摘要:

On the basis of magnitude and extent of mean gravity anomalies for 5° squares, 19 areas on the earth were selected as markedly positive; 14, as markedly negative; 10, as exceptionally ‘mild.’ Other geological and geophysical data for each of these 43 areas were examined. On the basis of certain patterns of correlation, eleven types of areas were defined. It was found that, where characteristics of different types appeared, certain characteristics were dominant over others. In general, characteristics associated with positive anomalies were dominant over ones associated with mild or negative anomalies, and characteristics associated with recent tectonics dominant over ones associated with ancient. The eleven types in order of dominance, with sign and a leading example of each given in parentheses, are: trench and island arc (+, Indonesia‐Philippines), Cenozoic oceanic flood basalts (+, Iceland‐North Atlantic), Cenozoic orogeny with Quaternary extrusives (+, Caucasus), Quaternary glaciation (−, Canadian shield), vigorous ocean rise (0, southeast Pacific), current orogeny without extrusives (−, Himalayas), ocean basin (−, Somali‐Arabian), continental basin (−, Parnaiba basin), pre‐Cenozoic orogeny (0, eastern United States), continental shield (0, Brazilian shield), pre‐Cenozoic oceanic floor basalts (0, Darwin rise). The strongest correlation found was between positive gravity anomalies and Quaternary volcanism. Positive correlation of gravity anomalies with topography residual to a fifth‐degree figure is almost universal. The extent to which the different area types relate to the global tectonics inferred from paleomagnetic and seismic data varies from strong (e.g., trench and island arc, vigorous ocean rise) to negligible (e.g., Cenozoic oceanic flood basalts, Quaternary glaciation). The lack of systematic correlation between temperature indicators (heat flow,Pnvelocities, seismic station delays) and gravity anomalies indicates that horizontal variations in p

ISSN:0148-0227    

DOI:10.1029/JB074i020p04807

年代:1969

数据来源: WILEY

摘要:

In some well‐surveyed areas, volcanic seamounts increase in size with distance from a midocean ridge crest. This suggests that they remain active as they drift with the crust. Hardly any volcanic islands rise above the oceanic crust produced during the last 10 million years by sea‐floor spreading. This indicates that active, drifting volcanoes rarely grow to the sea surface in less than that time. Guyots and atolls generally occur only on much older crust, which suggests that ocean basin volcanoes commonly remain active for tens of millions of years. Consequently the paleobathymetry of the Darwin rise is compatible with sea‐floor spreading. The discharge of lava in the Hawaiian Islands in historical times has been many times greater than average for an equivalent length of midocean ridge crest. These volcanoes in the center of the Pacific tectonic plate grow rapidly with minimum drifting before they reach the surface. Smaller volcanoes in the basin thus may be built in a single magnetic polarity period, as suggested by paleomagn

ISSN:0148-0227    

DOI:10.1029/JB074i020p04827

年代:1969

数据来源: WILEY

摘要:

An experimental investigation was conducted to determine the effects of varying gravity and atmospheric pressure on the size of small explosion craters formed in cohesionless sand. The explosives used were commercially available squibs and caps and a linear detonating cord. Gravity was varied by flying the test container in an aircraft through carefully controlled maneuvers to simulate 0.17, 0.38, and 2.5 times terrestrial gravity. Atmospheric pressure was controlled separately by means of a ground‐based vacuum chamber. The influence of boundary conditions, aircraft vibrations, changes in sand density and moisture content, and techniques of measurement were all considered and evaluated. It was concluded that both gravity and atmospheric pressure, when varied, will lead to significant changes in crater dimensions. The ratio of diameters is inversely proportional to the ratio of gravities to the powern, wherenincreases as the depth of burst of the explosive increases. Thenvalues are less than 0.16. Crater dimensions are less sensitive to changes in atmospheric pressure in the range from 5×10−4to about 300 mm Hg than in the range from about 300 to 1000 mm Hg. In the atmospheric pressure range from 300 to 1000 mm Hg, the ratio of diameters is inversely proportional to the ratio of pressures (mm Hg) to the powern, wheren= 0.044 ± 0.004. Diameters of craters formed at pressures between 1.0 and 5×10−4mm Hg were on the average 1.10±0.02 times the diameters of craters formed at pressures near terrestrial atmospheric. Cratering in layers of colored sand was also studied, and effects of pressure and gravity on crater morphology w

