ISSN:8755-1209    

DOI:10.1029/RG027i001p00001

年代:1989

数据来源: WILEY

摘要:

Sonic velocities of geologic fluids, such as volcanic magmas and geothermal fluids, can be as low as 1 m/s. Critical velocities in large rivers can be of the order of 1–10 m/s. Because velocities of fluids moving in these settings can exceed these characteristic velocities, sonic and supersonic gas flow and critical and supercritical shallow‐water flow can occur. The importance of the low characteristic velocities of geologic fluids has not been widely recognized, and as a result, the importance of supercritical and supersonic flow in geological processes has generally been underestimated. The lateral blast at Mount St. Helens, Washington, propelled a gas heavily laden with dust into the atmosphere. Because of the low sound speed in this gas (about 100 m/s), the flow was internally supersonic. Old Faithful Geyser, Wyoming, is a converging‐diverging nozzle in which liquid water refilling the conduit during the recharge cycle changes during eruption into a two‐phase liquid‐vapor mixture with a very low sound velocity. The high sound speed of liquid water determines the characteristics of harmonic tremor observed at the gyeser during the recharge interval, whereas the low sound speed of the liquid‐vapor mixture influences the fluid flow characteristics of the eruption. At the rapids of the Colorado River in the Grand Canyon, Arizona, the channel is constricted into the shape of a converging‐diverging nozzle by debris flows that enter from tributary canyons. Both subcritical and supercritical flow occur within the rapids. The transport capacity in the rapids can be so great that the river contours the channel to a characteristic shape. This shape can be used to interpret the flood history of the Colorado River over the past 10³–105years. The unity of fluid mechanics in these three natural phenomena is provided by the well‐known analogy between gas flow and shallow‐water flow in convergi

ISSN:8755-1209    

DOI:10.1029/RG027i001p00003

年代:1989

数据来源: WILEY

摘要:

The two Viking landers touched down on the surface of Mars in July and September of 1976. Viking Lander 1, renamed the Mutch Memorial Station (MMS), continued to operate for 3 Martian years in Chryse Planitia. The landing site is a rolling, sparsely cratered plain. Impact crater size‐frequency distributions demonstrate that only meters of vertical erosion have occurred over the lifetime of the surface. The site exhibits bedrock exposures, numerous rocks excavated during cratering events, indurated fine‐grained sediment (blocky material) mixed with dust and lithic fragments, and aeolian drifts that overlie the blocky material and which probably accumulated during global dust storms. Lander 2 operated for approximately 2 Martian years in Utopia Planitia, a region subjected to volcanic, impact, aeolian, and periglacial phenomena. Lander 2 is located west of the 100‐km‐diameter crater Mie, on or near the distal end of the continuous ejecta deposits. Polygonal ground suggests that desiccation or freeze‐thaw processes have occurred. The Lander 2 site exhibits numerous rocks excavated from craters and exposed by erosion, set in a matrix of crusty to cloddy sediment. Rare aeolian drifts overlie the crusty to cloddy material. Lander multispectral imaging of sediment exposures at both sites is best matched with iron‐rich, claylike weathering products called palagonites. The spectral characteristics are probably governed by thin deposits of dust from global and local dust storms. Darkest rocks have reflectance values that match iron‐rich igneous rocks, although most rocks are covered with indurated coatings of the palagonitelike products. Chemical compositions of sediments at the two landing sites are strikingly similar, suggesting an aeolian origin. The compositions suggest an iron‐rich igneous source rock and are matched by an assemblage of clays and salts. Samples of blocky materials were found to have about 50% more Cl and SO3than other samples, implying cementation by salts. The magnetic component in the sediment is silt‐sized or finer and may be maghemite or ultrafine‐grained hematite. No indigenous organic compounds were found. The materials were found to be highly reactive, releasing O2when H2O was added, oxidizing organic nutrients on board the landers and fixing atmospheric CO2. Clay‐ to silt‐sized particles were inferred for drift and crusty to cloddy materials. No terrestrial samples have been found that replicate all the attributes of the Viking samples. Approximately 10 µm of H2O ice accumulated during both winters at Lander 2 and each time evaporated over approximately 200 Martian days. Thin layers of bright red dust from global and local dust storms accumulated at both sites during the first 2 Martian years. During the third winter, strong winds generally scoured the MMS site, leaving behind a darker aeolian lag deposit. Bedrock at both sites formed billions of years ago, and rocks have been generated and weathered slowly over geologic time, but aeolian sediments accumulate, become cemented, and are subsequently eroded over time spans of millions of years or less. The lifetime of the sedimentary deposits at the landing sites may be governed by the 105‐ to 106‐year variations in climate due to quasi‐periodic changes in obliquity, eccentric

