can eruptions be forecast? chris newhall, us geological survey seattle
TRANSCRIPT
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Can eruptions be forecast?
Chris Newhall, US Geological Survey
Seattle
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Principal methods of volcano monitoring (All on surface, in lieu of an internal gas pressure gauge! Most, looking for repeated patterns of precursory change)
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For better, process-based forecasts, we need to track what
makes volcanoes erupt...GAS!
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magma input from depth
Gas leaks --> Lava domes
Viscous magma + Slow ascent to surface --> preeruption gas leakage --> low explosive potential.
H Mader/CN
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silicate melt + dissolved gases (H2O, CO2, S)
(+ crystals)
magma input from depth
• low viscosity magma• --> gas highly mobile and can escape easily
‘Effusive’ eruptions.
After H Mader
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Lava pond on Mauna Ulu (Hawaii) 1969 - bubble is ~ 5 metres across.
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• high viscosity magma• --> gas not very mobile and cannot escape easily
•IF ALSO rapid ascent, • --> high gas content persists to near surface• --> large potential expansion
silicate melt + dissolved gases (H2O, CO2, S)
(+ crystals?)
magma input from depth
Explosive eruptions.
After H Mader
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E.g., Pinatubo, Mount St. Helens
Popocatepetl (1994- ),Sakurajima, Mayon,Kilauea; Iwo-jima
Closed, Tight No convection in the conduit(s); incoming gas is stored
Open, LeakyConvection in the conduit, releasing most gas
Two modes of conduit, eruptive behavior
Strong eruption precursors; ample warnings
Weak and/or short eruption precursors; warnings difficult
Frequent, sm eruptions (or no eruptions at all)
Infrequent, large explosive eruptions
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Explosions/day
Earthquakes/day, < 1 km deep
Earthquakes/day, 1-15 km deep
(1987)
Open vent: Sakurajima Volcano, Japan
K. Kamo et al.
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Take home message #1:
Forecast success rates? ----------------------------------------------------• Volcano restless, eruption possible? >95%• Eruption next few days? >50% Hours?
<50%• Explosive potential? >95%• Actual explosive magnitude? Tough,
maybe 50%
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Take home message #2:
• Biggest uncertainty is the degree to which magma is still trapping gas as it nears the surface, i.e., how tight or leaky the volcano.
• Gas content, pressure in magma cannot be measured directly. Gas monitoring is relatively rare; more often, we rely on proxies like seismicity and ground deformation.
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Take home message #3:
• Big eruptions generally easier to forecast, with lead times of days to months.
• The smallest, most frequent eruptions, are tough to forecast with useful lead times.
• In all cases, eruption detection and tracking still critically needed!
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JB