Browsing University of Alaska Fairbanks by Subject "Volcanic eruptions"
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Causes and consequences of coupled crystallization and vesiculation in ascending mafic magmasTransitions in eruptive style and eruption intensity in mafic magmas are poorly understood. While silicic systems are the most researched and publicized due to their explosive character, mafic volcanoes remain the dominant form of volcanism on the earth. Eruptions are typically effusive, but changes in flow behavior can result in explosive, ash generating episodes. The efficiency of volatiles to degas from an ascending magma greatly influences eruption style. It is well known that volatile exsolution in magmas is a primary driving force for volcanic eruptions, however the roles vesicles and syn-eruptive crystallization play in eruption dynamics are poorly understood. Permeability development, which occurs when gas bubbles within a rising magma form connected pathways, has been suspected to influence eruption style and intensity. Numerous investigations on natural eruptive products, experimental samples, and analog experiments have extended the understanding of permeability development and fragmentation processes. However, these studies have focused on silicic, high viscosity, crystal-poor magmas. Little progress has been made in understanding fragmentation mechanisms in mafic or alkali magmas. Mafic systems involve lower viscosity magmas that often form small crystals, also known as microlites, during ascent. Because the merging of bubbles in magma is mitigated by melt viscosity, it is predicted that permeability development in mafic magma will occur at lower bubble volume fractions than in silicic magma. However, no study has been performed on experimental samples to provide evidence for this hypothesis. Furthermore, it is unknown how microlites affect the degassing process in terms of facilitating or hindering permeability development. This thesis employs experimental petrology to: 1) experimentally observe how melt viscosity alone affects permeability development, 2) Understand the effects of syn-eruptive crystallization in vesiculating mafic magmas and synergizes these results to 3) relate experimental findings to the 2008 eruption of Kasatochi volcano.
Detection, source location, and analysis of volcano infrasoundThe study of volcano infrasound focuses on low frequency sound from volcanoes, how volcanic processes produce it, and the path it travels from the source to our receivers. In this dissertation we focus on detecting, locating, and analyzing infrasound from a number of different volcanoes using a variety of analysis techniques. These works will help inform future volcano monitoring using infrasound with respect to infrasonic source location, signal characterization, volatile flux estimation, and back-azimuth to source determination. Source location is an important component of the study of volcano infrasound and in its application to volcano monitoring. Semblance is a forward grid search technique and common source location method in infrasound studies as well as seismology. We evaluated the effectiveness of semblance in the presence of significant topographic features for explosions of Sakurajima Volcano, Japan, while taking into account temperature and wind variations. We show that topographic obstacles at Sakurajima cause a semblance source location offset of ~360-420 m to the northeast of the actual source location. In addition, we found despite the consistent offset in source location semblance can still be a useful tool for determining periods of volcanic activity. Infrasonic signal characterization follows signal detection and source location in volcano monitoring in that it informs us of the type of volcanic activity detected. In large volcanic eruptions the lowermost portion of the eruption column is momentum-driven and termed the volcanic jet or gas-thrust zone. This turbulent fluid-flow perturbs the atmosphere and produces a sound similar to that of jet and rocket engines, known as jet noise. We deployed an array of infrasound sensors near an accessible, less hazardous, fumarolic jet at Aso Volcano, Japan as an analogue to large, violent volcanic eruption jets. We recorded volcanic jet noise at 57.6° from vertical, a recording angle not normally feasible in volcanic environments. The fumarolic jet noise was found to have a sustained, low amplitude signal with a spectral peak between 7-10 Hz. From thermal imagery we measure the jet temperature (~260 °C) and estimate the jet diameter (~2.5 m). From the estimated jet diameter, an assumed Strouhal number of 0.19, and the jet noise peak frequency, we estimated the jet velocity to be ~79 - 132 m/s. We used published gas data to then estimate the volatile flux at ~160 - 270 kg/s (14,000 - 23,000 t/d). These estimates are typically difficult to obtain in volcanic environments, but provide valuable information on the eruption. At regional and global length scales we use infrasound arrays to detect signals and determine their source back-azimuths. A ground-coupled airwave (GCA) occurs when an incident acoustic pressure wave encounters the Earth's surface and part of the energy of the wave is transferred to the ground. GCAs are commonly observed from sources such as volcanic eruptions, bolides, meteors, and explosions. They have been observed to have retrograde particle motion. When recorded on collocated seismo-acoustic sensors, the phase between the infrasound and seismic signals is 90°. If the sensors are separated wind noise is usually incoherent and an additional phase is added due to the sensor separation. We utilized the additional phase and the characteristic particle motion to determine a unique back-azimuth solution to an acoustic source. The additional phase will be different depending on the direction from which a wave arrives. Our technique was tested using synthetic seismo-acoustic data from a coupled Earth-atmosphere 3D finite difference code and then applied to two well-constrained datasets: Mount St. Helens, USA, and Mount Pagan, Commonwealth of the Northern Mariana Islands Volcanoes. The results from our method are within ~<1° - 5° of the actual and traditional infrasound array processing determined back-azimuths. Ours is a new method to detect and determine the back-azimuth to infrasonic signals, which will be useful when financial and spatial resources are limited.
