Explaining the Behaviour of Hot Springs on the Sea-Floor

One of the long-standing mysteries of the sea-floor has been solved. In this week's Nature, scientists from the University of Cambridge set out a model which shows why hot springs occurring deep beneath the ocean can never reach temperatures of more than 400 degrees centigrade.

For over twenty years, scientists have been observing the behaviour of "black smokers" - hot springs of mineral-rich water that spout from the seafloor at over 350 degrees.

This may seem very hot, but it struck researchers as being remarkably cold. The heat for black smokers comes from solidifying rock deep in the seafloor - a process which occurs at 1200 degrees. But black smokers always seem to be cooler than 400 degrees - a mystifying 800 centigrade cooler than their power source.

A black smoker, about the size of a domestic chimney, spurts particle-laden water through a hole of about 10cm across.

Now Tim Jupp and Adam Schultz, of the Institute of Theoretical Geophysics at the Earth Sciences Department, have found an explanation. They have produced a model which shows that the thermodynamic properties of water limit the temperature of the vent water to 400 degrees, regardless of the temperature of the heat source. This reason is that water at 400 degrees carries the most energy: if the temperature rises beyond this, the transfer of energy will actually be less efficient.

The model also predicts the temperatures in a cross-section of the seafloor. Temperatures vary according to the depth beneath the seafloor, but the flow pattern ensures that water above 400 degrees is trapped at the base of the system. This also fits with the existing empirical evidence.

Researcher Tim Jupp commented, "Our work confirms what researchers have long suspected: that the temperature of black smokers can be explained simply by the thermodynamic properties of water itself. After years of speculation, we finally have a quantitative explanation for this phenomenon."