Compound and simple lava flows and flood basalts     

G. P. L. Walker 《Bulletin of Volcanology》1971,35(3):579-590

Compound lava flows, defined as those lavas which are divisible into flowunits, commonly have a shield-like form and are thought to develop when the rate of extrusion of lava is relatively low.Simple lava flows, defined as those lavas which are not divisible into flow-units, are thought to form when the rate of extrusion of lava is relatively high. A logical definition oflava flow must embrace both simple lava sheets and substantial lava shields (compound lava flows) up to 600 m high. Flood basalt piles include both compound and simple flows, but the most extensive and far-reaching flows are simple and they are believed to form when the rate of extrusion of lava is particularly high.  相似文献   

WORLD DELTAS AND THEIR EVOLUTION   总被引：1，自引：0，他引：1  

H. Jesse Walker 《地理学报》1999,(1):30-41

In August 1998, an international symposium on the world deltas was held in New Orleans, Louisiana, USA. This symposium attracted discussion about more than 25 deltas from around the world with emphasis placed on those that are most densely populated and impacted by humans. Keynote papers printed details about the physical, biological, engineering and socioeconomic aspects of six deltas including the Mississippi, Nile, Ganges-Brahmaputra, Rhine-Meuse, Changjiang and Po. The main purpose of this symposium was to inform scientists, engineers and decision-makers about information that is currently available and to provide them a basis for working in such environments.  相似文献   

Samos 2 (1961 α 1): Orbit determination and analysis at 31 : 2 resonance     

Doreen M.C. Walker 《Planetary and Space Science》1980,28(11):1059-1072

Samos 2, 1961 α 1, launched on 31 January 1961, was the first satellite to enter a sun-synchronous orbit at an inclination of 97.4°. The initial perigee and apogee heights were 474 km and 557 km respectively, the initial period was 94.97 min and the satellite decayed on 21 October 1973 after more than 12 years in orbit.Samos 2 passed through the condition of 31 : 2 resonance in June 1971 and orbital parameters have been determined at 22 epochs from 1674 observations using the RAE orbit refinement program, PROP, between mid-April and Mid-September 1971. The variations of inclination and eccentricity during this time have been analysed and values for six lumped 31st-order harmonic coefficients in the geopotential have been obtained. These have been compared with those derived from the individual coefficients, of order 31 and appropriate degrees, from the most recent Goddard Earth Model, GEM 10C.The decrease in inclination between launch and 1971 has been examined: it is found to be caused mainly by a near-resonant solar gravitational perturbation.  相似文献   

Theoretical ion densities in the lower ionosphere     

R.W. Schunk  J.C.G. Walker 《Planetary and Space Science》1973,21(11):1875-1896

We have solved the coupled momentum and continuity equations for NO⁺, O₂⁺, and O⁺ions in the E- and F-regions of the ionosphere. This theoretical model has enabled us to examine the relative importance of various processes that affect molecular ion densities. We find that transport processes are not important during the day; the molecular ions are in chemical equilibrium at all altitudes. At night, however, both diffusion and vertical drifts induced by winds or electric fields are important in determining molecular ion densities below about 200 km. Molecular ion densities are insensitive to the O⁺ density distribution and so are little affected by decay of the nocturnal F-region or by processes, such as a protonospheric flux, that retard this decay. The O⁺ density profile, on the other hand, is insensitive to molecular ion densities, although the O⁺ diffusion equation is formally coupled to molecular ion densities by the polarization electrostatic field. Nitric oxide plays an important role in determining the NO⁺ to O₂⁺ ratio in the E-region, particularly at night. Nocturnal sources of ionization are required to maintain the E-region through the night. Vertical velocities induced by expansion and contraction of the neutral atmosphere are too small to affect ion densities at any altitude.  相似文献   

Analysis of the orbit of 1965-11D (COSMOS 54 rocket)     

D.G. King-Hele  Doreen M.C. Walker 《Planetary and Space Science》1973,21(7):1081-1108