ISSN:8755-1209    

DOI:10.1029/RG027i001p00039

年代:1989

数据来源: WILEY

摘要:

The increasing use of natural unsaturated zones as repositories for landfills and disposal sites for hazardous wastes (chemical and radioactive) requires a greater understanding of transport processes in the unsaturated zone. For volatile constituents an important potential transport mechanism is gaseous diffusion. Diffusion, however, cannot be treated as an independent isolated transport mechanism. A complete understanding of multicomponent gas transport in porous media (unsaturated zones) requires a knowledge of Knudsen transport, the molecular and nonequimolar components of diffusive flux, and viscous (pressure driven) flux. The constitutive equations relating these flux components are available from the “dusty gas” model of Mason et al. (1967). This review presents a brief discussion of the underlying principles and interrelationships among each of the above flux mechanisms. Some aspects of these transport mechanisms are, to our knowledge, generally unrecognized in the Earth science literature. The principles underlying the transport mechanisms are illustrated with binary systems; the constitutive equations are then cast in forms thought to be most useful for the study of natural unsaturated zones. The viscous and diffusive fluxes are coupled in the constitutive equations through the Knudsen diffusivities; a knowledge of Knudsen diffusivities is necessary to calculate the viscous component of flux and pressure gradients. The Knudsen diffusivities can be calculated from measurements of the Klinkenberg effect. Two examples are presented showing that in natural systems, very small pressure gradients (1 Pa/m or less) can produce viscous fluxes greater than or equal to diffusive fluxes and that, conversely, pressure gradients of this magnitude can be generated by diffusive processes. The example calculations show that major concentration gradients can be developed for stagnant (zero flux, nonreactive) gases. A method is presented for approximating the viscous and diffusive flux components of gases in a multicomponent system from a knowledge of the concentration profiles of stagnant gases. In subsoil environments, argon and nitrogen are considered to be stagnant gases. Fick's laws are essentially, by definition, inadequate to deal with stagnant gases. In the examples presented, the error associated with estimating the total fluxes of nonstagnant gases by Fick's law, relative to stationary coordinates, ranges from a few percent to orders of magnit

ISSN:8755-1209    

DOI:10.1029/RG027i001p00061

年代:1989

数据来源: WILEY

摘要:

Hole 504B is by far the deepest hole yet drilled into the oceanic crust in situ, and it therefore provides the most complete “ground truth” now available to test our models of the structure and evolution of the upper oceanic crust. Cored in the eastern equatorial Pacific Ocean in 5.9‐m.y.‐old crust that formed at the Costa Rica Rift, hole 504B now extends to a total depth of 1562.3 m below seafloor, penetrating 274.5 m of sediments and 1287.8 m of basalts. The site was located where the rapidly accumulating sediments impede active hydrothermal circulation in the crust. As a result, the conductive heat flow approaches the value of about 200 mW/m² predicted by plate tectonic theory, and the in situ temperature at the total depth of the hole is about 165°C. The igneous section was continuously cored, but recovery was poor, averaging about 20%. The recovered core indicates that this section includes about 575 m of extrusive lavas, underlain by about 200 m of transition into over 500 m of intrusive sheeted dikes; the latter have been sampled in situ only in hole 504B. The igneous section is composed predominantly of magnesium‐rich olivine tholeiites with marked depletions in incompatible trace elements. Nearly all of the basalts have been altered to some degree, but the geochemistry of the freshest basalts is remarkably uniform throughout the hole. Successive stages of on‐axis and off‐axis alteration have produced three depth zones characterized by different assemblages of secondary minerals: (1) the upper 310 m of extrusives, characterized by oxidative “seafloor weathering“; (2) the lower extrusive section, characterized by smectite and pyrite; and (3) the combined transition zone and sheeted dikes, characterized by greenschist‐facies minerals. A comprehensive suite of logs and downhole measurements generally indicate that the basalt section can be divided on the basis of lithology, alteration, and porosity into three zones that are analogous to layers 2A, 2B, and 2C described by marine seismologists on the basis of characteristic seismic velocities. Many of the logs and experiments suggest the presence of a 100‐ to 200‐m‐thick layer 2A comprising the uppermost, rubbly pillow lavas, which is the only significantly permeable interval in the entire cored section. Layer 2B apparently corresponds to the lower section of extrusive lavas, in which original porosity is partially sealed as a result of alteration. Nearly all of the logs and experiments showed significant changes in in situ physical properties at about 900–1000 m below seafloor, within the transition between extrusives and sheeted dikes, indicating that this lithostratigraphic transition corresponds closely to that between seismic layers 2B and 2C and confirming that layer 2C consists of intrusive sheeted dikes. A vertical seismic profile conducted during leg 111 indicates that the next major transition deeper than the hole now extends—that between the sheeted dikes of seismic layer 2C and the gabbros of seismic layer 3, which has never been sampled in situ—may be within reach of the next drilling expedition to hole 504B. Therefore despite recent drilling problems deep in the hole, current plans now include revisiting hole 504B for further drilling and experiments when the Ocean Drilling Program retur

