Explosive volcanic eruption processes: from mesoscopic simulations to constitutive laws

当火山爆发时，它们是所有自然灾害中最致命的。爆炸性喷发，就像1980年的圣海伦火山爆发一样，发生在熔融的岩石或岩浆喷发得如此猛烈，以至于岩浆被破碎成小碎片。这些碎片以火山灰的形式上升到空中，并以致命的火山碎屑流的形式从火山两侧流下。然而，并非所有的火山爆发都是爆炸性的。夏威夷的基拉韦厄火山已经连续喷发了二十多年，但在这段时间里没有爆发。相反，熔岩从火山缓缓流下，流入大海，对当地居民几乎没有造成危险;这是一次热情洋溢的喷发。有些火山可以表现出两种类型的行为：有时爆发，有时涌出。这些火山，其中圣海伦火山是一个例子，是特别危险的，因为它是非常难以预测的时候，喷发将切换从喷涌到爆炸。火山学的一个主要目的是了解是什么控制着喷发是爆发性的还是喷发性的，以及是什么使它在两者之间转换。我的研究将有助于回答这个问题。岩浆中含有许多气泡。火山学家认为，当气泡中的压力高到足以使周围的岩浆破裂成碎片时，就会发生爆炸性喷发。如果气泡变得足够大，它们可以合并并形成网络，气体可以沿着网络流动，从而使其从气泡中逸出。这意味着压力无法积聚，喷发更有可能是喷涌式的。对于火山学家来说，了解气体沿着这些气泡网络流动的容易程度至关重要。我正在开发一个计算机程序，LBFLOW，它可以模拟火山岩中气泡网络中的气体流动。我正在使用这个程序来计算不同类型的气泡网络中气体从气泡中逃逸的速度。这些信息将使我能够计算出压力从不同火山的岩浆中逃逸的速度。我已经使用LBFLOW来观察气体通过小气泡网络的流动。为了使结果尽可能有用，我想调整LBFLOW在“并行计算机”上运行，这是我们在剑桥大学BP研究所拥有的一种超级计算机，我将在那里进行这项工作。在这台计算机上运行LBFLOW将允许我模拟更大的网络，使结果更可靠。在使用计算机模型时，必须根据实验检查结果，以确保模型给出正确的答案。我将使用一台核磁共振成像仪，类似于医院里用来观察病人身体内部的仪器，来观察流经岩石中气泡网络的液体。MRI扫描将显示液体在网络不同部分的流动速度。通过在相同的气泡网络上运行LBFLOW，我可以将程序的结果与MRI结果进行比较，并确保它们是相同的。在这项研究中使用LBFLOW有几个主要优点。首先，由于计算机速度很快，我可以快速检查许多不同类型的气泡网络。另一个好处是，我可以精确地控制气泡网络是什么样的：e.气泡的大小和重叠程度这将使我的结果适用于广泛的火山。对火山学家来说，了解岩浆的粘性也很重要。如果岩浆是粘性的，它比流动的更容易爆发。LBFLOW还将显示气泡如何影响岩浆的粘度。我已经在实验室里做了实验来研究这个问题。我将使用LBFLOW来观察晶体如何影响岩浆的粘度。我的工作成果将有助于火山学家了解火山喷发的方式。它将帮助我们弄清楚火山可能会发生什么类型的喷发，以及什么会使它从喷涌式转变为爆炸式活动。

When volcanoes erupt explosively, they are amongst the deadliest of all natural hazards. An explosive eruption, like that at Mt St Helens in 1980, occurs when the molten rock, or magma, erupts so violently that the magma is broken into small fragments. These fragments rise into the air as volcanic ash and flow down the sides of the volcano as deadly pyroclastic flows. Not all volcanic eruptions are explosive, however. Kilauea in Hawaii has been erupting continuously for over twenty years, but has not exploded in this time. Rather, lava has flowed gently down the volcano and into the sea, causing little hazard to the local population; this is an effusive eruption. Some volcanoes can show both types of behaviour: sometimes erupting explosively, sometimes effusively. These volcanoes, of which Mt St Helens is an example, are particularly dangerous as it is very hard to predict when the eruption will switch from effusive to explosive. A main aim of volcanology is to understand what controls whether an eruption is explosive or effusive, and what makes it switch between the two. My research will help to answer this question. Magma contains many gas bubbles. Volcanologists believe that explosive eruptions occur when the pressure in the bubbles gets high enough to make the surrounding magma break into fragments. If the bubbles get big enough, they can merge and form networks along which gas can flow, allowing it to escape from the bubbles. This means that the pressure can't build up and the eruption is more likely to be effusive. It is vital for volcanologists to know how easily gas can flow along these bubble networks. I am developing a computer programme, LBFLOW that simulates gas flow through the bubble networks in volcanic rocks. I am using this programme to calculate how rapidly gas can escape from bubbles for different types of bubble network. This information will allow me to work out how rapidly the pressure can escape from magma at different volcanoes. I have already used LBFLOW to look at gas flow through small networks of bubbles. In order to make the result as useful as possible, I want to adapt LBFLOW to run on a 'parallel computer', a type of supercomputer that we have at the BP Institute at Cambridge University, where I will carry out this work. Running LBFLOW on this computer will allow me to simulate much larger networks, making the results more reliable. When using computer models, it is vital to check the results against experiments to ensure that the model gives the correct answer. I am going to use an MRI machine, similar to ones used in hospitals to look inside a patient's body, to look at fluid flowing through networks of bubbles in rock. The MRI scan will show how fast the fluid is flowing in different parts of the network. By running LBFLOW on the same bubble network, I can compare the programme's results with the MRI results and ensure that they are the same. There are a couple of major advantages to using LBFLOW for this research. Firstly, because computers are so fast, I can examine many different types of bubble network rapidly. The other advantage is that I can control exactly what the bubble networks are like: i. e. how big the bubbles are and how much they overlap. This will make my results applicable to a wide range of volcanoes. It is also important for volcanologists to know how sticky, or viscous magma is. If the magma is sticky, it is much more likely to erupt explosively than it if is runny. LBFLOW will also show how bubbles affect the viscosity of the magma. I have already performed experiments in the laboratory to look at this. I will use LBFLOW to look at how crystals affect magma's viscosity. The results of my work will help volcanologists to understand the way volcanoes erupt. It will help us to work out what type of eruption is likely to happen at a volcano, and what would make it switch from effusive to explosive activity.