EGU Blogs


Geology Photo of the Week #42

This week’s photo is a beautiful example of geochemistry in action. Briefly, travertine, which is composed of CaCO3 is often precipitated at hot springs as they emerge from the ground forming these gorgeous terraces. The reason for their formation is Henry’s gas law in action which states “the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas above the liquid” which in this case is the open air. At depth, the superheated water contains lots of dissolved carbon dioxide, which lowers the water pH and allows it to dissolve carbonate minerals it comes in contact with. However, once the water reaches the surface the CO2 degasses because of the much lower partial pressure (concentration) of CO2 in the atmosphere à la Henry’s law. This causes the pH to rise which in turn leads to precipitation of dissolved carbonates such as aragonite and calcite (travertine).

By the way, I have always, always wanted to swim/lounge in one of these sorts of places but have never had the chance. Maybe one day!

Mammoth Hot Springs – travertine terrace – Credit: Joern Behrens (distributed via

Geology Photo of the Week #13 – Nov 25-Dec 1

So the 13th photo of the week is this sweet as pic of boiling mud from New Zealand’s North Island. Both photos were taken by me in July of 2009. I had to stand at the ready for quite a while to try and get good pictures of the bubbles bursting and there were many failed attempts. On the whole I think I did pretty well.

(Photo: Matt Herod)

(Photo: Matt Herod)

Hope you like them! By the way, stay tuned for some an awesome guest post later this week.