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Graduate Student Supervision
Doctoral Student Supervision
Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.
It is increasingly recognized that the gravitational stability of volcanic jets is governed by complex ash-pumice-gas (multiphase) interactions and the mechanics of turbulent entrainment in the lower momentum-driven (fountain) and upper buoyancy-driven (plume) regions of these flows. We use analogue experiments on relatively dense particle-freshwater and particle- saltwater jets injected into a linearly-stratified saltwater layer to revisit, characterize and under- stand how transitions among Buoyant Plume (BP), Total Collapse (TC) and Partial Collapse (PC) multiphase jet regimes in a traditional source strength ( Ri0) - particle concentration ( phi0) parameter space are modified by particle inertial effects expressed through a Stokes number (St) and particle buoyancy effects expressed through a Sedimentation number (Sigma0). We show that “coarse particles” (0.1 ≤ St0 ≤ 10) modify significantly published conditions favouring BP and TC, causing the transition between these endmember regimes to occur smoothly over a PC regime that represents the majority of the Ri0 - phi0 parameter for eruptions. Large volume annular sedimentation waves excited periodically in PC and TC regimes produce terrace deposits and lead to “phoenix clouds” spreading at multiple altitudes. Consistent with most eruptions having fluctuating source strengths, we carry out additional experiments on “unsteady” jets. We identify an additional key source Pulsation number Pu0 and develop a new Ri0 - phi0 - Pu0 parameter space for jet regimes. Applied to volcanic jets generally, and to data we recover from Doppler radar monitoring of two erupting volcanoes, we develop a new set of conceptual models for steady jets in the BP, TC and PC regimes and for unsteady jets in newly defined “Connected Thermals” and Discrete Thermal” regimes, all of which make readily-tested links among source parameters, column heights, sedimentation processes, cloud structures and deposit architectures. The predictions for cloud structures and deposit architectures agree with field-based and remote-sensing observations made for well-studied historic and pre-historic eruptions and explain the origin of common but enigmatic near-field features of explosive eruption deposits. The predictive power of our new Ri0 - phi 0 - Pu0 classification for explosive eruptions provides exciting new pathways for future observational and modelling studies. Supplementary materials available at: http://hdl.handle.net/2429/79887
The surface of Mars is incised with hundreds of ancient valley networks, the physical record of flowing liquid water during the early stages of Mars evolution (3.5-3.8 Byr ago). Their remarkable similarity to terrestrial rivers has historically motivated their interpretation in terms of rainfall and surface runoff, indicating that Mars’ climate was significantly warmer than present day. Protracted surface liquid water stability is, however, hard to reconcile with results from state of the art Global Climate Models, which predict that under a fainter young Sun and a thicker primitive atmosphere, the Martian southern hemisphere would be largely under ice cover. Distinguishing whether early Mars harbored surface water or was covered by an extensive cryosphere is key to understanding the nature of any habitable environments. The goal of this dissertation is to reconcile the climate and geomorphological characterizations of early Mars by establishing quantitative constraints on the origin of the Martian valley networks. In Chapter 2, I develop a methodology to quantitatively characterize valley networks in terms of their predominant erosional mechanism, including fluvial, glacial, sapping, and subglacial regimes. Chapter 3 uses constraints from a detailed field characterization of subglacial channels to establish their reliable identification from remote sensing data. In Chapter 4, I present the main results: the identification of subglacial channels among the Martian valley networks. These results support climate model predictions and are consistent with morphological observations. Chapter 5 builds on Chapter 4 to further understand the dynamics of landscape evolution on early Mars. In particular, I demonstrate that only a small fraction of valley networks are in a steady-state, and that erosion rates were likely very low on early Mars. I conclude (1) that subglacial erosion is widespread on the Martian Highlands and best explains the puzzling characteristics of valley networks, (2) that fluvial erosion was short-lived and only concentrated in narrow topographic corridors, and (3) that glacial and sapping erosion were rare on early Mars. In marked contrast to the popular view that Mars was "warm and wet'', my results show that early Mars had a climate akin to Antarctica: extensive ice sheets with localized melting. Supplementary materials: http://hdl.handle.net/2429/67020
Volcanic plumes rising above the tropopause inject SO₂ directly into the stratosphere, where itforms sulfate aerosols that modulate Earth’s radiative balance. Stratospheric volcanic sulfateaerosol forcing reduces Earth’s surface temperature and is a predominant driver of climatevariability. The processes that govern the volcanic injection of SO₂ into the stratosphere arecontrolled to a large extent by climate. Thus, climate changes may affect stratospheric volcanicSO₂ inputs, volcanic forcing and climate, in turn. The assessment of this potential feedbackis hindered by difficulties in understanding and constraining observationally the key processesgoverning plume rise.To address this challenge, we compile a new exhaustive database of eruption source parameters,along with their uncertainties (Aubry et al., 2017b). We apply these data along withthe results of laboratory experiments to compare the performances of our newly proposed andpublished scalings for predicting volcanic plume heights. We demonstrate that plume heightsare captured better