There is a maximum size for the first detaching bubble,

Bubble Rise and Break-up in Volcanic Conduits A. Soldati 1, K. V. Cashman 2, A. C. Rust 2, M. Rosi 1

1 Department of Earth Sciences, University of Pisa (Italy) 2 School of Earth Sciences, University of Bristol (United Kingdom)

Conclusions • System geometry controls rise

velocity, which in turn controls

break-up

• There is a specific size to the

bubbles a system of a certain size

and geometry can deliver

• Being able to measure that size

(through geophysical techniques)

we can invert it to infer upper

conduit geometry

Bubble rise through magma

inside a volcanic conduit is

effectively described by an

appropriate flow regime,

therefore eruptive dynamics

can be studied in the

framework of two-phase flow.

e.g. slug flow will result into

Strombolian paroxysms.

strombolian

paroxysm

slug

flow

Flow Regimes

How: Volume Distribution

1st daughter bubble: 50-95% total volume

2nd daughter bubble: 5-45%

micro bubble: up to 5%

but dependent on the slope geometry

0

20

40

60

80

100

9 11 13 15

dau

gh

ter

bu

bb

les

vol

(%)

original slug volume (ml)

bigger bubble

more even partition

Bubble vs. Slug Size (%)

independent of the original slug size

100

125

150

175

200

100 125 150 175 200 225

up

per

dau

gh

ter

are

a (

mm

2)

original slug area (mm2)

Bubble vs. Slug Size (abs)

Rationale

Existing models of volcanic degassing consider

the feeder conduit to be cylindrical, while

there is strong evidence that it is flattened

instead, like a dyke; such approximation

affects our interpretation and understanding

of deep magmatic processes.

This study aims at determining the impact of

system geometry on active degassing dynamics

in terms of bubbles’ rise velocity and stability.

Analogue Modelling

Flexible set-up

Analogue melt:

golden syrup-water

mixtures (17<η<59 Pa∙s)

Analogue volcanic gas:

air bubbles manually

injected with a syringe

Why: Gas Motion Why: Syrup Motion

0

15

30

45

60

0 3 6 9 12 15 18

slop

e (°

)

original bubble volume (ml)

60

45

30

15

Regime Diagram

stability

field

break-up

field

time

Break-up

undisturbed

rise

head

bulging

middle

thinning

break-up

nose

pointing

Bubbles rise faster in wider spaces.

The shallower the slope, the higher the difference in velocity

between the head and tail of the bubble.

Shallower slopes promote a faster syrup re-occupation

of the channel cross sectional area after the slug passage.

-10

-8

-6

-4

-2

0

0 15 30 45 60 75

bre

ak

-up

lev

el (

mm

)

slope angle (°)

Where

The syrup displaced by the slug, descending along the slope, has

to change direction at the slope-break level to enter the channel;

however, the actual break-up occurs slightly below it.

A steeper slope determines a steeper downward syrup flux,

therefore the steeper the slope, the deeper the break-up level.

Scaling

0

50

100

150

200

0 15 30 45 60

are

a (m

m2

)

slope (°)

15 30 45 60

200

150

100

50

0

slope angle (°)

up

per

dau

gh

ter

size

(m

m2)

Bubble Size vs. Slope

0

10

20

30

40

50

60

70

0 0,5 1 1,5

tim

e (s

ec)

film width (cm)

15p15

30p15

45p15

60p15

Film Width (at y=0)

Temporal Evolution

film

wid

th i

n t

he

chan

nel

-0,5

0,0

0,5

1,0

1,5

0 5 10 15 20 25

Δv (

nm

m/s

ec)

time (sec)

15° 30° 45° 60°

Δvt-h

Geometrical Slug volume → slug lenght/channel width

→ slug lenght/channel cross sectional area

Slope → 1/sin(slope)

Wall Roughness Effect (neglegible)

infrasounds

Experimental Work

Model

Field

Measurements

Better understanding

of volcanic degassing

Improved risk and

hazard assessment

…

Applications

infrasounds

measure emerging

bubble size

invert for

conduit geometry

established correlation between

system geometry and bubble size Kynematical Newtonian Fluid → Non-Newtonian Fluid

Pure Silicate Melt → Real Magma

Golden Syrup → + sugar xls, bubbles

ρ w v Re = ———— < 2300 η

η-dominated

system

laminar flow

conditions

Dynamical