The satellite 1965-11D was the final-stage rocket used to launch Cosmos 54, 55 and 56 into orbit on 21 February 1965. The orbit of 1965-11D was inclined at 56° to the Equator, with an initial perigee height of 280 km; the lifetime was nearly 5 yr, with decay on 23 December 1969. The orbit has been determined at 75 epochs during the life, using the RAE orbit determination program PROP with over 4000 observations, photographic, visual and radar. Observations from the Hewitt camera at Malvern were available for 34 of the 75 orbits and typical accuracies for these orbits are 0.0005° in inclination and 100 m in perigee height.The variations in perigee height have been analyzed to determine reliable values of density scale height, at heights between 240 and 360 km. The analysis also revealed a rapid decrease of 5 km in perigee distance early in 1966, attributed to the escape of residual propellants.The variations in orbital inclination have been analyzed to determine upper-atmosphere zonal winds and 15th-order harmonics in the geopotential. The region of the upper atmosphere traversed by 1965-11D near its perigee is found to have had an average rotation rate of 1.10 ± 0.05 rev/day in 1966–1967, and 1.00 ± 0.03 rev/day between March 1968 and May 1969. In late 1969 there were probably wide variations in zonal winds, with east-to-west winds of order 100 m/s followed by west-to-east winds of order 200 m/s. The changes in inclination at the 15th-order resonance in July 1969 have been analyzed to give the first accurate values of lumped 15th-order harmonics obtained from a high-drag satellite. This success points the way towards similar analyses of the many other high-drag satellites that pass through 15th-order resonance, to evaluate individual geopotential coefficients of order 15 and even degree.  相似文献   

HST colour–magnitude diagrams of six old globular clusters in the LMC     

K. A. G. Olsen  P. W. Hodge  M. Mateo  E. W. Olszewski  R. A. Schommer  N. B. Suntzeff  & A. R. Walker 《Monthly notices of the Royal Astronomical Society》1998,300(3):665-685

We report on HST observations of six candidate old globular clusters in the Large Magellanic Cloud (LMC): NGC 1754, 1835, 1898, 1916, 2005 and 2019. Deep exposures with the F555W and F814W filters provide us with colour–magnitude diagrams that reach to an apparent magnitude in V of ∼25, well below the main-sequence turn-off. These particular clusters are associated with significantly high LMC field star densities and care was taken to subtract the field stars from the cluster colour–magnitude diagrams accurately. In two cases there is significant variable reddening across at least part of the image, but only for NGC 1916 does the differential reddening preclude accurate measurements of the CMD characteristics. The morphologies of the colour–magnitude diagrams match well those of Galactic globular clusters of similar metallicity. All six have well-developed horizontal branches, while four clearly have stars on both sides of the RR Lyrae gap. The abundances obtained from measurements of the height of the red giant branch above the level of the horizontal branch are 0.3 dex higher, on average, than previously measured spectroscopic abundances. Detailed comparisons with Galactic globular cluster fiducials show that all six clusters are old objects, very similar in age to classical Galactic globulars such as M5, with little age spread among the clusters. This result is consistent with ages derived by measuring the magnitude difference between the horizontal branch and main-sequence turn-off. We also find a similar chronology by comparing the horizontal branch morphologies and abundances with the horizontal branch evolutionary tracks of Lee, Demarque &38; Zinn. Our results imply that the LMC formed at the same time as the Milky Way Galaxy.  相似文献   

The spatial structure of the dayside ionospheric trough     

S. E. Pryse  L. Kersley  M. J. Williams  I. K. Walker 《Annales Geophysicae》1998,16(10):1169-1179

Tomographic imaging provides a powerful technique for obtaining images of the spatial distribution of ionospheric electron density at polar latitudes. The method, which involves monitoring radio transmissions from the Navy Navigation Satellite System at a meridional chain of ground receivers, has particular potential for complementing temporal measurements by other observing techniques such as the EISCAT incoherent-scatter radar facility. Tomographic reconstructions are presented here from a two-week campaign in November 1995 that show large-scale structuring of the polar ionosphere. Measurements by the EISCAT radar confirm the authenticity of the technique and provide additional information of the plasma electron and ion temperatures. The dayside trough, persistently observed at high latitudes during a geomagnetically quiet period but migrating to lower latitudes with increasing activity, is discussed in relationship to the pattern of the polarcap convection.  相似文献   

