Bain Amelia A, Calder Eliza S, Cortés Joaquín A, Cortés Gloria Patricia, Loughlin Susan C
1Grant Institute, The University of Edinburgh, Edinburgh, UK.
2Department of Geography, Edge Hill University, Ormskirk, UK.
Bull Volcanol. 2019;81(1):1. doi: 10.1007/s00445-018-1260-y. Epub 2018 Dec 7.
Hazardous sequences of vulcanian explosions are thought to result from the repeated emplacement and destruction of degassed, highly crystalline magma plugs in the shallow conduit of arc volcanoes. The processes governing the timing and magnitude of these explosions are thought to be related to magma ascent rate and efficiency of degassing and crystallisation. We study a rare suite of time-constrained ballistic bombs from the 2004-2010 period of activity of Galeras volcano to reconstruct magma plug architecture prior to six individual vulcanian explosions. We find that each plug was vertically stratified with respect to crystallinity, vesicularity and melt volatile content, melt composition and viscosity. We interpret this structure as resulting from multiple bubble nucleation events and degassing-driven crystallisation during multi-step ascent of the magma forming the plug, followed by spatially variable crystallisation within the plug under contrasting conditions of effective undercooling created by degassing. We propose that the shallow conduit evolved from more open degassing conditions during 2004-2008 to more closed conditions during 2009-2010. This resulted in explosions becoming smaller and less frequent over time during 2004-2008, then larger and more frequent over time during 2009-2010. This evolution was controlled by changing average ascent rates and is recorded by systematic changes in plagioclase microlite textures. Our results suggest that small volume vulcanian explosions (~ 10 m) should generally be associated with longer repose times (hundreds of days) and produce ballistics characterised by small numbers of large, prismatic plagioclase microlites. Larger volume vulcanian explosions (~ 10 m) should be associated with shorter repose times (tens of days) and produce ballistics characterised by high numbers of small, more tabular plagioclase microlites.
武尔卡诺式火山爆发的危险序列被认为是由弧火山浅部管道中反复安置和破坏已脱气、高度结晶的岩浆塞造成的。控制这些火山爆发时间和强度的过程被认为与岩浆上升速率以及脱气和结晶效率有关。我们研究了一套来自加勒拉斯火山2004 - 2010年活动期的罕见的有时间限制的抛射弹，以重建六次单独的武尔卡诺式火山爆发之前的岩浆塞结构。我们发现每个岩浆塞在结晶度、气孔率、熔体挥发物含量、熔体成分和粘度方面都是垂直分层的。我们将这种结构解释为在形成岩浆塞的岩浆多步上升过程中，由多次气泡成核事件和脱气驱动的结晶作用导致的，随后在脱气产生的有效过冷的对比条件下，岩浆塞内部发生空间可变的结晶作用。我们提出浅部管道在2004 - 2008年期间从更开放的脱气条件演变为2009 - 2010年期间更封闭的条件。这导致在2004 - 2008年期间火山爆发随着时间推移变得更小、更不频繁，而在2009 - 2010年期间随着时间推移变得更大、更频繁。这种演化受平均上升速率变化控制，并通过斜长石微晶纹理的系统变化记录下来。我们的结果表明，小规模的武尔卡诺式火山爆发（约10米）通常应与更长的静止期（数百天）相关联，并产生以少量大型棱柱状斜长石微晶为特征的抛射物。规模较大的武尔卡诺式火山爆发（约10米）应与更短的静止期（数十天）相关联，并产生以大量小型、更板状斜长石微晶为特征的抛射物。
