排序方式: 共有2条查询结果,搜索用时 78 毫秒
1
1.
In the Upper Cretaceous Gross Brukkaros Volcanic Field, southern Namibia, a radial dyke system surrounds a dome structure
and its 74 closely related carbonatite diatremes. This paper focuses on volcanological features which seem to be typical for
a low-viscosity melt in various settings such as dykes, sills and diatremes. The total or near absence of vesicles in carbonatite
ash grains and lapilli inside the diatremes is evidence against explosive exsolution of volatile phases and in favour of a
phreatomagmatic fragmentation mechanism and thus for a phreatomagmatic eruption mechanism of the carbonatite diatremes.
Received: 15 August 1996 / Accepted: 13 January 1997 相似文献
2.
Rapid injection of particles and gas into non-fluidized granular material, and some volcanological implications 总被引:3,自引:3,他引:0
Pierre-Simon Ross James D. L. White Bernd Zimanowski Ralf Büttner 《Bulletin of Volcanology》2008,70(10):1151-1168
In diatremes and other volcanic vents, steep bodies of volcaniclastic material having differing properties (particle size
distribution, proportion of lithic fragments, etc.) from those of the surrounding vent-filling volcaniclastic material are
often found. It has been proposed that cylindrical or cone-shaped bodies result from the passage of “debris jets” generated
after phreatomagmatic explosions or other discrete subterranean bursts. To learn more about such phenomena, we model experimentally
the injection of gas-particulate dispersions through other particles. Analogue materials (glass beads or sand) and a finite
amount of compressed air are used in the laboratory. The gas is made available by rapidly opening a valve—therefore the injection
of gas and coloured particles into a granular host is a brief (<1 s), discrete event, comparable to what occurs in nature
following subterranean explosions. The injection assumes a bubble shape while expanding and propagating upwards. In reaction,
the upper part of the clastic host moves upward and outward above the ‘bubble’, forming a ‘dome’. The doming effect is much
more pronounced for shallow injection depths (thin hosts), with dome angles reaching more than 45°. Significant surface doming
is also observed for some full-scale subterranean blasts (e.g. buried nuclear explosions), so it is not an artefact of our
setup. What happens next in the experiments depends on the depth of injection and the nature of the host material. With shallow
injection into a permeable host (glass beads), the compressed air in the “bubble’ is able to diffuse rapidly through the roof.
Meanwhile the coloured beads sediment into the transient cavity, which is also closing laterally because of inward-directed
granular flow of the host. Depending on the initial gas pressure in the reservoir, the two-phase flow can “erupt” or not;
non-erupting injections produce cylindrical bodies of coloured beads whereas erupting runs produce flaring upward or conical
deposits. Changing the particle size of the host glass beads does not have a large effect under the size range investigated
(100–200 to 300–400 μm). Doubling the host thickness (injection depth) requires a doubling of the initial gas pressure to
produce similar phenomena. Such injections—whether erupting or wholly subterranean—provide a compelling explanation for the
origin and characteristics of multiple cross-cutting bodies that have been documented for diatreme and other vent deposits. 相似文献
1