The eruption was preceeded by an increase in fumarolic activity and a swarm of earthquakes on the 27th April which were sourced at ~4-6 km below the surface. Closer to the 4th the earthquakes began to increase in intensity and were also present at shallower levels - indicating the passage of magma up the conduit.
A minor eruption occurred in 1990 at Cordón-Caulle however a larger VEI3 eruption occurred in 1960. This has been attributed in various sources to the Valdivia earthquake of 1960 the most powerful earthquake ever recorded on the Earth at magnitude 9.5. The most recent (confirmed) eruption of the Puyehue volcano according to the Global Volcanism Program was in AD 860 (+/-75 years).
The Puyehue-Cordón Caulle is a volcanic chain which trends NW to SE (see below) with Puyehue at the most South-Easterly point and the Cordón Caulle fissure system stretching to the NW. The Cordón Caulle fissure system is the most geothermally active area in Chile.
The interesting thing about the Puyehue volcano is the diversity of its geochemical characteristics which range from Basaltic to Rhyolitic.