Relevant Degree Programs
Graduate Student Supervision
Doctoral Student Supervision (Jan 2008 - Mar 2019)
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 (2010-2017)
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.