DeutschClear Cookie - decide language by browser settings
Richard, Dominique (2015): Crossing the glass transition during volcanic eruptions: a matter of time scale and magma rheology. Dissertation, LMU München: Faculty of Geosciences



Predicting the occurrence and the evolving nature of volcanic eruptions remains an outstanding challenge. The complexity of volcanic Systems requires the use of many different approaches to gain a more profound understanding of the interplay of parameters such as magma temperature, composition, volatile content, cooling rate and viscosity as they interactively control the rheology of magma. This study focusses on three different scenarios in which the glass transition, a kinetic boundary distinguishing between solid-like and liquid-like behavior of melt in response to applied stresses, is crossed. (1) Bubbly magma fragments due to rapid decompression. The yield strength of the magma is overcome and the magma behaves in a brittle manner. Here, the influence of permeable gas flow, inside porous and permeable rocks, on the speed at which magma fragmentation occurs was investigated experimentally. (2) Spatter from a lava fountain agglutinates and coalesces upon landing. A spatter-fed flow forms, progressively thickens and lengthens, and results in a complex accumulation of glassy layers mingled with scoria, which grades into a lava-like section at the front of the flow. Because it is likely that individual layers forming the spatter-fed flow remain in the glass Transition interval for extended periods of time, the effect of thermal annealing on glasses was tested. Conditions conducive to thermal annealing during emplacement are discussed. (3) A pyroclastic deposit is emplaced in a hot and viscous state via fall or flow processes and consists of vitrified material and obsidian. The eruptive scenario is unclear; here the cooling history as well as microscopic and macroscopic textures help constraining the nature of the deposit’s components and identify the last few processes by which this deposit was emplaced. Permeable flow of volatiles through the porous and permeable network of a bubbly magma may influence the speed of magma fragmentation. Experiments were performed to reproduce the fragmentation of magma using a shock tube apparatus at room temperature and natural pyroclastic material with a connected porosity ranging from 15% to 78%. For each sample series, the initial pressure required to initiate Magma fragmentation was determined. Furthermore, sample permeability was measured and the samples were classified into: (a)dome/conduit wall rocks and (b) pumice/scoria. Results confirm that substantial outgassing during fragmentation leads to higher fragmentation thresholds. In addition, experimental fragmentation speeds are unexpectedly significantly higher than the modeled fragmentation speeds for high-permeability dome/conduit wall rocks, but lower for high-permeability pumices. Low-porosity, low-permeability, altered dome/conduit wall rocks fragment at significantly higher speeds than expected. Because fragmentation threshold and fragmentation viii speed are among the determining parameters for the initiation, sustainment and cessation of an eruption, outgassing should be considered in the modeling of magma fragmentation dynamics. The spatter-fed rheomorphic deposit from Cala di Tramontana, on the island of Pantelleria, Sicily, resulted from the mildly explosive eruption of pantelleritic magma as a lava fountain. This peralkaline rhyolitic magma has exceptionally low viscosities due to its high alkali, halogen and iron contents. Microscopic textural, geochemical and thermal analyses have helped setting better constraints on the cooling history and emplacement of this ca. 7 m thick deposit. Peak glass transition temperatures from the glassy layers are very low and range from 512 to 571 °C for a heating rate of 10 K min-1. Cooling rate estimates are obtained from the modeling of heat capacity curves using the enthalpy relaxation geospeedometry method, which follows the Tool-Narayanaswamy-Moynihan (TNM) method. Cooling rates range from 10-1 to ca. 10-7 K min-1 and are dispersed inconsistently throughout the flow deposit. For the first time, the effect of thermal annealing on cooling rate estimates has been tested. Thermal annealing experiments on remelted pantelleritic glasses from the same flow reveal that cooling rate estimates can be reduced by up to 3 log units when the investigated glasses are maintained at 450 °C for 1 day Prior to the modeling with the TNM method. The individual layers most likely remained at temperatures high enough to cause thermal annealing by which additional relaxation of the glasses could occur. Results support an emplacement model in which several hot-melt layers were emplaced gradually in a complex aggradation process and provided heat to underlying layers to allow for thermal annealing within the glass transition interval. Caution is necessary when interpreting cooling rate estimates. The phonolitic glassy base of a sequence of pyroclastic deposits from the top of the Guajara Formation along the Las Cañadas caldera, Tenerife, in the Canary Islands, exhibits unusual textures that sparked our interest. The ca. 3 m thick volcanic deposit is mostly composed of obsidian with light-green material dispersed in the obsidian. A cooling rate profile from the base up to about the middle of the deposit suggests higher cooling rates at the base (10-3 K s-1) than in the middle of the deposit (10-6 K s-1). The textural analyses reveal that the green Phase is composed of welded and devitrified ash and geochemical data indicate that the Major element composition of the obsidian and the green phase is identical within the accuracy of the measurements. An additional convincing piece of evidence consists of fragments of the green phase that detached and moved a short distance from their probable original location. Based on this analysis, it is likely that a pyroclastic flow was emplaced, followed by partial devitrification during cooling as the melt, now obsidian, was still viscous.