Jury:Dr. Luca Caricchi

Supervisors:Prof. Costanza BonadonnaDr. Laura PioliDr. Paul Cole (Plymouth University)

Ash aggregation during the 11th February 2010 partial dome collapse of the Soufrière Hills

Volcano, Montserrat

https://feww.wordpress.com

Fiona Burns

Tephra plume developed at 13:27 on 11 Feb 2010

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What is ash aggregation?

Fine airborne volcanic particles (< 63 µm) adhere to each other:

electrostatic forces (Gilbert and Lane 1994)

moist adhesion (James et al 2002)

hydrometeor formation (Durant et al. 2009)

… and form ash aggregates.

http://ehp.niehs.nih.gov/121-a197

Consequence : reduction atmospheric residence time of fine ash.

Increase fine ash fallout 10 -100 km from volcano

Reduction airborne fine ash 1’000 km distance (Brown et al. 2012)

http://www.arctic-dreams.com/en/living_earth

Eyjafjallajökull 2010

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Motivation for the study of ash aggregation

Aggregation is not fullly understood

Modelling of tephra fall

Improve hazard assessment :

‒ Health impact (particles < 10 µm)

‒ Economic impact (risk to aviation)

Tephra fall , Eyjafjallajökull 2010http://icelandinpictures.com

https://rainbowwarrior2005.wordpress.com

http://www.strangedangers.com

Aim of the study

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New info about aggregation by correlating aggregates characteristics with source

Variation in size and shape of aggregates with distance from source

Internal structure and internal grain size of aggregates

Presentation overview

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Introduction

11 Feb 2010 partial dome collapse

Methodology

Results

Conclusion

Perspectives

https://volcanosss.wordpress.com

Soufriere Hills Volcano, Montserrat

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Introduction 11 Feb 2010 Methodology Results Conclusion Perspectives

http://www.earthweek.com

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Geological setting

Introduction 11 Feb 2010 Methodology Results Conclusion Perspectives

17 active volcanoes

Lesser Antilles Volcanic Arc

Subduction of Atlantic oceanic lithosphere beneath the Caribbean plate (Kokelaar 2002)

http://www.earth-of-fire.com

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Volcanic history

Introduction 11 Feb 2010 Methodology Results Conclusion Perspectives

Pelean volcano, andesitic magmas (58-64% SiO2) (Sparks & Young 2002)

Type of activity: dome collapses and Vulcanian explosions

Renewal of activity in 1995 2/3 of population left the island Current population 5’000 people

60% of the island : exclusion zone (access forbidden ) (Kokelaar 2002)

http://commons.wikimedia.org/wiki/File:Montserrat

11 February 2010

partial dome collapse

Block-and-ash flow at Soufriere Hills, Montserrathttp://www.photovolcanica.com

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Terminology

Introduction 11 Feb 2010 Methodology Results Conclusion Perspectives

Tephra : fragmented material ejected

during explosive volcanic eruption

Tephra plume: buoyant volcanic plume

Tephra fall deposit: tephra settled

from the volcanic plume

Pyroclatic Density Current (PDC) fast-moving current (several 100 km/h)

of hot gas (up to 1000°C) and tephra

Co-PDC plume : ash cloud derived from PDC by elutriation of fine particles and air entrainment (Bonadonna et al 2002)

Topinka, USGS, 1997, modified from : Gardner et al , 1995

http://ffden-2.phys.uaf.edu

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Introduction 11 Feb 2010 Methodology Results Conclusion Perspectives

Piecemeal collapse of the NE flank of

the dome, lasted 107 minutes and

removed 50 x 106 m3 of material

6 peaks in PDC generation

2 Vulcanian explosions

PDCs entered the sea adding 1 km2 of

land to the NE coast

Co-PDC plume rose to 3 km altitude

Tephra plume rose to 15 km altitude (Stinton et al. 2014)

11 February 2010 partial dome collapse

https://feww.wordpress.com

http://www.montserratvolcano.org

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Deposits associated with the 11 February 2010 dome collapse

Modified from Cole et al. 2014

Methodology

Field work near Trants, Montserrat

http://blogs.agu.org/magmacumlaude/2011/04/12/soufriere-hills-volcano-15-years-on-conference

