ISSN:0148-0227    

DOI:10.1029/93JB03265

年代:1994

数据来源: WILEY

摘要:

We have found that image data acquired with NASA's airborne Thermal Infrared Multispectral Scanner (TIMS) can be used to make estimates of the SO2content of volcanic plumes. TIMS image data are most applicable to the study of partially transparent SO2plumes, such as those released during quiescent periods or nonexplosive eruptions. The estimation procedure is based on the LOWTRAN 7 radiative transfer code, which we use to model the radiance perceived by TIMS as it views the ground through an SO2plume. The input to the procedure includes the altitudes of the aircraft and ground, the altitude and thickness of the SO2plume, the emissivity of the ground, and altitude profiles of the atmospheric pressure, temperature, and relative humidity. We use the TIMS data to estimate both ground temperatures beneath a plume and SO2concentrations within a plume. Applying our procedure to TIMS data acquired over Mount Etna, Sicily, on July 29, 1986, we estimate that the SO2flux from the volcano was approximately 6700 t d−1. The use of TIMS to study SO2plumes represents a bridge between highly localized methods, such as correlation spectroscopy or direct sampling, and small‐scale mapping techniques involving satellite instruments such as the Total Ozone Mapping Spectrometer or Microwave Limb Sounder. We require further airborne experiments to refine our estimation procedure. This refinement is a necessary preparation for the scheduled 1998 launch of the Advanced Spaceborne Thermal Emission and Reflectance Radiometer, which will allow large‐scale multispectral thermal infrared image data to be collected over virtually any volcano on Earth at least once every 16

ISSN:0148-0227    

DOI:10.1029/93JB02062

年代:1994

数据来源: WILEY

摘要:

Computer simulations of the paths of flowing lava are achieved using a program, FLOWFRONT, that describes the behavior of flow and digital models of the terrain. Two methods of application of simulations to the hazards posed by lava flows are described. The first, deterministic, method requires that program parameters such as vent position, minimum flow thickness, and thickness/slope relationship be based on the ambient eruptive conditions so that the future course of a specific lava flow can be simulated. This is illustrated using retrospective modeling of the first 21 days of the eruption of an andesitic lava flow at Lonquimay volcano, Chile, in 1988–1989. The usefulness of this method for real‐time predictive modeling is likely to be limited by the lack of accurate field data on flow characteristics, the simple nature of the model, and the sensitivity to parameter choice of the final planimetric form of the model flow. The second application is probabilistic in nature and creates a map of the likelihood of inundation by lava flows that is useful for long‐term land use planning. This method uses the historical record of past eruptions to constrain a series of Monte Carlo simulations and is illustrated using data from Etna volcano in Sicily. A multivariate statistical analysis of nine parameters for the 1763–1989 eruption catalog using simulated annealing permitted a classification of Etna's flank eruptions into two types: A and B. Type A eruptions are short‐lived and produce linear lava flows; type B eruptions are long‐lived, and produce lava flows that are much broader in shape, and their vents are restricted to the eastern flank of the volcano. The simulation method consists of creating a probability surface of the location of future eruption vents and segmenting the region according to the most likely historical eruption on which to base the simulation. Analysis of the autocorrelation of the historical eruptions shows that type A eruptions are strongly autocorrelated but type B eruptions are only weakly autocorrelated. A library is created of simulation parameters that give reasonable retrospective fits to each of the historical lava flows. Monte Carlo sampling of the simulation space using this library and the above spatial constraints produces a map of probability of lava

ISSN:0148-0227    

DOI:10.1029/93JB01561

年代:1994

数据来源: WILEY

摘要:

This contribution is concerned with the fundamental thermodynamic aspects of solid‐fluid phase transformations in stressed rocks, specifically in the context of pressure solution. We concentrate in particular on the formulation of a kinetic law governing the migration of stressed and curved solid‐fluid phase boundaries, an objective that is achieved by using the methods of the thermodynamics of irreversible processes. We then apply our result to the study of the geometrical evolution of a fluid‐filled cylindrical pore embedded in an isotropic, linear elastic solid that is subject to a hydrostatic remote stress, assuming that the interface kinetics controls the phase boundary migration and allowing for the effects of capillarity. On the basis of this investigation, we obtain an analytical expression for the pore's growth and show that phase equilibrium along the cylindrical solid‐fluid phase boundary is possible only when the pore pressure exceeds a critical value. The phase equilibrium is found to be kinetically unstable: when subjected to a small perturbation of its radius, the pore will either grow or shrink. The nature of this instability is further explored in the companio

ISSN:0148-0227    

DOI:10.1029/93JB02573

年代:1994

数据来源: WILEY

摘要:

