Reconstructing eruptive processes from volatile distribution in volcanic glass

火山喷发有两大类：爆发式和喷发式。爆炸性喷发会产生高大的火山灰和浮石云，这些云可能会落在广阔的地区，还会产生高温、快速流动的火山碎屑流，对当地社区构成严重危害。喷出式喷发产生缓慢流动的熔岩流和圆顶，通常危险性要小得多。许多火山，如圣海伦火山（美国）、默拉皮火山（印度尼西亚）和苏弗里耶尔火山（蒙特塞拉特，英国在西印度群岛的领土），可能会喷发或爆发，并可能在一次喷发中在这些风格之间切换。这是特别危险的，因为这些开关可能在没有警告的情况下发生，使得有效的危险管理非常困难。因此，了解火山喷发风格转变的原因是火山学最重要的目标之一。这项研究将开发一种新的工具，帮助火山学家了解火山喷发风格之间的转换。该工具可用于所谓的“火山”，具有特殊类型的岩浆是非常粘稠（粘性）。由于岩浆是如此粘稠，这些火山能够产生最猛烈的喷发类型。阿拉斯加的诺瓦鲁普塔火山就是这样一座火山--它在1912年的喷发是20世纪世纪最强大的一次，也是过去1000年来第四次最强大的火山。我们将把研究重点放在这次喷发上，因为它多次转换喷发方式，而且它喷发的岩浆沉积物保存得非常完好。火山喷发是由浮力驱动的，当水最初溶解在岩浆中时，它储存在地下，从溶液中出来并形成蒸汽气泡-就像香槟中气泡的形成一样，软木塞一开就会喷出来我们的新工具的工作原理是测量岩浆冷却时还剩多少水，以及岩浆冷却时水如何分布在“冻结”的气泡周围。该研究小组成员进行的一项博士研究表明，通过测量水在气泡周围的分布方式，可以重建岩浆样本在喷发期间经历的压力和温度变化。我们使用光谱技术来做这件事，这种技术可以绘制出精确到千分之几毫米的水分布图。通过了解爆发和溢出的岩浆样品的压力和温度历史的差异，我们可以确定是什么物理差异导致了喷发风格的变化。我们将使用实验室生产的人造浮石样品，因此具有非常好的约束“喷发”条件，来测试和完善我们的工具。然后，我们将把它应用于1912年诺瓦鲁普塔火山爆发的样本。有几个具体的问题我们想回答，其中最重要的是：火山喷发方式的转变是由气体从岩浆中出来的方式的变化引起的吗？如果是这样的话，是什么导致了这种变化，以及这种变化发生在火山的管道系统的哪里？找到这些问题的答案不仅有助于我们理解诺瓦鲁普塔火山爆发，而且有助于我们理解为什么在喷发和爆炸之间转换的火山爆发在火山中如此常见。这将产生远远超出我们研究范围的影响。它将帮助火山学家确定一次新的火山爆发是否可能改变风格;它甚至可能让我们找到什么迹象来寻找风格即将改变的迹象。最终，这将有助于保护面临风险的社区免受最严重的自然灾害之一的影响。

There are two broad categories of volcanic eruption: explosive and effusive. Explosive eruptions produce tall clouds of ash and pumice, which may fall over a wide area, and hot, fast-moving pyroclastic flows, which pose an acute hazard to local communities. Effusive eruptions produce slow-moving flows and domes of lava and are usually much less hazardous. Many volcanoes, such as Mt St Helens (USA), Merapi (Indonesia) and Soufrière Hills (Montserrat, a UK territory in the West Indies) may erupt either effusively or explosively, and may switch between these styles during a single eruption. This is particularly hazardous because these switches may happen with no warning, making effective hazard management very difficult. Consequently, understanding the causes of transitions in eruption style is one of the most important goals in volcanology.This study will develop a new tool that will help volcanologists to understand why volcanoes switch between eruption styles. The tool can be used at so-called "silicic" volcanoes that have a particular type of magma that is very sticky (viscous). Because the magma is so viscous, these volcanoes are capable of producing the most violent types of eruption. One such volcano is Novarupta in Alaska - its eruption in 1912 was the most powerful of the 20th century and the fourth most powerful in the last 1000 years. We will focus our study on this eruption because it switched eruption style several times, and because it has exceptionally well-preserved deposits of the magma that it erupted.Volcanic eruptions are driven by buoyancy forces that arise when water, which is initially dissolved in the magma while it is stored underground, comes out of solution and forms bubbles of steam -much like the formation of bubbles in champagne, which cause it to spray out when the cork is popped. Our new tool works by measuring how much water is left in the magma when it cools, and how it is distributed around the bubbles that are 'frozen' in when the magma cools. A PhD study conducted by members of this research team has shown that it is possible to reconstruct the changes in pressure and temperature that a sample of magma experienced during eruption by measuring the way that the water is distributed around the bubbles. We do this using a spectroscopic technique that can make maps of the water distribution that are accurate to a few thousandths of a millimetre. By understanding the differences in pressure and temperature history of magma samples that were erupted explosively and effusively, we can determine what physical differences caused the change in eruption style.We will use artificial pumice samples, which are produced in the laboratory and therefore have very well constrained 'eruption' conditions, to test and perfect our tool. We will then apply it to samples from the eruption of Novarupta in 1912. There are a few specific questions that we want to answer, the most important of which are: Are the switches in eruption style caused by changes to the way the gas comes out of the magma? If so, what causes the change, and where in the volcano's plumbing system does this happen?Finding the answers to these questions won't just help us to understand the Novarupta eruption, but to understand why eruptions that switch between effusive and explosive are so common at silicic volcanoes. This will have impact well beyond our study. It will help volcanologists to work out whether a new eruption is likely to switch in style; it may even allow us to work out what signs to look for that a change in style is imminent. Ultimately, this will help to protect at-risk communities from one of the most serious natural hazards.