[2] This segment is 100 kilometres (62 mi) long and extends between the volcanoes Isluga in the north and Irruputuncu in the south.
[3] The lack of volcanism in the Pica gap appears to be due to unfavourable mantle flow patterns, which prevent the ascent of magma.
[4] Geologically, the Pica gap lies between two crustal domains which are defined by their divergent lead isotope ratios,[5] the northerly Arequipa-Antofalla block of Proterozoic age and the southern Chilenia terrane of Paleozoic age.
[6] Other phenomena which coincide with the Pica gap are the absence of tectonic horsts west of the Andes,[2] an area where a zone of high electrical conductivity in the deep crust beneath the forearc does not extend to the actual volcanic arc (and thus may reflect the absence of molten magma),[7] the lack of a low seismic velocity zone in the crust,[8] a symmetry plane of the Andes, the Salar de Uyuni basin behind the arc and the location of the actual bend in the Andes that is usually placed at the Arica Bend.
[3] The Cerro Porquesa lava dome and ignimbrite are the only volcanic centres with late Pleistocene activity in the area.