When we gaze upon the perfect cone of a smoking mountain, we are only seeing the tip of the iceberg, though some of its surface features may reveal something about the 90% of the magmatic system lying below. A lava chamber lies below, cooling and giving off heat and gradually degassing a mixture of water, carbon dioxide and sulphur dioxide, waiting as the pressure builds up or new hot lava approaches from below to trigger an eruption.
The photo shows a solfatara, a fumarole that is discharging sulphur rich hot gas, evidenced by the bright yellow crystals and surrounding orange crust. I'm not sure what the white material is, but it could be silica sinter, another common exudation of hot volcanic waters. These gaseous emanations play an important role in the process of volcano monitoring, being able to reveal by changes in their composition some idea of what is happening below. When an edifice starts pumping out alot more sulphur dioxide than usual, it is a signal that fresh lava has arrived in the chamber and is degassing. As it rises, so the ratios of different gases change, allowing vulcanologists to track, however vaguely, developments deep below.
In combination with seismic patterns, records of ground deformation (as the edifice swells as the magma rises towards the vent) and other data, researchers can now forecast with some degree of accuracy when a smoker is about to blow. This allows civil authorities to prepare during the early phases of the process when the gas ratios first change for the evacuations that may be needed later on. Routine monitoring using spectrometers, sometimes from the geologist's car window is now a vital element in volcano forecasting worldwide.
Image credit: Banco de Imagenes Geologicas
