Sillar is a variety of

rhyolite Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral ...

, which is a type of

volcanic rock Volcanic rock (often shortened to volcanics in scientific contexts) is a rock formed from lava erupted from a volcano. In other words, it differs from other igneous rock by being of volcanic origin. Like all rock types, the concept of volcanic r ...

. Although sillar is of rhyolitic composition, it has been erupted from

volcano A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often found where tectonic plates are ...

es which mostly erupt

andesite Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predomi ...

lava Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or un ...

, and sillar contains small fragments of andesite. A pink variety of sillar owes its colour to

crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...

s of

hematite Hematite (), also spelled as haematite, is a common iron oxide compound with the formula, Fe2O3 and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of . ...

within the rock. A white variety lacks these hematite crystals. Sillar is found as

pyroclastic flow A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud) is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that flows along the ground away from a volcano at average speeds of bu ...

deposits of

tuff 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 lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock cont ...

near volcanoes in southern

Peru , image_flag = Flag of Peru.svg , image_coat = Escudo nacional del Perú.svg , other_symbol = Great Seal of the State , other_symbol_type = Seal (emblem), National seal , national_motto = "Fi ...

, for example the now-extinct

Chachani Chachani is a volcanic group in southern Peru, northwest of the city of Arequipa. Part of the Central Volcanic Zone of the Andes, it is above sea level. It consists of several lava domes and individual volcanoes such as Nocarane, along with ...

volcano which erupted flows of sillar during the

Pleistocene The Pleistocene ( , often referred to as the ''Ice age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fina ...

epoch In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured. The moment of epoch is usually decided by ...

. Sillar

facies In geology, a facies ( , ; same pronunciation and spelling in the plural) is a body of rock with specified characteristics, which can be any observable attribute of rocks (such as their overall appearance, composition, or condition of formatio ...

Orvieto-Bagnoregio

Ignimbrite Ignimbrite is a type of volcanic rock, consisting of hardened tuff. Ignimbrites form from the deposits of pyroclastic flows, which are a hot suspension of particles and gases flowing rapidly from a volcano, driven by being denser than the surrou ...

(black blocks of

scoria Scoria is a pyroclastic, highly vesicular, dark-colored volcanic rock that was ejected from a volcano as a molten blob and cooled in the air to form discrete grains or clasts.Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, eds. (2005) ''G ...

in red tuff) occurs at

Civita di Bagnoregio Civita di Bagnoregio is a town in the Province of Viterbo in central Italy, a ''suburb'' of the comune of Bagnoregio, east from it. It is about north of Rome. The only access is a footbridge from the nearby town, with a toll introduced in 20 ...

in the

Vulsini Vulsini, also known as Volsini volcano, Vulsini Volcanic District, Vulsini Volcanic Complex and the Vulsinian District, is a circular region of intrusive igneous rock in Lazio, Italy, about to the north northwest of Rome, containing a cluster ...

volcanic district of central Italy.Palladino D.M. et al. (2014) ''Bolsena, Latera and Vico calderas, central Italy'', IAVCEI Field Guide Series 3, page 29

Building stone

Sillar has been used as a building stone in Peru. Many colonial buildings in the city of

Arequipa Arequipa (; Aymara and qu, Ariqipa) is a city and capital of province and the eponymous department of Peru. It is the seat of the Constitutional Court of Peru and often dubbed the "legal capital of Peru". It is the second most populated city ...

are made of sillar, for example, the arches of the "Mirador of Yanahuara" in Arequipa, from which the entire city can be viewed.

References

Composition of sillar
from Maricopa Center for Learning and Instruction

Volcanic rocks Building stone Architecture in Peru {{Petrology-stub es:Toba volcánica#Sillar de Arequipa

Building stone

See also

References