# Presentation: 497197

## Can eruptions be forecast?

- <date/time>

**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

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

- H Mader/CN

**Notes:**
In these eruptions, very viscous magma is extruded very slowly at the surface. The magma viscosity is very high because of the loss of gas (as we shall discuss this in more detail shortly) and crystallisation. Much of the gas escapes from the system. Lava dome eruptions are subject to: spurts of growth, cycles of dome growth and subsidence, sudden onset of violent explosive activity and disintegration of lava during formation of pyroclastic flows. These dramatic changes occur in over 95% of cases.

## ‘Effusive’ eruptions.

**silicate melt **
**+ dissolved gases (H****2****O, CO****2****, S) **
**(+ crystals)**

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

- ‘Effusive’ eruptions.

- After H Mader

**Notes:**
Effusive eruptions are characterised by lava flows and fire fountains. 

Silicate melt with dissolved gases at depth (6km ish) in a magma chamber. The confining ‘lithostatic’ pressure increases with depth (note: the ‘increasing pressure’ label on the image relates solely to this lithostatic pressure gradient, not the local pressures in the flow, which are a result of the flow dynamics). Nucleation of bubbles occurs at some depth. The bubbles grow by decompression and diffusion. Crystals can also nucleate and grow.

The chemical composition of the magma in such systems is often low in silica (SiO2).

## 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?)**

- After H Mader

**Notes:**
In an explosive eruption, the process is essentially the same. However, the process is more violent and results in ‘fragmentation’ leading to a two-phase mixture of fragments of bubbly magma being carried along in a continuous phase of hot gas. The mixture can form eruption columns and pyroclastic flows.

The magma in these eruptions generally has a high silica (SiO2) content.

## Two modes of conduit, eruptive behavior

- 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, Leaky**
- Convection 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

## Slide 9

## Open vent:  Sakurajima Volcano, Japan

- <date/time>

*Explosions/day*

*Earthquakes/day, < 1 km deep*

*Earthquakes/day, 1-15 km deep*

-  (1987)

- K. Kamo et al.

## Take home message #1:  

- <date/time>

- 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:

- <date/time>

- 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:

- <date/time>

- 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