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

John Stix

doi:10.3389/feart.2018.00056

Abstract

This paper examines the behaviour of fast volcanoes that erupt quickly with paroxysmal explosive eruptions, and slow volcanoes that erupt over extended periods without such paroxysmal activity. I review activity at four fast and four slow volcanoes, highlighting the main events and commonalities in behaviour among the different systems. In terms of forecasting, fast volcanoes typically have short 1-3 month precursory periods prior to the climactic eruption, while slow volcanoes commonly have an extended period of considerable uncertainty regarding the presence or absence of new magma, as well as unanticipated accelerations in activity. Fast volcanoes are associated with magmas having elevated volatile contents (up to ~7 wt. % H2O), rapid magma ascent rates, and rapid declines in activity after the climactic eruption. Fast volcanoes also exhibit well defined magma plumbing systems with mobile volatile-rich magma, with the plumbing system sealed between the top of the shallow magma reservoir and the surface prior to the climactic eruption. Slow volcanoes have complex plumbing systems comprising cracks, fractures, dykes, and sills and magmas that are crystal-rich, partly degassed, and rheologically sluggish. Slow 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. 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 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, Vp /Vs 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.

Highlights

Volcanoes exhibit a range of eruptive styles
Volcanoes exhibiting slow behavior escalate their activity over years to decades to their point of “peak” activity, implying slow, fitful rise of magma which may never reach the surface
A period of high dome growth and associated pyroclastic flow activity occurred in July-August 1996, followed by the first magmatic explosive eruption on 17 September 1996 after 40% of the dome collapsed (11.7 × 106 m3) (Robertson et al, 1998)

Summary

Introduction

Volcanoes exhibit a range of eruptive styles. One type of end-member behavior, as exemplified by volcanoes such as Hekla in Iceland and Cerro Negro in Nicaragua, awaken nearly instantaneously with essentially no warning. Volcanoes with fast activity exhibit a period of precursory unrest lasting several months prior to the principal eruption or eruptions (Figure 1), implying rapid magma emplacement and/or ascent. Likewise, these volcanoes return rapidly to a state of quiescence, commonly over timescales of weeks or months. Volcanoes exhibiting slow behavior escalate their activity over years to decades to their point of “peak” activity, implying slow, fitful rise of magma which may never reach the surface Such volcanoes may not necessarily show a climactic phase; instead, periods of enhanced activity, such as higher rates of dome growth or explosive activity, are more common. These slow volcanic systems commonly require years to decades before they can be considered inactive

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