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 (~ 105 m3) 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 (~ 106 m3) should be associated with shorter repose times (tens of days) and produce ballistics characterised by high numbers of small, more tabular plagioclase microlites.

中文翻译：

---

2004-2010 年哥伦比亚加莱拉斯火山火山爆发之前安山岩塞埋置的结构和地球化学限制

火山爆发的危险序列被认为是由于弧火山浅管道中脱气的高度结晶岩浆塞的反复就位和破坏造成的。控制这些爆炸的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的效率有关。我们研究了 2004 年至 2010 年加勒拉斯火山活动期间的一套罕见的限时弹道炸弹，以重建六次火山爆发之前的岩浆塞结构。我们发现每个插塞在结晶度、气泡度和熔体挥发物含量、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形成栓塞的岩浆多步上升过程中的多个气泡成核事件和脱气驱动的结晶，随后在脱气产生的有效过冷的对比条件下，栓塞内发生空间可变的结晶。我们认为，浅管道从 2004 年至 2008 年期间更开放的脱气条件演变为 2009 年至 2010 年期间更封闭的条件。这导致 2004 年至 2008 年期间爆炸规模逐渐缩小且频率降低，而 2009 年至 2010 年期间爆炸规模逐渐扩大且更加频繁。这种演化是通过改变平均上升速率来控制的，并通过斜长石微晶石纹理的系统变化来记录。我们的结果表明，小体积的火山爆炸（~ 105 m3）通常应与较长的静止时间（数百天）相关，并产生以少量大型棱柱形斜长石微晶石为特征的弹道。较大体积的火山爆炸（约 106 立方米）应与较短的静止时间（数十天）相关，并产生以大量小型、更板状的斜长石微晶石为特征的弹道。