Tuff

Tuff (from the Italian tufo) is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is compacted into a solid rock in a process called consolidation. Tuff is sometimes called tufa, particularly when used as construction material, although tufa also refers to a quite different rock. Rock that contains greater than 50% tuff is considered tuffaceous.

Tuff is a relatively soft rock, so it has been used for construction since ancient times. Since it is common in Italy the Romans used it often for construction. The Rapa Nui people used it to make most of the moai statues in Easter Island.

Tuff can be classified as either sedimentary or igneous rocks. They are usually studied in the context of igneous petrology, although they are sometimes described using sedimentological terms.

The material that is expelled in a volcanic eruption can be classified into three types:

Tephra is made when magma inside the volcano is blown apart by the rapid expansion of hot volcanic gases. It is common for magma to explode as the gas dissolved in it comes out of solution as the pressure decreases when it flows to the surface. These violent explosions produce solid chunks of material that can then fly from the volcano. When these chunks are smaller than 2 mm in diameter (sand-sized or smaller) they are called volcanic ash. It is made of small, slaggy pieces of lava and rock that have been tossed into the air by outbursts of steam and other gases.

Among the loose beds of ash that cover the slopes of many volcanoes, three classes of materials are represented. In addition to true ashes of the kind described above, there are lumps of the old lavas and tuffs forming the walls of the crater, etc., which have been torn away by the violent outbursts of steam, and pieces of sedimentary rocks from the deeper parts of the volcano that were dislodged by the rising lava and are often intensely baked and recrystallized by the heat to which they have been subjected.

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