ISSN:8755-1209    

DOI:10.1029/RG027i001p00079

年代:1989

数据来源: WILEY

摘要:

This review traces early research on the Earth's magnetic environment, covering the period when only ground‐based observations were possible. Observations of magnetic storms (1724) and of perturbations associated with the aurora (1741) suggested that those phenomena originated outside the Earth; correlation of the solar cycle (1851) with magnetic activity (1852) pointed to the Sun's involvement. The discovery of solar flares (1859) and growing evidence for their association with large storms led Birkeland (1900) to propose solar electron streams as the cause. Though laboratory experiments provided some support, the idea ran into theoretical difficulties and was replaced by Chapman and Ferraro's notion of solar plasma clouds (1930). Magnetic storms were first attributed (1911) to a “ring current” of high‐energy particles circling the Earth, but later work (1957) recognized that low‐energy particles undergoing guiding center drifts could have the same effect. To produce the ring current and aurora, plasma cloud particles required some way of penetrating the “Chapman‐Ferraro cavity”: Alfvén (1939) invoked an electric field, but his ideas met resistance. The picture grew more complicated with observations of comets (1943, 1951) which suggested a fast “solar wind” emanating from the Sun's corona at all times. This flow was explained by Parker's theory (1958), and the permanent cavity which it produced around the Earth was later named the “magnetosphere” (1959). As early as 1905, Birkeland had proposed that the large magnetic perturbations of the polar aurora reflected a “polar” type of magnetic storm whose electric currents descended into the upper atmosphere; that idea, however, was resisted for more than 50 years. By the time of the International Geophysical Year (1957–1958), when the first artificial satellites were launched, most of the important features of the magnetosphere had been glimpsed, but detailed understanding had to

ISSN:8755-1209    

DOI:10.1029/RG027i001p00103

年代:1989

数据来源: WILEY

摘要:

The presence of radiatively active gases in the Earth's atmosphere (water vapor, carbon dioxide, and ozone) raises its global mean surface temperature by 30 K, making our planet habitable by life as we know it. There has been an increase in carbon dioxide and other trace gases since the Industrial Revolution, largely as a result of man's activities, increasing the radiative heating of the troposphere and surface by about 2 W m−2. This heating is likely to be enhanced by resulting changes in water vapor, snow and sea ice, and cloud. The associated equilibrium temperature rise is estimated to be between 1 and 2 K, there being uncertainties in the strength of climate feedbacks, particularly those due to cloud. The large thermal inertia of the oceans will slow the rate of warming, so that the expected temperature rise will be smaller than the equilibrium rise. This increases the uncertainty in the expected warming to date, with estimates ranging from less than 0.5 K to over 1 K. The observed increase of 0.5 K since 1900 is consistent with the lower range of these estimates, but the variability in the observed record is such that one cannot necessarily conclude that the observed temperature change is due to increases in trace gases. The prediction of changes in temperature over the next 50 years depends on assumptions concerning future changes in trace gas concentrations, the sensitivity of climate, and the effective thermal inertia of the oceans. On the basis of our current understanding a further warming of at least 1 K seems likely. Numerical models of climate indicate that the changes will not be uniform, nor will they be confined to temperature. The simulated warming is largest in high latitudes in winter and smallest over sea ice in summer, with little seasonal variation in the tropics. Annual mean precipitation and runoff increase in high latitudes, and most simulations indicate a drier land surface in northern mid‐latitudes in summer. The agreement between different models is much better for temperature than for changes in the hydrological cycle. Priorities for future research include developing an improved representation of cloud in numerical models, obtaining a better understanding of vertical mixing in the deep ocean, and determining the inherent variability of the ocean‐atmosphere system. Progress in these areas should enable detection of a man‐made “greenhouse” warming within the next

ISSN:8755-1209    

DOI:10.1029/RG027i001p00115

年代:1989

数据来源: WILEY

摘要:

