El Chichon, Mexico - 1982


El Chichon was the first major eruption to have its atmospheric effects studied in detailed by modern instruments. Although the volume of the eruption was small (< 1 cubic km of alkalic trachyandesite tephra, similar in volume to Mount St. Helens), El Chichon was also notable because it released an unusually large volume of aerosols (7 Mt of SO2 compared to 1.0 for Mount St. Helens). The ash contained up to 2 weight percent sulfate. Anhydrite (made of CaSO4) crystals were also in the deposits. Anhydrite is rare in volcanic rocks.

El Chichon had three Plinian eruptions, each sending gas and dust to the stratosphere. The eruption cloud was carried west, reaching the Philippines in 10 days and circling the globe at returning to Mexico in 20 days. Atmospheric circulation cells kept the cloud at about 30 degrees north latitude for more than six months after the eruption. All of the gaseous SO2 ejected into the stratosphere had been converted to sulfuric acid aerosol within six months.

LIDAR measurements showed that the atmosphere was 140 times more dense than after the Mount St. Helens

Balloon measurements discovered that a month after the eruption 20 million tonnes of sulfuric acid remained in the atmosphere. After one year, less than 8 million tonnes remained.

El Chichon produced some climate effects. The temperature of the stratosphere increased by 4 degrees C. This was caused by the absorption of some of the incoming solar radiation. This was the greatest increase since measurements began in 1958. Impact on ground temperatures is harder to quantify but temperatures in the Northern hemisphere may have been 0.2 degree C less about 2 months after the eruption.