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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. == Volcanic ash == The material that is expelled in a volcanic eruption can be classified into three types: # Volcanic gases, a mixture made mostly of water vapour, carbon dioxide, and a sulfur compound (either sulfur dioxide, SO2, or hydrogen sulfide, H2S, depending on the temperature). #lava, the name of magma when it comes out to the surface. #and tephra, chunks of solid material of all shapes and sizes that come flying out of the volcano. Tephra is made when magma inside the volcano is blown apart by coming into contact with the 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 then fly away 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 magma and rock that have been tossed into the air by outbursts of steam and other gases. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「tuff」の詳細全文を読む スポンサード リンク
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