Processes, time scales and unrest of monogenetic volcanism

Albert Mínguez, Helena

2016-A
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Resumen

Las erupciones volcánicas están generalmente precedidas por la actividad sísmica, la deformación y la desgasificación (unrest). El éxito en la predicción del evento volcánico depende de la calidad de la red de vigilancia para detectar cualquier cambio en el comportamiento del volcán. Para interpretar los precursores geoquímicos y geofísicos correctamente es importante entender los procesos volcánicos que ocurren antes y durante las erupciones volcánicas. El conocimiento en detalle de la estructura interna del volcán, la reología de los magmas, las escalas de tiempo de los procesos que ocurren en profundidad y las características de los episodios pasados de unrest, debe combinarse con una red de vigilancia adecuada para mejorar el pronóstico de los eventos volcánicos. Sin embargo, estos aspectos han recibido poca atención en los volcanes monogenéticos.

El objetivo de mi tesis doctoral es mejorar nuestra comprensión sobre el vulcanismo monogenético, sus causas y su dinámica, con el objetivo de mejorar la posibilidad de anticiparse a la actividad volcánica. Me he centrado en tres aspectos principales de este problema. El primero es el cálculo de las propiedades reológicas de los magmas durante los eventos de mezcla. El segundo aspecto es el estudio de los procesos, junto con sus escalas temporales, que llevan a erupciones monogenéticas con el fin de interpretar mejor la actividad volcánica y mejorar los pronósticos de una erupción. Por último, he investigado los períodos de unrest sísmico de erupciones monogenéticas históricas en todo el mundo mediante una compilación de documentos históricos. Los resultados proporcionan un marco conceptual que permite mejorar la predicción de erupciones monogenéticas y deberían conducir a mejores estrategias para mitigar sus peligros y riesgos asociados.

Abstract

Seismic, deformation, and gas activity (unrest) typically precedes volcanic eruptions.

Successful volcanic event forecasting depends on the quality of the surveillance

network for detecting any changes in the volcano behaviour. To interpret

the geochemical and geophysical precursors correctly it is important to

understand the volcanic processes that occur prior and during volcanic eruptions.

Detailed knowledge of the volcano internal structure, the rheology of the magmas,

the time scales of the processes occurring at depth and the characteristics of

past unrest episodes, must be combined with an adequate monitoring network to

improve the volcanic hazard forecast. However, these aspects have received little

attention in monogenetic volcanoes. The aim of my PhD Thesis is to improve

our understanding on monogenetic volcanism, its causes and dynamics, and to

help anticipating the volcanic activity. I have focused on three main aspects of

this problem.

The first one is the calculation of the rheological properties of magmas during

mixing. I have analysed samples from the Monta˜na Reventada eruption (Tenerife,

Canary Islands). This is an example of magma mingling and mixing in

which the eruption was triggered by intrusion of basanitic magma into a phonolitic

reservoir. The phonolitic lava flow is characterized by the presence of mafic

enclaves. The morphology of each enclave is different, varying from rounded to

complex finger- like structures usually with cuspate terminations. I have quantified

the textural heterogeneity of the enclaves by applying fractal geometry methods

to obtain the viscosity ratio between the phonolitic magma and the enclaves.

These results enable me to infer the water content of the basanitic magma and the

enclaves (1.5–2 wt %).

The second aspect of monogenetic volcanism that I have addressed are the processes

and time scales that lead to monogenetic eruptions with the aim to better

interpret volcanic unrest and improve eruption forecasts. I have conducted a

geochemical and petrological study of the historical eruptions of Siete Fuentes,

Fasnia, and Arafo (Tenerife, Canary Islands). All the erupted magmas were basanitic.

I have identified four olivine compositional populations that are either

unzoned, normally or reversely zoned in major and minor elements. The variety

of olivine core populations and zoning patterns reflects mixing and mingling

between different magmas. Modelling of the zoning profiles of olivine shows

that early mixing occurred between two relatively evolved magmas, probably at

shallow depths one year prior to the eruption. Another mixing event between

two more primitive magmas stored presumably at deeper levels occurred less

than two months prior to the eruption. Finally, movement from depth of the

pre-mixed primitive melts and interaction with the also pre-mixed shallower and

more evolved melts occurred only two weeks before the eruption. The shorter

transport times of weeks are also seen in the compilation of the historical accounts

of seismicity associated with these eruptions.

The third aspect of monogenetic volcanism I have investigated are the seismic

unrest periods of historical monogenetic eruptions from a compilation of historical

accounts worldwide. Eruptions from monogenetic volcanoes are particularly

difficult to anticipate since they occur at unexpected locations (e.g. Paricutin,

1943) and there is very limited instrumental monitoring data. I have gathered the

available instrumental data of unrest episodes and combined it with new historical

accounts of seismicity. I find that there is a commonality in the seismic activity

preceding monogenetic eruptions, with clusters at around one or two years, two

or three months, and one or two weeks. The petrological and geochemical characteristics

of these eruptions show that multiple magma batches interacted in a

subvolcanic reservoir, and multiple intrusions occurred on similar time scales to

the seismicity. I propose a general model where early dike intrusions in the crust

do not erupt and create small plumbing systems (i.e. stalled intrusions). These

intrusions are probably instrumental in creating a thermal and rheological pathway

for later dikes to be able to reach the surface. These observations provide a

conceptual framework for better anticipating monogenetic eruptions and should

lead to improved strategies for mitigation of their associated hazards and risks.

