Geologically, Sangay marks the southern boundary of the Northern Volcanic Zone, and its position straddling two major pieces of crust accounts for its high level of activity.

Sangay's approximately 500,000-year-old history is one of instability; two previous versions of the mountain were destroyed in massive flank collapses, evidence of which still litters its surroundings today.

Due to its remoteness, Sangay hosts a significant biological community with fauna such as the mountain tapir, giant otter, Andean cock-of-the-rock and king vulture.

Folklore sourced from native people in the region compared Sangay to a lighthouse surrounded by a sea, which represented the dense jungle.

[12] The principal rivers draining Sangay are the Llushin, Palora, and the Upano, each of which have numerous smaller tributaries on the middle and upper slopes.

Sangay is above a seismogenic tectonic slab about 130 km (80 mi) beneath Sabancaya, reflecting a sharp difference in the thermal character of the subducted oceanic crust, between older rock beneath southern Ecuador and Peru (dated more than 32 million years old), and younger rock under northern Ecuador and Colombia (dated less than 22 million years old).

Its oldest edifice, formed between 500,000 and 250,000 years ago, is evidenced today by a wide scattering of material opening to the east, defined by a crest about 4,000 m (13,120 ft) high, pockmarked by secondary ridges; it is thought to have been 15–16 km (9–10 mi) in diameter, with a summit 2 to 3 km (1 to 2 mi) southeast of the present summit.

The curved shape of the remnants of this first structure shows that it suffered a massive flank collapse, scattering the nearby forest lowlands with debris and causing a large part of its southern caldera wall to slide off the mountain, forming an embayment lower on its slopes.

This 400 m (1,312 ft) thick block, the best-preserved specimen of Sangay's early construction, consists of sequentially layered breccias, pyroclastic flows, and lahar deposits.

[17] Sangay currently forms an almost perfect glacier-capped cone 5,286 m (17,343 ft) high, with a 35° slope and a slight northeast-southwest tilt.

Sangay has been active in its current form for at least 14,000 years, and is still filling out the area left bare by its earlier incarnations, being smaller than either of them.

The earliest report of a historical eruption was in 1628;[4] ash fell as far away as Riobamba, located 50 km (31 mi) northwest of Sangay, and was severe enough to cover pastures and starve local livestock.

[4] Despite its activity, Sangay is located in a remote, uninhabited region; only a large Plinian eruption could threaten occupied areas 30–100 km (19–62 mi) to its west.

The area's isolation has allowed it to maintain a pristine ecology relatively untouched by human interaction, and the park hosts a biome ranging from alpine glaciers on the volcanoes' peaks to tropical forest on their flanks.

Altitude and rainfall are the most significant local factors affecting fauna, and therefore the lushest ecosystems are found on the wetter parts of the volcano's eastern slope.

Below this lies a zone of small trees and shrubs which develops into montane forest, principally in western valleys and on well-irrigated eastern slopes, which occurs below 3,750 m (12,303 ft).

In addition, the volcano is located in a remote region with poor roads and is difficult to access, and periods of heavy rainfall can flood rivers and cause landslides, rendering the mountain routes impassable.

Nonetheless, the Instituto Ecuatoriano Forestal y de Areas Naturales, which maintains an office near the mountain, facilitate such activities by providing local guides and rooms for rent to visitors.