During the week Guagua Pichincha continued to display seismic unrest, typically with several earthquakes per day but including 16 long-period earthquakes on 26 April. During 23, 24, 26, and 27 April observers on the mountain typically smelled sulfur, but qualitative assessments of fumarolic output varied. In conditions of poor visibility, observers sometimes heard the sound of gases escaping in the crater. In overview, the volcano's activity has not entirely tapered off.

Geological Summary. Guagua Pichincha and the older Pleistocene Rucu Pichincha stratovolcanoes form a broad volcanic massif that rises immediately W of Ecuador's capital city, Quito. A lava dome grew at the head of a 6-km-wide scarp formed during a late-Pleistocene slope failure ~50,000 years ago. Subsequent late-Pleistocene and Holocene eruptions from the central vent consisted of explosive activity with pyroclastic flows accompanied by periodic growth and destruction of the lava dome. Many minor eruptions have been recorded since the mid-1500's; the largest took place in 1660, when ash fell over a 1,000 km radius and accumulated to 30 cm depth in Quito. Pyroclastic flows and surges also occurred, primarily to then W, and affected agricultural activity.

Sources: Washington Volcanic Ash Advisory Center (VAAC), Instituto Geofísico-Escuela Politécnica Nacional (IG)