The present study deals with the morphological alterations resulting from the pressure solution of fluid‐filled tubular pores embedded in a stressed solid matrix. Building upon previous work concerning transformation kinetics and the kinetically unstable nature of cylindrical solid‐fluid phase boundaries subjected to nonhydrostatic stress, we investigate the response of such a pore to circumferential and longitudinal perturbations in its shape by means of a linear stability analysis. We obtain criteria for the onset of instability, determine the wavenumber of the disturbance with fastest growth, and demonstrate that with increasing pore pressure the dominant instability changes from a circumferential to a longitudinal mode. This last feature could distinguish interface migration that is limited by the dissolution/precipitation kinetics from that controlled by diffusion through the pore fluid. We finally obtain estimates of the number and spacing of the inclusions formed when a cylindrical pore breaks up following unstable circumferential or longitudinal gro

ISSN:0148-0227    

DOI:10.1029/93JB02572

年代:1994

数据来源: WILEY

摘要:

Adjacent segments of the Pacific‐Antarctic ridge display significantly different morphologies and depth‐age relationships over seafloor younger than 36 Ma. The spreading corridor southwest of Fracture Zone XII is characterized by a rift valley and an usually small subsidence constant of 226±13 m/m.y.½, while the two spreading corridors immediately northeast of Fracture Zone XII have an axial high and a subsidence constant consistent with the global average. This abrupt variation in ridge morphology is not usually characteristic of medium‐rate spreading centers, nor is such an abrupt variation expected of adjacent ridge segments that are spreading at the same rate. We suggest that a thermal anomaly beneath the ridge may influence the first‐order effects of spreading rate and lithospheric cooling enough to produce the observed rift valley and axial high and the different subsidence constants. Although we are not certain what would produce the thermal anomaly here, we speculate that when the spreading rate on the Pacific‐Antarctic ridge increased from slow to intermediate rates since 20 Ma, so did the need for materials for accretion, which may be supplied in part by along‐axis asthenospheric flow from hotspots or a hot region to the northeast. A sufficient supply of hot asthenosphere may still be lacking in the ridge segment with the axial valley to the southwest, leaving it cooler and starved for accretion

ISSN:0148-0227    

DOI:10.1029/93JB02760

年代:1994

数据来源: WILEY

摘要:

A detailed bathymetric, side scan, and magnetic survey of the East Pacific Rise out to a seafloor age of 1 Ma has been carried out between 18° and 19°S. It reveals that some left‐stepping axial discontinuities have been migrating southward at rates an order of magnitude faster than the spreading rates (1000 mm/a or higher). These rapid migration events have left on the Nazca plate discordant features striking nearly parallel to the ridge axis. A discontinuity with an offset of several kilometers has migrated in two stages at around 0.45 and 0.3 Ma, and has left two large discordant zones consisting of a series of unfaulted, hummocky basins bounded to the east by short ridges oriented about N‐S, oblique to the ambient 013° fabric. The morphology and reflectivity characteristics of these discordant zones are akin to the overlap basins and abandoned ridge tips which make up the migration trails of large, slowly‐migrating overlapping spreading centers. Between 18°35′ and 19°03′S, the ridge axis is flanked a few kilometers to the east by a prominent, sedimented ridge previously recognized as a recently abandoned ridge axis. The present ridge segment steadily deepens and narrows southward, which suggests the abandoned ridge has been rafted onto the Nazca plate during the ultrafast southward propagation of the ridge segment rather than by one discrete ridge jump. By transferring Pacific lithosphere to the Nazca plate, these migration events account for most of the asymmetric accretion observed (faster to the east). This process is consistent with the features common to asymmetric spreading, namely the sudden onset or demise of asymmetric spreading, and the ridge segment to ridge segment variability. Because the discordant zones left by these rapid migration events are near‐parallel to the ambient seafloor fabric, they are unlikely to be detected by conventional bathymetry or magnetic surveys, and so‐called “ridge jumps” may actually often represent ultrafast propagation of a ridge segment. Variations in fault azimuth with age show there has not been any significant change in spreading direction over the past 0.8 m.y. Instead, the counterclockwise trend of the East Pacific Rise relative to the Brunhes/Matuyama reversal (0.78 Ma) mostly reflects that ultrafast propagation of ridge segments has transferred a larger amount of the Pacific lithosphere to the Nazca plate at 18°S than at 19°S. In keeping with the regional features of the magnetic anomalies, we propose that an 8 to 10 km left‐stepping discontinuity which was located between 17° and 17°30′S at 0.78 Ma has been recently redistributed into the present staircase of small left‐stepping discontinuities between 16° and 19°S. This smoothing of the ridge geometry probably occurred through repeated small lateral steps of the ridge segments inside of the discontinuities during ultra‐fast propagation episodes, and may be the consequence of a significant replenishment of the magma reservoir between 1

