Historic pattern of activity
Similar to seismic gap theory, look at the intervals between eruptions
- if they are regular then it may be possible to find the average time between them ad calculate future eruptions
- sequence of movement; are the eruptions moving north or south?
- time between eruptions; the larger the gap the more explosive the eruption
It is VERY approximate, no reliability
Shown in eruptions in decade volcanos e.g. Vesuvius
Changes in ground level using tilt meters
Active volcanos expand in volume due to magma rising from below
An increase in steepness or bulging of a volcano's slope may signal an impending eruption
Tiltmetres are used to detect an increase in steepness of a volcano. They can measure a change in tilt of milemetres.
Satellite monitoring of volcano cone temperature
As magma rises, the cone of the volcano begins to heat up
The temperature increase is monitored by satellite
The satellite infrared heat sensors detect even the slightest change in surface temperature.
This is an early warning for many of the world's 600 active volcanos.
Changes in gas composition and volume
Before a volcanic eruption, magma rises towards the surface. There is a reduction in pressure and therefore gases tend to escape.Increase in emissions before an eruption (but a decrease in gas emissions suggests a blocked vent)
As a result of this, sulpher dioxide and carbon dioxide emissions are continuously monitored on active or potentially active volcanoes
Escape of gases is related to supersaturation
- sulphur dioxide is the prominent volcanic gas
- gases are supersaturated in the magma - normally the solvent magma would not be able to hold such high quantities, but under high temperature and pressure they are successfully dissolved.
- sights of gas release on the slopes of any active volcanoes
- before an eruption fumarole activity will increase greatly
Precursor earthquake tremors
Seismomoters are used to monitor many volcanoes.
As magma rises the greater pressure forces aside fractured rock, enlarging the cracks as it rises.This creates a distinctive pattern of earthquake tremors called harmonic tremors.
Foci get progressively shallower as magma rises.