ISSN:0148-0227    

DOI:10.1029/JB074i020p04838

年代:1969

数据来源: WILEY

摘要:

New spectral reflectivity curves for the light and dark areas of Mars and for seasonal changes of the dark regions were modeled in the laboratory. Light areas are more oxidized and are probably composed of finer particles than the dark areas. Curves for both regions indicate a combination of ferric iron oxide and mafic silicate rock such as basalt. Seasonal changes in the spectral reflectivity of Syrtis Major are not compatible with wind transport of materials or with surface chemical changes. Models of local seasonal condensation of ice or moisture or of growth of macroscopic very dark gray vegetation satisfy the spectral data.

ISSN:0148-0227    

DOI:10.1029/JB074i020p04851

年代:1969

数据来源: WILEY

摘要:

Paleomagnetic studies of 54 superposed lavas from the northwest wall of Mokuaweoweo, Mauna Loa, Hawaii, show that changes in direction of the geomagnetic field were very small during the time of extrusion of these lavas. After reasonable estimates are made for the time involved (a period of several thousands of years, occurring sometime after 10,000 years ago), it is tentatively concluded that secular variation due to nondipole field changes in the central Pacific Ocean is very small and can be described by an angular deviation for virtual poles of only 0.05 radians (3°). In combination with other Hawaiian paleomagnetic studies representing longer time intervals, this leads to an estimated angular deviation of 0.20 radians (12°) for ‘wobble’ of the main dipole during the Brunhes magnetic polarity

ISSN:0148-0227    

DOI:10.1029/JB074i020p04857

年代:1969

数据来源: WILEY

摘要:

Recent magnetometer measurements over the Mason‐Raff type of magnetic anomalies made close to the ocean floor reveal anomalies of shorter wavelength and higher amplitude than are measured at the ocean surface. These anomalies have been shown to be lineated and parallel to the topography and anomalies at the ocean's surface in an area in the north‐eastern Pacific. Magnetic model studies in this area show that they are caused by slowly and continuously changing magnetization within the oceanic second layer, in a direction perpendicular to the lineations. These fluctuations in magnetization are recorded in the oceanic crust through the processes of sea‐floor spreading at oceanic rise crests. They are due to time variation either in the paleofield intensity or in the properties of the injected magmas; variations in the properties of the magma result in petrologic variations in the oceanic crust. Emprical evidence from other authors and current theories on the behavior of the earth's magnetic field suggest that paleofield fluctuations are the most likely cause. If the oceanic crust faithfully records these paleofield fluctuations, the history of the behavior of the earth's field (most probably in a statistical sense) and thus the earth's interior may be studied by magnetic observations near the ocean

ISSN:0148-0227    

DOI:10.1029/JB074i020p04869

年代:1969

数据来源: WILEY

摘要:

A method is suggested for using a three‐channel inertial navigator in a ship or aircraft to provide accurate information on the direction and magnitude of the earth's gravity field. The principles are similar to those used to determine the earth's gravity field by observations of a satellite. The inertial navigator considered here includes a provisional model of the earth's gravity field from which the three components of gravitational acceleration are derived as functions of position. The velocity and position are outputs of the system in its normal mode of operation and they themselves, or functions of them, can be compared with corresponding values obtained from noninertial sources such as Loran. The parameters of the gravity computations and the initial conditions are then modified to produce the best agreement between computed and observed quantities. The connections between this method and existing gravimetric and astrogeodetic techniques are establishe

ISSN:0148-0227    

DOI:10.1029/JB074i020p04882

年代:1969

数据来源: WILEY

摘要:

Comparisons of longitudinal (compressional) wave velocity with structural subfabrics have been made for Yule marble, Newberry Crater pumice, and Salisbury granite. Velocities were (1) measured omnidirectionally in spherical specimens by an ultrasonic pulse technique and (2) plotted and contoured on equal area nets. These contour patterns show velocity anisotropy that can be interpreted in terms of elastic symmetry. Microstructural subfabrics were determined from oriented thin sections of the specimens. Subfabrics were then displayed on equal area nets for comparison of their symmetries with those of the longitudinal wave velocities. The two structural subfabric elements selected for comparison were crystallographic orientation of constituent anisotropic minerals and shape and orientation of pores or cracks. The longitudinal velocity symmetry in Yule marble appears to be axial, whereas in both Newberry Crater pumice and Salisbury granite the pattern is orthorhombic. The pattern of longitudinal velocity anisotropy is associated with preferred orientation of calcite optic axes in Yule marble, preferred orientation of elongate vesicles in the Newberry Crater pumice, and preferred orientation of microfractures in quartz in the Salisbury granite. A prospectus given suggests that the petrophysical technique described can be used to determine elastic symmetry, orient rock to inherent elastic symmetry axes, indicate sample homogeneity, and supplement conventional petrofabric techniques.

ISSN:0148-0227    

DOI:10.1029/JB074i020p04897

年代:1969

数据来源: WILEY

摘要:

The increased refractive index of diopside and albite glass, quenched from liquidus temperatures at pressures to 50 kb, can be accurately calculated from compressibility data on the crystals at 25°C and the Lorentz‐Lorenz refraction law. The results indicate that the pressure effect is locked in by ‘configurational trapping’ on cooling through the glass point, but thermal relaxation takes place with the thermal expansion coefficient characteristic of the applied pressure. The resulting permanent compression is therefore that predicted at 25°C and load pressure. Permanent compression of glasses at temperatures below the glass point should be that predicted at load pressure and (1) 25°C, if temperature is released before pressure, or (2) run temperature, if pressure is released before temperature, because thermal relaxation then takes place with the low‐pressure thermal expansion coefficient. For SiO2the glass compressibility is known as a function of temperature, and the overlap field parameter (β = 1.37) can be established from other data. Refractive indices of SiO2glass compressed at various P andTwith varying quenching cycles are consistent with the values calculated for permanent compression whenT>∼500°C. At lower temperatures the compression is partly elastic and the resultant indices are thus lower than expected. In all these glasses, configurational trapping of pressure deformation is adequate to explain the permanent compression. The direct relationship of permanent compression with compressibility shows that the model of H. M. Cohen and R. Roy (1961, 1965), based on second‐order structural changes at highPfollowed by elastic decompression, is not necessary to explain any of t

ISSN:0148-0227    

DOI:10.1029/JB074i020p04910

年代:1969

数据来源: WILEY

摘要:

Experiments on oriented cores from a single crystal of quartz show that the temperature of the α‐β transition is raised 10.6±0.4°C/kb and 5.0±0.4°C/kb by uniaxial compression perpendicular and parallel, respectively, to the optic axis at a confining pressure of 3 kb. Approximately the same results are indicated for confining pressures ranging from 1 to 5 kb. There is no detectable curvature of the phase boundary for uniaxial stresses σ between 0 and 10 kb: |(d2Tα‐β/dσ2)|<0.05°C/kb2. The increase of transition temperature with hydrostatic pressure of 25.8±0.3°C/kb between 1 and 5 kb also determined in these experiments is consistent with previous determinations. These results and others for quartz are analyzed assuming the α‐β inversion is either a λ transition or a first‐order transition characterized by a small reversible transformation strain. Although both hypotheses are roughly consistent with most of the results, the hysteresis in the transition suggests that the second may be preferable, and a theory is developed that describes the effect of general nonhydrostatic str

ISSN:0148-0227    

DOI:10.1029/JB074i020p04921

年代:1969

数据来源: WILEY