Volcanic Ash Clouds Jeremy Phillips Costanza Bonadonna , Rose Burden, Andrew Hogg,

1. Volcanic Ash Clouds Jeremy Phillips CostanzaBonadonna, Rose Burden, Andrew Hogg, Steve Sparks, Mark Woodhouse

2. Outline 1. Explosive Volcanic Eruptions and Generation of Ash Clouds 2. Gravity Current Models of Ash Cloud Spreading and Tephra Deposition 3. Inversion of Deposits to Infer Eruption Conditions 4. Ongoing Research into Wind-affected Eruptions 5. Summary

3. Explosive Volcanic Eruptions • Explosive volcanic eruptions are caused by overpressurization of highly- • viscous magmas by volatile gases (H2O, CO2, SO2, HCl etc.)

4. Explosive Volcanic Eruptions • Volcanic ash results from fragmentation of vesicular magma

5. Explosive Volcanic Eruptions 1024 mm 32 mm 1 mm 32 mm 1 mm

6. Explosive Volcanic Eruptions • Initial temperature in the range 300 to 800 oC • Initial mass flux in the range 104 to 109 Kg s-1 • Initial vertical velocity in the range 50 to 300 m s-1 • Eruptions can be sustained for 10s of hours – quasi steady • Movies • Explosion at Soufriere Hills Volcano Montserrat 1997 • Laboratory experiment showing continuous buoyant plume in a stratified • environment

7. Explosive Volcanic Eruptions Redoubt 1989

8. Explosive Volcanic Eruptions Shiveluch 2007

9. Explosive Volcanic Eruptions Puyehue 2011

10. Gravity Current Models • Comparing the characteristic vertical velocity in the plume to the • atmospheric wind speed allows classification into ‘strong’ and ‘weak • plumes • (B0is the initial buoyancy flux in the plume and N2is the buoyancy • frequency of the atmosphere) • For strong plumes, the plume is assumed to be unaffected by the wind, and • the spreading ash cloud takes the form of a gravity current propagating in a • stratified ambient fluid, driven by input from a turbulent buoyant plume • The ash cloud can be affected by the wind

11. Density profile z h r Gravity Current Models • The ash cloud is driven by the density difference due to mixing up a • stratified environment around a level of neutral buoyancy • r h Thickness of the ash cloud determines excess pressure above hydrostatic balance Particle volumetric concentrations are small (initially < 0.03 in plume) so contribution of their mass to density difference is small

12. Gravity Current Models • The ash cloud is a dilute turbulent gravity current – particle concentration • varies according to Hazen’s Law • (Martin and Noakes 1988) • (A = base area, C0 = initial concentration)

13. Gravity Current Models • We can solve for mass deposited from the ash cloud • Far from the ash cloud front where the flow is approximately steady

14. Gravity Current Models • The radial velocity can be determined from the volumetric flux from plume • to ash cloud (which is itself simply related to the plume source flux) • So the total ash accumulation on the ground is • (following Bursik et al 1992)

15. Gravity Current Models • The ash cloud shape depends on downwind and crosswind velocities • Different approaches have been taken: • (Bursik et al 1992) (Bonadonna and Phillips 2003) • u = downwind velocity, uw = wind speed, uc = crosswind velocity

16. Gravity Current Models • Model testing on Fogo A eruption (Bursik et al 1992)

17. Gravity Current Models • Model testing on explosive eruptions (Bonadonna and Phillips 2003)

18. Gravity Current Models • Model testing on explosive eruptions (Bonadonna and Phillips 2003) • The buoyant component of spreading velocity can be significant far from • the source

19. Applications • Volcanologists map deposit thickness (isopach) and largest particle • (isopleth)

20. Applications • Volcanologists map deposit thickness (isopach) and largest particle • (isopleth) • Isopleth for Pinatubo 1991 pumice

21. Applications • Inferring eruption plume height from the position of the largest particle in a • deposit (Carey and Sparks 1986; Burden et al 2011) • Pinatubo 1991 • (Burden et al 2011)

22. Applications • Inferring eruption plume height from the position of the largest particle in a • deposit (Carey and Sparks 1986; Burden et al 2011) • There is a relationship between plume height and source volumetric flux (in the absence of wind) • (Sparks 1986)

23. Applications • Inferring eruption phase duration from layer thicknesses and grainsize • distribution at two locations (Koyaguchi 1994; Burden et al in prep) • The grainsize at a given location depends on the total deposition of all • particle sizes in the ash cloud, and the ash cloud volumetric flux Q • ( f(vt) is the grainsize distribution at r ) • The time to deposit a layer • ( d is the layer thickness and r is the ash deposit density ) • Fine layers in tephra deposits reflect changes at the volcanic source

24. Applications • Source volumetric flux and eruption duration enable eruption volume to be • estimated • Constraining eruption volume enables the impact of volcanic eruptions on • climate and society to be quantified

25. Wind-affected eruptions • The dynamics of the interaction of weak plumes with the atmospheric wind • requires modified approaches • Wind influences plume height for a given source mass flux • Ash cloud dynamics are sensitive to wind interactions • These effects have been highlighted by the 2010 eruption of Eyjafjallajokull

26. Wind-affected eruptions

27. Wind-affected eruptions

28. Summary • Explosive volcanic eruptions create ash clouds whose dynamics depend • on the relative velocities of the eruption plume and the atmospheric wind • When the plume velocity is greater than that of the wind, the plume • provides a source to an ash cloud that can be described as a turbulent • gravity current spreading in a continuous stratification • Models of ash clouds as gravity currents have successfully predicted the • variation of deposit grainsize and thickness with distance from the source • These models have formed the basis of methods for inverting deposits to • infer eruption plume height, source mass flux and duration • Modified approaches are required for weak plumes and this is an area of • current research effort