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Characterization of the juvenile pyroclasts from the Kos-Nisyros Volcanic Complex (south Aegean Arc, Greece): a tool for understanding the dynamics of explosive rhyolitic eruptions

English title Characterization of the juvenile pyroclasts from the Kos-Nisyros Volcanic Complex (south Aegean Arc, Greece): a tool for understanding the dynamics of explosive rhyolitic eruptions
Applicant Bonadonna Costanza
Number 124820
Funding scheme Project funding
Research institution Département des sciences de la Terre Université de Genève
Institution of higher education University of Geneva - GE
Main discipline Mineralogy
Start/End 01.05.2009 - 30.04.2010
Approved amount 59'440.00
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Keywords (12)

X-ray microtomography; pumices; volcanic conduits; modelling; volcanic eruption; effusive; explosive; Kos Plateau Tuff; conduit; outgassing; fragmentation; pumice

Lay Summary (English)

Lead
Lay summary
Volcanic eruptions can erupt in many different styles, from the effusion of lava flows to explosive eruptions which can eject material into the atmosphere for days. 160 000 years ago, a very large explosive volcanic eruption occurred near the island of Kos (Greece), known as the Kos Plateau Tuff or KPT eruption. An eruption which would have devastating effects if it occurred today not only on a regional but also on a global scale. This project focuses on the physical controls that will define the style of the eruption by looking at natural products (pumices) of the KPT eruption and tying it to computer simulations of eruptions.Volcanoes comprise of a magma chamber, which is connected to the Earth's surface through a volcanic conduit. An eruption is triggered when the magma chamber is decompressed, which pushes the magma into the conduit. Volatiles such as water are exsolved, bubbles nucleate and grow, and accelerate the magma upward. When bubbles start coalescing they may form a permeable network through the magma and let gas escape. This process is called outgassing and evacuates energy that drives the eruption and thus triggers effusive behavior. If the conditions are such that bubble expansion is rapid, the magma can disintegrate into hot gases and volcanic particles. This process is called fragmentation and is the key feature of an explosive eruption. The focus of this study lays on the complex interplay between outgassing and fragmentation that define the outcome of an eruption. By looking at pumices we can quantify the outgassing efficiency at the moment of fragmentation and use it as bench marks for computer models simulating volcanic eruptions.As the human population rapidly expands and the slopes of dangerous volcanoes become densely inhabited the problem of volcanic eruptions becomes increasingly important. Therefore the understanding of these complex natural phenomena is crucial to assess its risks and reduce the damage it may cause.
Direct link to Lay Summary Last update: 21.02.2013

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Associated projects

Number Title Start Funding scheme
111709 Characterization of the juvenile pyroclasts from the Kos-Nisyros Volcanic Complex (south Aegean Arc, Greece): a tool for understanding the dynamics of explosive rhyolitic eruptions 01.04.2006 Project funding
111709 Characterization of the juvenile pyroclasts from the Kos-Nisyros Volcanic Complex (south Aegean Arc, Greece): a tool for understanding the dynamics of explosive rhyolitic eruptions 01.04.2006 Project funding

