Blowing Off Steam: Tuffisite Formation As a Regulator for Lava Dome Eruptions
Blowing Off Steam: Tuffisite Formation As a Regulator for Lava Dome Eruptions
复制标题
DOI：
10.3389/feart.2015.00002
复制
发表时间：
2016-04-22
影响因子：
2.9
通讯作者：
Vasseur, Jeremie
中科院分区：
文献类型：
--
作者：
Kendrick, Jackie E.;Lavallee, Yan;Vasseur, Jeremie
超级喷发产生数百立方公里的火山碎屑密度流，通常记录厚，焊接和熔岩状熔结凝灰岩。尽管这种类型的喷发推断出巨大的环境影响，但人们对沉积和沉积后流动的时间尺度知之甚少。没有这些时间尺度，任何环境影响的持续时间及其对地球系统的影响的严重性的关键问题，我们就无法解决。熔结凝灰岩的喷发和熔结需要三个玻璃化转变断面。岩浆需要：（1）在上升过程中碎裂，（2）在沉积、凝集和焊接（烧结）过程中膨胀和松弛，以及（3）通过冷却到玻璃态而淬火。在这里，我们表明，焊接是一个快速的，同沉积过程，焊接的熔结凝灰岩片可能会流动长达几个小时，然后通过玻璃化转变的最后一次。地质速度测定法显示，格雷兰丁熔结凝灰岩的基底玻璃斑岩在870摄氏度时以近似0.1摄氏度·分（-1）的速率经历了玻璃化转变;也就是说，比喷发前的地质温度估计值低30 - 180摄氏度。1-D冷却模型的应用约束的沉积，凝集和焊接的基底玻璃斑岩的时间尺度小于1小时，甚至可能几十分钟。烧结过程的热机械迭代表明，在966摄氏度的玻璃斑岩就位的最佳温度解决方案。玻璃体揭示了牛顿流变高达46 MPa，这表明，完全退火的灰颗粒在焊接过程中，粘性能量耗散是不可能从单独的加载条件，除非剪切应力施加的上覆灰流是过高的，并持续很长的距离。研究结果强调了“熔岩样”流的价值，以描述蛇河型熔结凝灰岩的流变学，完全符合典型的lithophaseobserved.Tuffisites是脉的烧结，火山碎屑颗粒，形成在导管和熔岩圆顶作为一个结果，在气体和火山灰爆炸的局部破碎事件。在2012年活跃的Volcan de Colima熔岩穹丘上原位观察到的那些，从内部碎屑的空隙（显示出很少的运动，被解释为失败核）到烧结颗粒状聚集体的平行透镜（被解释为破碎层位），再到填充裂缝（有粘性再活动的证据）。所有凝灰岩都显示出烧结的迹象。进一步检查复杂的火山岩和通道模式揭示了周围岩浆的粘性回填，这表明熔岩破碎后，随着凝灰岩的形成，应力松弛和持续的粘性变形。天然凝灰岩比寄主安山岩更具有渗透性，并且具有宽范围的孔隙度和渗透性，而寄主岩石的孔隙度和渗透性较窄，并且从其在整个圆顶上的显著分布来衡量，我们认为凝灰岩脉可能充当重要的出气通道。为了研究凝灰岩的形成，我们将安山岩从熔岩穹丘中压碎并过筛，然后在岩浆温度下烧结不同的时间。然后，我们通过测量孔隙率和渗透性来评估愈合和密封能力，表明烧结会随着时间的推移而降低。在烧结过程中，由于相邻颗粒之间形成粘性颈，孔隙率-渗透率降低，这一过程由Frenkel（1945）的颈形成模型描述。这一过程导致颗粒状的起始材料的孔隙度-渗透率制度预期的涌出熔岩，并描述了自然宿主熔岩以及最不透水的天然凝灰岩。这表明Volcan de Colima的凝灰岩构造构造了一个可渗透的网络，使气体能够被动地从岩浆中渗出。我们假设，这逐渐降低了熔岩圆顶的密封能力和建立压力，驱动爆炸。事实上，2007 - 2011年期间爆炸之间的时间间隔逐渐增加，然后从2011年6月开始进入平静期。我们认为，在凝灰岩形成过程中的渗透性演变有重要的影响，气体和火山灰爆炸，常见的圆顶形成的火山建模。
Super-eruptions generating hundreds of cubic kilometers of pyroclastic density currents are commonly recorded by thick, welded and lava-like ignimbrites. Despite the huge environmental impact inferred for this type of eruption, little is yet known about the timescales of deposition and post-depositional flow. Without these timescales, the critical question of the duration of any environmental impact, and the ensuing gravity of its effects for the Earth system, eludes us. The eruption and welding of ignimbrites requires three transects of the glass transition. Magma needs to: (1) fragment during ascent, (2) liquefy and relax during deposition, agglutination and welding (sintering), and (3) quench by cooling into the glassy state. Here we show that welding is a rapid, syn-depositional process and that the welded ignimbrite sheet may flow for up to a few hours before passing through the glass transition a final time. Geospeedometry reveals that the basal vitrophyre of the Grey's Landing ignimbrite underwent the glass transition at a rate of similar to 0.1 degrees C.min(-1) at 870 degrees C; that is, 30-180 degrees C below pre-eruptive geothermometric estimates. Application of a 1-D cooling model constrains the timescale of deposition, agglutination, and welding of the basal vitrophyre to less than 1 h, and possibly even tens of minutes. Thermo-mechanical