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!

Gas leaks --> Lava domes

magma input from depth

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

‘Effusive’ eruptions.

• • low viscosity magma --> gas highly mobile and can escape easily

silicate melt + dissolved gases (H 2 O, CO 2 , S) (+ crystals) magma input from depth

After H Mader

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

Explosive eruptions.

• • 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 (H 2 O, CO 2 , S) (+ crystals?) magma input from depth

After H Mader

Two modes of conduit, eruptive behavior Closed, Tight

No convection in the conduit(s); incoming gas is stored Infrequent, large explosive eruptions

Open, Leaky

Convection in the conduit, releasing most gas Frequent, sm eruptions (or no eruptions at all) Strong eruption precursors; ample warnings E.g., Pinatubo, Mount St. Helens Weak and/or short eruption precursors; warnings difficult Popocatepetl (1994- ), Sakurajima, Mayon, Kilauea; Iwo-jima

Open vent: Sakurajima Volcano, Japan

Explosions/day Earthquakes/day, < 1 km deep Earthquakes/day, 1-15 km deep

(1987) 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