Can eruptions be forecast?

Chris Newhall, US Geological Survey

Seattle

Principal methods of volcano monitoring (All on surface, in lieu of an internal gas pressure gauge! Most, looking for repeated patterns of precursory change)

For better, process-based forecasts, we need to track what

makes volcanoes erupt...GAS!

magma input from depth

Gas leaks --> Lava domes

Viscous magma + Slow ascent to surface --> preeruption gas leakage --> low explosive potential.

H Mader/CN

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

Lava pond on Mauna Ulu (Hawaii) 1969 - bubble is ~ 5 metres across.

• 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

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

Explosions/day

Earthquakes/day, < 1 km deep

Earthquakes/day, 1-15 km deep

(1987)

Open vent: Sakurajima Volcano, Japan

K. Kamo et al.

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%

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.

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!

JB