The effects of liquid immiscibility and thermal diffusion on oxygen isotopes in silicate liquids   总被引：2，自引：1，他引：1  

T. K. Kyser  C. E. Lesher  D. Walker 《Contributions to Mineralogy and Petrology》1998,133(4):373-381

Differences between the δ¹⁸O values of Si- and Fe-rich immiscible liquids in the system Fe₂SiO₄-KAlSi₂O₆-SiO₂ (Fa-Lc-Q) in isothermal experiments at 0.1 MPa have been determined experimentally to be 0.6 permil. The observed partition of ¹⁸O into the Si-rich liquid is consistent with previous experience with the preferential partition of ¹⁸O into Si-rich minerals in isothermal equilibrium with minerals of less polymerized structure. Crystallochemical principles affect the distribution of oxygen isotopes in coexisting isothermal liquids in the same way as they apply to isothermally coexisting crystals. The effects of Soret (thermal) diffusion on the distribution of oxygen isotopes in silicate liquids above the solvus in the system Fa-Lc-Q under conditions of an imposed temperature gradient of ca. 250 °C over 4 mm and at 2 GPa have also been investigated experimentally. Both the magnitude and the direction of separation of oxygen isotopes as a result of Soret diffusion are unexpected. For each of the silicate liquids, the cold end of the charge is enriched in ¹⁸O by up to 4.7 permil, and the highest δ¹⁸O values are associated with the most silica-poor compositions. The distribution of oxygen isotopes appears to be similar in each liquid, regardless of their chemical compositions, which is in contrast to the behaviour of cations whose distributions are compositionally dependent and characterized by strong crystallochemical effects wherein network-forming species such as Si and Al separate to the hot end and Mg, Fe and Ca are segregated preferentially to the cold end. Structural units in the melts are evidently less selective between oxygen isotopes than between cations, because oxygen redistribution over all possible sites in these units proceeds according to mass. Self-diffusion coefficients of oxygen in basaltic liquids estimated from the Soret experiments are in accord with those from other isotope tracer experiments, and comparable to those of Si. The possible effects of Soret diffusion on the oxygen isotopic composition of metasomatic veins in the mantle are examined in light of these data, and indicate that decay of the thermal gradients in the veins exceeds that of the diffusion of oxygen needed to produce variations in the δ¹⁸O values of mantle minerals. Variations in oxygen isotope ratios in most natural systems as a result of Soret effects are unlikely. Received: 6 January 1997 /  Accepted: 28 June 1998  相似文献   

Infrared mapping of the dust disc around Vega     

Heinrichsen  Walker  & Klaas 《Monthly notices of the Royal Astronomical Society》1998,293(4):L78-L82

ISOPHOT has been used to perform high-resolution 60-μm scans of Vega, and these have been compared with those from γDra, to obtain a Gaussian width of 22±2 arcsec. The dust disc around Vega has been mapped, resolving it at 60 and 90 μm with ISOPHOT. At 90 μm a Gaussian width of 36±3 arsec has been derived. In addition, multi-filter photometry is presented, at 25, 60, 80, 100, 120, 150, 170 and 200 μm. The data are fitted by a modified blackbody with a temperature of 73 K [ Q (λ)∝1/λ^1.1]. The dust disc has a luminosity L _IR/ L _*∼3×10⁻⁵. Using a distance of 7.8 pc, 22 arcsec corresponds to a distance of 86 au, and 36 arcsec to a distance of 140 au.  相似文献   

Pyroclastic geology of the rhyolitic volcano of La Primavera,Mexico     

G. P. L. Walker  J. V. Wright  B. J. Clough  B. Booth 《International Journal of Earth Sciences》1981,70(3):1100-1118

Explosive eruptions of this large Quaternary rhyolitic volcano have produced voluminous pumice fall deposits and ignimbrites. A stratigraphie study reveals more than 30 fall deposits, totalling more than 90 km³, which have come from many different vents. Some are of plinian type, and one is among the largest known in the world. The main ignimbrite (Rio Caliente) is of intra-plinian type. In all, 140 km³ of rhyolitic material were erupted during the past 10⁵ years, of which 50 km⁵ are dispersed well outside the volcano. The dense rock equivalent volume is 60 km³, of which fall deposits, ignimbrites and lava bodies comprise 45 %, 25 % and 30 % respectively. The average output rate of 0.06 km³ per century is nearly an order of magnitude less than for the most productive rhyolitic volcanoes known. A circular area 10 km across in the centre of La Primavera contains updomed lacustrine ashes and associated sediments, including a remarkable giant pumice bed, which probably accumulated in a caldera lake. La Primavera has no record of historical eruptions, and hot springs are the only signs of present activity, but the updoming is thought to be due to the uprise of a new acid pluton beneath the volcano and future eruptions are probable.

