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

火山本质上是不稳定的地质特征，它们的坍塌会造成地球表面一些最大的山体滑坡。这种类型的滑坡最著名的例子之一是1980年圣海伦斯火山喷发时发生的滑坡。这一事件产生了一种快速移动的岩石混合物，称为碎片雪崩，体积接近3立方千米。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，在这些情况下，山体滑坡的规模可能比圣海伦斯火山的碎片雪崩大100倍以上。火山岛上的碎片雪崩可能会引发非常大的海啸。然而，海啸的规模取决于山体滑坡的移动方式。例如，一场分几个不同阶段发生的滑坡，或者涉及周围海底深处物质的滑坡，产生的海啸将比同样体积的单一滑坡产生的海啸小得多。现代社会没有观察到大规模的火山岛坍塌。然而，过去山体滑坡的几个海底沉积物表明，它们普遍存在。同样的沉积物提供了了解这些山体滑坡如何移动的关键，从而准确地模拟了与之相关的海啸危险。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。具体地说，这项研究将表明：火山岛上的碎片雪崩通常发生在单个阶段还是多个阶段；是什么控制着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山体滑坡。这项研究将集中在西印度群岛蒙特塞拉特附近的两个滑坡沉积物上。每个矿藏的大小都与圣海伦斯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截然不同。其中一次山体滑坡移动得更远，而且似乎也涉及大量来自海底的物质，而不是来自火山的物质。在其他火山岛周围也可以找到与蒙特塞拉特矿床类似的例子。因此，蒙特塞拉特沉积物为了解火山岛周围滑坡的发育过程提供了一般信息。要全面了解蒙特塞拉特滑坡沉积物是如何形成的，需要将地球物理数据与实际沉积物的样本结合起来。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构造在滑坡运动方面的含义。国际大洋钻探计划(IODP)将于2012年3月采集这些沉积物的样本。这将是第一次对这种类型的沉积物进行直接采样。因此，这一独特的样本集为我们提供了一个显著提高我们对火山岛周围山体滑坡的认识的机会。第一部分将根据IODP样本重新解释地球物理数据。这将有助于显示滑坡是如何在水下移动的。第二阶段将调查滑坡沉积物边缘沉积的沉积物，使用物理和化学测量来显示滑坡是否发生在多个阶段。第三阶段将确定滑坡沉积物的成分，以测量有多少沉积物是由海底物质形成的。最后阶段将直接确定较年轻的滑坡堆积物的年代，然后使用已经收集的样本和数据进一步调查其成分和崩塌阶段。总之，这项工作将提供对蒙特塞拉特滑坡沉积物的详细了解，并将为更好地了解其他火山岛周围的滑坡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.