This review presents results from magnetovariation fields recorded by two‐dimensional arrays of magnetometers. The emphasis is on the conductive structures mapped and studied and their tectonic implications. Eleven arrays were operated in North America between 1967 and 1985. A region of highly conductive uppermost mantle and/or lower crust, extending at least from 33°N to 54°N, has been shown to extend beneath much of the continent west of the Rocky Mountains. Two recent investigations are discussed in more detail: those of conductive structures under the Canadian Rockies and EMSLAB (Electromagnetic Sounding of the Lithosphere and Beyond) array results for Washington and Oregon, including a conductive strip beneath the Cascades volcanoes. Correlations with high heat flow and seismic parameters make it reasonable to attribute these regional conductors to partial melting and/or hot saline water. Within the craton the North American Central Plains conductor is discussed; this narrow, crustal feature is associated with a major fracture zone and age boundary, which may be a plate boundary of Proterozoic time. An array beneath the auroral electrojet, for study of the external currents, is noted. Results from four array studies in southern Africa are considered. Two of these relate to the Southern Cape Conductive Belt, which correlates not with high heat flow but with a large static magnetic anomaly. The magnetovariation, magnetic and gravity anomalies, and geological data can be accounted for if an accumulation of ophiolitic rocks including serpentine is present in the lower crust. Such an accumulation may have formed at a subduction in Proterozoic time. Two other arrays delineated a conductive belt associated with the southwestern end of the African rift system, in Namibia. Subsequent resistivity soundings located a conductor of resistivity 10 ohm m at less than 4 km depth. Two array studies in India follow, one in the north and one in the south. The array in the Himalayan foothills and Indo‐Gangetic plain mapped a conductor transverse to the regional strike; this may be a fracture zone produced in the interplate collision, with a rise of asthenospheric material such as fluids into the fractures. The southern Indian array reveals crustal conductors under the Indo‐Sri Lankan graben and another under the Comorin Ridge southwest of the tip of the subcontinent. Array studies in central and southeastern Australia and in Fennoscandia, in regions without large local conductivity anomalies, have been used to study conductivity variation with depth. A note on the Carpathian crustal anomaly is included as an example of combination of response parameters secured at different times to form a “virtu

ISSN:8755-1209    

DOI:10.1029/RG027i001p00141

年代:1989

数据来源: WILEY

摘要:

El Niño events—anomalous warmings of the tropical Pacific with associated climatic and economic impacts around the globe—have occurred at several‐year intervals since before written records began with the logs of Francisco Pizarro in 1525. In this review, the history of El Niño research is traced from its beginnings through the key innovations of Bjerknes and Wyrtki to the unusual 1982–1983 event. Recent research is then reviewed, with detailed discussions of two important processes: instability growth and vacillation between climate states. Throughout the paper there are adjunct discussions of extraregional teleconnections, ecological impacts, and research on El Niño in the ancient record. The final section discusses the present paradigm for vacillations between El Niño and non‐El Niño states and speculates on the possibly chaotic nature of El Niño. El Niño and its atmospheric counterpart, the Southern Oscillation, appear to occur as an internal cycle of positive and negative feedbacks within the coupled ocean‐atmosphere climate system of the tropical Pacific, although hypotheses based on external forcing also exist. All events are preceded by westerly wind anomalies on the equator near the date line. Baroclinic equatorial Kelvin waves are generated, propagating eastward toward South America where they depress the thermocline and raise sea level, while the deep, upper ocean reservoir of warm water in the western Pacific is depleted. Sea surface temperature (SST) anomalies in the cool eastern Pacific occur primarily because the normal source of cold water is depressed below the reach of mixing and upwelling processes. In the central equatorial Pacific, eastward advection by anomalous zonal flows is the principal mechanism. Nonlinear heat transfer to the lower atmosphere creates a positive ocean‐atmosphere feedback resulting in the unstable growth of anomalies along the equator. Much of the present research aims at determining how the ocean‐atmosphere system vacillates between the El Niño and non‐El Niño states. Coupled models suggest that a longer time scale, negative‐feedback process produces the transitions: at the apex of an El Niño development an anomalous atmospheric convection above the areas of maximum SST produces areas of reduced upper layer thickness in the off‐equatorial ocean, which slowly propagate westward to the western boundary as Rossby waves and back to the central equatorial Pacific as upwelling Kelvin waves, reestablishing the normal cooling process. A similar negative feedback of opposite sign completes the second half of an oscillation, returning again to the El Niño state. However, the notion that El Niño‐Southern Oscillation variability results only from an internal feedback process is still highly contentious, and a number of external fo

ISSN:8755-1209    

DOI:10.1029/RG027i001p00159

年代:1989

数据来源: WILEY