Why do some volcanoes fizz away quietly for years while others explode with such violence their names are remembered with fear for centuries? Krakatau and Tarawera are two fine examples of volcanoes behaving badly, pyrotechnically speaking.

Researchers from Otago's Department of Geology are exploring how eruptions change when hot magma meets groundwater near the surface to create a series of violent steam explosions.

When Mt Tarawera erupted in 1886, the eruption started on the mountain itself, says vulcanologist Associate Professor James White. But later a fissure extended into the geothermal area, producing a series of violent explosions that excavated the deep basin now filled by Lake Rotomahana, demolishing the world-famous Pink and White Terraces in the process.

White's PhD students are working closely with colleagues at the University of Wuerzburg, in Germany, to replicate in the laboratory the explosive effects of injecting water at various rates into magma, made from crushed volcanic rock re-melted at extreme temperatures, albeit on a smaller scale than Tarawera and company.

"The challenge is to understand why some explosions are so violent and some are not," White says. It would be very useful if scientists could be more specific about the potential magnitude of explosions when magma meets groundwater at certain depths.

An understanding of the pyrotechnic properties of magma when encountering various combinations of sediments and water could help geologists map hazard areas of well-known volcanic fields like those, for example, under Auckland, New Zealand's largest city.