ISSN:0148-0227    

DOI:10.1029/93JB02382

年代:1994

数据来源: WILEY

摘要:

The results of heat balance calculations for a single‐pass hydrothermal system overlying an axial magma body, based on a given rate of heat output of 102–103MW at a time t0, predict that vent temperatures should decay rapidly fort>t0, as the magma freezes and the boundary layer between the hydrothermal system and liquid magma thickens. The model may describe a declining phase of high‐temperature, high‐heat‐output hydrothermal activity. The model shows that for systems with heat output ∼100 MW or greater, the boundary layer between the magma and hydrothermal system must remain thin, if vent temperatures remain relatively constant on a decadal timescale. A thin boundary layer can be maintained as a result of downward migration of the hydrothermal system into freshly frozen magma or by some mechanism that maintains high heat flux from liquid magma to the base of the boundary layer. Some combination of these factors is likely to operate. Downward migration of the hydrothermal system into freshly frozen magma may occur in conjunction with fracturing resulting from dike injection and the propagation of these cracks laterally away from the dike as a result of thermal stresses. High heat flux from liquid magma to the base of the hydrothermal system cannot be maintained simply by convection within the magma chamber. High heat flux might be maintained as a result of magma chamber replenishment or by latent heat transfer during the formation of a cumulate mush at the base of the magma chamber, however. A hydrothermal system, in which the permeability decreases with time, can maintain relatively constant vent temperatures even though the thermal output declines. Better time series data on thermal output of the vents, and not just on the vent temperature, could help distinguish whether the permeability is decreasing or whether heat flux as well as vent temperatures are relative

ISSN:0148-0227    

DOI:10.1029/93JB02568

年代:1994

数据来源: WILEY

摘要:

The first controlled source electromagnetic experiment directly on a ridge, with the potential to identify the presence of an axial melt body beneath a fast‐spreading center, was conducted at 13°N on the East Pacific Rise (EPR) in 1989. Transmission for 36 hours was achieved by a deep towed horizontal electric dipole source, of moment 6000 Am, operating at frequencies between 1/4 and 8 Hz. Signals from the source were recorded by seven seafloor electric field receivers positioned both along the ridge crest and 5 km to the east on 100,000‐year‐old crust. Data above ambient noise levels were obtained at ranges of up to 10 km. The results of modeling observed electric field amplitudes reveal that resistivities in the uppermost crust are very low (∼1 Ω), indicating a heavily fractured, high‐porosity surficial layer. Below this topmost layer, the upper 2 km of crust is found to be moderately resistive (∼100 Ωm). We find no evidence for a large conductive axial melt body with dimensions on the order of kilometers in the middle or upper crust. If a partial melt body is present, which is continuous along strike and which comprises a connected, and therefore conductive, melt texture, it must be of very limited volumetric extent. This picture is consistent with recently proposed models of a thin sill‐like melt lens with across strike dimensions of no more than 1 km and probably with smaller vertical extent. The larger region below the sill, characterized by low seismic velocities, must contain at best a very small melt fraction distributed in isolated pockets, providing further evidence that the EPR at 13°N is currently in a state of relative ma

ISSN:0148-0227    

DOI:10.1029/93JB02577

年代:1994

数据来源: WILEY

摘要:

Many numerical models have stressed the key importance of processes operative within magma‐hydrothermal transition zones of spreading centers, but few studies have attempted a geologic characterization of the key physical parameters of this zone. With this in mind, fracture‐related data have been collected in the Samail ophiolite (Oman) within gabbro/dike transition zones and major plagiogranite plutons, and at each site the nature, abundance, orientation, filling, and attitude of the fractures were determined. Two main hydrothermal vein groups can be distinguished: an amphibole vein system and a quartz‐epidote‐sulfide vein system. Amphibole veins are restricted to the gabbro, whereas quartz‐epidote‐sulfide veins mainly appear in the sheeted‐dike complex and plagiogranite. The amphibole vein system is strongly anisotropic (perpendicular to the layering and subparallel to the sheeted dikes) and heterogeneous. The quartz‐epidote‐sulfide veins, too, are subparallel to the sheeted dikes. They are characterized by a sharp density decrease of macroscopic veins: from an average 0.5 m−1(sheeted dikes) to 0.2 m−1(transition zone) to less than 0.0l m−1(gabbro). In plagiogranite, where no concentric or radiating vein systems were seen, they are 0.1 to 0.3 m−1. The ubiquitous presence of ridge parallel hydrothermal veins implies a major contribution of regional tectonic stresses associated with spreading in addition to other mechanisms involved in fracture generation: thermal cracking, volatile‐rich magma expansion, differenti

ISSN:0148-0227    

DOI:10.1029/93JB02569

年代:1994

数据来源: WILEY