Michael R, of King's High School, asks :-

How do hot mudpools form and work?

Ron Keam, a physicist at the University of Auckland and the author of the book "Tarawera", responded.

Thermal mud pools are found where steam and other thermal gases discharge in small depressions where rain-water tends to collect.

There are two fundamental conditions: first, the elevation must be higher than can be reached by the mineralised water that discharges in flowing thermal springs - in other words the fluid in the mud pool is `perched' and has no connection with the mineralised water; second, there must have been chemical interaction between the thermal gases, such as hydrogen sulphide, sulphur dioxide, or resulting acids such as sulphurous acid or sulphuric acid, and the ground, to result in the formation of clay minerals. The upflowing gases (including steam) mechanically churn through the wet chemically attacked ground, and over the course of a few decades produce the characteristic `plopping' features we know as mud pools. Rainwater is not absolutely essential because condensed steam can provide sufficient moisture to get the process started, but once there is a hollow formed rainwater will inevitably collect in it.

I have been watching the gradual growth and extension of a series of mud pools in the floor of Black Crater, Waimangu. These now occupy parts of a circle about 10 metres in diameter. In 1951 when I first examined this area there were no mud pools, only a small area (about 3 metres across) where vegetation was absent, and the activities have been gradually extending and becoming more intense during the intervening years.