Uh-oh! Unrest at a long-dormant volcano

Chris Newhall,Colleagues of Nanyang Geoscience Roundtable, and other Colleagues

A “magma marathon” view of possible outcomes of intrusions

HSL

WEAK FINISH

HITTING THE WALL

Marathon MAGMA finishes can be:

• False starts (magma starts the race too soon, without adequate preparation, and is unable to proceed)

• “Hitting the wall” (magma intrusion freezes just before reaching surface)

• Weak finish (magma just barely erupts, as sluggish dome)

• Strong (vigorous magmatic eruption)HSL

Examples of “false start” or “insufficient preparation”

• Other examples: • Mt. Baker, 1975-76• Long Valley, 1980 1989 present• Iwate, 1995-1998 (complicated by lateral migration at shallow depth)• Fuji, 2001

Note: consistent theme: intrusions of small volume

Illustrated: Three Sisters, 1998-present

Examples of near-eruptions(“close but not quite”)

• Illustrated: Soufriere Guadeloupe 1976• Other examples:

– Akutan, 1996

Examples of weak marathon finish

Illustrated: Pinatubo Aug 1992

Other examples: Unzen 1991-95; Usu 2000; Mount St Helens 2004-08

Examples of strong marathon finishes

• Pinatubo 1991• Mayon 1814• Taal 1965• Chaiten 2007• Merapi 2010• Soufriere Hills, 1997-present

From Moran et al., BV, 2011

Or, in more “scientifically proper” terminology

From Moran et al., BV, 2011

Pinatubo – 20th anniversary1st Nanyang Geoscience

Roundtable (EOS/PHIVOLCS)

“Can plinian eruptions be forecast?”

• What conditions in magma and its surrounding environment create the long-term POTENTIAL for a plinian eruption? (prerequisites)

• What additional conditions must develop over the short term to TRIGGER, ESCALATE and SUSTAIN a plinian eruption?

What makes an eruption large and explosive?

• Rapid and sustained fragmentation of gas-rich magma… with magma rising fast enough during eruption to match or nearly match the downward progression of the fragmentation front.

• Optimal conditions:– Large volume of volatile-rich magma (best if supra-saturated, with

excess “free” volatile phase even at depth)– Viscosity low enough that volatiles can diffuse quickly through melt

and into bubbles, yet high enough to impede gas loss and to allow pressurization and then rapid fragmentation.

– Pre-eruption ascent rate high enough that magma can reach surface without significant degassing

– Structural and stress setting that maintains plug for centuries, but then facilitates rapid magma ascent when magma does start to rise

A new paradigm from Pinatubo (and El Chichón)

• Excess (free) volatile phase, far in excess of saturation. CO2-H2O-SO2 rich

• Discovered because SO2 release during eruptions (measured by TOMS) >> predicted from volume of erupted magma

• Luhr et al. & TOMS, from El Chichón 1982; • Reinforced by TOMS team and Gerlach,

Westrich, Wallace from Pinatubo 1991

Giant SO2 cloud, 17 Mt, largest since satellite coverage began in 1979; ~ ## x that predicted

Long-range indicators and precursors?

• Geological and historical evidence of previous large explosive eruptions. The facts of such eruptions are obvious from topography and stratigraphy though volumes, intensities, and dates of past eruptions must still be determined. – Textural and other indicators of high intensity in past eruptions, e.g.,

pulverization of phenocrysts by extreme shear in the conduit. – Petrologic signs of high volatile contents, especially in discrete volatile-

rich bubbles at depth. Signs include already-degassed glass inclusions in phenocrysts, and presence of primary anhydrite and sulfides.

• PLUGGING AND TIME help… • Known structural and alteration-induced weakness so that magma

COULD rise and depressurize quickly … e.g., risk of sector collapse• Leaks of silicic magma out of arcuate fracture zones, sometimes

years and even centuries ago, indicating stress dominated by pressurization of the reservoir.

3500 years b.p.

2300 years b.p.

500 years b.p.

After Koyama and Yoshida, 1994

What can we tell from repose and cumulative eruptive volume pattern?

E.g., constant threshold P, open vent E.g., plugged conduit

Examples?

