Siliceous rhizoliths and rhizocretions, which developed in diatomaceous sediments at Loop Road hot springs on the North Island of New Zealand, started to form while the plants were alive or shortly after their demise. The "porous" and "solid" laminae in the rhizoliths and rhizocretions incorporate well-preserved filamentous microbes, spores, and diatoms. The silica needed for the formation of these laminae came from the hydrothermal fluids that permeate through the sediments. Silica precipitation was triggered by degradation of the plant tissue, which caused a local increase in CO 2 and a decrease in pH. During the initial phase of rhizolith development, the plant tissues acted as a template for silica precipitation. The silica precipitated during these early phases provided the sites for later, more extensive silica precipitation.