A direct conduit to the mantle
Villarrica is a 2,860-meter-tall stratovolcano with a persistent lava lake that has been active since at least 1986. This lake, typically 30 to 60 meters in diameter, sits within the volcano's 200-meter-wide summit crater. Its basaltic-andesitic lava maintains a temperature that has been measured between 1,149°C and 1,186°C. The volcano is known in the local Mapuche language as Rucapillán, meaning "great spirit's house."
The system is described as an "open conduit," which allows for continuous degassing. This steady release of volcanic gases like sulfur dioxide (SO₂), water vapor, and carbon dioxide prevents pressure from building to the point of a massive explosion. Instead, the volcano's typical activity consists of mild strombolian eruptions—small, intermittent bursts that eject incandescent lava fragments. The level of the lava lake itself fluctuates, sometimes rising high within the crater and other times subsiding to more than 80 meters below the rim. These fluctuations are closely tied to the intensity of gas emissions and seismic tremors.
Villarrica is one of Chile's most active volcanoes, with more than 74 documented eruptions since 1558. Major eruptions, like those in 1964 and 1971, have demonstrated the primary hazard associated with the volcano. The heat from lava flows can rapidly melt sections of the 40-square-kilometer summit glacier, generating dangerous volcanic mudflows called lahars that sweep down the mountain's flanks.
The Rucapillán natural laboratory
The constant activity and relative accessibility make Villarrica an exceptional site for volcanological study. Scientists use an array of instruments to monitor its behavior and understand the processes driving its persistent lava lake. Instruments like Multi-GAS analysers are used to measure the ratios of gases in the volcanic plume. Changes in the carbon dioxide to sulfur dioxide (CO₂/SO₂) ratio, for example, can signal the arrival of new, gas-rich magma from deep within the earth, can be a precursor to larger eruptions. In the months before the significant eruption on March 3, 2015, scientists observed the CO₂/SO₂ ratio increase from a baseline of 1-2 to as high as 9.
Seismic monitoring is also constant. A network of seismometers detects the harmonic tremors associated with the movement of magma and gas bubbles within the conduit. Infrasound sensors, which detect low-frequency sound waves inaudible to humans, track the bubble-bursting explosions at the lava lake surface. By measuring the time delay between a seismic signal and its corresponding infrasound signal, researchers can even calculate the depth of the lava lake within the crater. This continuous stream of data from different sources allows for real-time tracking of the volcano's dynamic inner workings.