“Sillar” is the name used locally in Arequipa to designate the block of rock typically and historically used for construction in the city. It is due to its use as a building and ornamental material that Arequipa is known as the “White City.”
Geological studies have dubbed the sillar the “Arequipa Airport Ignimbrite” because this rock is found throughout almost the entire Arequipa Depression, on which most of the city is built, and most of the quarries are located near Rodríguez Ballón Airport. In geology, the term ignimbrite is used to describe a pyroclastic density current (pyroclastic flow) deposit containing abundant pumice fragments of various sizes, ash, crystals, and rock fragments torn from the conduit and ground that are ejected at high temperatures during highly explosive volcanic eruptions. When emplaced, this material is very hot, and when it cools, it can solidify, forming rock. At the time of emplacement, ignimbrites can reach distances of tens or hundreds of kilometers from their source.
The Arequipa Airport Ignimbrite, or “sillar,” was formed 1.65 million years ago by very violent explosive eruptions generated by a volcano that existed before the Chachani and Misti volcanoes. These explosive eruptions generated two clearly distinguishable units: the well-solidified white ignimbrite, which can be seen in the ravines, forming vertical walls, and the less compact pinkish (salmon) ignimbrite found above the white unit. The eruption of a large amount of magma (~18 km3) would have led to the formation of a caldera (a topographic depression resulting from the collapse of the roof of the magma chamber) in the area.
Approximately 1 million years ago, this caldera was covered by the eruptions that began to build the Chachani Volcanic Complex. Later, 112,000 years ago, the Misti volcano began to form, also building its volcanic edifice through several eruptions that accumulated lava deposits, pyroclastic flows, and so on, until it reached the conical shape seen today. Lahars and mudslides also covered the ignimbrite in the lower reaches; however, over time, water eroded these layers, forming the Añashuayco, Culebrillas, Chullo, and other ravines that descend from the volcanoes and cross different areas of the city.
The Arequipa Airport Ignimbrite, or “sillar,” can be found in an area exceeding 600 km2, in areas that currently comprise the districts of Yura, Cerro Colorado, Cayma, Yanahuara, Alto Selva Alegre, Sachaca, and Uchumayo. In this way, Ingemmet, through its Volcanological Observatory (OVI), develops research projects to understand the geological history of southern Peru, with the goal of understanding volcanic activity and generating scientific information for disaster risk management, protecting the population and infrastructure.
In addition, it has been working in cooperation with other institutions, such as the University of Clermont Auvergne (France) and UNESCO, to enhance geological knowledge in the Añashuayco quarries and promote geotourism through the “Geoheritage for Resilience to Geological Hazards” project.
Sillar Extraction
The sillar extraction process is one of the main cultural attractions of the Sillar Tourist Route in the Cerro Colorado district of Arequipa. You can observe the artisanal process of sillar extraction and carving, which uses ancient techniques that master stonemasons have learned from generation to generation.
First, the master ashlar cutter selects the primary block to be extracted from the quarry and then cuts it out. Then, once the ashlar block is on the ground, the master calculates how many blocks it can be divided into. They then locate the cutting points and divide it. Finally, they shape the block and remove any imperfections it may have.
If you decide to visit the Sillar Tourist Route, we recommend bringing the following:
Comfortable hiking clothing
Mosquito repellent
Sunglasses
Sunscreen
Bottled water
Building with Sillar
Asillar can be cut into various shapes and sizes during construction, as it is very flexible to fine carving with a chisel. There are two types of ashlar: fine-grained and more porous. The former is used for openings, lintels, and doorways, while the latter is used for foundations and walls.
Churches, convents, and mansions were built with ashlar during the Hispanic era. It allows for a fine finish on facades, vaults, capitals, columns, domes, and archways. Due to its flexibility in carving, figures of saints and virgins were created alongside themes from American flora and fauna, combined with heraldic symbols.
Don’t miss the carved arches of the cloister of the Society of Jesus, on General Moran Street, the Church of Cayma, and the entrance to the Casa del Moral—they’re truly impressive!