Understanding Fast and Slow Unrest at Volcanoes and Implications for Eruption Forecasting

Abstract

This paper examines the behavior of volcanoes that erupt quickly with paroxysmal explosive eruptions, and other volcanoes that erupt over extended periods without such paroxysmal activity. "Fast" activity typically occurs over the course of months to years, including precursory unrest, the paroxysmal eruption itself, and post-paroxysmal activity. "Slow" activity comprises extended restlessness over the course of decades, and eruptions are typically small and sometimes uncommon. I review activity at eight volcanoes with fast and slow activity, highlighting the main events, and commonalities in behavior among the different systems. In terms of forecasting, volcanoes with fast unrest typically have short 1-3 month precursory periods prior to the climactic eruption, while volcanoes with slow unrest commonly have an extended period of considerable uncertainty regarding the presence or absence of new magma, as well as unanticipated accelerations in activity. Volcanoes with fast behavior are associated with magmas having elevated volatile contents (up to ∼7 wt. % H 2 O), rapid magma ascent rates, and rapid declines in activity after the climactic eruption. These volcanoes also exhibit well-defined magma plumbing systems containing mobile volatile-rich magma, with the plumbing system often sealed between the top of the shallow magma reservoir and the surface prior to the climactic eruption. Volcanoes with slow behavior have complex plumbing systems comprising cracks, fractures, dykes, and sills and magmas that are crystal-rich, partly degassed, and rheologically sluggish. These volcanoes experience a progressive opening of their systems as magma intrudes and fractures country rock, allowing degassing to occur. The degree to which a system is opened is determined by the rate at which new magma is emplaced at shallow levels. As such, magma emplacement rates which are fast, intermediate, or slow should produce unrest on similar timescales. Slower rates of emplacement enhance the opening process due to a cumulatively high number of fractures and increased fracture density which develop during the extended period of unrest. Many systems both fast and slow receive inputs of more mafic magma which can drive activity seen at the surface. A series of recently developed tools is examined and discussed in order to provide an improved means of forecasting activity at both types of volcanoes. These include assessment of early phreatic activity and associated gases, V p /V s ratios of magma by seismic tomography, and estimates of magma volume from precursory seismicity. What is required now are protocols which integrate these approaches in a manner which is useful for accurate forecasting.

Explosive Eruptions With Little Warning: Experimental Petrology and Volcano Monitoring Observations From the 2014 Eruption of Kelud, Indonesia

“…The rapid onset of unrest prior to the Reventador 2002 eruption was attributed to its low silica, volatile‐rich, fluid magma, making it ascend aseismically (Hall et al, ). Stix () also suggests “fast” volcanoes, those with onset of unrest times of months to years prior, are derived from volatile‐rich, mobile magma, with fast ascent rates, whereas “slow” volcanoes, those displaying restlessness over the course of decades have complex plumbing systems, erupted magmas that are partly degassed and rheologically sluggish. Absolute volatile contents or magma water contents are difficult parameters to compare, as these relate to the solubility limit and therefore the depth at which the magma resides, making magma water content comparisons meaningless unless corrected for depth, and we show here that magma storage depth has little effect on onset time and eruptive style (Figure ).…”

A review framework of how earthquakes trigger volcanic eruptions

“…The sensitivity of hydrothermal systems make them the key third parameter to be. It is interesting to note that this choice of parameters which naturally arises from our analysis resembles other recent volcano classifications 168,169 , despite the different objectives. Our classification currently focuses on subaerial volcanism only.…”

“…Lava lakes or Strombolian-style volcanoes are typical examples of open systems. Many very explosive eruptions show a phase of closed-system degassing with low levels of degassing prior to eruption 169 . It is very common for volcanoes to transition from open to closed through time, and even within a single eruptive phase, via a wide range of processes 154,[179][180][181][182][183] .…”

“…acidic crater lakes, and fumaroles—whereas others show barely any visual sign of activity. For instance, it is quite common for smaller hydrothermal systems to dry due to proximity with magmatic bodies 169 . Thus, for the purpose of this classification, we will consider that a developed hydrothermal system is either present or absent.…”

Mechanisms of Unrest and Eruption at Persistently Restless Volcanoes: Insights From the 2015 Eruption of Telica Volcano, Nicaragua

“…The processes that drive persistently restless volcanism are poorly understood, as our knowledge of PRVs is currently based on a small number of case studies (e.g., Kumagai et al, ; Park et al, ; Petersen et al, ) using disparate geophysical and geochemical observational techniques. Stix () proposed that activity at PRVs (“volcanoes with slow unrest” in his study) involves slow, fitful rise of magma which may never reach the surface, an absence of a well‐defined reservoir with magma storage and transport instead occurring in a complex of dikes and sills, and a relatively open connection between the shallow magmatic system and the surface. While this model is consistent with observations of several recent magmatic eruptions (e.g., Turrialba Volcano, Costa Rica and Soufriere Hills Volcano, Montserrat), it does not fully explain observations at PRVs characterized by dominantly phreatic eruptions with little or no evidence of shallow magma emplacement and thus motivates detailed multiparameter and comparative analyses of long‐term activity at these PRVs to further develop a general model for both long‐ and short‐term unrest and eruption in these systems.…”