iteration of the sintering process indicates an optimal temperature solution for the emplacement of the vitrophyres at 966 degrees C. The vitrophyres reveal a Newtonian rheology up to 46 MPa, which suggests that the ash particles annealed entirely during welding and that viscous energy dissipation is unlikely from loading conditions alone, unless shear stresses imposed by the overlying ash flow were excessively high and sustained over long distances. The findings underline the value of the term "lava-like" flow to describe the end rheology of Snake River-type ignimbrites, fully consistent with the typical lithofacies observed.Tuffisites are veins of variably sintered, pyroclastic particles that form in conduits and lava domes as a result of localized fragmentation events during gas-and-ash explosions. Those observed in-situ on the active 2012 lava dome of Volcan de Colima range from voids with intra-clasts showing little movement and interpreted to be failure-nuclei, to sub-parallel lenses of sintered granular aggregate interpreted as fragmentation horizons, through to infilled fractures with evidence of viscous remobilization. All tuffisites show evidence of sintering. Further examination of the complex fracture-and-channel patterns reveals viscous backfill by surrounding magma, suggesting that lava fragmentation was followed by stress relaxation and continued viscous deformation as the tuffisites formed. The natural tuffisites are more permeable than the host andesite, and have a wide range of porosity and permeability compared to a narrower window for the host rock, and gaging from their significant distribution across the dome, we posit that the tuffisite veins may act as important outgassing pathways. To investigate tuffisite formation we crushed and sieved andesite from the lava dome and sintered it at magmatic temperatures for different times. We then assessed the healing and sealing ability by measuring porosity and permeability, showing that sintering reduces both over time. During sintering the porosity-permeability reduction occurs due to the formation of viscous necks between adjacent grains, a process described by the neck-formation model of Frenkel (1945). This process leads the granular starting material to a porosity-permeability regime anticipated for effusive lavas, and which describes the natural host lava as well as the most impervious of natural tuffisites. This suggests that tuffisite formation at Volcan de Colima constructed a permeable network that enabled gas to bleed passively from the magma. We postulate that this progressively reduced the lava dome's ability to seal and build pressure that drives explosions. Indeed, the time interval between explosions during 2007-2011 gradually increased before the onset of a period of quiescence starting in June 2011. We suggest that the permeability evolution during tuffisite formation has important consequences for modeling of gas-and-ash explosions, common at dome-forming volcanoes.