The volcano was active within the last one million years, but not within recent times considering the heavy glacial erosion of the mountain and the widespread periglacial modifications.

[14] About 50 different volcanoes and geothermal features have been active in the Central Andes during the Holocene,[15] with earthquakes observed at Guallatiri, Irruputuncu, Isluga, Lascar, Olca, Parinacota and Putana.

[16] Most volcanoes of the Central Volcanic Zone (CVZ) are relatively poorly researched and many exceed 5,000 metres (16,000 ft) of elevation.

The volcano was the source of 30–90 metres (98–295 ft) thick lava flows that reached lengths of about 14–5 kilometres (8.7–3.1 mi)[29] and valleys occur all around it.

[8] Farther west lie the Cerros de Quimsachata which form a volcanic chain with Sillajhuay.

[10][30][8] Firn including penitentes occurs on the mountain at elevations of over 5,750 metres (18,860 ft)[31] and is visible over large distances[32] but there are no presently active, moving glaciers[31] unless they are buried beneath a snow cover.

[36] In the past during the Late Quaternary, the mountain was more extensively glaciated, with about nine glaciers surrounding it including a subsidiary summit to the south.

[43] Solifluction landforms and other surfaces generated by periglacial processes are common on the southern and northern-northwestern flanks of the massif.

During the Eocene-Oligocene Incaic deformation phase this basement was uplifted and eroded and subsequently covered by rhyolitic ignimbrites called the Utayane Formation.

Eventually large central volcanoes developed during the Miocene and Pliocene and are mostly uneroded and not affected by tectonic deformation.

[54] The regional geography is characterized by north–south trending mountain chains which are separated by relatively flat plains covered by Quaternary sediments.

[62] Tectonic stress during the subduction process has led to the development of a horst that Sillajhuay is part of, perpendicular to the main strike of the Andes[1] where magma formation was increased.

[61] Isotope ratios of volcanic rocks indicate a strong crustal influence on the magmas that were erupted at Sillajhuay.

[9] Grasses and shrubs with rare trees form the vegetation,[51] mostly on the eastern flank,[68] and reach elevations exceeding 5,200 metres (17,100 ft).

[68] The dry climate is caused by the South East Pacific High and compounded by the Humboldt Current off the coast, which cools the atmosphere and reduces evaporation.

[79] The whole volcano is considered to be of Pliocene-Pleistocene age, although the lack of detailed study precludes a precise dating of volcanic activity.

[80] Very young activity may have formed some gravel plains in the river valleys, when the heat from the eruption melted the permafrost of the summit region.