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81.
The 1783–1784 Laki tholeiitic basalt fissure eruption in Iceland was one of the greatest atmospheric pollution events of
the past 250 years, with widespread effects in the northern hemisphere. The degassing history and volatile budget of this
event are determined by measurements of pre-eruption and residual contents of sulfur, chlorine, and fluorine in the products
of all phases of the eruption. In fissure eruptions such as Laki, degassing occurs in two stages: by explosive activity or
lava fountaining at the vents, and from the lava as it flows away from the vents. Using the measured sulfur concentrations
in glass inclusions in phenocrysts and in groundmass glasses of quenched eruption products, we calculate that the total accumulative
atmospheric mass loading of sulfur dioxide was 122 Mt over a period of 8 months. This volatile release is sufficient to have
generated ∼250 Mt of H2SO4 aerosols, an amount which agrees with an independent estimate of the Laki aerosol yield based on atmospheric turbidity measurements.
Most of this volatile mass (∼60 wt.%) was released during the first 1.5 months of activity. The measured chlorine and fluorine
concentrations in the samples indicate that the atmospheric loading of hydrochloric acid and hydrofluoric acid was ∼7.0 and
15.0 Mt, respectively. Furthermore, ∼75% of the volatile mass dissolved by the Laki magma was released at the vents and carried
by eruption columns to altitudes between 6 and 13 km. The high degree of degassing at the vents is attributed to development
of a separated two-phase flow in the upper magma conduit, and implies that high-discharge basaltic eruptions such as Laki
are able to loft huge quantities of gas to altitudes where the resulting aerosols can reside for months or even 1–2 years.
The atmospheric volatile contribution due to subsequent degassing of the Laki lava flow is only 18 wt.% of the total dissolved
in the magma, and these emissions were confined to the lowest regions of the troposphere and therefore important only over
Iceland. This study indicates that determination of the amount of sulfur degassed from the Laki magma batch by measurements
of sulfur in the volcanic products (the petrologic method) yields a result which is sufficient to account for the mass of
aerosols estimated by other methods.
Received: 30 May 1995 / Accepted: 19 April 1996 相似文献
82.
The 1999 basaltic eruption of Shishaldin volcano (Alaska, USA) included both Strombolian and Subplinian activity, as well as a “pre-Subplinian” phase interpreted as the local coalescence within a long foam in the conduit. Although few visual observations were made of the eruption, a great deal of information regarding gas velocity, gas flux at the vent and plume height may be inferred by using acoustic recordings of the eruption. By relating acoustic power to gas velocity, a time series of gas velocity is calculated for the Subplinian and pre-Subplinian phases. These time series show trends in gas velocity that are interpreted as plumes or, for those signals lasting only a short time, thermals. The Subplinian phase is shown to be composed of a thermal followed by five plumes with a total expelled gas volume of .The initiation of the Subplinian activity is probably related to the arrival of a large overpressurised bubble close to the top of the magma column. A gradual increase in low-frequency (0.01–0.5 Hz) signal prior to this “trigger bubble” may be due to the rise of the bubble in the conduit. This delay corresponds to a reservoir located at ≈3.9 km below the surface, in good agreement with studies on other volcanoes.The presence of two thermal phases is also identified in the middle of the pre-Subplinian phase with a total gas release of and . Gas velocity at the vent is found to be and for the Subplinian plumes and the pre-Subplinian thermals respectively.The agreement is very good between estimates of the gas flux from modelling the plume height and those obtained from acoustic measurements, leading to a new method by which eruption physical parameters may be quantified. Furthermore, direct measurements of gas velocity can be used for better estimates of the flux released during the eruption. 相似文献