Emplacement dynamics of debris avalanches and submarine landslides at Soufrière Hills volcano, Montserrat

火山是天生不稳定的地质特征，它们的崩溃会产生地球表面最大的滑坡。这类山崩最著名的例子之一是1980年圣海伦火山爆发时发生的山崩。这一事件产生了一个快速移动的岩石混合物，称为碎片雪崩，体积近3立方公里。类似的山体滑坡发生在许多火山岛周围，如夏威夷和特内里费岛，在这些环境中，山体滑坡的规模可能比圣海伦斯山的碎片雪崩大100倍以上。火山岛上的碎片雪崩可能会产生非常大的海啸。然而，海啸的规模取决于滑坡如何移动。例如，一次滑坡发生在几个不同的阶段，或者涉及到周围海底深处的物质，所产生的海啸要比一次同样体积的、都来自火山的滑坡小得多。然而，过去滑坡的几个海底沉积物表明它们广泛存在。同样的沉积物提供了了解这些山体滑坡如何移动的关键，因此可以准确地模拟其相关的海啸危险。了解火山岛周围滑坡的形成和发展是本研究项目的主要目的。具体而言，研究将显示：火山岛上的碎片雪崩通常是单阶段还是多阶段发生的;是什么控制了滑坡在水下移动的距离;以及碎片雪崩一旦降落在海底是否会引发进一步的滑坡。每一个存款的大小都与圣海伦火山的碎片雪崩相似。该地区的地球物理数据显示，这两个矿床的形状和结构非常不同。其中一次山体滑坡传播得更远，似乎还涉及大量来自海底的物质，而不是来自火山。在其他火山岛周围也可以找到类似蒙特塞拉特矿床的例子。因此，蒙特塞拉特矿床为了解火山岛周围的山崩发展过程提供了一般性资料，要充分了解蒙特塞拉特山崩矿床的形成，就必须将地球物理数据与实际矿床样品相结合。这些样本将显示滑坡沉积物的不同部分是由什么组成的，并将揭示地球物理数据中看到的滑坡运动的结构。国际大洋钻探计划将于2012年3月收集矿床样本。这将是第一次对这类矿床进行直接取样。因此，这一独特的样本集提供了一个机会，大大提高我们对火山岛周围山体滑坡的认识。第一次将根据IODP样品重新解释地球物理数据。这将有助于显示滑坡如何在水下移动。第二阶段将调查滑坡沉积物边缘的沉积物，使用物理和化学测量来显示滑坡是否在多个阶段发生。第三阶段将确定滑坡沉积物的成分，以测量有多少存款是由海底物质形成的。最后阶段将直接测定较年轻的滑坡存款的年代，然后利用已经收集的样本和数据进一步调查其成分和崩塌阶段。这项工作将使人们详细了解蒙特塞拉特的山崩沉积物，并将为更好地了解其他火山岛周围的山崩过程和海啸危险提供重要成果。

Volcanoes are inherently unstable geological features, and their collapse produces some of the largest landslides on Earth's surface. One of the best known examples of this type of landslide is that which accompanied the 1980 eruption of Mount St. Helens. This event generated a rapidly moving mixture of rock, known as a debris avalanche, with a volume of nearly three cubic kilometres. Similar landslides have occurred around many volcanic islands, such as Hawaii and Tenerife, and in these settings can be over 100 times larger than the Mount St. Helens debris avalanche. Debris avalanches on volcanic islands may potentially generate very large tsunamis. However, the size of the tsunami depends on how the landslide moves. For example, a landslide that occurs in several distinct phases, or which involves material from the deep, surrounding seafloor, will produce much smaller tsunamis than a single landslide of the same volume that all derives from the volcano.No large-scale collapses of volcanic islands have been observed in modern times. However, several submarine deposits from past landslides indicate their widespread occurrence. The same deposits provide the key to understanding how these landslides moved, and for therefore accurately modelling their associated tsunami hazard. Understanding the formation and development of landslides around volcanic islands is the major aim of this research project. Specifically, the research will show: whether debris avalanches on volcanic islands typically occur in single or multiple stages; what controls how far the landslide moves underwater; and whether debris avalanches trigger further landslides once they land on the seafloor.This research will focus on two landslide deposits off Montserrat, in the West Indies. Each deposit is a similar size to the Mount St. Helens debris avalanche. Geophysical data from the area shows that the two deposits have very different shapes and structures. One of the landslides travelled much further and also appears to have involved a large volume of material from the seafloor, rather than from the volcano. Examples similar to both the Montserrat deposits can be found around other volcanic islands. The Montserrat deposits therefore provide general information for understanding the processes of landslide development around volcanic islands.Fully understanding how the Montserrat landslide deposits formed requires combining the geophysical data with samples of the actual deposits. These samples will show what different parts of the landslide deposits are made of, and will reveal what structures seen in the geophysical data mean in terms of landslide movement. Samples from the deposits will be collected in March 2012 by the International Ocean Drilling Program (IODP). This will be the first time that deposits of this type have been directly sampled. This unique sample set therefore provides an opportunity for significantly advancing our knowledge of landslides around volcanic islands.This research project involves four stages. The first will reinterpret the geophysical data in light of the IODP samples. This will help show how the landslide moved underwater. The second stage will investigate the sediment deposited around the margins of the landslide deposits, using physical and chemical measurements to show if the landslides occurred in multiple phases. The third stage will determine the constituents of the landslide deposits, to measure how much of the deposit was formed by material from the seafloor. The final stage will directly date the younger landslide deposit, and then further investigate its constituents and stages of collapse, using samples and data that have already been collected. Together, this work will provide a detailed understanding of the Montserrat landslide deposits, and will provide important results for a better understanding of landslide processes and tsunami hazards around other volcanic islands.