The influence of phenocrysts in silicic magma degassingUnderstanding the degassing process in magma is an important goal because of the first-order control it exerts on determining eruption style. Degassing in high viscosity magmas is of particular interest since these magmas tend to erupt explosively. However, the role of phenocrysts in the degassing process is still poorly constrained, though recent data indicate that the presence of phenocrysts should promote permeability development at lower porosities than in crystal-free magmas. This study specifically examined the effect of phenocrysts in a rhyolitic magma, but the results can also be applied to crystal-rich intermediate magmas that have rhyolitic matrix melts. Isothermal decompression experiments were conducted using powdered rhyolite (76 wt. % SiO2) and seeded with corundum (Al2O3) crystals to approximate magmas with 20 and 40 vol. % phenocrysts. Experiments were saturated at 900˚C and 110 MPa then continuously decompressed to final pressures between 75 and 15 MPa. Percolation threshold was determined by measuring permeability on a benchtop permeameter and measuring porosity from reflected light images. Additionally, vesicle structure was assessed by measuring pore throat radii from back-scattered electron images and plotting bubble size distributions. Finally, degassing state was checked by measuring dissolved water contents in the glass with Fourier Transform Infrared (FTIR) spectroscopy analyses. The addition of at least 20 vol. % phenocrysts resulted in a decrease in percolation threshold from 70-80 vol. % porosity in crystal-free rhyolites to 55 vol. % porosity. Bubble size distribution patterns indicate that coalescence was more widespread as final pressure decreased and crystal content increased. Minimum pore throat radii in the 40 vol. % phenocryst series were larger than in the 20 vol.% phenocryst and crystal-free series. The dissolved water measurements indicate that these experiments degassed in equilibrium even at the fast decompression rate of 0.25 MPa/s. Calculations of the magnitude of outgassing from the decreased percolation threshold and timescales of pressure dissipation indicate that the presence of phenocrysts plays a role in the effusive-explosive cyclicity of Vulcanian-style eruptions.
A multi-sensor approach to determining volcanic plume heights in the North PacificDuring a volcanic eruption, accurate height information is necessary to forecast a volcanic plume's trajectory with volcanic ash transport and dispersion (VATD) models. Recent events in the North Pacific (NOPAC) displayed significant discrepancies between different methods of plume height determination. This thesis describes two studies that attempted to resolve this discrepancy, and identify the most accurate method for plume height determination. The first study considered the 2009 eruption of Redoubt Volcano. This study found that the basic satellite temperature method, in which satellite thermal infrared temperatures are compared to temperature-altitude profiles, vastly underestimates volcanic plume height due to decreased optical depth of plumes soon after eruption. This study also found that the Multi-angle Imaging SpectroRadiometer (MISR) produced very accurate plume heights, even for optically thin plumes. The second study investigated the application of MISR data to multiple eruptions in the NOPAC: Augustine Volcano in 2006, Okmok, Cleveland, and Kasatochi volcanoes in 2008, and Redoubt and Sarychev Peak volcanoes in 2009. This study found that MISR data analysis retrieves accurate plume heights regardless of grain size, altitude, or water content. Exceptions include plumes of low optical depth over bright backgrounds. MISR is also capable of identifying ash clouds by aerosol type.