Abstract

The project “Characterization of the juvenile pyroclasts from the Kos-Nisyros Volcanic Complex (south Aegean Arc, Greece): a tool for understanding the dynamics of explosive rhyolitic eruptions” (proposal No 200021-111709) started the 1st of April 2006 and has funding until the 31st of March 2009. Wim Degruyter started as a PhD candidate on this project the 1st of August 2006 leading to a slight delay in the beginning of data acquisition. The principal supervisor for this project has changed in 2007. Olivier Bachmann was hired as an assistant professor at the University of Washington (Seattle), starting from the 1st of September 2007 and resigned from his position at the University of Geneva the 31st of July, 2007. As a result Wim Degruyter has joined the research group of Prof. Costanza Bonadonna, who is now the main supervisor of this project. Olivier Bachmann is however still actively involved in the research. We propose to continue this project for one extra year until the 31st of March 2010, in collaboration with our colleagues from the Georgia Institute of Technology (Prof. J. Dufek), and CNRS Orléans (Dr. A. Burgisser).Approximately 160,000 years ago, one of the largest Quaternary explosive eruptions occurred off shore of the eastern Mediterranean island of Kos, during which ~150 km3 of volcanic ash and pumice were emplaced as pyroclastic fall and flow deposits over an area >2000 km2. In comparison to the Minoan eruption on the nearby island of Santorini, the Kos Plateau Tuff (KPT) was almost five times larger in volume. Since the KPT eruption, several smaller but similarly explosive events occurred in the same area, indicating that the system continues to generate significant volcanic hazards. Although the repose periods for such volcanic systems are on the order of 103-104 years (increasing with increasing erupted volume), similarly cataclysmic eruptions will occur again in the Mediterranean basin and elsewhere, with devastating consequences on a regional or even hemispheric scale. Whether a volcanic eruption of viscous, water-rich magma occurs as a (relative slow) effusion of lava or as a highly hazardous explosive burst of hot gases and volcanic particles depends primarily on how the magma behaves in the conduit prior to reaching the earth’s surface. If the conditions are such that bubble expansion is rapid, the magma may reach fragmentation (the process by which viscous, bubbly magmatic foam disintegrate into a low-viscosity particle-laden hot gas), triggering an explosive event. Understanding how magma ascends in conduits and how it fragments as a function of its physico-chemical characteristics is therefore fundamental to predict the impact of the eruption on human infrastructures and populations. Despite the fact that this area of research has been very active over the last decade, much remains to be done. A particularly important aspect is to better understand how outgassing (gas separating from the magma through permeable bubble networks) influences magma fragmentation Juvenile clasts (pumices) produced by explosive silicic eruptions are the only available natural sample that preserves information on the state of magma at fragmentation. In the past two years, we have characterized pumices from the largest explosive eruption of the South Aegean volcanic island arc (the KPT). This unit is exceptional mine of textural information due to abundance of non-welded juvenile material, and permit a detailed comparison of pumices from different eruptive styles, including (1) phreato-magmatic activity (early and late part of the KPT), (2) a magmatic, central-vent, pre-caldera phase of the KPT, and (3) the magmatic, caldera-forming phase (climactic part of the KPT). Eruptive products were described using the following methods: porosity (total and connected) and permeability were measured both in-situ (by Hg-porosimetry and gas permeametry) and by image analysis in 2-D (thin section observation, SEM). Currently the 2-D textural data is being complemented by 3-D reconstruction and statistical analysis of the pore space structure using high resolution X-ray computed microtomography. We selected two different types of pumices (one with highly elongated bubbles, tube pumice; and the other with near spherical bubbles, frothy pumice) to tie micro- and macroscopic textures and quantify differences of the bubble network geometry. We show that shear stress, assumed to be the cause of deformation inside tube pumices, creates more connected paths per unit volume in the tube-like bubble networks and that these paths are straightened in the direction of elongation. Throat size distributions provide estimates of mean bubble aperture size, similar for both types of pumices. These measurements explain the range of laboratory permeability values, which showed that tube pumices are more permeable in the direction of bubble elongation than frothy pumices despite lower porosities. After the preliminary data collection using desktop X-ray tomography at the Universities of Lausanne and Ghent, it was concluded that higher resolution images were necessary to test the proposed hypotheses. Therefore, proposals were sent out to the largest synchrotron X-ray tomography facilities in Europe (Elettra in Italy, the SLS in Switzerland, and the ESRF in France) to obtain these images. All synchrotrons accepted this project for beam time and images are currently being acquired. These data will provide a leap forward for this project as successful 3D images with a resolution of ~1-2 microns were obtained each of the pumice types. However, they were not foreseen in the original time table and image processing is time-consuming. Finally, numerical simulations of 2-D magma flow in the volcanic conduit and 3-D gas flow through the porous pumice will be undertaken. These simulations are the most powerful tools available to better understand: (1) how magma fragmentation proceeds during large, explosive, silicic, volcanic eruptions and (2) how gas escape from the volatile-rich magma controls the transition from explosive to effusive eruptive style. Using the textural information gathered on the KPT pumices, Wim Degruyter will develop, with the help of the team of experts involved in this project, a state-of-the-art numerical model that will include permeability variations within the magma column. We expect to gain new insight into the velocity profile across the conduit, and shed some light on how and where magma fragmentation occurs. Considering that Wim Degruyter started the 1st of August 2006 and the considerable amount of additional work underway, we ask for an extra year of funding for this project.
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