Explosive or effusive style of volcanic eruption determined by magma storage conditions

Abstract

Most volcanoes erupt both effusively and explosively, with explosive behaviour being responsible for most human fatalities. Eruption style is thought to be strongly controlled by fast conduit processes, limiting our ability for prediction. Here we address a critical question in the quest to develop timely forecasting of eruptive behaviour: are there conditions in which the outcome of an eruption is predetermined by the state of the magma in the subvolcanic reservoir? We analyse the pre-eruptive storage conditions of 245 units from volcanoes around the world. We show that pre-eruptive crystallinity, dissolved water content and the presence of exsolved volatiles in the chamber exert a primary control on eruptive styles. Magmas erupt explosively over a well-defined range in dissolved water content (~4–5.5 wt%) and crystallinity (less than 30 vol%). All other conditions, namely higher crystallinity, dissolved water contents below 3.5 wt% and, counterintuitively, in excess of 5.5 wt%, favour effusive activity. Between these ranges, there is a narrow field of transitional storage properties that do not discriminate between eruptive styles, and where the conduit exerts the main control on eruptive behaviour. Our findings suggest that better estimates of crystallinity and water content in subvolcanic chambers are key to forecasting eruptive style.

Full text

Articles
https://doi.org/10.1038/s41561-021-00827-9
1Institute of Geochemistry and Petrology, ETH Zürich, Zürich, Switzerland. 2Department of Earth, Environmental & Planetary Sciences, Brown University,
Providence, RI, USA. ✉e-mail: razvan.popa@erdw.ethz.ch
Volcanic eruptions often lead to fatalities, but explosive
behaviour on its own accounts for more than 95% of human
casualties1. In light of this, over the past decades, the scien-
tific community has made substantial progress in unravelling how
syneruptive and conduit processes influence the eruptive behav-
iour of volcanoes2–22. To a first order, eruptive style is thought to
depend dominantly on conduit processes, namely on whether the
gas remains trapped in the magma or escapes and outgases during
ascent3. In the first case, the trapped gas bubbles expand, acceler-
ate and fragment the magma column, releasing the energy required
for explosive activity. In the second case, outgassing neutralizes the
explosive potential of the magma, resulting in effusive eruptions.
However, most volcanoes are known to manifest both effusive and
explosive behaviour, sometimes simultaneously10,23, and a clear
understanding of the factors that control transitions between effu-
sive and explosive eruptions remains elusive.
In this study, we focus on the question: are there conditions on
the state of the magma stored before an eruption that predetermine
whether the next event will be effusive or explosive? If so, what are
the parameters that one should constrain? Can the same conceptual
framework explain the common occurrence of effusive precursors
observed at the onset of highly explosive events, including cal-
dera collapses (for example, volcan Quizapu24, Quilotoa volcano15,
Mount Pinatubo25, the Fish Canyon Tuff sequence26 or sequences of
the Aira caldera27)?
To analyse the role that various pre-eruptive parameters have
on eruption behaviour, we perform a survey of the pre-eruptive
magma chamber conditions that were prevalent when effusive and
explosive eruptions initiated at various volcanoes around the globe
(Fig. 1). We have mostly considered arc volcanoes, which gener-
ally show highly variable volatile contents, favouring a broad range
of eruptive styles. We have selected volcanic eruptions involving
intermediate to silicic magmas (andesites to rhyolites), which are
expected to have broadly similar rhyodacitic to rhyolitic melts, and
inherently comparable compositional effects on viscosity and water
saturation levels. For representability, we selected volcanoes with
subvolcanic storage regions located at around 2 kbar, which is the
most common pressure for upper-crustal magmatic storage in such
settings28. We restrict the storage pressure to avoid variations in the
water saturation level caused by this parameter.
Rationale and investigated parameters
We reconstruct a snapshot of the pre-eruptive conditions for 245
eruptive events, based on previously published data (Supplementary
Data 1). Our goal is to evaluate pre-eruptive (1) storage tempera-
tures, (2) dissolved water contents and (3) crystallinities. We cor-
relate these properties with eruptive styles (here categorized as
effusive or explosive) and with the potential pre-eruptive presence
of a water-dominated magmatic volatile phase (exsolved ‘gas’), to
highlight their effect on effusive–explosive transitions. In some
instances, specifically when both types of eruption occurred simul-
taneously, defining an eruption style might be ambiguous. In the
case of contemporaneous eruptions, we make this distinction based
on the style of eruption that initiated the event, and for older erup-
tions based on the type of deposit that was analysed. In the special
case of dome or sector collapse events, the eruptive style is still con-
sidered effusive because the explosion is a secondary surface effect
caused by gravitational processes.
Storage temperature is defined here as the temperature of the
eruptible batch of magma before eruption triggering. This is an
essential parameter that constrains the dissolved water content and
the water saturation level of the melt. To avoid the potential reheating
effect of mafic recharge, which is one of the most common processes
leading to eruptions29, we consider the pre-recharge, pre-reheating
temperature recorded by minerals crystallized in the subvolcanic
reservoir. As a first choice, we applied the amphibole-plagioclase
thermometer30, which we used throughout the dataset for consis-
tency. Where amphibole did not crystallize in equilibrium with
the pre-eruptive mineral assemblage, we relied on the pyroxene
thermometers31. We would like to stress that Fe-Ti oxides, used
Explosive or effusive style of volcanic eruption
determined by magma storage conditions
Răzvan-Gabriel Popa 1 ✉ , Olivier Bachmann1 and Christian Huber 2
Most volcanoes erupt both effusively and explosively, with explosive behaviour being responsible for most human fatalities.
Eruption style is thought to be strongly controlled by fast conduit processes, limiting our ability for prediction. Here we address
a critical question in the quest to develop timely forecasting of eruptive behaviour: are there conditions in which the outcome
of an eruption is predetermined by the state of the magma in the subvolcanic reservoir? We analyse the pre-eruptive storage
conditions of 245 units from volcanoes around the world. We show that pre-eruptive crystallinity, dissolved water content and
the presence of exsolved volatiles in the chamber exert a primary control on eruptive styles. Magmas erupt explosively over a
well-defined range in dissolved water content (~4–5.5 wt%) and crystallinity (less than 30 vol%). All other conditions, namely
higher crystallinity, dissolved water contents below 3.5 wt% and, counterintuitively, in excess of 5.5 wt%, favour effusive activ-
ity. Between these ranges, there is a narrow field of transitional storage properties that do not discriminate between eruptive
styles, and where the conduit exerts the main control on eruptive behaviour. Our findings suggest that better estimates of
crystallinity and water content in subvolcanic chambers are key to forecasting eruptive style.
NATURE GEOSCIENCE | VOL 14 | OCTOBER 2021 | 781–786 | www.nature.com/naturegeoscience 781
Content courtesy of Springer Nature, terms of use apply. Rights reserved