Short-period deformation at a lava dome volcano

当火山喷发时，就在岩浆或熔岩到达地表产生熔岩和火山灰之前，它会变得更加粘性，不愿流动。岩浆性质的这种变化反过来又影响了许多其他过程--高压的形成，气体流动的变化，以及岩浆本身的流动速度变得可变。有时，这些变化每隔几个小时就会有系统地变化，从而产生周期性的行为。能够测量这种周期性行为对火山天文台的科学家来说非常有用，原因有两个。首先，在这段时间内的某些时间更容易发生爆炸和危险流动，因此能够预测它们的发生是有用的。其次，通过观察各种现象在每个周期中的变化，可以了解导致周期性流动的条件。这反过来又可以更好地预测火山的长期行为，使受影响的人受益。在这个项目中，我们将提高我们对蒙特塞拉特苏弗里尔山火山这种行为的理解。这种周期性行为可能在更危险的火山类型中很常见，因为火山的岩浆中含有丰富的二氧化硅。然而，它很难观察，因此没有得到很好的理解。这是因为与它相关的一些信号仅限于火山喷口附近，很难测量。苏弗里雷尔山就是测量到这种周期性信号的地方之一。在1997年初的几个月里，测量地面倾斜度的倾斜仪记录了一系列引人注目的地面运动上下循环，周期约为9小时。不幸的是，倾斜仪被火山摧毁，随后该位置太危险，无法重新安装新的倾斜仪。我们计划在雷丁大学的一个为期两年的项目中引入一项新技术来解决这个问题，该项目将应用于蒙特塞拉特火山天文台。我们不会使用埋在地下的仪器测量地面运动，而是使用雷达干涉测量法在安全距离内测量。从几公里外，我们将测量苏弗里尔山火山口内熔岩穹顶周围地面的向外和向内运动。我们预计这个周期将在几个小时内进行测量，对于十倍于此的信号，其精度为几毫米。一种便携式的陆基雷达干涉仪已经为这种类型的任务而开发出来，我们将第一次在这样的火山上使用它。由于该仪器给出的是地面位移的图像，而不是读数，因此它将能够通过从不同的角度进行测量来测量运动的空间模式。这将使新的测量能够检验一种假设，即将岩浆输送到苏弗里尔山火山下方地表的管道的形状类似于一个垂直圆柱体，连接到大约一公里深的裂缝上。自1997年以来，蒙特塞拉特火山观测站常规测量整个岛屿的地震、气体和更大范围形变的技术有了很大进步，特别是测量频率。我们将使用这些频繁的(非常小时或更短的)周期测量来与计算机模拟的充满岩浆的管道进行比较。这将帮助我们更好地了解管道的行为方式以及它未来可能的行为方式。

At erupting volcanoes, just before magma, or molten rock, arrives at the surface to produce lava and ash, it can become much more viscous and reluctant to flow. This change in character of the magma in turn affects a number of other processes - high pressures build, gas flows change and the rate of flow of the magma itself becomes variable. Sometimes these changes vary systematically every few hours to produce a periodic behaviour. Being able to measure such periodic behaviour is very useful to scientists in volcano observatories for two reasons. Firstly, certain times in the period are much more prone to explosions and hazardous flows, and so being able to forecast their occurrences is useful. Secondly, by observing how a variety of phenomena change during each cycle allows the conditions that give rise to the periodic flow to be understood. This in turn allows the longer-term behaviour of the volcano to be better anticipated, with benefits to people affected. In this project we will improve our understanding such behaviour at Soufriere Hills Volcano, Montserrat. This type of periodic behaviour is probably common at the more dangerous type of volcano with magma rich in silica. However, it is very difficult to observe and as a consequence not well understood. This is because some of the signals associated with it are restricted to near the vent of volcano and are difficult to measure. One place where such periodic signals were measured is Soufriere Hills. Over an interval of a few months in early 1997, tiltmeters that measure the inclination of the ground surface, recorded a remarkable series of cycles of ground motion up and down with a period of about 9 hours. Unfortunately, the tiltmeters were destroyed by the volcano and the location was subsequently too dangerous to re-install new ones. We plan to bring a new technology to bear on this problem in a 2-year project based at the University of Reading and applied at the Montserrat Volcano Observatory. Rather than measure the ground movement using an instrument buried in the ground we will do so from a safe distance using radar interferometry. From a few kilometres away we will measure the outward and inward movement of the ground around the lava dome growing within the crater at Soufriere Hill. We expect the cycle to be measured over a few hours and to an accuracy of a few millimetres for a signal ten times as large. A portable, ground-based radar interferometer has been developed for this type of task, and we will be the first to use it on a volcano like this. Because the instrument gives an image of the ground displacement rather than a point reading it will be able to measure the spatial pattern of motion, by making measurements from different viewpoints. This will enable the new measurements to test a hypothesis that the conduit feeding the magma to the surface below Soufriere Hills Volcano has a shape like a vertical cylinder joined onto a fissure below depths of about one kilometre. The technology of the measurements of earthquakes, gas and wider deformation of the whole island routinely made by the Montserrat Volcano Observatory has advanced greatly since 1997, particularly the frequency of measurements. We will use these frequent (very hour and less) measurements of the cycle to compare with a computer simulation of the magma-filled conduit. This will help us to understand better how the conduit behaves and how it might behave in the future.