Susan Kidd, of Ilam school, asks :-

Can we tell beforehand whether or not a volcano will erupt?

Peter Otway, a volcanologist with the Institute of Geological and Nuclear Sciences, Taupo, responded.

Sometimes. Certain types of eruptions at well monitored volcanoes, such as White Island and Ruapehu, can be predicted with varying degrees of accuracy. However, almost any volcano can sometimes erupt in such a way that prediction would be very difficult no matter how closely the volcano had been monitored.

Firstly we determine the past history of a volcano's eruptions by dating the layers of lava and ash that have built up around the volcano. The more frequent the eruptions in the recent past, the more frequent they are likely to be in the future.

More precise information about its present state of activity - or "health" - can be obtained by recording any earthquakes (seismicity) under the volcano, by measuring the temperature and analysing the gas releases at any hot spots (particularly in the crater), and by surveying for small ground movements (earth deformation) due to underground movement of the magma (lava full of explosive gas).

A major eruption will often be preceded by an increase in the seismicity, heat, gas output and/or deformation as the magma forces its way upwards towards the crater.

Our unofficial predictions of small eruptions are regarded as only one step towards devel- oping the best possible detection system for really serious eruptions. We are steadily working towards that goal.