A 9,000-year slumber ends
Before May 2008, the Chaitén volcano was considered dormant, having last erupted around 7420 BC. Located just 10 kilometers from the town of the same name, it was not monitored by Chile's geological survey. The situation changed. After a day of escalating earthquake swarms, Chaitén reawakened on May 2, 2008, sending a plume of ash and gas over 17 kilometers into the atmosphere. This initial explosive phase lasted for nearly two weeks. The eruption was the first major explosive eruption of rhyolite magma—a viscous, silica-rich type—since Novarupta's 1912 event in Alaska.
The eruption prompted the rapid evacuation of about 4,000 residents from the town of Chaitén and surrounding areas. Ash blanketed the region, spreading across Argentina to the Atlantic Ocean, contaminating water supplies and closing airports hundreds of kilometers away. Following the explosive bursts, heavy rains mixed with the thick ash deposits on the volcano's flanks. This created destructive volcanic mudflows, known as lahars. Beginning around May 12, these lahars inundated the town, filling the Chaitén River channel and carving a new path directly through the community. Buildings were buried in up to 1.5 meters of mud and sediment.
A dome rises
After the initial explosive phase, the eruption's nature shifted. By May 21, 2008, scientists observed a new lava dome forming in the caldera. This marked the beginning of an effusive phase, where thick, rhyolitic lava oozed from the vent rather than exploding. The dome's growth was exceptionally fast. For the first four months, the effusion rate averaged 45 cubic meters per second, among the highest rates ever measured for a silicic lava eruption. The lava, a phenocryst-poor obsidian and microcrystalline rhyolite with 75.3% SiO2, extruded at a high temperature, which contributed to its rapid ascent and effusion.
This event was an opportunity for volcanology. It was the first eruption of high-silica rhyolite to be monitored with modern scientific instruments, including satellite data and aerial photogrammetry. These tools allowed for detailed documentation of the dome's growth, which eventually reached a total volume of approximately 0.8 cubic kilometers. The dome complex grew to exceed the height of the original, pre-2008 dome inside the caldera. The growth involved both the surface extrusion of lava (exogenous growth) and internal expansion (endogenous growth), as well as the formation of a prominent, unstable spine that later collapsed. The government initially planned to relocate the town permanently to a new site 10 km away, but many residents resisted and eventually began rebuilding in the less-damaged northern part of the original town.