indicate that the thickness of the earth9s crust under the valley is less than 12 miles. Thus, the anomalous mass is in the lower part of the earth9s crust and is conceivably related to the more mafic rocks of the earth9s upper mantle.

Anomalous gravity field in east-central california

During the past 10 years, personnel of the U. S. Geological Survey have made about 14,000 observations of the Earth's gravity field in California, about 13,000 in Nevada, 3,400 in Oregon, and about 1,000 in the Hawaiian Islands. The total number of stations established in the four states is slightly in excess of 30,000.

Much of this work has been done in support of the Geological Survey's basic programs of investigating natural resources, mapping regional geology, and studying geological processes [U. S. Geological Survey, 1963]. Several gravity investigations have also been conducted at the request of other Federal and state agencies to choose sites for underground nuclear weapons tests or to discover sources of such strategic commodities as uranium, boron, and water.

Howard W. Oliver

Papers

Origin of high mountains in the continents: The Southern Sierra Nevada

Gravity and magnetic investigations of the Sierra Nevada batholith, California

Standardization of gravimeter calibrations in the geological survey

Anomalous gravity field in east-central california

The U. S. Geological Survey's Gravity program in California, Hawaii, Nevada, and Oregon