E.g., constant threshold P, open vent E.g., plugged conduit

Geysers, Stromboli, Pinatubo, MSH Vesuvius 1632-1944 Vesuvius today

Long-term eruption rate at Mount St. Helens

Calendar year

600 800 1000 1200 1400 1600 1800 2000 2200

Cu

mu

lativ

e V

olu

me

, D

RE

(1

0^7

m^3

)

0

100

200

300

400

500

Step-wise increase in volume of extruded magma

MSH gets plugged at end of an eruption Long repose, Large and explosive eruption Then less and less explosive “residue”

Magma goes “flat,” plugs up again for another century+.

Mount St. Helens long-term – a good example of volume AND VEI predictability

Credit – check PP#

How to detect a separated fluid phase?

• Large bubble-to-glass ratio (bubbles up to 50+ vol% of the inclusion) indicates that the bubbles cannot simply be due to contraction.• The presence of multiple large bubbles in a single inclusion also suggests trapping of the fluid.

• PRESENCE proves gas phase• ABSENCE proves nothing, since the bubbles can “explode” their way out

Fidel Costa

What if you can’t find volatile bubbles in melt inclusions?

• Other indicators: gas-depleted melt inclusions, high S minerals e.g., anhydrite, sulfides, apatite, hb as phenocrysts or inclusions in MI’s

• Best evidence for modern eruptions is the SO2 release during eruption, relative to erupted melt volume

Highfracture

permeabilityneeded for ores.

might also be needed for

plinian eruptions?

Dick Henley, ANU

Intermediate- and short-range precursors-- General case for any size intrusion/ eruption

after a long repose

Intermediate* --• Months- years of slow

preparation for eruption… sometimes starting right after previous eruption

• InSAR-detected deep recharge, deep LP eqs, CO2 gas anomalies

• Mainly, resupply of the reservoir

Short*--• Hours to a few months of

shallow VT seismicity, shallow deformation, increased SO2 flux. May have DVT’s.

• Mainly, pressurization of reservoir, hydrothermal system, and conduit as magma ascends

* Termed long- and short- by S. McNutt

Various lengths of eruption precursors…

Short-range (immediate) precursors for plinian events?

• Early-erupted glass or gas flux measurements that indicate exceptional S or CO2 content of rising magma.

• Monitoring and early-erupted petrologic evidence of magma ascending at rates of > 0.2 or 0.3 m/s, especially if relatively viscous.

• Increasingly frequent subplinian eruptions, especially if accompanied by RAPIDLY ACCELERATING seismic or ground deformation indicators of runaway pressurization.

At Pinatubo, June 1991: Subplinian eruptions (June 12-15, spikes) and, starting June 14, dramatic increase in shallow low-frequency earthquakes (curve)

12 13 14 15 JUNE 1991

Kilo

met

er0

10

20

Non-diagnostic for large explosive eruption

• Deep LP earthquakes• Strong seismicity, deformation, gas• Evidence of magma mixing• Phreatic and phreatomagmatic eruptions• Dome extrusion, even from multiple vents,

arcuate or radial.• Unusually rapid depressurization, as during

giant landslides or major faulting.

Are there diagnostic signs that magma will NOT erupt???

• Constant (?) or slowly decreasing ( ) rates of unrest; progressively longer pauses

(1)Unrest

RestlessVolcano

Magmaticintrusion

Tectonic orhydrothermalno magmaintrusion

(2)Origin of Unrest

(3)Eruption

Magmatic

Phreatic only

None

Phreatic

None

(4)ExplosiveMagnitude

VEI 4

VEI 3

VEI 1-2

VEI 0

(5) EruptivePhenomena

pyroclastic flow

pyroclastic surge

tephra fall

lahar

lava flow

lahar

lava flow

(same as VEI 3)

(same as VEI 3)

(6)Sector

8 sectors

(7)Distance (km)

0-5

5-10

10-15

15-20

>20

(8)Exposure

Pulling Everything Together using a Bayesian Event Tree

0= none

to

1=full

(9)Vulnerability

0= none

to

1=highlyvulnerable

TIME and S PECIFICITY

Grazie!