The Old Cone was active over one hundred thousand years ago and was eventually truncated by a giant landslide that removed its northwestern side.

San Pedro is located in northern Chile, in the Ollagüe commune, El Loa Province, Antofagasta Region;[4] the border to Bolivia is 35 kilometres (22 mi) away.

[5] The whole region is remote and thinly populated;[6] towns in the area include Ascotán, Cupo, Inacaliri and Paniri;[7] and the San Pedro railway station lies southwest of the volcano.

[9] A number of lava flows of volumes between 0.1–1.7 cubic kilometres (0.024–0.408 cu mi) make up part of the edifice.

[23] The upper part of the Younger Cone is covered with such lava flows, which have steep fronts[2] and are up to 100 metres (330 ft) thick.

[26] Scars at elevations of 5,500 metres (18,000 ft) on the edifice have been the origin of mudflows; the largest is known as Estación flow and covers much of the southern and western foot of San Pedro to distances of 30 kilometres (19 mi);[21] the debris apron reaches the Loa River and San Pedro River.

[28] Hot avalanche deposits or pyroclastic flows are also found on the volcano; the largest of which covers large parts of the edifice and has a total volume of approximately 1.5 cubic kilometres (0.36 cu mi).

[29] Their formation was probably due to the steep incline of San Pedro's slopes which tended to destabilize lava bodies high on the edifice,[30] similar to regular landslides.

[32] This Plinian eruption was accompanied by the formation of an ignimbrite that covers much of the southern, southwestern and western slopes of San Pedro and reaches thicknesses of 3 metres (9.8 ft).

[21] This avalanche deposit covers a surface area of 120 square kilometres (46 sq mi) and its front is 35–40 metres (115–131 ft) high[5] in the Pampa de la Avestruz.

A 250 metres (820 ft) high scarp west of the eastern summit was left by the collapse of San Pedro; otherwise much of the evidence was buried by later volcanic activity.

[21] As with the mudflows, the steep slopes of San Pedro probably facilitated the onset of the sector collapse,[35] which descended over an elevation difference of about 2,845 metres (9,334 ft).

[36] A parasitic vent named La Poruña lies on the western foot of San Pedro,[8] its formation was probably influenced by a normal fault that runs in that area.

[39] In addition, a lava dome at an elevation of 5,000 metres (16,000 ft) lies on the southwestern flank of San Pedro and also seems to be a parasitic vent.

Evidence of such glaciation is found especially on the southern side of the Old Cone and it includes moraines at elevations of over 4,400 metres (14,400 ft) as well as other glacially modified surfaces such as rock pavements and striated boulders.

[47] In the San Pedro region, this volcanic activity migrated east from its point of inception but recently has moved back westward.

[14] West of San Pedro lies the Pampa del Tamarugal and the Coastal Cordillera, neither of which show evidence of recent volcanic activity.

However, in the area of San Pedro this basement is entirely buried beneath volcanites of Miocene age, including ignimbrites, remnants of composite volcanoes[14] and volcanic debris.

[5] After activity of the Old Cone ceased, glacial and fluvial erosion dissected San Pedro until the large sector collapse occurred.

The so-called "white airfall" with a volume of 2.5 cubic kilometres (0.60 cu mi) as well as pyroclastic flows on the western summit may have been erupted during the collapse but this is uncertain.

[28] Radiometric and surface-exposure dating has yielded ages of between 110,000 and 56,000 years ago for La Poruña;[62] it was previously proposed that this cone formed in the late 19th century.

[68] The entry of mafic magma at depth about 100,000 years ago may have stimulated the occurrence of eruptions not only at San Pedro but also at neighbouring volcanoes.

[73] Presently, fumaroles in the eastern summit crater[22] are the only ongoing activity at San Pedro; a steam plume is barely visible.

The first is the formation of long debris avalanches or pyroclastic flows that could reach the International Route CH-21 [es] which is just 9 kilometres (5.6 mi) away from the volcano.