---

Zusammenfassung Explosionsartige Eruptionen des gro\en quartÄren Rhyolith-Vulkans La Primavera haben mÄchtige Bimstuffe und Ignimbrite aufgehÄuft. Eine stratigraphische Untersuchung hat über 30 Aschelagen erkennen lassen, die aus verschiedenen Schloten stammen und insgesamt mehr als 90 km³ an Volumen ausmachen. Einige davon gehören zum plimanischen Typ, von denen eine sogar die grö\te bisher bekannte Lage der Erde ist. Der Haupt-Ignimbrit (Rio Caliente) gehört zum intraplimanischen Typ. Insgesamt wurden in den letzten 100 000 Jahren 140 km³ rhyolithisches Material ausgeworfen, von denen 50 km³ deutlich au\erhalb des eigentlichen Vulkans liegen. Das Äquivalentvolumen des dichten Gesteins betrÄgt 60 km³, von dem 45 % auf Tuffe, 25 % auf Ignimbrite und 30 % auf Lavakörper entfallen. Die durchschnittliche Auswurfrate von 0,06 km³ pro Jahrhundert liegt fast eine Grö\enordnung unter den meisten bekannten aktiven Rhyolith-Vulkanen. Ein kreisrundes Gebiet von 10 km Durchmesser im Zentrum des La Primavera zeigt aufgewölbte lakustrine Aschen und damit assoziierte Sedimente, darunter eine bemerkenswerte riesige Bimslage, die wahrscheinlich in einem Caldera-See abgelagert wurde. Der Vulkan La Primavera ist in historischer Zeit nicht ausgebrochen und hei\e Quellen sind die einzigen Anzeichen heutiger AktivitÄt, aber die Aufwölbung wird als Hinweis auf das Hochdringen eines neuen sauren Plutons unter dem Vulkan angesehen, so da\ zukünftige Eruptionen wahrscheinlich sind.

---

Résumé Des éruptions explosives de ce grand volcan rhyolitique quaternaire ont produit de volumineux dépÔts de ponces et d'ignimbrites. Une étude stratigraphique fait apparaÎtre plus de 30 dépÔts totalisant plus de 90 km³ provenant de nombreuses bouches volcaniques différentes. Certaines sont du type plinien, dont l'une est parmi les plus grandes connues au monde. L'ignimbrite principale (Rio Caliente) est du type intraplinien. En tout 140 km³ de matériaux rhyolitiques ont été rejetés au cours des 10⁵ dernières années, dont 50 km³ ont été dispersés bien au delà du volcan. Le volume de roche compact équivalent est de 60 km³, parmi lequel les tuffs, les ignimbrites et les laves occupent respectivement 45%, 25% et 30%. L'émission moyenne, de 0,06 km³ par siècle, est d'un ordre de grandeur moindre que pour la plupart des volcans rhyolitiques les plus productifs actuellement connus. Une région circulaire de 10 km de diamètre dans le centre de La Primavera contient des domes de cendres lacustres et autres sédiments associés, et parmi eux une couche géante de ponce remarquable, qui s'est probablement accumulée dans un lac-caldère. On ne connaÎt pas d'éruptions historiques de la Primavera, et seules des sources chaudes témoignent d'une activité actuelle; toutefois on pense que l'allure en dome est due à la poussée d'un nouveau pluton acide sous le volcan, et que des éruptions sont probables dans le futur.

---

- . 30 , ; 90 ³. , . - - - . 3 100 000 140 ³ , 50 ³ . 60 , 45% , 25% 30% . 0,06 ; , . 10 - , , , . - , , , , .

  相似文献