Showing papers in "Annual Review of Earth and Planetary Sciences in 1978"

State of Stress in the Earth's Crust

Abstract: Measurements ofthe stress field within the crust can provide perhaps the most useful information concerning the forces responsible for various tectonic processes, such as earthquakes Advances in knowledge of the state of stress at mid-crustal depths are essential if further progress is to be made toward solving a broad class of problems in geodynamics Most stress measurements have been, and will continue to be, motivated by engineering needs rather than the needs of geologists engaged in fundamental research Knowledge of the state of stress is critical to the design of underground excavations for mining and for nuclear waste disposal (eg Jaeger & Cook 1969, pp 435-64) The massive hydraulic fracturing of formations in oil and gas fields to stimulate production is another application for which knowledge of the stress field at depth is very important and, in fact, many of the deeper stress determinations have been by-products of these "hydrofrac" operations (eg Howard & Fast 1970) A recent and exciting application of hydraulic fracturing is the Hot-Dry-Rock Geothermal Energy Program (Aamodt 1977) Heat is extracted from the rock by circulating fluid down a pipe into hot rock and then up through a second pipe A large fracture connecting the two pipes serves as the heat exchanger Knowing the state of stress is critical in the solution to the problem of creating and maintaining such a crack There is little argument about the applicability of information on the state of stress to these and many other engineering problems The application of stress measurements to the solution of problems in tectonics is not so straightforward as in engineering design Whereas the engineer is concerned with the stress field affecting the rock, the geologist attempts to deduce the processes that might have caused the stresses Before the measured stress field can be related

The Interstellar Wind and its Influence on the Interplanetary Environment

Abstract: The existence of diffuse matter between the stars has been known for decades. How­ ever, only recently have we known of its presence in the immediate environs of the solar system. The discovery in 1970 of neutral interstellar gas pervading the solar system was a by-product of space exploration -specifically of the study of planetary atmospheres by ultraviolet photometry and spectroscopy. Because most of the work in this field was done by aeronomers using techniques borrowed from studies of the ultraviolet airglow, these new findings have not yet been widely disseminated in the astronomical community. However, studies of the interstellar medium have much to gain from these new results. To illustrate this point, wc considcr the hierarchy of sizes of neutral interstellar gas in numbers of parsecs ( 1 pc = 3 X 1018 cm): the galaxy ( 104), large interstellar clouds and spiral arms ( 102-103), small clouds (10°), intercloud eddies (10-2-10-1), and the mean free path ( 10-3-10-2). Radio­ astronomical techniques probe the largest scales ( 102-104), and absorption spectroscopy applies to smaller distances (101_103). In contrast, the ultraviolet back­ scattering of solar resonance lines from the local gas constitutes literally a micro­ scopic probe on a scale of 10-4 pc. It is as if a geologist were suddenly given the magnifying power to examine an individual atom of a rock sample, where before he was able to view only the individual grains. Unfortunately, we have only a single sample of the interstellar gas that we can study with such microscopic precision. Offsetting this disadvantage is the fact that for this "tiny" scale, the relevant laws of physics are simply those of single-particle motion (this is not true of the ionized component of the gas), and it is straightforward to infer the dynamical and chemical properties of the neutral gas. These properties can in turn be extrapolated to larger scales to infer the gas temperature and the chemical composition of at least the smallest irregularity in which the Sun is imbedded. This extrapolation is far safer

Chemical Exchange Across Sediment-Water Interface

Abstract: The sediment-water interface separates a mixture of solid sediment and interstitial water from an overlying body of water. Wherever sediment accumulates, growth of the sediment pile is achieved by sedimentation of solid particles and inclusion of water in the pore spaces among the particles. Growth or erosion of sediments on the bottom of a body of water results in a rise or fall of the sediment-water interface relative to an observer on shore. If, however, an imaginary observer were positioned at the sediment-water interface, he would observe the sediment particles and water moving downward past him when the sediment column grows, or moving upward when the sediment is being eroded. In considering various chemical, physical, and biological processes taking place near the sediment-water interface, it is often more convenient to regard the interface as a plane of reference, as if one were an imaginary observer who sees sediment particles and water flow by him while he balances him­ self on the interface. Arrival of sediment particles and inclusion of water in the interparticle pores space are the two major fluxes of materials across the sediment-water interface. The rates of sediment deposition vary from the low values of millimeters per 1000 years in the pelagic ocean up to 1 centimeter per 1 year in lakes and near-shore oceanic areas. Sediments near the interface commonly contain 70--90 vol % of water, but upon com­ paction the volume fraction of water (called the sediment porosity) usually decreases to 40-60% within a few tens of centimeters to a few meters below the interface. Representative values of the mass fluxes of solids and dissolved materials across the sediment-water interface can be estimated for areas of low and high deposition rates as follows. The slower sedimentation rates in the ocean are of the order of 5 x 10-4 cm yr-1 (5 mm 1000 yr-1), and the higher rates in lakes and coastal sections are