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Sampling

4 locations

Harris (2.5 km)

Farm (4 km)

Spanish Point (5 km)

Trants (6 km)

HARRIS

FARM

TRANTS

SPANISH POINT

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Preparation and analysis methods

Introduction 11 Feb 2010 Methodology Results Conclusion Perspectives

109876543210-1-2-30

10

20

30Sp-02A

Whole ALdeposit

φ

wt %

Aggregates = Accretionary lapilli (AL)

Manual separation , wt%, dimensions

impregnation with araldite resin

Grainsize distribution of deposit:

Sieving (particles > 0.5 mm)

Laser diffraction (part. < 0.5 mm)

Internal structure and GS of aggregates:

SEM (Scanning Electron Microscope)

Image analysis (J Micro Vision)

Results

Lava dome of Soufriere Hills volcano, 18 January 2010

http://www.photovolcanica.com

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Abundance of accretionary lapilli in different deposits

Introduction 11 Feb 2010 Methodology Results Conclusion Perspectives

PDC deposits sampled low abundances in AL

(0.3 wt % BAF, 2 wt % surge , 3 wt % pumice flow)

Co-PDC fallout deposit: 13 wt % abundance in AL

The study focuses on AL from the co-PDC fallout deposit

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Isopach map for co-PDC fallout Isopleth map for AL of co-PDC

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Internal grain size distribution of aggregates

Introduction 11 Feb 2010 Methodology Results Conclusion Perspectives

Nearly identical distributions:

Aggregation is a stable and size

selective process

Selects particles comprised

(< 1mm and 1μm), majority

fine particles (< 63 μm )-1 0 1 2 3 4 5 6 7 8 9 10

0

5

10

15

20

25

30

35Ha-02

Ha-03

Fa-01A

Fa-02

Tr-02

Sp-01A

Sp-01C

Sp-02B

Sp-05

φ

wt %

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Single layer aggregates: coarse grained core coated by a thin layer of finer ash

Internal structure of aggregates

Introduction 11 Feb 2010 Methodology Results Conclusion Perspectives

Multiple layer aggregates: 2 – 3 extra layers accreted over the core

Internal grain size per layer

Introduction 11 Feb 2010 Methodology Results Conclusion Perspectives

Core: main part of aggr.

main stage in

history of formation

Similar Mdɸ , thickness:

Core and L2

L1 and L3

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Interpretative model for ash aggregation in the co-PDC plume

Introduction 11 Feb 2010 Methodology Results Conclusion Perspectives

Conclusion

Soufriere Hills, PDC © Dr Paul Cole

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Introduction 11 Feb 2010 Methodology Results Conclusion Perspectives

Aggregation selects same range of particles independent of type of event

High abundance (13 wt%) aggregates in co-PDC fallout favorable dynamics within the co-PDC plume

Estimated volume co-PDC fallout deposit: 2 x 106 m3

Volume 260 000 m3 aggregated = 0.5 % of tot collapsed volume

Conclusion

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Abundant vapour : larger particles are aggregated by moist adhesion: core, L2

Scarce vapour : finer particle are accreted by electrostatic forces: L1, L3

Size and number of layers of aggregates: indicates conditions in thermally

stratified plume.

Role of water vapour in the mechanism of aggregation

Introduction 11 Feb 2010 Methodology Results Conclusion Perspectives

http://www.dunhilltraveldeals.com

Perspectives

Plymouth, Montserrat http://abandoned.photos/post/69080596492

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Introduction 11 Feb 2010 Methodology Results Conclusion Perspectives

Perspectives

Chemical analysis aggregates (NaCl) sea water main source humidity?

Parameters which control, enhance aggregation to improve models which estimate tephra fall dispersal

More precise analysis of the hazards

Aggregation mobilizes the fine ash: reduces the health impact

http://www.photovolcanica.com

http://www.montserratvolcano.org

Thanks for Your Attention

Tambora caldera, Indonesia 

http://commons.wikimedia.org/wiki/File:Caldera_Mt_Tambora_Sumbawa_Indonesia.jpg