The 1966 eruption of Izalco Volcano, El Salvador

During October–November 1966 900,000 m 3 of olivine basalt flowed from the flank of Izalco volcano, El Salvador. The total heat energy was approximately 10 15 calories. No measurable changes in gravity occurred at stations on the active cone between August 1964 and August 1967. In the summit crater fumaroles have surface temperatures as high as 540°C. The cooling rate of these fumaroles was 18°C/yr before the eruption and 45°C/yr after. Yearly temperature cycles due to wet and dry seasons are superimposed on the general cooling trend. The rate of gas emission at four fumaroles in November 1967 was 86 g/sec. The data from fumaroles and the volume of the flank eruption indicate that the volume of the high‐level magma storage beneath the crater was 3.8×10 6 metric tons before the eruption and 1.4×10 6 metric tons after. Four of the larger hot fumaroles contribute at least 10% of the heat loss from the high‐level magma storage, whereas heat conduction accounts for more than half the total loss.