Mantle Convection Models

Abstract: The term thermal convection implies a flow dri"ven by thermal density variations, these variations being in turn maintained against the action of diffusion by the advection of heat by the flow itself. The energetics is a balance between viscous dissipation and the release of potential energy by the rising and sinking of warm and cold material. Convection connotes a buoyancy-driven flow in a fluid, but it can apply equally well to a solid medium so long as the solid can be continuously deformed. There is ample evidence that earth-forming materials at temperatures above about lOOO°C are readily deformed on geologic time scales, and therefore there is no obvious inconsistency in applying concepts derived from an understanding of convection to flows in the earth's interior. In fact, there seems to be little alternative to convection as the source of the large-scale motions described by plate tectonics. The simplest argument to this effect is that the release of potential energy by a con­ vective process is the only recognized energy source large enough to balance the dissipation associated with plate motions. Potential energy can be continuously released only if the system is in some way being heated, and in the earth, this is accomplished in large part by radiogenic heating. Energy arguments suggest that plate tectonics is a surface expression of convec­ tion in the earth but gives no indication as to the thermal and flow structure within the system. In the absence of direct measurements, such specific information requires explicit models. It is evident from the discussion below that our present understanding of plate tectonics as a convective process is not based on any single model or simulation, but instead depends on a synthesis of ideas derived from a variety of models, each treating a specific aspect of the larger problem. The purpose of this review then is to discuss the different types of models that contribute to this synthesis. The reader should realize that in the absence of a definitive model, the choice of topics and discussion that follows is perforce somewhat subjective.

The Probable Metazoan Biota of the Precambrian as Indicated by the Subsequent Record

Abstract: In recent years Precambrian sedimentary rocks have been receiving much attention in our efforts to trace the beginnings and understand the development of the diverse biota known from the Phanerozoic. Because shelly fossils suddenly appeared at the beginning of the Cambrian, most of them without recognizable ancestors in the immediately preceding Precambrian, many investigators, SUGh as Cloud (1968, 1976) and Stanley (1973, 1976), have suggested that eucaryotes and subsequently metazoans did not appear until relatively late in the earth's history. Cloud (1976) concluded that the first metazoans appeared about 700 m.y. ago, and the eucaryotic cell prob­ ably about 1300 m.y. ago. In contrast to these late dates he further suggests that the first autotrophs may have appeared as early as about 3800 m.y. ago. Only a few workers have suggested that the Metazoa appeared earlier than con­ cluded by Cloud. Glaessner (1972, p. 46), after noting that the Ediacaran metazoan fauna has been shown to have a time range of about 600 to 700 m.y. BP, com­ mented, " . . . their differentiation indicates a long history of coelomates preceding that date" and "The well defined biostratigraphic, chronostratigraphic and evolu­ tionary identity of the Ediacara fauna confirms the view that a sequence of three stages in the early history of the Metazoa has to be explained rather than the supposed sudden appearance of most of the known phyla at the beginning of Cambrian or of Phanerozoic time." I have argued (Durham 1971a) on theoretical grounds that the deuterostomate metazoans may have appeared "between 800 million years as a minimum and about 1700 million years as a maximum before the present . . .. " Thus two strongly contrasting viewpoints are held. The scarcity of conventional evidence of metazoans in pre-Ediacaran sediments has overawed most investigators into concluding that no metazoans were present. Their conclusions became widely accepted when this evidence was added to the prevalent ideas about

Quantifying biostratigraphic correlation

Abstract: The historical development of concepts of biostratigraphic correlation has been reviewed recently by Hancock (1977). Stratigraphic correlation had its origin in the very practical problems faced by the British civil engineer William Smith in the course of his work in Somerset. From 1793 he was busy classifying the strata in the region of Bath and relating them to the sequence in the north of England, using fossils as indicators to establish the original continuity of lithologic units. Smith's interest in fossils was wholly practical ; they enabled him to discriminate between units having similar appearance, and to determine what the sequence might be in places where only small exposures were available to view. By 1799 he had prepared the first stratigraphic table of the English Mesozoic, and in 1815 he issued a geologic map of England in 15 sheets. George Cuvier, a French contemporary of Smith's, provided the concept of catastrophism as the logical basis for stratigraphic correlation of France. Cuvier's idea of catastrophism was based chiefly on observations in the Paris Basin, where the successive marine invasions during the Tertiary each left homogeneous assemblages of organisms as a paleontologic record (Cuvier & Brongniart 1808). Within each of the transgressive-regressive sequences, there is no evidence for the gradual evolution of species, but between the cycles there are wholesale changes in fauna. It was a logical conclusion on Cuvier's part to suggest a series of global catastrophies that annihilated life, and a series of creations that produced new forms of l ife. Cuvier's theory of catastrophism would remain merely an interesting relict were it not for the fact that he exerted a considerable influence on the development of geologic thought in France and on that of d'Orbigny in particular, so that it became an integral part of the legacy of stratigraphy. Alcide d'Orbigny created the term "stage" in 1851; he envisioned it as a particular landscape in the earth's history. By this he meant a particular distribution of land and

The Galilean Satellites of Jupiter: Four Worlds

Abstract: A broad survey is presented of our current knowledge of the Galilean satellites. Attention is given to the physical properties (size, masses, densities, and rotation) and to the surface properties (albedo, surface markings, composition, and physical state) of the satellites. In particular, Io's atmosphere is considered with emphasis on the sodium, hydrogen and sulfur clouds and the ionosphere. The atmospheres of the other satellites are examined briefly and consideration is given to models of planetary origin, evolution and interior structure.

