A geochemical cascade
The Petrifying Well of Knaresborough is a geological feature showing rapid mineral deposition. The process is not petrification, where organic material is replaced by minerals, but rather a rapid encrustation. Water sourced from an underground lake flows through an aquifer, dissolving minerals from gypsum and limestone deposits of the Permian period. Specifically, the spring water emerges from the Cadeby Formation dolomite but has high sulfate levels from the overlying Edlington Formation gypsum.
This water has a high concentration of dissolved calcium carbonate. When the spring water emerges and is exposed to the air, it releases carbon dioxide. This change in gas pressure reduces the water's ability to hold dissolved calcite, causing the minerals to precipitate out of the solution. The water deposits a thin layer of this mineral, a form of calcite called tufa, on any surface it touches. The process is fast enough to visibly coat objects in weeks. A small teddy bear can be covered in a hard, stone-like shell in just three to five months. Larger, non-porous items might take up to 18 months to be fully encased. The constant flow rate of 3,200 liters per hour ensures the process is continuous.