Tom Osborne, of King's High School, asks :-
From how deep underground does the water thrown from the Rotorua geysers come?
Ron Keam, a geophysicist at the University of Auckland and the author and distributor of the award winning book 'Tarawera' about the 1886 volcanic eruption, responded.
Quite surprisingly, most of the water discharged is believed to have been rain water originally.
This is supported by the the fact that very small quantities of radioactive elements, which could have come only from nuclear bomb tests conducted in the atmosphere in the 1950s and 1960s, have been found in some of the water.
Gravity measurements indicate that the country beneath the New Zealand thermal region to depths of several kilometres consists of volcanic rock, much of which is fairly porous or else is cracked and creviced. The space comprising the pores and cracks is almost completely filled with water - dominantly at one time rain-water. Beneath the porous region lies hot and even molten rock. Heat from this conducts to the water-saturated porous region and sets up a convective circulation resembling the way that water in an electric jug convects above the hot electric element. It is the rising hot water that boils and discharges from the geysers. The circulation time depends on the precise water-flow path and can be anything from a few months to thousands of years.
A very tiny proportion of the water probably comes from much deeper, from the molten material.