Micro-organisms and sinter
When hot springs overflow they often form layers of sinter – a rock made of very fine-grained silica – that takes the form of flats, terraces and mounds. Sinter terraces are one of the most distinctive features of geothermal areas, and provide evidence for past geothermal activity.
Sinter deposits are covered with a wide variety of complex textural features such as spicules (spike-like growths of silica) and mini-terraces. Their surfaces are also extensively colonised by micro-organisms, which in many geothermal areas show their presence by colouring the sinter. The coloration of the Pink Terraces, obliterated in the 1886 eruption of Mt Tarawera, was probably due to the presence of extensive growths of a pigmented thermophilic bacterium such as Thermus ruber.
How sinter forms
When high-temperature geothermal fluids reach the surface, they undergo drastic cooling. Much of the mineral material dissolved in these fluids can no longer remain in solution and begins to precipitate as the fluid cools.
The most common precipitate is amorphous silica. This is composed of silicon dioxide (SiO2), and has no regular crystal structure. Amorphous silica forms spheres so small they cannot be seen with a microscope. These spheres stick together to coat surfaces. They continue to increase in size, forming a continuous coating of silica much like a very thin layer of glass. Amorphous silica will coat any surface, including twigs, feathers, pine cones, newspaper, bottles and micro-organisms.
Other less common minerals that can be found in New Zealand deposits include calcite (calcium carbonate), gypsum (calcium sulfate), pyrite (iron sulfide) and other metal sulfides.
Sinter and the origin of life
There is continuing debate among scientists about the contribution of micro-organisms to the growth and development of the many textural varieties of sinter deposits. Detailed study of sinters has shown that they are composed of thin layers of chemically precipitated silica interleaved with silicified mats and clots of thermophilic organisms. Do the organisms actively encourage silica precipitation, or are they just passive recipients of the silica?
The resolution of this controversy is of great importance. The discovery of biologically generated (biogenic) sinter textures in the ancient fossil record of the earth could help us to understand the origin and evolution of life. In addition, the discovery of preserved hydrothermal deposits containing biogenic textures on extraterrestrial bodies may provide us with evidence of life beyond earth.