Índice

Contents

List of Figures xiii

List of Tables xix

1 Introduction 1

1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.3 Thesis structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2 State of the Art 7

2.1 Monogenetic volcanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.1.1 Monogenetic volcanism in Tenerife (Canary Islands) . . . . . . . . . 8

2.1.1.1 Monta˜na Reventada eruption . . . . . . . . . . . . . . . . 10

2.1.1.2 Historical volcanism in Tenerife . . . . . . . . . . . . . . . 11

2.2 Magma interactions and mixing . . . . . . . . . . . . . . . . . . . . . . . . 12

2.2.1 Fractal analysis of mixing textures . . . . . . . . . . . . . . . . . . . 12

2.2.2 Time scales of magma mixing . . . . . . . . . . . . . . . . . . . . . 14

2.3 Unrest data of monogenetic eruptions . . . . . . . . . . . . . . . . . . . . . 15

3 Results 17

3.1 Fractal analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3.1.1 Fractal dimension . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3.1.2 Constraining of rheological properties from the fractal analysis results 19

3.1.2.1 Viscosity . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.1.2.2 Water content estimation . . . . . . . . . . . . . . . . . . 23

3.2 Historical eruptions of Siete Fuentes, Fasnia and Arafo . . . . . . . . . . . . 24

3.2.1 Whole-rock chemistry and petrology . . . . . . . . . . . . . . . . . . 24

CONTENTS

3.2.2 Mineral composition and chemical zoning . . . . . . . . . . . . . . . 25

3.2.2.1 Olivine . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3.2.2.2 Clinopyroxene . . . . . . . . . . . . . . . . . . . . . . . . 29

3.2.3 Assessing the effects of crystal accumulation and

geothermobarometrical calculations . . . . . . . . . . . . . . . . . . 29

3.2.4 Magma mixing processes and their time scales . . . . . . . . . . . . 32

3.2.4.1 Magma plumbing system . . . . . . . . . . . . . . . . . . 32

3.2.4.2 Time scales of magmatic processes . . . . . . . . . . . . . 33

3.3 Unrest data of monogenetic eruptions . . . . . . . . . . . . . . . . . . . . . 38

3.3.1 Details of the seismicity and brief petrological review . . . . . . . . . 38

3.3.1.1 Canary Islands (Spain) . . . . . . . . . . . . . . . . . . . 38

3.3.1.2 Michoacan-Guanajuato monogenetic volcanic field (Mexico) 42

3.3.1.3 Higashi-Izu monogenetic volcanic field (Japan) . . . . . . 42

3.3.1.4 Goropu Mountains (Owen Stanley Ranges, Papua) . . . . . 43

3.3.1.5 Heimaey (Iceland) . . . . . . . . . . . . . . . . . . . . . . 43

3.3.2 Comparison between factual accounts and monitored unrest data . . . 43

4 Discussion 45

4.1 The role of coherent and active regions in the mixing area . . . . . . . . . . . 46

4.2 Integration of petrological and unrest data . . . . . . . . . . . . . . . . . . . 48

4.3 General model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

5 Conclusions and Future work 53

5.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

5.2 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

References

A Historical accounts of unrest

B Fractal analysis of enclaves as a new tool for estimating rheological

properties of magmas during mixing: The case of Monta˜na Reventada

(Tenerife, Canary Islands) 83

C Timing of magmatic processes and unrest associated with mafic historical

monogenetic eruptions in Tenerife Island 85

CONTENTS

D Years to weeks of seismic unrest and magmatic intrusions precede

monogenetic eruptions 87

Conclusiones

Conclusions and

Future Work

The main goal of the Thesis is to improve our knowledge on the pre-eruptive processes,

their time scales, and how may these relate to seismic unrest of monogenetic eruptions. I

have highlighted the commonality of the magma mixing processes preceding monogenetic

eruptions by studying samples from Tenerife and by compiling information from other petrologically

studied monogenetic volcanoes around the world. The study of the monogenetic

eruption of Monta˜na Reventada has allowed me to develop a method for the calculation of the

water content of the basanite, at depth, from the fractal study of the enclaves. Knowledge of

the water content of the basanitic magmas from Tenerife is very important for assessing the

geothermobarometrical conditions at which olivine crystals reside in these melts. The study

of the zoning patterns of olivine crystals due to the mixing between different magmas has

allowed me to calculate the time scales of these processes in three historical eruptions from

Tenerife. Combining the obtained time scales, and others from the literature, with the unrest

information available for some historical monogenetic volcanoes around the world I provide

a framework for anticipating the monitoring data that can be expected during the unrest episodes.

I also suggest in this chapter some future research directions that could complement the

work presented and discussed in the previous chapters.

