Experiments on gas-ash separation processes in volcanic umbrella plumes

被引:51
作者
Holasek, RE
Woods, AW
Self, S
机构
[1] UNIV HAWAII,DEPT GEOL & GEOPHYS,HONOLULU,HI 96822
[2] UNIV HAWAII,HAWAII CTR VOLCANOL,HONOLULU,HI 96822
[3] INST THEORET GEOPHYS,DEPT APPL MATH & THEORET PHYS,CAMBRIDGE CB3 9EW,ENGLAND
关键词
D O I
10.1016/0377-0273(95)00054-2
中图分类号
P [天文学、地球科学];
学科分类号
07 ;
摘要
We present a series of analogue laboratory experiments which simulate the separation of ash and gas and the formation of secondary intrusions from finite volcanic umbrella plumes. We examined the lateral spreading of mixtures of freshwater and particles released into a laboratory tank containing a uniformly stratified aqueous solution. For times smaller than the sedimentation time of particles through the intrusion, the current remains coherent and intrudes laterally. As some of the particles settle into the underlying ambient fluid, a layer of particle-depleted fluid develops below the upper surface of the current and the density of the residual fluid is reduced. Over longer times, the intrusion ceases to be coherent, with small fingers of relatively buoyant, particle-depleted fluid rising from the upper part of the intrusion into the overlying fluid. Meanwhile, the lateral motion of the injected solution induces a return flow in the ambient fluid which sweeps some of the particles sedimenting from the lower surface of the intrusion inwards. As a result, relatively dense particle-laden fluid collects below the intrusion and then sinks into the underlying fluid. Eventually this fluid reaches a new neutral buoyancy height, where it intrudes to form a second laterally spreading current below the original intrusion. The process then repeats to form further weaker intrusions below. These results of the separation of the ash and volcanic gas in an umbrella plume are consistent with field observations at Sakurajima volcano where positively charged plumes, thought to consist of volcanic gas, have been observed above negatively charged plumes of ash. This work also suggests that volcanic aerosols may form up to a kilometer above the original injection height of the ash. In a strong wind shear, this could result in very different trajectories of the ash and gas and so be important for evaluating the impact of ash plumes on both aviation safety and volcanic aerosol formation. The separation of the ash from the umbrella plume, after a time of order 10(3)-10(4) s, corroborates recent models of the May 18, 1980 eruption at Mount St. Helens that, after a few hours, the motion of the ash was simply controlled by wind dispersal and gravitational settling. The complex motion of the sedimenting ash may also explain some of the observations of multiple intrusions in short-lived umbrella plumes such as observed at Redoubt, Alaska in April 1990.
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页码:169 / 181
页数:13
相关论文
共 28 条
[1]   GRAVITATIONAL COLLAPSE OF A MIXED REGION INTO A LINEARLY STRATIFIED FLUID [J].
AMEN, R ;
MAXWORTHY, T .
JOURNAL OF FLUID MECHANICS, 1980, 96 (JAN) :65-&
[2]  
[Anonymous], NAT HAZARDS, DOI DOI 10.1007/BF00144974
[3]  
[Anonymous], 1992, FAA AVIATION SAFETY
[4]   A GRAVITY CURRENT MODEL FOR THE MAY 18, 1980 MOUNT-ST-HELENS PLUME [J].
BURSIK, MI ;
CAREY, SN ;
SPARKS, RSJ .
GEOPHYSICAL RESEARCH LETTERS, 1992, 19 (16) :1663-1666
[5]   SEDIMENTATION OF TEPHRA BY VOLCANIC PLUMES .1. THEORY AND ITS COMPARISON WITH A STUDY OF THE FOGO-A PLINIAN DEPOSIT, SAO-MIGUEL (AZORES) [J].
BURSIK, MI ;
SPARKS, RSJ ;
GILBERT, JS ;
CAREY, SN .
BULLETIN OF VOLCANOLOGY, 1992, 54 (04) :329-344
[6]   ON CONVECTION AND MIXING DRIVEN BY SEDIMENTATION [J].
CARDOSO, SSS ;
WOODS, AW .
JOURNAL OF FLUID MECHANICS, 1995, 285 :165-180
[7]   EXPERIMENTAL STUDIES OF PARTICLE-LADEN PLUMES [J].
CAREY, SN ;
SIGURDSSON, H ;
SPARKS, RSJ .
JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH AND PLANETS, 1988, 93 (B12) :15314-15328
[8]  
GILL AE, 1981, ATMOSPHERE OCEAN DYN
[9]   ON THE LONG-TERM VARIATION OF STRATOSPHERIC AEROSOL CONTENT AFTER THE ERUPTION OF VOLCANO EL CHICHON - LIDAR MEASUREMENTS AT NAGOYA, JAPAN [J].
HAYASHIDA, S ;
IWASAKA, Y .
JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN, 1985, 63 (03) :465-473
[10]  
HAZEN A, 1904, T AM SOC CIVIL ENG, V53, P45