Photochemistry and Dynamics of the Ozone Layer

Abstract: The paper presents a broad review of the photochemical and dynamic theories of the ozone layer. The two theories are combined into the MIT three-dimensional dynamic-chemical quasi-geostrophic model with 26 levels in the vertical spaced in logarithmic pressure coordinates between the ground and 72-km altitude. The chemical scheme incorporates the important odd nitrogen, odd hydrogen, and odd oxygen chemistry, but is simplified in the sense that it requires specification of the distributions of NO2, OH and HO2. The prognostic equations are the vorticity equation, the perturbation thermodynamic equation, and the global mean and perturbation continuity equations for ozone; diagnostic equations include the hydrostatic equation, the balance condition, and the mass continuity equation. The model is applied to the investigation of the impact of supersonic aircraft on the ozone layer.

Organic matter in the earth's crust

Abstract: There are about 830 x 1015 g of living matter on the land surface of the Earth and in the oceans. Annual productivity is estimated at 78 x 1015 g (Revelle & Munk 1977). We do not know how long this productivity has been maintained or how uniform the annual rate has been. A rough guess is that the total produced during the Earth's history has been between 5 x 1024 and 5 x 1025 g. Most of this has been consumed and returned to CO2, but about 19 x 1021 g of organic or elemental carbon may be found in the crust of the Earth (Hunt 1972, 1977). Most of this carbon is in sedimentary rocks. Some has been exposed for long periods of time to temperatures above 200°C and has been converted to graphite. Part has been deposited in relatively concentrated forms such as peat, lignite, or coal. Other portions of a different composition have given rise to petroleum. The processes involved are quite inefficient. Hunt (1972) estimated that only 2% of the carbon in sedimentary rocks becomes the carbon of petroleum. Of this, only 0.5% of the petroleum finds its way to a reservoir accumulation. Thus, the overall efficiency is only 0.01%. Nevertheless, even with so tiny an efficiency, reservoirs contain nearly 100 billion tons, with a value of trillions of dollars. Obviously, the efficiency of producing recoverable petroleum from organic matter varies with circumstances; in some instances nature is more efficient than in others. Under­ standing of such processes is intellectually and economically challenging. About one part in a thousand of the living matter produced annually escapes being converted to CO2• A small fraction of the chemicals persist relatively un­ changed for long periods of time. They have been called biochemical fossils. Among the compounds that have sufficient stability to last are, for example, fatty acids. During the late 1950s and most of the 1960s a principal activity of organic geochemists was to discover biochemical fossils and to identify the chemical processes involved when compounds known to be present in living matter were converted to somewhat altered, but recognizable, forms. For example, in nature, chlorophyll is rapidly degraded. However, two of its components, phytol and the porphin structure, are preserved in only slightly modified form for hundreds of millions of years. Similarly slightly modified forms of sterols and terpenes can

Effects of Waste Disposal Operations in Estuaries and the Coastal Ocean

Abstract: Since about 1800, human activities have significantly altered shorelines, bottom topography, sediment characteristics, and marine life in estuarine and coastal waters. The first man-induced changes of thc ocean were restricted to nearshore waters and most were small, scattered, and primarily associated with food production or port development (Klimm 1956). Sediment deposition caused by erosion of agricultural lands resulted in extensive delta building and shoreline changes in the Persian Gulf, the Adriatic Sea, and the Mississippi Delta, to name a few examples (Davis 1956). Diking and draining of wetlands, shallow ocean areas, and lakes to form agricultural land has greatly altered shorelines in the Netherlands and England (Davis 1956). Not all wastes are placed directly in the ocean. Some are brought there by normal sediment transport processes. For example, mining is a prolific sediment producer and has also caused extensive changes in wetlands and shorelines due to downstream sediment deposition. Perhaps the best studied case is the hydraulic mining of gold in California's Sierra Nevada (Gilbert 1917). Between 1850 and 1914, 1.8 x 109 m3 of debris was mobilized by mining and erosion in the San Francisco Bay drainage system. About 1.1 x 109 m 3 was deposited in the bay system or on wetlands (Gilbert 1917). Movement and deposition of this material apparently persisted for approxi­ mately 50 years after cessation of mining (Smith 1965). More recently, rapid growth of coastal cities and associated industry has led to greatly incrcascd construction, demolition, and dredging, and the disposal of wastes produced from these activities has emerged as a geologic process causing significant changes in coastal areas. Because waste solids have caused the most obvious geologic changes, this review deals primarily with these materials. Effects of dissolved wastes such as nutrients have been extensively discussed elsewhere (NAS 1969, Likens 1972).