There are three classes of volcanoes:
Class A volcanoes are often commonly held to be extinct, but to assess volcanic activity periods as long as hundreds or even thousands of years are required. An example that illustrates this is the eruption of Pinatubo on the Philippines. Before the eruption in 1991, the last time it became active was 600 years previously. The volcano El Chichon in Mexico was considered to be totally extinct before it erupted in 1983.
There are several risk factors associated with volcanoes, the principal being:
These phenomena vary from volcano to volcano. Whereas ashfalls and tidal waves can cause damage over relatively large areas, the other phenomena only present a danger to the area in the immediate vicinity of the volcano and so are easier to record. The spread of ash depends on the direction and force of the wind and so the risk for regions further afield is difficult to estimate. The impact of tidal waves, caused by volcanic eruptions under lakes, seas and on the coast is comparable with that of seismic sea waves. All the phenomena that have been referred to have a great potential to cause damage as the history of natural catastrophes tells us.
It is however, very difficult to assess and, as is the case with earthquakes, classify the actual exposure. On the one hand, eruptions are usually too rare for reliable statistical analysis and on the other classification in terms of the latest instruments to make short and medium term predictions would seem to be considerably more promising than the same approach with earthquakes as a few successful cases (Rabaul, Montserrat) show.
