# Properties of the Ignimbrites in the Architecture of the Historical Center of Arequipa, Peru
## 1 Introduction  
Arequipa ignimbrite began to be used in vaulted constructions in the 17 th century [1], although the city was founded in the middle of the 16 th century during the Spanish viceroy-alty. The city is surrounded by three volcanoes--Misti, Chachani and the extinct Pichu--in the central volcanic zone of the Andes in southwestern Peru. The abundance of white ignimbrite called \"sillar de Arequipa\" [2] made it possible to cut ashlars, voussoirs and pavement tiles measuring 40 \(\times\) 40 \(\times\) 20 cm\({}^{3}\), which were dimensions based on the Spanish yard (vara de Castilla, 0.835905 m). The characteristics of the carving of the church and stately house facades that represent the Hispanic American Baroque have been extensively studied, with its architecture identified by \"very white asilar stone\" [3] (Figure 1a,b). Genetically, the ignimbrites are associated with volcanic trends of variable petrological series in terms of their composition [4; 5; 6], which affects their petrophysical and mechanical properties. These properties are in turn conditioned by their degree of consolidation [7], which determines their suitability as construction materials [8; 9; 10]. Currently, areas of the now-dormant quarry fronts are open to tourists. The city is located in a seismically active zone, and earthquakes have caused significant damage for centuries; one of the oldest was recorded in 1471 [11], and the most recent of greater intensity in 2001. The last major reconstruction of the main monuments in the historical center lasted two decades after earthquakes in 1958 and 1960. In the second half of the 19 th century, with the arrival of the railway to the city, ignimbrite was used on flat roofs with ashlars between cast iron joists. Starting in the 1870s, the use of face-ashlar became popular, due to the attractive white color, without thepigmented lime paint traditionally used in cochineal red and indigo blue colors, especially in residential buildings. This article compares the differences of the white, beige and pink ignimbrites (Figure 1c,d) that were used in the historical center (declared a World Heritage Site by UNESCO in 2000), which require ongoing building conservation and restoration works. It can contribute to aging studies [8] and the application of treatments to eliminate deposits due to environmental contamination [12] on the surfaces of the pink and beige varieties, which have been little studied. There are several ignimbrite quarries, but the largest is in the Anashuayco stream and is also called AAI (Arequipia Airport Ignimbrite); others are RCI (Rio Chili Ignimbrite), YT (Yura Tuffs) and LJI (La Joya Ignimbrite) [6] (Figure 2a). The Arashuayco quarry to the south of the Chachani volcano is one of eleven streams with volcanic deposits [13] in the surroundings of the potentially active volcano with an altitude of 6057 masl, which is made up of several stratovolcano. AAI is composed of two units--white ignimbrite and pink ignimbrite--covered by some alluvial deposits on top (Figure 2b). The quarry location is set between UTM coordinates 8,192,365 North and 223,806 East at 2545 masl on the slope of the volcano and UTM 8,182,237 North and 214,495 East at 1969 masl [14], along 18 km at the SW direction down to the Uchumayo zone. Geomorphologically, they are planar beds or alluvial deposits that overlap each other, which means that, topographically, the oldest series in age are found higher up the volcano (white ignimbrite), close to the mouth or vent, and are overlapped by the younger series (pink ignimbrite). The average age of the white unit using fission track dating is 2.41 \(\pm\) 0.08 Ma [2] and 1.65 \(\pm\) 0.04 Ma (on biotite) for pink ones from the isotope ratios\({}^{40}\)Ar-\({}^{39}\)Ar [6]. Figure 1: (**a**) Carved white ignimbrite facade of La Companía de Jesus church, ca. 1698; (**b**) restored cloister after 1960 earthquake; (**c**) pink ignimbrite pilasters, corbels and moldings; (**d**) neogothic Bishop’s Palace (the latter two from the first quarter of the 20 th century).