by scalings accounting for atmospheric conditions (Aubry et al., 2017b).Furthermore, we evaluate 1D models of volcanic plume using the experimental and naturaleruption datasets. We show that these new datasets enable reliable constraints on processescritical to plume rise including the rate of entrainment of atmosphere as well as the role condensationof water vapor (Aubry et al. (2017a) and Chapter 4). Significant limitations in thecompiled data remain and we identify future improvements required to improve plume modelsevaluation.Next, we explore the impacts of climate projections for ongoing global warming on therise height of volcanic plumes and SO₂ injection into the stratosphere. Our results reveal anovel feedback where global warming will reduce stratospheric injections of SO₂ by explosiveeruptions (Aubry et al., 2016). This would lead to reduced volcanic forcing and surface cooling,and enhance global warming, in turn. To test this feedback, we develop a new idealized model ofvolcanic aerosol forcing and show that the proposed feedback may have important implicationsif greenhouse gas concentrations continue to increase at currents rates (Chapter 6). An excitingfuture direction is to assess interactions among the proposed feedback with other publishedclimate-volcano feedbacks. Supplementary materials available at: http://hdl.handle.net/2429/66192
Volcanic tremor, a seismic signal with longer durations and lower frequency content compared to local earthquakes, is often observed before or during eruptions and may consequently be useful for eruption forecasting. However, the processes generating volcanic tremor are still poorly understood. The main goal of this thesis is to assess systematic similarities and differences among tremor from a global sample of volcanoes, which is crucial to successfully constrain plausible source mechanisms. Using time series analysis of seismic signals accompanying three eruptive episodes at Kīlauea Volcano, Hawai‘i, I show that two characteristic phases of seismicity accompany dike intrusions, and that a different type of tremor occurs during a period of explosive activity. The signals differ in their spatial, temporal, and most strongly in their spectral properties. I thus construct a synthetic dataset of spectra that mimic the different spectral shapes observed in Hawai‘i. I use this dataset to evaluate the performance of two pattern recognition algorithms that may facilitate a global comparison of volcanic tremor spectra. A variety of tests with the synthetic spectra including different numbers and character of spectral patterns, as well as increasing levels of noise reveal that Principal Component Analysis and hierarchical clustering, in combination with a newly developed criterion to determine the ideal number of groupings in the data, can successfully identify the correct number and character of the known spectra. I thus develop a detection algorithm for volcanic tremor and apply the pattern recognition approach to detect patterns in tremor spectra from Kīlauea, Okmok, Pavlof, and Redoubt volcanoes. By analyzing the station network for each volcano individually, I show that tremor has distinct spatial and temporal characteristics for each of the volcanic settings. A subsequent comparative analysis suggests that several volcanic settings share common spectral tremor characteristics. I identify at least four types of volcanic tremor with systematic variations among the four settings, which indicates relationships to volcanic controls such as magma storage depth and viscosity. Further analysis of tremor from a larger sample of volcanoes will help to constrain plausible source processes and ultimately improve eruption forecasting.
Understanding the growth and differentiation of silicic magma chambers is a central issue in volcanology. Specifically, the injection, deformation and breakup of new pulses of magma can influence how the chamber evolves thermally and chemically, as well as the potential for eruption. Magmatic structures (e.g. enclaves, ladder dikes, and schlieren) preserved in plutonic and volcanic rocks record information about the physical processes that occur within the chamber prior to solidification. A key outstanding issue is how to use magmatic structures to extract information about magma rheology and host chamber dynamics within the chamber and during magma ascent--processes that are inherently inaccessible to direct observation. This thesis is an attempt to elucidate the fundamental physics that governs the breakup of an injected magma into a preexisting chamber. One major obstacle for the popular model that mafic inputs trigger big eruptions (Pallister et al., 1992, Murphy et al., 1998) and govern the long-term growth of silicic chambers is the way the new magma is injected. In particular, the scale length at which thermal and compositional heterogeneity is introduced controls how efficiently heat is transferred and the extent to which chamber convection causes mixing. This thesis provides a new understanding of how injections breakup to such small sizes, which can lead to a greater efficiency for mixing and remobilization of an otherwise immobile magma. I use field and experimental studies to investigate specific magmatic features preserved in plutonic and volcanic rocks that can be used to constrain the magma rheology within the chamber at the time of deformation. First, I use experiments and scaling theory to investigate the mechanical and rheological conditions leading to the deformation and breakup of analog crystal-rich dikes. Second, I use field observations of ``ladder dikes'' from the Tuolumne Intrusive Suite, together with experiments and scaling theory to demonstrate that prior to solidification, these features are deformed and broken by shearing motions in the magma chamber. And third, using experimental results along with thermodynamic and modeling constraints on key physical properties of the injected and host magmas, I use size distributions of enclaves preserved in lava flows to characterize the flow regime governing enclave formation.
