After yesterday's retrospective on the 30th anniversary of the eruption of El Chichón in México, I thought I'd follow up with a bit about what is going on at the today and what the future activity might be for the volcano. For volcanoes with long repose time between eruptions, it isn't too surprising how little has happened at El Chichón since 1982's events. Inputs of new magma might be widely spaced in time or accumulate slowly under such volcanoes, so after the 1982 eruption was declared over in September of the same year, the most obvious sign of heat beneath the volcano has come in the form of intense hydrothermal activity in the crater and the acidic crater lake (see above). The hydrothermal activity at El Chichón typically manifests itself as fumarolic activity in the crater lake area (see below) and the changing acidity of the crater lake (although this can also be affected by the changing volume of the lake.) Soon after the lake first formed, it was highly acidic and hot - upwards of 58ºC with a pH of 0.5 (1983). In the decades that followed, the lake has become less acidic, with a pH closer to 2-2.5 and the temperature has cooled to 25-30ºC. This might reflect the waning influence of the magma from the 1982 eruption. The changing volume of the lake reflects mostly input from a neutral pH and saline boiling spring that is likely rainwater that has percolated through the rocks at the crater and is being released in the spring. Overall, the heat and volcanic gases being released at El Chichón are thought to be rooted in the magmas from one of the older domes that were destroyed in the 1982 eruption and the residual magma from the 1982 events. There are also occasional earthquake swarms and tremor felt at El Chichon, again betraying the presence of magma under the volcano.
As for the future activity at El Chichón, the likelihood of another catastrophic explosive eruption in the near-term is relatively low, at least if the volcano follows its pattern of hundreds of years between major explosive eruptions. What is more likely is the growth of domes in the crater, much like the domes that filled the crater before they were destroyed during the 1982 events. There is some suggestion that monitoring the temperature and chemistry of the lake waters at the crater might hint at whether new magma for a dome eruption might be on its way, so regular monitoring of the crater lake is a must at El Chichón. Dome growth eruptions would likely be less destructive, however, as with any volcanic dome, the hazard of pyroclastic flows is high if the dome grows large and can gravitationally collapse (see Soufriere Hills in Montserrat for an example). Ash fall and lahars are also potentially significant hazards for the over 70,000 people who live within 40 km of the volcano, along with potential catastrophic release of the crater lake waters if the crater walls were to breached, an event that may have happened in the past, at least according to stories from the Chamula Indians. Overall, El Chichón might not be the most potentially hazardous and active in México today (those titles likely belong to Popocatépetl and Colima, respectively). However, as I mentioned yesterday, even volcanoes that appear to be tranquil and dormant can unleash a tremendous eruption.
