The 3-kilometre (2 mi)-wide caldera was formed by the collapse of this dacite volcano following a catastrophic VEI-6 eruption about 800 years ago, which produced pyroclastic flows and lahars that reached the Pacific Ocean, and spread an airborne deposit of volcanic ash throughout the northern Andes.

[3] The caldera has since accumulated a 250-metre-deep (820 ft) crater lake, which has a greenish color as a result of dissolved minerals.

Fumaroles are found on the lake floor and hot springs occur on the eastern flank of the volcano.

Camping is permitted at the bottom of the crater, but there is no potable water (except half-litre bottles sold at the hostel)[4] The lake surface is located at 3,500 metres (11,483 ft) asl.

The total volume of water stored in Lake Quilotoa is 0.35 cubic kilometres (0.084 cu mi).