Tufa

    Tufa is the name for an unusual geological form of calcite rock.

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Tufa is a rough, thick, rock-like calcium carbonate deposit that forms by precipitation from bodies of water with a high dissolved calcium content. Tufa is not to be confused with tuff which is volcanic.

Tufa deposition occurs in seven known ways:

• Mechanical precipitation by wave action against the shore. This form of tufa can be useful for identifying the shoreline of extinct lakes (for example in the Lake Lahontan region).
• Precipitation from supersaturated hot spring water entering cooler lake water.
• Precipitation in lake bottom sediments which are fed by hot springs from below.
• Precipitation from calcium-bearing spring water in an alkaline lake rich in carbonates.
• Precipitation throughout the lake as the lake dries out.
• Through the agency of algae. Microbial influence is often vital to tufa precipitation.
• Precipitation from cold water springs (for example in the foothills of the Rocky Mountains near Hinton, Alberta)

There are some prominent towers of Tufa at Mono Lake and Trona Pinnacles in California, formed by fourth method mentioned above. Tufa is also common in Armenia and central-southern Italy (Campania, Lazio and Tuscany).  

Usage note: The rock type "tufa" is commonly confused in name by laypersons with the rock type "tuff", which is a rock formed from welded volcanic ash. These rocks are nothing like each other.

Practical uses

In the ancient world, tufa's relative softness meant that it was commonly used for construction where it was available. Tufa is common in Italy, and the Romans used it for many buildings and bridges. The Servian Wall, built to defend the city of Rome in the 4th century BC, is built almost entirely from tufa. The Romans also cut tufa into small rectangular stones that they used to create walls in a pattern known as opus reticulatum.

Tufa is today occasionally shaped into a planter. Its porous consistency makes tufa ideal for alpine gardens. A concrete mixture called hypertufa is used for similar purposes.

The Romans thought bees nested in tufa. The substance is mentioned in the Aeneid (Book XII, ln 805).