5. CONCLUSIONS AND FUTUREWORK

5.1 Conclusions

The results of the fractal study conducted on Monta˜na Reventada show that enclaves with

different fractal dimension (Dbox) —and hence different composition— represent different

degrees of mixing in the system, even though all could have been generated at the same time

and with the same percentage of mafic magma. Fractal analysis of enclaves of Monta˜na Reventada

offers clues to constraining the dissolved water content and the viscosities of the

enclaves and the basanitic magma. The calculated H2O contents for the basanite and the enclaves

range between 1.5 and 2 wt %. These values were considered to constrain the physical

conditions of the Siete Fuentes, Fasnia and Arafo erupted magmas.

The three historical mafic monogenetic eruptions of Siete Fuentes, Fasnia and Arafo that

occurred within two months of each other and more than 10 km apart on Tenerife were fed

by the same magma that accumulated crystals with time. The three eruptions record the involvement

of two distinct magma storage regions in which the more evolved mafic magmas

resided and pre-mixed, before a final mixing event that brought them to the surface. The first

mixing and mingling event likely occurred at shallow depths and about one year prior to the

first eruption. This probably involved differentiated magmas from a more primitive parent

that were not able to erupt, most likely because of buoyancy constraints, but they could have

been instrumental in pre-conditioning the crust for future eruptions. A second magma mixing

and mingling event between two more primitive magmas, and probably located at deeper

levels, occurred about two months before the first eruption. A final event of magma transport

and mingling occurred because of the movement of the deeper magma to the shallow storage

reservoir, followed by transport to the surface in the last two weeks before each eruption. The

reported seismicity, compiled from historical documents, lasted on the order of a week and

matches my shorter calculated times for the last mixing event and magma transport. The observation

that early mixing of more differentiated magmas occurred before the deeper mixing

suggests that for monogenetic eruptions to occur it might be necessary to prepare and precondition

the crustal column with a series of ‘failed eruptions’ before the magmas can actually

reach the surface. My findings agree well with the instrumental seismic monitoring of the recent

El Hierro eruption and show that a few days to a few months before an eruption a modern

seismic network should be able to detect such magma mixing events. The shorter times of a

few weeks between the final mixing and transport to the surface should be useful for emergency

planning if a new period of magmatic unrest should occur on the island of Tenerife and

perhaps also in other monogenetic fields.

5.1 Conclusions

A review of the historical documents and the literature shows that there is a commonality

in the seismic activity preceding monogenetic eruptions, with clusters at around one or

two years, two or three months, and one or two weeks. The petrological and geochemical

characteristics of these eruptions also show that the magmas were affected by mixing in a

subvolcanic reservoir and multiple intrusions on similar time scales to the seismicity (Albert

et al., 2015a; Johnson et al., 2008; Kl¨ugel et al., 2000; Longpr´e et al., 2014; Mart´ı et al.,

2013a; Rowe et al., 2011). I propose a general model where early dike intrusions in the crust

do not erupt and create small plumbing systems (i.e. stalled intrusions, Moran et al., 2011).

These intrusions are probably instrumental in creating a thermal and rheological pathway for

later dikes to be able to reach the surface (Strong &Wolff, 2003). These observations provide

a conceptual framework for better anticipating monogenetic eruptions and should lead to improved

strategies for mitigation of their associated hazards and risks.

5. CONCLUSIONS AND FUTUREWORK

5.2 Future work

The findings in this PhD are significant advancements in understanding the processes and

times scales that lead to monogenetic eruptions and how may these be related to seismic

unrest. However, as in any other study there are new or unresolved issues that have also

appear and here I propose what could complement and improve the results obtained in this

Thesis.

I have focused on the petrological study of historical eruptions that have available unrest

information. Based on the petrological and unrest data I have proposed a model that provides

useful information for monogenetic eruptions forecasting. The systematic study of the preeruptive

processes and their time scales of additional monogenetic eruptions will test and

improve the model. One avenue for future research is to study monogenetic volcanoes located

within different tectonic settings. This would allow testing the role of the upper crustal stress

in monogenetic eruptions, since an extensional regime should lead to easier magma ascent

(e.g. lower energy to propagate a dike). In this case shorter times of unrest and of magma

interactions could be anticipated. It would also be interesting to check any variability for

monogenetic volcanoes within the same field but with different ages. In the particular case of

Tenerife, along with the study of other historical eruptions I consider particularly interesting

the study of the Monta˜na Mostaza (15 ka, Ablay & Mart´ı, 2000) eruption.

I propose the study of Monta˜na Mostaza because this eruption represents the post-collapse

intracaldera part of the NE rift of Tenerife (Carracedo et al., 2007a). I have already started

an exploratory geochemical and petrological study of these deposits that shows the presence

of zoned olivine crystals suggesting magma mixing. It would be interesting to compare the

olivine zoning patterns and the time scales of the open-system processes between this eruption

and the three historical eruptions of Siete Fuentes, Fasnia and Arafo (also located in the NE

rift).

Along with the geochemical and petrological studies, experiments and numerical models

of dike migration will improve the proposed general model of the plumbing system in monogenetic

volcanoes. These experiments and models will test the importance of the repetitive

intrusions of magma in the crust (stalled intrusions) in allowing mafic magma to reach the

surface.