Mark Jellinek

Explosive eruptions impact global climate through stratospheric injection of SO₂ to form sulfate aerosols.The stratospheric delivery and life time of sulfur is sensitive to eruption height and cloud chemistry which are, in turn, influenced by plume water and fine ash concentration.Diverse processes that result from the interaction of magma with surface water give rise to ash columns that are abundant in water and fine ash, have characteristically unsteady source conditions, and are prone to gravitational collapse.All of these effects have significant consequences for plume rise height, stratospheric delivery, and the chemical and microphysical evolution of SO₂ and aerosols, but associated relationships have not been systematically explored and the climate impacts of hydrovolcanic eruptions remain poorly constrained.To address this knowledge gap, I build a novel 1-dimensional model for hydrovolcanic eruptions simulating magma ascent in the conduit, magma-water interaction in a subaqueous pyroclastic jet, and subaerial plume rise.Critically, I make predictions of the water depths through which eruptions of a given magnitude can penetrate to form buoyant, ash-laden eruption columns, as well as the abundance of fine ash and water mass in the resulting eruption clouds.A water layer of 50 to 70 m thickness overlying a volcanic vent is sufficient to increase by an order of magnitude the mass eruption rate required for buoyant stratospheric plumes. Next, I apply the hydrovolcanic model to reconstruct the 1918 subglacial eruption of Katla volcano.Combined with a model governing the ice melt and drainage from the subglacial eruption site and constrained by eyewitness accounts, I show that the timing of emergence of the subaerial eruption column required early drainage of the englacial cauldron by subglacial pathways.Finally, I develop a machine learning algorithm to track coherent vortices in thermal imagery of unsteady eruption plumes from Sabancaya Volcano, Peru, quantifying their time-evolving thermal mixing behavior.Tracking results demonstrate evolution of entrainment and mixing behavior between predictions for sustained and instantaneous plume sources.Analysis of unsteady plume sources leads to a preliminary framework for quantitative definitions of source unsteadiness and its impact on hydrovolcanic and other unsteady explosive eruption plumes.

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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.

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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.

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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.

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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.

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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.

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