Records of and controls on temporal variations in activity at arc volcanoes

309 Pages·2015·13.32 MB·English

by  Stefan M. Lachowycz

Records of and controls on temporal
variations in activity at arc volcanoes

Stefan M. Lachowycz
University College

Thesis submitted in accordance with the requirements of the
University of Oxford for the degree of Doctor of Philosophy

Hilary Term 2016
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Records of and controls on temporal variations in activity at arc volcanoes
Stefan M. Lachowycz (University College)

Many attributes of volcanic activity, whether physical, geochemical/petrological, or geophysical,
change over timescales from minutes to millions of years. Understanding the nature of these
variations and their controls is essential both for hazard assessment and to understand volcanic
and magmatic processes. Arc volcanism entails a relatively broad range of eruption styles and
magma compositions, and thus wide-ranging hazards and a variety of eruption records. Despite
their significance, records of activity for many arc volcanoes are inadequately characterised, so
any temporal variability is poorly constrained. In this thesis, I investigate temporal variations in
different styles of activity (dome-forming, Plinian-style, and effusive/glaciovolcanic) on a range of
timescales (days–years, millennia, and hundreds of ky, respectively), from diverse records
(seismicity, tephra deposits, and effusive eruption products), at four arc volcanoes. I analyse
seismic time-series from Soufrière Hills Volcano (Montserrat) and Volcán de Colima (Mexico)
using two statistical techniques, identifying temporal variation in the extent of long-range
correlations and randomness in these data, which has potential to inform real-time monitoring
and constrain eruptive processes. I also present new composition data for and review previous
studies of tephra deposits from Volcán Hudson (Chile), revising its explosive eruption history and
finding previously unrecognised shifts in the magma composition erupted through the Holocene. I
also describe and analyse sequences of lithofacies from effusive eruptions of a range of magma
compositions from Volcán Sollipulli (Chile), inferring varying extents of interaction with ice during
their emplacement, and so changes in eruption style through time. These case studies highlight
the importance of detailed characterisation of temporally varying records of volcanic activity, and
some of the limitations of and uncertainties in these records and their interpretation.
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Extended abstract

Records of and controls on temporal variations in activity at arc volcanoes
Stefan M. Lachowycz (University College)

Advances in monitoring and studies of the eruptive history of volcanoes in recent decades are
producing an increasing amount and variety of information on the timing of volcanic activity.
Temporal variations have been identified in many attributes of volcanism on a wide range of
timescales, from precursory changes in seismicity only minutes to hours before eruptions, to
cycles in eruption frequency or style over hundreds to thousands of years, and shifts in magma
composition and vent location due to tectonic changes over millions of years. Understanding the
nature of and controls on these variations is essential for hazard assessment and/or provides
invaluable insight into volcanic and magmatic processes. Arc volcanism entails a relatively broad
range of eruption styles and magma compositions, and thus wide-ranging hazards and a variety of
eruption records. The potential for and implications of temporal variation in arc volcanism are
exemplified by recent eruptions in Chile (Calbuco, Chaitén, and Cordón-Caulle volcanoes) and
Indonesia (Kelud, Merapi, and Sinabung volcanoes), which included rapid escalations in activity
and/or changes in eruption style. In some of these cases and at many other arc volcanoes,
investigation and interpretation of their temporal variability has been hindered by limited
monitoring and insufficient characterisation of eruption records. In this thesis, I explore some of
the challenges and opportunities in constraining temporal variations in arc volcanism, by
investigating such variations in different styles of activity (dome-forming, Plinian-style, and
effusive/glaciovolcanic) on a range of timescales (days–years, millennia, and hundreds of ky,
respectively), from diverse records (seismicity, tephra deposits, and effusive eruption products),
at four volcanoes (Soufrière Hills Volcano (Montserrat) and Volcán de Colima (Mexico), Volcán
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Hudson (Chile), and Volcán Sollipulli (Chile)). New information is obtained in each case either by
statistical analysis or detailed characterisation of eruption records; in some cases this provides
insight into the nature of and controls on the temporal variations in activity at these volcanoes.

Understanding the underlying structure of data from volcano monitoring is essential to identify
changes in eruptive activity and their precursors and controls. However, effective analysis of
longer-term trends in monitoring data is challenging as these data are not necessarily statistically
stationary or linear, particularly those from lava dome-forming volcanoes, which are commonly
characterised by pulsatory eruptive activity. In Chapter 2, I use detrended fluctuation analysis, a
statistical technique previously applied to nonstationary data, to identify long-range (slowly
decaying, e.g. power-law) correlations in a number of time-series of volcano seismicity recorded
during the recent dome-forming eruptions of Volcán de Colima, Mexico, and Soufrière Hills
Volcano, Montserrat. For all the time-series analysed, correlation strength varies through time
and/or on different timescales; in some cases, this variation is periodic, seasonal, and/or related
to activity. These results may provide new insights into eruptive processes and possibly further
constrain the generation mechanisms of a number of the volcano-seismic event classes analysed.
Furthermore, the correlation properties of real-time seismic measurements are shown
(retrospectively) to contain information valuable to real-time volcano monitoring that is not
identifiable by conventional techniques. This demonstrates that long-range correlation analysis
may be useful for extracting additional information from monitoring data at dome-forming or
similar volcanoes.

