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首页> 外文期刊>Journal of Volcanology and Geothermal Research >Crystal-rich lava dome extrusion during vesiculation: An experimental study
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Crystal-rich lava dome extrusion during vesiculation: An experimental study

机译：结晶过程中富含晶体的熔岩穹顶挤出：一项实验研究

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Lava dome-forming eruptions represent a common eruptive style and a major hazard at numerous active volcanoes worldwide. The extrusion mechanics of crystal-rich lava domes and the influence of volatiles on the transition from viscous to brittle behaviour during lava dome extrusion remain unclear. Understanding how gas exsolution and crystallinity control effusive versus explosive eruption behaviour is essential. Here, we present new experimental results on the rheology of synthesised, crystal-rich (50 to 80 vol% quartz crystals), hydrous (4.2 wt% H2O in the glass) dacite samples, which vesiculate from 5 to 27 vol% gas bubbles at high temperatures (from glass transition temperature to 797 degrees C) during deformation conducted in a parallel plate viscometer (constant stress at 0.63-0.64 MPa, and variable strain-rates ranging from 8.32.10(-8) to 3.58.10(-5) s(-1)). The experiments reproduce certain aspects of lava dome deformation in volcanic conduits during vesiculation of the residual melt, instigated in the experiments by increasing temperature. During gas exsolution (i.e. nucleation and growth of gas-pressurised bubbles) and volume inflation, we find that the rheological lubrication of the system during deformation is strongly dictated by the initial crystallinity. At crystal contents 60 vol%, gas bubbles form and coalesce during expansion and viscous deformation, favouring strain localisation and gas permeability within shear bands, which control the overall sample rheology. At crystallinities of 60 to 70 vol%, gas exsolution generates pressurisation (i.e. pore pressure increase) within the bubbles trapped in the solid crystal clusters, and embryonic formation of microscopic fractures through melt and crystals drives the system to a brittle behaviour. At higher crystallinity (80 vol%) vesiculation leads to large pressurisation, which then triggers extensive brittle fragmentation. Through macroscopic fractures, outgassing determines the rheological stalling of the system. In the light of these results we propose a rheological description of crystal-rich lava dome mechanics. The contrasting experimental behaviours at different crystallinities have implications for the style of slow-ascending dome-forming eruptions. All other factors being equal, our experiments suggest that crystal-poor magmas will undergo efficient outgassing, reducing the potential for an explosive eruption. Conversely, crystal-rich magmas may experience limited outgassing and larger gas over-pressures during vesiculation, therefore increasing the potential for an explosive eruption. (C) 2017 Published by Elsevier B.V.
机译：熔岩穹顶形成的喷发代表着一种普遍的喷发形式，并且是全球众多活跃火山的主要危害。熔岩穹顶挤出过程中，富含晶体的熔岩穹顶的挤出机理以及挥发物对从粘性到脆性转变的影响尚不清楚。了解气体释放和结晶度如何控制喷发与爆炸性喷发是至关重要的。在这里，我们介绍了合成的，富含晶体（50％至80％（体积）石英晶体），含水（玻璃中4.2％（重量）H2O））石样品的流变学的新实验结果，这些样品在5％至27％（体积）的气泡中呈泡状在平行板粘度计中进行变形时的高温（从玻璃化转变温度到797摄氏度）（0.63-0.64 MPa的恒定应力，可变应变率范围从8.32.10（-8）到3.58.10（-5 ）s（-1））。实验重现了残余熔体气泡化过程中火山管道中熔岩穹顶变形的某些方面，这是在实验中通过提高温度来实现的。在气体析出（即气体加压气泡的成核和增长）和体积膨胀期间，我们发现系统在变形过程中的流变润滑主要受初始结晶度的支配。当晶体含量小于60 vol％时，在膨胀和粘性变形过程中会形成气泡并聚结，有利于剪切带内的应变局部化和透气性，从而控制了整个样品的流变性。在60％至70％（体积）的结晶度下，气体逸出会在固体晶体簇中捕获的气泡内产生压力（即孔隙压力增加），并且通过熔体和晶体形成的微观裂缝的胚状形成使系统趋于脆性。在较高的结晶度（80％（体积））下，囊泡化会导致压力升高，从而引发大量脆性碎裂。通过宏观裂缝，除气作用决定了系统的流变性。根据这些结果，我们提出了富含晶体的熔岩穹顶力学的流变学描述。不同结晶度的对比实验行为对缓慢上升的圆顶形成喷发的样式有影响。在所有其他因素相同的情况下，我们的实验表明，晶体贫乏的岩浆将进行有效的除气，从而减少发生爆发性喷发的可能性。相反，富含晶体的岩浆可能在脉动期间经历有限的放气和较大的气体超压，因此增加了爆发性喷发的可能性。 （C）2017由Elsevier B.V.发布

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    Pistone Mattia; Whittington Alan G.; Andrews Benjamin J.; Cottrell Elizabeth;

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    Smithsonian Inst, Natl Museum Nat Hist, Dept Mineral Sci, 10th St & Constitut Ave NW, Washington, DC 20560 USA;

    Univ Missouri, Dept Geol Sci, 101 Geol Sci, Columbia, MO 65211 USA;

    Smithsonian Inst, Natl Museum Nat Hist, Dept Mineral Sci, 10th St & Constitut Ave NW, Washington, DC 20560 USA;

    Smithsonian Inst, Natl Museum Nat Hist, Dept Mineral Sci, 10th St & Constitut Ave NW, Washington, DC 20560 USA;

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