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
HITTING
THE WALL
WEAK FINISH
HSL
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
Illustrated:
Three Sisters, 1998-present
Note: consistent theme: intrusions of small volume
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
Or, in more
“scientifically
proper”
terminology 
From Moran et al., BV, 2011
From Moran et al., BV, 2011
Pinatubo – 20th anniversary
1st 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 volatilerich 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.
500 years b.p.
2300 years b.p.
3500 years b.p.
What can we tell from repose and
cumulative eruptive volume pattern?
E.g., constant threshold P, open vent
E.g., plugged conduit
After Koyama and Yoshida, 1994
Examples?
E.g., constant threshold P, open vent
Geysers, Stromboli,
Vesuvius 1632-1944
E.g., plugged conduit
Pinatubo, MSH
Vesuvius today
eruption rate –ataMount
Helens
Mount St. Long-term
Helens long-term
goodSt.example
of
volume AND VEI predictability
Cumulative Volume, DRE (10^7 m^3)
500
400
300
200
100
0
600
800
1000
1200
1400
1600
1800
2000
2200
Calendar year
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+.
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
High
fracture
permeability
needed for ores.
might also be
needed for
plinian
eruptions?
Dick
Intermediate- and short-range precursors-General case for any size intrusion/ eruption
after a long repose
Various lengths of eruption precursors…
Intermediate* --
Short*--
• 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
• 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
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.
0
Kilometer
10
20
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
JUNE 1991
15
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
Pulling Everything Together using a Bayesian Event Tree
(1)
(2)
(3)
Unrest Origin of Unrest Eruption
(4)
Explosive
Magnitude
(5)
Eruptive
Phenomena
(6)
Sector
(7)
(8)
Distance Exposure
(km)
(9)
Vulnerability
0-5
(same as VEI 3)
None
VE I 4
Magmatic
intrusion
Phreatic
only
VE I 3
Restless
Volcano
pyroclastic flow
to
pyroclastic surge
15-20
1=full
tephra fall
>20
lava flow
None
VE I 1-2
Tectonic or
hydrothermal
no magma
intrusion
(same as VEI 3)
Phreatic
VE I 0
TIME and S PEC IF IC ITY
lahar
lava flow
8 sectors
0= none
10-15
lahar
Magmatic
5-10
0= none
to
1=highly
vulnerable
Grazie!