In Chapter 3, I examine the utility of another statistical analysis technique, Shannon entropy (a
measure of the randomness of a signal), for analysis of monitoring data by its application to the
same time-series of volcano seismicity analysed in Chapter 2. Entropy may be computed in real-
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time by the method used even if the signal is sampled at a high frequency and/or discontinuously,
so this technique is potentially widely applicable to monitoring time-series. At the two volcanoes
studied, the entropy of real-time seismic measurements and the count rate of certain volcano-
seismic event types is found to vary through time, generally being elevated (often close to
maximal) during phases of greater volcanic activity. High entropy indicates greater uncertainty in
the value taken by the variable, i.e. that the time-series is less predictable. In some instances, the
entropy of a signal shifts prior to or coincident with changes in seismic or eruptive activity, some
of which were not clearly recognised at that time by real-time monitoring. I examine the nature of
the entropy variations by testing for uniform data distribution and comparison to the coefficient
of variation. This demonstrates the sensitivity of the entropy to the data distribution, and that it is
distinct from measures of data dispersion. Therefore, entropy analysis may have potential as a
tool to extract additional information in real-time from volcano monitoring data streams.

Volcán Hudson, in the Andean Southern Volcanic Zone, has been identified as the source of four
eruptions of >1 km3 of tephra (volcanic ash and lapilli) and tens of smaller explosive eruptions in
the past 20 ky. The resulting tephra deposits have great potential as chronological markers in
studies of the palaeoenvironment of the region, and to assess any variation in eruption frequency.
However, the characteristics and timing of many of the eruptions of Volcán Hudson are poorly
constrained, inhibiting precise tephrochronology and hazard assessment. In Chapter 4, I present
matrix glass and whole rock composition analyses of Volcán Hudson tephra units in relatively
complete terrestrial sections, and catalogue occurrences of tephra previously attributed to
eruptions of Volcán Hudson, in order to correlate the tephras analysed and critically evaluate
previous tephra correlations. The Volcán Hudson tephras characterised generally have a narrow
range in their glass composition, so in most cases can be distinguished from the other tephras of a
similar age, which often have a similar composition. I find previously unrecognised shifts in
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erupted magma composition during the Holocene, from silicic to mafic after the H2 eruption
(~3.9 cal ka BP), and then to intermediate compositions for the last ~1 ky. The new geochemical
data and tephrostratigraphy reanalysis in this study provide a framework for future
tephrochronology and studies of the record of eruptions in the region. In addition, they highlight
the importance of detailed tephra characterisation in developing correlations between and age
models for palaeoenvironmental records, or accurate constraint of eruption frequency.

Magma-ice-meltwater interactions produce diverse landforms and lithofacies, reflecting the
multitude of factors that influence glaciovolcanism, including both magmatic (e.g., composition,
eruption rate) and glacial (e.g., ice thickness, thermal regime) conditions. This is exemplified by
the walls of the partly ice-filled summit caldera of Volcán Sollipulli, a stratovolcano in southern
Chile, which include lithofacies from eruptions of a wide range of magma compositions beneath
or in contact with ice. In Chapter 5, I analyse these lithofacies and hence propose new
interpretations of the eruptive and glacial history of Volcán Sollipulli. The facies include a thick,
laterally extensive sequence of fragmental glaciovolcanic deposits, comprising massive, mafic lava
pillow-bearing hyaloclastite overlain by sills and then hyaloclastic debris flow deposits (similar to
Dalsheidi-type sequences). The distribution and thickness of these units indicate an unusual
abundance of magma-meltwater interaction for an arc stratovolcano in temperate latitudes,
perhaps due to eruptions beneath a thick ice cap. Coherent lava coulées, domes, lobes, and stacks
of basaltic andesite–trachydacite composition are present around the top of the caldera rim;
these display morphologies and fracture patterns on caldera-facing margins that indicate that the
caldera was filled with ice when these lavas were erupted. The lithofacies characterised
demonstrate the diversity of glaciovolcanism that is possible at arc stratovolcanoes capped by
temperate ice or with ice-filled calderas, and the potential for uncertainties in inference of the
palaeoenvironmental conditions of their emplacement.
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The case studies presented in this thesis identify new changes or trends in distinct styles of arc
volcanism on different timescales, and provide additional information on the nature of those
recognised by previous work. Some of the data are suggested to further constrain (or provide
evidence of previously unknown) controls on the volcanic activity studied. This work also
contributes to the development of new techniques and understanding to analyse and interpret
some aspects of the types of volcanism examined. Although there are many challenges and
limitations to constraining temporal variations in arc volcanism from its records across the whole
range of timescales, this thesis highlights the potential to better understand such variability and
its controls through detailed characterisation and statistical analysis of records of volcanic activity.
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