Uh-oh! Unrest at a long-dormant volcano

1. Uh-oh! Unrest at a long-dormant volcano Chris Newhall, Colleagues of Nanyang Geoscience Roundtable, and other Colleagues

2. A “magma marathon” view of possible outcomes of intrusions HITTING THE WALL WEAK FINISH HSL

3. 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

4. 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

5. Examples of near-eruptions(“close but not quite”) • Illustrated: Soufriere Guadeloupe 1976 • Other examples: • Akutan, 1996

6. Examples of weak marathon finish Illustrated: Pinatubo Aug 1992 Other examples: Unzen 1991-95; Usu 2000; Mount St Helens 2004-08

7. Examples of strong marathon finishes • Pinatubo 1991 • Mayon 1814 • Taal 1965 • Chaiten 2007 • Merapi 2010 • Soufriere Hills, 1997-present

8. Or, in more “scientifically proper” terminology  From Moran et al., BV, 2011

9. From Moran et al., BV, 2011

10. Pinatubo – 20th anniversary 1stNanyangGeoscience 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?

11. 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

12. 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

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

14. 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.

15. 500 years b.p. 2300 years b.p. 3500 years b.p.

16. 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

17. Examples? E.g., plugged conduit E.g., constant threshold P, open vent Geysers, Stromboli, Pinatubo, MSH Vesuvius 1632-1944 Vesuvius today

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

19. 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

20. 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

21. High fracture permeability needed for ores. might also be needed for plinian eruptions? Dick Henley, ANU

22. Intermediate- and short-range precursors-- General case for any size intrusion/ eruption after a long repose Intermediate* -- 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 Various lengths of eruption precursors… • 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 * Termed long- and short- by S. McNutt

23. 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.

24. At Pinatubo, June 1991: Subplinian eruptions (June 12-15, spikes) and, starting June 14, dramatic increase in shallow low-frequency earthquakes (curve) Kilometer 0 10 20 12 13 14 15 JUNE 1991

25. 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.

26. Are there diagnostic signs that magma will NOT erupt??? • Constant (?) or slowly decreasing ( ) rates of unrest; progressively longer pauses

27. Pulling Everything Together using a Bayesian Event Tree ( 1 ) ( 2 ) ( 3 ) ( 4 ) ( 5 ) ( 6 ) ( 7 ) ( 8 ) ( 9 ) U n r e s t O r i g i n o f U n r e s t E r u p t i o n E x p l o s i v e E r u p t i v e S e c t o r D i s t a n c e E x p o s u r e V u l n e r a b i l i t y M a g n i t u d e P h e n o m e n a ( k m ) 0 - 5 ( s a m e a s V E I 3 ) N o n e 5 - 1 0 0 = n o n e 0 = n o n e V E I 4 p y r o c l a s t i c f l o w 8 s e c t o r s 1 0 - 1 5 t o t o M a g m a t i c P h r e a t i c p y r o c l a s t i c s u r g e 1 5 - 2 0 1 = f u l l 1 = h i g h l y i n t r u s i o n o n l y v u l n e r a b l e t e p h r a f a l l > 2 0 V E I 3 R e s t l e s s l a h a r V o l c a n o M a g m a t i c l a v a f l o w N o n e V E I 1 - 2 T e c t o n i c o r ( s a m e a s V E I 3 ) h y d r o t h e r m a l n o m a g m a i n t r u s i o n l a h a r V E I 0 P h r e a t i c l a v a f l o w T I M E a n d S P E C I F I C I T Y a

28. Grazie!