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Understanding basement processes in sub-volcanic settings by laboratory measurement

Student thesis: Doctoral Thesis

ºVolcanic eruption and flank collapse are among the greatest hazards volcanoes pose. Detailed field analysis, numerical modelling and experimental work are the key tools to study how such events can arise. This thesis focuses on the latter, and shows how laboratory techniques can be employed to enhance our understanding of volcanic processes.
When rocks are exposed to high temperatures, easily found in volcanic systems, their mechanical behaviour can change. This can lead to a scenario where rocks no longer are able to support the load caused by the volcanoe, and subsequently destablize it. Here it is assessed how physical properties, such as mechanical strength and permeability of volcanic basement rocks,vary due to the exposure to high temperatures and pressures.
For this purpose, this wirk focuses on two volcanoes as case studies Mt. Etna (Southern Italy) and Snæfellsjökull (Western Iceland). As representative samples were subjected to high pressures and tempertures by using a Paterson-type gas pressure medium deformation apparatus. Samples were held at pressure of 50 to 150 MPa, corresponding to approximately 2-6 km depth, and temperatures ranging from room temperature to 1000ºC. Mechanical properties were measured by deforming a suit samples at constant or varying strain rate (10-5 to 10-4s-1). Permeabilites were evaluated at hydrostatic conditions.Limestone forms a substantial pressure in the basement of Mt. Etna. When such rocks are exposed to high temperatures (>600ºC), the calcite minerals that make up the rock can decompose to CO2 and lime. The results in this thesis suggest that, when rocks are exposed to high pressures as well as hig temperature, these reactions are halted due to the closure of pore space. At temperatures above 600ºC the rock strength is significantly decreased such that the confining pressure causes rocks to compact, also in hydrostatic conditions. This is evidenced by the permanent decrease of permeability with temperature.The sub-Etnean limestone shows a switch in brittle to ductile behaviour at 350 +/-50ºC (at a strain of 10-5 s-1). The edifice of Mt. Etna consists of basalt, which is noably stronger, and has brittle to ductile transition at 987 +/-12ºC at the same strain rate. To evaluate how strong the edifice is, a rheological profile is constructed. In the top (balsaltic) part, deformation will likely take place on existing fractures. Here an experimentally derived friction coefficient of 0.704 is found, which is assumed to be independent of strain rate and temperature. For the deeper parts a constitutive law relating stress, temperature and strain rate is developed, based on experiments with varying strain rate. The results show two flow regimes, grain size independent power law creep can occur at low temperatures, and a similar power law creep at high temperatures (>750ºC), where grain size dependent processes could be active but are not observed. Using these laws structural weaknesses in the basement of Mt. Etna are identified and quantified.One of the key manners in which magma is transported through the crust is dyking. Such events consist of two stages: the opening of a fracture; and the infill of that fracture with magma. The process of dyking can be reproduced in the lab by modifying a traditional compression setup. By recording acoustic emissions during dyking , it is found that the two two events have a different type of acoustic signature, which, when scaled to natural cases, can be interpreted as two different types of earthquakes. Such results can be used for future early-warning systems for volcanic eruptions.High temperatures change the elastic properties of rocks, particularly around the brittle to ductile transition temperature. Country rocks surrounding the magma chamber are hot, and because of that, relatively soft (i.e. low Young's modulus) acts as a dampening effect for pressure variations caused by unloading effects.The results of this dissertation provide new insights and accurate numbers at volcano-tectonic pressure and temperature conditions. Both can be adopted in field studies and numerical models to improve their predictive ability, and ultimately  lead to accurate forecasting of volcanic hazards.
Original languageEnglish
Awarding Institution
  • Swiss Federal Institute of Technology
  • J. P. Burg (External person) (Supervisor)
  • Dr Philip Benson (Supervisor)
  • Marie E.S Violay (External person) (Supervisor)
  • Ernest Rutter (External person) (Supervisor)
Award date2016
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