An eruption column rises above Guagua Pichincha volcano, Ecuador, minutes after an explosive eruption began in the summit crater (not visible from this view). The column rose to a height of  about 16.5 km above sea level. Ash fell in Quito soon after, forming a layer 1-3 mm thick in northern parts of the capital city. This explosion and a similar one two days earlier blasted away part of the volcano's summit lava dome and sent pyroclastic flows 4-5 km down the volcano's west flank. During the month of October, 53 explosions sent smaller columns of ash into the atmosphere, nearly double the number of explosions during pervious months.

During most of 1999, activity at Guagua Pichincha consisted of many steam-driven explosive eruptions and visible steam plumes that rose tens to hundreds of meters above the dome. The activity became potentially more hazardous on September 23 when magma erupted in the volcano's horseshoe-shaped summit crater. The magma added new material to the existing lava dome and led to stronger explosive activity and a few pyroclastic flows. In November and December, the growing dome produced more explosions, lava extrusions, and many rockfalls.