Master's Student Supervision
Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.
Explosive volcanic plume rise is governed by the rate at which ambient air is ingested and heated by turbulent entrainment and mixing processes. A daunting observational challenge is to constrain the character of the underlying physical processes and their dependence on complex particle-particle and particle-gas interactions. Important clues may lie in the particle-particle momentum exchange that gives rise to lightning ﬂashes and related electrical discharges near the vent during supersonic eruptions where they occur. Recent laboratory studies of positive and negative shock tube-generated volcanic discharges show a correlation between ﬁne ash concentration and the magnitude and number of positive discharges . Charge generation via collisions (triboelectriﬁcation) is hypothesized to be more eﬃcient with high ash concentrations and at high decompression rates because collisions between particles become more frequent under these conditions . To test this hypothesis and understand the experimental data in greater detail, we develop and implement a regression-based random forest algorithm to quantitatively constrain concentrations of ﬁne ash using discharge count, magnitude and polarity as predictors. We bin experiments into subsets containing 1. all (ALL), 2. low ( 22 g) mass experiments. We deﬁne a metric for variable importance and ﬁnd in ALL, HP, and HM subsets that positive discharge properties are more important predictors of ﬁne ash concentration than negative discharge properties, consistent with triboelectriﬁcation as the predominant process under these conditions. This mechanism is not constrained for LP conditions, suggesting a potential threshold decompression rate condition for this class of charging and insight into why near-vent lightning is not a ubiquitous feature of explosive eruptions.
Pre-eruptive seismic tremor with similar spectral properties is observed at active volcanoes with widely ranging conduit geometries and structures. Accordingly, the ``magma wagging'' model introduced by Jellinek & Bercovici and extended by Bercovici et al. hypothesizes an underlying mechanism that is only weakly-sensitive to volcano architecture: Within active volcanic conduits, the flow of gas through a permeable foamy annulus of gas bubbles excites and maintains an oscillation of a central magma column through a well-known Bernoulli effect. In this thesis, we carry out a critical experimental test of this underlying mechanism for excitation. We explore the response of analog columns with prescribed elastic and linear damping properties to forced annular airflows. From high-speed video measurements of linear and orbital displacements and time series of accelerometer measurements we characterize and understand the excitation, evolution, and steady-state oscillating behaviors of analog magma columns over a broad range of conditions. We identify three distinct classes of wagging: i. rotational modes which confirms predictions for whirling modes by Liao et al.; as well as newly-identified ii. mixed-mode; and iii. chaotic modes. We find that rotational modes are favored for symmetric, and high intensity forcing. Mixed-mode responses are favored for a symmetric and intermediate intensity forcing. Chaotic modes occur in asymmetric or low intensity forcing. To confirm and better understand our laboratory results, and also extend them to conditions beyond what is possible in the laboratory, we carry out complementary two-dimensional simulations of our analog experiments.Our combined experimental and numerical results can be applied to make qualitative predictions for natural testable in future studies of pre- and syn-eruptive volcano seismicity. Long before an eruptive phase, the total gas flux is low and we expect magma wagging in a chaotic mode, independent of the spatial distribution of the gas flux. At a pre-eruptive state signaled by gas flux increasing, if the distribution of gas flux is approximately symmetric, we expect a transition to mixed and possibly rotational wagging modes. During an eruption, fragmentation and explosions within the foamy annulus can cause spatial heterogeneity in permeability resulting in non-uniform gas flux that favors chaotic wagging behavior.
Striking textural and petrologic evidence for mixing of basalt into silicic melts occurs in volcanic arc settings worldwide. These textures (e.g. mingled/banded pumices, inclusions), along with chemical signatures of mixing (e.g. zoning in solid solution minerals) found in many eruption products, support a popular hypothesis that magma mixing can trigger explosive eruptions through a variety of possible mechanisms. However, rigorous observational constraints on the nature of the underlying thermal and mechanical processes remain elusive. A method is developed based on Nomarski differential interferometry to image and quantitatively characterise quasi-periodic zoning in plagioclase at very high spatial resolution. Applied to individual crystals, variations in zoning with crystallographic direction confirm experimental measurements of anisotropic diffusivities for Ca²⁺Al³⁺ and Na⁺Si⁴⁺. When applied to the AD 1315 Kaharoa eruption of Tarawera Volcano, Taupo Volcanic Zone, New Zealand, periodic zoning at scale lengths of 3.1–12.4 μm is consistent with quasi-periodic convective motions within the magma chamber acting on timescales of 39 days to 1.8 years prior to eruption. The structure of the zoning is inconsistent with a single large basalt injection causing eruption. Rather, this eruptive episode may have been preceded by several small volume basaltic inputs, consistent with observations from mafic-silicic layered intrusions.
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