Emplacement dynamics Montserrat - Seb Watt - NERC

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.