Tuff
Channel: 宇宙のすべての知識 プリンシピア
Date: 2019-03-19

Tuff, by Wikipedia https://en.wikipedia.org/wiki?curid=44481 / CC BY SA 3.0

#Tephra
#Tuff_cones
#Volcanic_rocks
Tuff

Tuff (from the Italian "tufo"), also known as volcanic tuff, 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 erroneously called "tufa", particularly when used as construction material, but properly speaking, tufa is a limestone precipitated from groundwater. 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 rock. 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. Magma commonly explodes 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. Chunks smaller than 2 mm in diameter (sand-sized or smaller) are called volcanic ash.

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, lumps of the old lavas and tuffs form the walls of the crater, 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 recrystal...