Tufa

Tufa is a porous, lightweight variety of limestone made of calcium carbonate that precipitates from cool, calcium-rich fresh water around springs, streams, waterfalls and lakes. It is highly porous and often spongy or honeycombed, with a rough, irregular surface.

Tufa commonly forms around and over plants, moss, algae and other organic material, which it encrusts and sometimes preserves as molds and casts. It is generally white to cream or tan, soft, and chalky.

It should not be confused with tuff, a completely different volcanic rock; tufa is purely a chemical and biochemical carbonate deposit.

Tufa precipitates when groundwater rich in dissolved calcium bicarbonate emerges at springs or flows through streams and lakes. As carbon dioxide escapes from the water, or as plants and microbes remove it during photosynthesis, calcium carbonate becomes oversaturated and crystallizes out as calcite.

Much of this precipitation is biologically mediated by mosses, algae and bacteria, which provide surfaces and chemistry that promote growth. Spectacular tufa forms at places like Mono Lake in California, Pamukkale in Turkey, and Plitvice Lakes in Croatia. Tufa forms at ambient temperatures, distinguishing it from the warmer-water deposit travertine.

Tufa is recognized by being very porous, light and often spongy or full of plant-shaped holes, with a white to tan color. It fizzes vigorously in dilute acid, confirming its carbonate composition.

It is soft enough to scratch with a knife and may crumble. Its closest relative is travertine, which is denser, banded and forms from hot springs; tufa is more porous, irregular and cool-water formed. Unlike volcanic tuff (which does not react with acid and contains volcanic fragments), tufa effervesces strongly. The combination of high porosity, light weight, plant imprints and acid reaction is diagnostic.

Tufa has long been used as a lightweight, easily worked building stone; the Romans used it extensively, and it appears in churches, walls and aqueducts. Its porosity makes it light and a good insulator, though it is relatively soft and weathers over time.

Gardeners prize tufa for rock gardens and alpine plantings because its porous surface holds moisture and lets roots penetrate, making it ideal for growing alpines and mosses. It is also used decoratively in landscaping and aquariums, and tufa deposits are of interest to geologists and ecologists studying past climates.